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Old January 10th, 2014, 01:29 AM   #1
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Default REALLY GOOD THEORY that SMELL is the FIRST SENSE?

A looongie but goodie. I know I didn't have the stamina to read through it when Jasmin originally assigned it to me as essential reading, but nevertheless it's really really good!

Check the link here: Smell - Neurobiology of Sensation and Reward - NCBI Bookshelf

All credit given where credit is due. We thank the authors for their exhaustive work on the subject and post it here in hopes to raise awareness for this work. --JSLV


Chapter 5 Smell

Jay A. Gottfried and Donald A. Wilson.

To speak generally then, things that have been cooked, delicate things, and things which are least of an earthy nature have a good odour, (odour being a matter of exhalation), and it is obvious that those of an opposite character have an evil odour.
Theophrastus c. 300 BC
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5.1 INTRODUCTION

Smell is arguably the most ancient sense. Consider for example a bacterial prokaryote, monocellular, anuclear, flagella-rotating, as it tumbled through the Archaean (Archaeozoic) seas roughly 2?3 billion years ago. For such an organism, chemotaxis?the ability to redirect its movements in the presence of chemical gradients?was synonymous with survival. This most rudimentary sense of smell was critical for finding chemoattractants like organic nutrients (Figure 5.1a; Adler 1969) and evading chemorepellents like excreted waste (Figure 5.1b; Tso and Adler 1974; Adler 1978). In this manner, the sense of smell was rooted at the earliest evolutionary stages with the machinery of affective processing, to the extent that chemical sensing was indistinguishable from the sensing of biological imperatives.
FIGURE 5.1

Olfactory hedonics in bacteria. (a) Chemosensory attraction. E. coli bacteria are using chemo-reception to migrate towards the open end of a capillary tube that has been filled with aspartate, an amino acid with nutritive value (Adler 1969). (Reprinted (more...)


Interestingly, the key features of bacterial chemotaxis presage many of the same principles guiding olfactory processes in higher animals (Koshland 1980; Kleene 1986; Baker, Wolanin, and Stock 2006). First, the bacterial chemoreceptor complex is highly localized into clusters at the cell poles (Figure 5.2; Maddock and Shapiro 1993). This anatomical polarity effectively segregates sensory detection into spatially discrete regions of the organism, an arrangement that reaches its full eukaryotic expression in the form of antennae, noses, and olfactory epithelia.
FIGURE 5.2

A primordial nose. Immunoelectron microscopy shows that the chemoreceptor complex of E. coli bacteria is clustered at a discrete polar location of the cell?s inner membrane (arrow) (Maddock and Shapiro 1993). This cellular sequestration of chemoreceptive (more...)


Second, bacteria adapt to their chemical environment. Directional movement through a chemical gradient relies on concentration changes, rather than on absolute concentration per se, so that chemotaxis ceases once the bacterium finds itself in an isotropic solution. Reduced responsiveness to an unvarying chemical background closely parallels sensory adaptation in the mammalian olfactory system, which, as described later in this chapter, is an important mechanism underlying odor discrimination.
Third, bacteria learn from experience. Wild-type Escherichia coli show exuberant chemotaxis toward maltose when grown in a medium containing maltose, but respond minimally to the same attractant when grown in a maltose-free medium (Adler, Hazelbauer, and Dahl 1973). As a basic example of sensory-specific plasticity, this experience-dependent gain in chemical sensitivity is also instantiated in vertebrate and invertebrate olfaction, whereby learning and experience robustly modify how smells are perceived. Remarkably, human neonates respond selectively to the odor of familiar (vs. unfamiliar) amniotic fluid (Schaal, Marlier, and Soussignan 1998) and they prefer food flavors that had been experienced prenatally by way of maternal consumption (Mennella, Jagnow, and Beauchamp 2001). That human fetuses ?grown? in carrot-containing amniotic fluid are attracted to carrot flavor postnatally (Mennella, Jagnow, and Beauchamp 2001) curiously mirrors the growth-dependent inducibility of chemotaxis in prokaryote species. The role of experience in modulating odor coding is addressed in detail in Section 5.4.
Fourth, bacterial chemotaxis is synthetic. In other words, the decision to swim or tumble is based on the integration of chemical inputs. Simultaneous exposure to two different chemicals often elicits chemotactic behaviors that cannot be simply predicted from the mere sum of the individual components (Rubik and Koshland 1978). Even in Salmonella typhimurium, which contains a humble armament of five chemoreceptor genes (Wadhams and Armitage 2004; Hazelbauer, Falke, and Parkinson 2008), response potentiation (supra-additivity) to a combination of chemoattractants can be observed. The idea that behavioral non-linearities emerge in the presence of chemical mixtures has interesting implications for odor quality perception, multisensory integration, and flavor processing in rodents and humans, topics that are examined in Section 5.4, and throughout the sensory chapters of this book.
Fifth, and last, bacteria have a memory for smells, albeit on an extremely short timescale. The very fact that a 2-?m bacterium successfully swims through a chemical gradient to pinpoint a food source suggests that it must be able to retain a memory for what just came before (Macnab and Koshland 1972). Without the means to compare chemical concentrations from adjacent time frames (i.e., temporal integration), the bacterium would soon founder. This feature is not meant to imply that the olfactory systems of ?higher? animals have adopted the same molecular cellular strategies to remember smells. Rather the point is that already from a very primordial moment in animal evolution, the same functional strategies were in place to maximize contact with behaviorally beneficial, or rewarding, chemical stimuli.
It is thus perhaps no accident that the human sense of smell remains so closely aligned with emotional processes, anatomically, physiologically, and psychologically. To our predecessors the bacteria, chemical sensation was fundamentally intertwined with the acquisition of rewards and the avoidance of threats. The operating principles by which bacteria achieved better living through chemistry?anatomical segregation of chemical detectors, response adaptation to static environments, sensory plasticity, signal integration, and chemical memory?continue to define and constrain the ways that the olfactory systems of higher organisms, including humans, contend with the odor landscape.
An olfactory enthusiast might plausibly go so far as to state that all biological subsystems are essentially a spin-off of the chemical detector apparatus. Since Buck and Axel first identified a large multigene family of G-protein-coupled receptor genes on rat olfactory sensory neurons (OSNs) (Buck and Axel 1991), it has become apparent that the G-protein-coupled conformation of the olfactory receptor shares much in common with receptors that bind a great many other critical biological ligands, including neurotransmitters, neuromodulators, tastants, light, hormones, growth factors, chemokines, cell adhesion molecules, and chemotactic peptides (Dryer 2000; Fredriksson et al. 2003). Laurence Dryer (2000) has nicely captured the idea that olfaction is a virtual microcosm of the human nervous system:
This functional and structural diversity [within the olfactory receptor gene family] is not surprising if one bears in mind that there is no essential difference between neurotransmitter binding and odorant detection. Whether they are expressed at the synapse or in sensory cells, chemoreceptors serve the same function, that is, the chemical detection of a ligand carried by extracellular or external space.
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5.2 THE ODOR STIMULUS SHAPES THE OLFACTORY SYSTEM

If a sensory system is to extract meaning from a sensory stimulus, what is the natural form of that stimulus in the real-world environment that a brain is likely to confront? For the olfactory system, a general answer to this question would be to state that olfactory percepts arise from physical and chemical properties of odor molecules, just as visual percepts arise from wavelengths of light. * This is correct, though troublingly vague, and indeed much of our current scientific understanding of olfactory systems in insects, rodents, and primates stems from research using monomolecular odorants (L-carvone, amyl acetate, etc.) with well-defined physical features, helping to ensure experimental control over the sensory input and to promote experimental replicability across different laboratories and species.
But it is critical to consider that the ecological context of an odor source defines what type of odor signal will be transmitted to a receiver. (See Dusenbery 1992 for an extensive and insightful overview of the sensory ecology of chemical signals, as well as thermal, light, sound, and mechanical signals.) The important biological implication is that, as an outcome of natural selection, how an odor is realistically encountered in the environment will ultimately shape how an olfactory nervous system evolves to optimize feature detection (Gottfried 2009).
The first point to keep in mind is that most real-world odors are not monomolecular compounds courtesy of the Sigma-Aldrich Flavors & Fragrances catalog. Single-molecule, single-meaning odorants are extremely rare in nature. The majority of emitted odors making contact with a chemical biosensor are mixtures of tens to hundreds of different odorant molecules. For example, microwave-popped popcorn contains at least 56 distinct volatile compounds (Walradt, Lindsay, and Libbey 1970), pressure-cooked pork liver liberates 179 volatile components (Mussinan and Walradt 1974), and the smell of chocolate is a blend of over 600 chemical constituents (Counet et al. 2002). Even the release of a potent moth sex attractant in the wild is often packaged together with many additional volatile components (Linn, Campbell, and Roelofs 1987; Dusenbery 1992). The compound nature of most environmental odors suggests that an olfactory system with the ability to encode information about the overall configuration of an emitted chemical stimulus would have a sensory discriminatory advantage. This issue is discussed at length in the final section of this chapter.
The second point is that odors are highly subject to the whims of their natural settings. The same odor may be experienced in the context of different background smells, potentially distorting how the odor is perceived. The direction or strength of the wind may make all the difference to an organism using airborne odor cues to locate its lunch. The length of time that has elapsed since the origination of an odor at its source will determine whether that odor cue still contains reliable predictive information, for example, if the odor source has run away (in the case of an animal), washed away (in the case of rain), or chemically transformed (in the case of a decomposing fruit). Each of these ecological scenarios means that an animal may have partial or corrupted access to an olfactory signal. Therefore, an olfactory system that reconstructs odor meaning?i.e., species, gender, sexual receptive state, social dominance, edibility?from sensory fragments would provide maximal adaptability to an organism.
The last point is that earthly transmission of an odor message markedly differs from the transmission of the other distance senses. In vision and audition, spatial patterns in the external world provide important sources of information that the brain can sample, extract, and codify in the form of spatial patterns of peripheral receptor activation. Critically, it is the spatial fidelity of visual and auditory stimuli?all the way from point source to receiver?that makes possible the use of spatial ?codes? within these sensory modalities. However, in contrast to sights and sounds, smells diffuse rather less predictably and less quickly from their sources. As mentioned above, air currents rapidly shear an odor message away from its spatial site of origin. Indeed, this carries certain advantages: olfactory signals can defy mundane physical obstacles, such as trees, bushes, hilltops, that would quickly extinguish a visual signal unable to bend around these foes. Thus, with the possible exception of localizing smells to one nostril or the other (Wilson 1997; Porter et al. 2007), spatial information is not a feature with which the olfactory system has evolved to contend. It is true that, as described in the next section, odorant-specific spatial patterns of neural activity are evoked within the olfactory bulb, but these patterns probably have more to do with encoding of odor identity, as opposed to odor space per se. Actually the spatial organization of receptor projections and bulb activity patterns are lost en route to olfactory cortex, and at this level temporal codes may be more critical for information processing (Haberly 2001).
In concluding this section, a proviso is in order. Tremendous advances in our neuroscientific understanding of the visual and auditory systems have been possible because the fundamental physical ?primitives? underlying visual and auditory processing have been precisely identified. That is, receptive fields of color in vision or pitch in audition can be mapped precisely by varying the wavelength of light or sound along a linear dimension. * However, a precise characterization of odorant receptive fields remains elusive, given that the specific metric along which olfactory space is measured is not clear. For example, the physicochemical dimensions along which piperonyl acetate or 4-hydroxy-3-methoxy-benzaldehyde lie are still rather obscure; of equal perplexity is the lack of consensus regarding the perceptual dimensions along which the smell of cherry or vanilla lie. In spite of these uncertainties, much contemporary olfactory research, with modest successes, has placed an emphasis on hydrocarbon chain length and chemical functional group as two putative dimensions of odorant space, which will become evident from the following discussion.

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5.3 THE OLFACTORY SYSTEM: ANATOMY AND PHYSIOLOGY

The olfactory system shares many functional properties with its counterpart sensory systems, yet is organizationally unique among mammalian sensory pathways. Broadly speaking, the functional anatomy of all sensory systems begins with an analysis of complex information streams into component features, local circuit enhancement of contrasts between features, and subsequent synthesis of those features into perceptual objects. Especially for complex organisms, the neurobiological assembly of sensory inputs into sensory ?outputs? (i.e., perceptually meaningful objects) is optimized when sensory systems have an opportunity to integrate ascending and descending information flow, contextual and state-dependent modulatory effects, and multisensory interactions. Current evidence suggests that olfaction shares these functional properties with each of the other sensory systems.
A canonical mammalian sensory nervous system includes peripheral receptors, initial local circuit processing (e.g., retinal bipolar cells, auditory brainstem nuclei, dorsal root ganglia), a thalamic nucleus processing center, and a primary sensory cortex with reciprocal corticothalamic connections. Higher-order cortical areas may be specialized for specific, behaviorally relevant information content within one sensory modality, for example, faces, speech, or pain. Many sensory systems include parallel processing streams, wherein different aspects of the same stimulus are simultaneously captured and processed through anatomically distinct subsystems. Finally, it is becoming apparent that the opportunity for multimodal sensory convergence occurs at least as early as primary sensory cortex, an area traditionally considered to be a unimodal processing stream (e.g., Lakatos et al. 2007). Again, olfaction shares many of these anatomical characteristics with the other sensory systems, but there are important differences, as outlined below.
The anatomy of the initial stages of the olfactory pathway suggests a hierarchical, combinatorial processing of odorants, with initial feature extraction in the periphery and subsequent convergence and blending of features through subsequent cortical stages. The following sections trace the encoding and transfiguration of an odor message as it moves through the olfactory system, beginning with the nose, olfactory epithelium, and OSNs, then pausing at the olfactory bulb, glomeruli, and mitral and tufted cells (the second-order neurons), and finishing at the olfactory cortex, including piriform cortex, and higher-order brain regions. The reader is referred to many comprehensive reviews (Price 1990; Carmichael, Clugnet, and Price 1994; Shipley and Ennis 1996; Haberly 1998; Cleland, Linster, and Doty 2003; Wilson, Sullivan, and Doty 2003; Zelano and Sobel 2005; Gottfried 2006; Gottfried, Small, and Zald 2006a) for in-depth discussions of olfactory system anatomy and physiology across different species.
5.3.1 Peripheral Considerations

The mammalian olfactory system begins at the olfactory receptor sheet, where it lines the deeper recesses along the nasal passage. The sheltered location of the olfactory epithelium means that stimulus sampling is tied to airflow, if not absolutely dependent on it: in the ?orthonasal? direction, inspiration draws in odorous molecules (odorants) from the outside environment and across the receptor layer; in the ?retronasal? direction, expiration forces out odorant molecules from the inside environment (typically, food-based substances residing in the oral cavity) across the receptor layer. Thus, as Rozin (1982) has argued, olfaction is the only dual sensory system, operating both as a distance sense and as a contact sense.
The route of stimulus access can affect stimulus perception (e.g., King et al. 2006) and central brain circuit activation (Small et al. 2005). Detection thresholds and perceptual qualities for a given odorant are both dependent on the pathway by which that stimulus is delivered. These route-specific differences may arise because of variations in stimulus access to different zones within the olfactory receptor sheet (Scott and Brierley 1999; Scott et al. 2007) or because of gas chromatographic effects as the odorant molecules diffuse through the mucus overlying the sensory receptor cilia (Mozell 1991). Odor pleasantness, irritancy, and intensity, as well as state effects related to arousal and attention, can influence sniffing rate and volume (Mainland and Sobel 2006; Wesson et al. 2008), with further potential for modulating olfactory perception. Odor sampling behavior itself has an impact on olfactory coding: high-frequency sniffing vs. low-frequency passive respiration has been shown to filter out static background odors, enhancing an animal?s ability to detect changes in the olfactory landscape (Verhagen et al. 2007).

5.3.2 Contact: Olfactory Sensory Neurons

Initial contact between an odor stimulus and the olfactory system occurs at the receptor endings of OSNs. Each OSN expresses one specific G-protein-coupled receptor gene (Buck and Axel 1991), of which there are approximately 1000 in rodents, 350 in humans, and 100 in fish (Firestein 2001). These receptors are studded along 20?30 long dendritic cilia that poke into the overlying mucus of the olfactory epithelium. Interestingly, OSNs expressing the same receptor gene do not segregate together within the epithelium, but instead are widely dispersed throughout different epithelial zones (Ressler, Sullivan, and Buck 1993). The lack of a systematic topographical organization within the olfactory receptor sheet already marks a fundamental difference between olfaction and the other distance senses (vision and audition).
The G-protein-coupled receptors may play a role not only in stimulus binding on the receptor cilia, but also in helping target the sensory neuron axons to discrete locations in the olfactory bulb called glomeruli (Vassalli et al. 2002). It is important to bear in mind that olfactory receptors do not bind and transduce ?smells? (e.g., banana, rose), or even entire molecules (e.g., isoamyl acetate, phenyl ethyl alcohol), but sub-molecular moieties (Araneda, Kini, and Firestein 2000). Furthermore, an individual odorant molecule can serve as a full agonist of an olfactory receptor, a partial agonist, or even an antagonist (Gentilcore and Derby 1998). Thus, some interaction between odorant molecules occurs at the receptor sheet.
Once bound to receptor proteins, odorants evoke neural activity in OSNs, which transmit that activity into the central nervous system. Like neurons in other sensory systems, OSNs and their central targets respond to only a subset of all possible odorants (Duchamp-Viret, Chaput, and Duchamp 1999; Malnic et al. 1999), which is termed their molecular receptive range or, in the vernacular of sensory neuroscience, their odorant receptive field. The receptor ligand binding site on the OSN appears to be selective for molecular constituents of an odorant. In consequence, a given monomolecular odorant may bind to multiple different odorant receptors and a given receptor may bind multiple different odorants. Therefore, a natural scent, potentially composed of many different monomolecular odorants, will activate a unique combination of OSNs. Despite these guiding principles, recent work suggests that, at least in Drosophila, some individual receptor neurons may exhibit a highly narrow tuning profile for biologically salient odors such as pheromones (Schlief and Wilson 2007).
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Old January 10th, 2014, 01:30 AM   #2
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Default Re: REALLY GOOD THEORY that SMELL is the FIRST SENSE?

5.3.3 First Synapse: Olfactory Bulb Glomerulus

The OSNs send afferent projections directly into the central nervous system, terminating in a fore-brain structure called the olfactory bulb. Olfactory glomeruli, lying near the surface of the olfactory bulb, are the principal functional units of information exchange between first-order neurons (OSNs) and second-order neurons (mitral and tufted cells). Individual glomeruli receive axons from OSNs all expressing the same receptor protein, which in turn form excitatory synapses onto mitral/tufted cell dendrites (see below). In the rodent olfactory system, the axons from several thousand OSNs (of the same receptor type) project to just one or two glomeruli (Firestein 2001) and a single mitral or tufted cell innervates just one glomerulus. This architecture ensures that each second-order neuron is targeted by a homogenous population of OSNs, providing an opportunity for dense anatomical convergence. On the other hand, in humans, with only 350?400 different olfactory receptor genes, this pattern breaks down, as there may be more than 10,000 glomeruli (Maresh et al. 2008). Nonetheless, given the homogenous receptor input to a given glomerulus, odorant stimulation evokes a stimulus-specific, combinatorial spatial pattern of glomerular activation across the olfactory bulb.
While there is debate over whether the glomerular spatial patterns of sensory input of odor-induced activity constitute a veridical map of odor perception (Laurent 1997), regional variation within the bulb in response to different molecular functional groups (e.g., alcohols or aldehydes; Johnson and Leon 2007) is certainly evident (though this chemotopy may not be preserved on a finer scale; Soucy et al. 2009). Furthermore, there may be regions that are more responsive to intrinsically meaningful odors (fox urine) and others more responsive to novel odors (Schaefer, Young, and Restrepo 2001; Kobayakawa et al. 2007). Recent work suggests that intact sensory input to the dorsal olfactory bulb is required for the expression of fear-based behavioral responses to innately aversive odors, whereas the ventral bulb appears sufficient to support learned odor aversions (Kobayakawa et al. 2007). This functional dissociation between innate and learned odors may be associated with differential output projections, and thus could comprise an important avenue of parallel processing.

5.3.4 Olfactory Bulb Output: Mitral and Tufted Cells

Mitral and tufted cells comprise the second-order olfactory neurons, whose cell bodies reside deep in the glomerular layer of the olfactory bulb. The dendrites arising from approximately 5?25 mitral/ tufted cells typically innervate a single glomerulus (Firestein 2001), where they receive excitatory input from the OSN axons, as noted above. Given the precise spatial organization of odorant molecular information at the input (OSN) side of the glomerulus (Rubin and Katz 1999; Wachowiak et al. 2000; Johnson and Leon 2007), it is not surprising that the output side also shows a high degree of spatial patterning for odorant molecular features, with, for example, mitral/tufted cells in dorsomedial olfactory bulb responsive to aldehydes (Imamura, Mataga, and Mori 1992) and cells in ventrolateral bulb responsive to aromatic (benzene-like) compounds (Katoh et al. 1993). Generally speaking, mitral and tufted cells are broadly responsive, or ?tuned,? to the presence of specific chain lengths or functional groups (Luo and Katz 2001; Fletcher and Wilson 2003; Igarashi and Mori 2005; Tan et al. 2010) with a specificity that resembles the receptive fields of the OSNs synapsing on them.
The hierarchical elaboration of odorant receptive fields from sensory neurons to central neurons shows broad similarities to processing hierarchies in the visual system. A few studies comparing odorant receptive fields across a variety of central olfactory pathway stages indicate that stimulus encoding becomes sparser, and receptive fields become more selective, as information ascends through the system (Tanabe et al. 1975a; Litaudon et al. 2003). For example, in monkeys, olfactory bulb neurons are broadly tuned, responding to an average of three to five stimuli within a fixed set of eight molecularly diverse odorants. To the same set of odorants, neurons in the amygdala and piriform cortex responded to an average of three odorants, and neurons in the orbitofrontal cortex (OFC) responded to one (Tanabe et al 1975a). Neurons within lateral hypothalamus are about as selective as olfactory bulb neurons (Scott and Pfaffmann 1972; Kogure and Onoda 1983).
An important organizational feature of the olfactory bulb is that second-order neurons located near each other express similar odorant receptive fields. As in other sensory systems, this arrangement provides an opportunity for lateral and feedback excitation and inhibition, which could contribute to lateral interactions, contrast enhancement, and gain control (Wilson and Leon 1987; Urban 2002; McGann et al. 2005; Olsen and Wilson 2008). A large population of GABAergic inhibitory interneurons, granule cells, forms dendrodendritic synapses with the output neurons in the bulb and probably helps mediate local interactions by serving as an inhibitory link among neighboring glomeruli. In fact, single-unit physiological processes resembling lateral inhibition have been demonstrated in mitral cell responses to odors and may even contribute to behavioral discrimination (Yokoi, Mori, and Nakanishi 1995; Luo and Katz 2001). Extensive mixture suppression is observed throughout the olfactory pathway (Kadohisa and Wilson 2006a; Lei, Mooney, and Katz 2006), which may reflect interactions at the sensory neuron level or through lateral inhibitory interactions within central circuits. Adaptation to one stimulus within the receptive field produces broad cross-adaptation to other odorants within the field, and stimuli evoking stronger initial responses induce greater cross-adaptation (Fletcher and Wilson 2003). These results again are consistent with a feature detection process, wherein the mitral or tufted cell responds to many odorants as long as they contain a particular feature.
A potential form of parallel processing may actually arise from the two main classes of glomerular output neurons. Mitral cell projections disseminate extensively throughout olfactory cortex and as far caudal as the entorhinal cortex, whereas tufted cell projections are limited largely to the most rostral portions of olfactory cortex (Scott 1981). Tufted cells have also been found to have lower thresholds for olfactory nerve activation than mitral cells (Harrison and Scott 1986; Nagayama et al. 2004).

5.3.5 Olfactory Cortex

Afferent output from the olfactory bulb targets a large forebrain area called the olfactory cortex, a relatively simple trilaminar paleocortex that is devoid of classic cortical columns. The primary recipient of bulbar projections is called the piriform (?pear-shaped?) cortex. In rodents this structure is found at the ventrolateral margin of the cortex and superficially borders the amygdala sub-nuclei for much of its length; in primates it is centered at the junction between the basal frontal and medial temporal lobes. Other direct recipients of olfactory bulb input include the anterior olfactory nucleus (AON), the olfactory tubercle, cortical nuclei of the amygdala, and entorhinal cortex. All of these regions, apart from the tubercle, project back to the bulb, allowing for descending modulation of bulb activity (Price 1990; Carmichael, Clugnet, and Price 1994; Shipley and Ennis 1996; Haberly 1998). The AON is the major intermediary of hemispheric cross-talk between olfactory structures, connecting the two olfactory bulbs through inhibitory (granule cell interneuron) relays (Yan et al. 2008), and linking left and right piriform cortices, perhaps as a critical link for transfer of odor information and memories during brief periods of unilateral blockade of nasal airflow (Kucharski and Hall 1987; Yeshurun, Dudai, and Sobel 2008). Whether these crossed pathways are functionally active in the human olfactory system is unclear.
Cortical afferents from the olfactory bulb project widely across piriform cortex, terminating in broad patches (Ojima, Mori, and Kishi 1984; Buonviso, Revial, and Jourdan 1991), in the absence of clear topographical organization. The relative lack of odor-specific spatial patterning within piriform cortex is distinctly different from the precise spatial arrangement of odor-specific activity within the olfactory bulb. It is hypothesized that the axon terminals from different populations of mitral cells, each conveying unique information about specific receptor activation, overlap in this region. In this manner, multiple features of an odorant, or of multiple odorants, extracted by disparate receptors may converge onto individual cortical pyramidal cells. Diverse techniques including c-fos immunohistochemistry (Datiche, Roullet, and Cattarelli 2001; Illig and Haberly 2003), voltage-sensitive dyes (Litaudon et al. 1997), microelectrode arrays (Rennaker et al. 2007), and optical imaging (Stettler and Axel 2009) have separately confirmed a diffuse pattern of odor-evoked activity throughout rodent anterior piriform cortex. The use of cortical flattening algorithms to generate two-dimensional ?flat? maps of odor-evoked fMRI activity in the human brain has identified an equally diffuse, distributed projection pattern in posterior piriform cortex (Howard et al. 2009). Albeit at a more macroscopic (millimeter) level of resolution, these imaging findings show that the same piriform voxel may respond to more than one odor quality category, and neighboring voxels may respond to different odor categories, further reinforcing the idea of a non-topographical organization in piriform cortex.
Importantly, piriform cortex contains an extensive associational fiber system (Johnson et al. 2000; Yang et al. 2004). Individual cortical pyramidal cells make excitatory connections with several thousand other pyramidal cells, greatly expanding the associative convergence of information from different olfactory receptors. Further potential for information exchange occurs via extracortical connections that reciprocally join piriform cortex with higher-order areas such as amygdala, entorhinal cortex, and prefrontal cortex (Johnson et al. 2000). These association fiber synapses are highly plastic, allowing formation of templates of previously experienced patterns of afferent input in a content-addressable format (Haberly 2001). Computational models suggest that a key benefit of experience-dependent templates is efficient reconstruction of odor object percepts from patterned input, even if those inputs are degraded or fragmented (Hasselmo et al. 1990; Hopfield 1991; Haberly 2001).
The complex assortment of patchy mitral cell input and dense associational connections (both intracortical and extracortical) suggests that odorant receptive fields of neurons in the olfactory cortex may reflect an integrated, non-linear combination of features that cannot be predicted from afferent (?bottom-up?) inputs alone (Lei, Mooney, and Katz 2006; Yoshida and Mori 2007; Barnes et al. 2008; Howard et al. 2009). For example, in some neurons of the anterior olfactory nucleus, responses to mixtures exceed the algebraic summation of the response to individual components (Lei, Mooney, and Katz 2006). Additionally, in both anterior olfactory nucleus (Lei, Mooney, and Katz 2006) and anterior piriform cortex (Yoshida and Mori 2007; Barnes et al. 2008), single neurons respond to a molecularly diverse range of odorants, while their mitral and tufted cell afferents respond to a more narrow range of molecular moieties (Lei, Mooney, and Katz 2006). Finally, olfactory cross-adaptation paradigms indicate that anterior piriform cortical neurons can learn to discriminate between mixtures and their elemental components, unlike olfactory bulb neurons that treat the parts and the whole much the same (Wilson 2000a, 2000b). These latter results suggest that, in contrast to mitral and tufted cells, piriform cortical neurons treat mixtures as unique objects, distinct from their components. Indeed, odor categorical perception can be estimated from distributed ensemble patterns of fMRI activity in human posterior piriform cortex (Howard et al. 2009), such that odorants more (or less) similar in perceptual quality exhibit more (or less) fMRI pattern overlap in this region, but not in anterior piriform cortex, OFC, or amygdala. Taken together these findings imply that piriform cortex is a critical repository for the encoding, retrieval, and modulation of odor objects (Wilson and Stevenson 2003; Gottfried 2010). This topic will be further examined in Sections 5.4 and 5.5.

5.3.6 Higher-Order Projections

A rich array of regions receives input from olfactory cortex, including most of the limbic system? amygdala, hypothalamus, entorhinal cortex?as well as OFC, perirhinal neocortex, and mediodorsal thalamus. As with the olfactory bulb and cortex, many of these areas are reciprocally connected. Indirect connections include the insula, cingulate cortex, nucleus accumbens, ventral putamen, and hippocampus. Thus, the olfactory system has strong links with circuits involved in emotion, hedonics, memory, and decision making, and these systems in turn feed back to very early stages of olfactory processing.
The OFC is the principal olfactory neocortical projection site. The location of odor-responsive cells in OFC appears to vary across species: areas LO and VLO in rodent orbital cortex (Carmichael, Clugnet, and Price 1994); posterior orbital segments of Walker?s area 13 in non-human primates (Tanabe et al. 1975b; Yarita et al. 1980; Carmichael, Clugnet, and Price 1994); and more anterior orbital segments adjacent to Walker?s area 11 in humans (Ongur, Ferry, and Price 2003; Gottfried and Zald 2005). These anatomical differences may reflect biological differences in the way each of these species incorporates odors into their behavioral repertoires. Neurons in OFC respond not only to specific odors, but also to odor-contextual cues and odor cue-outcome contingencies (Schoenbaum and Eichenbaum 1995; Critchley and Rolls 1996a; Ramus and Eichenbaum 2000). In primate OFC both the identity of an odor and its reinforcement value can be extracted from the spike trains of individual neurons (Rolls et al. 1996), and similar results have been seen in rat OFC, leading to the idea that the OFC integrates sensory representations with associated reward value to help guide motivated behavior (Schoenbaum et al. 2003). This interpretation from single-unit recordings is supported by lesion studies in animals (Otto and Eichenbaum 1992; Schoenbaum et al. 2003) and functional neuroimaging studies in humans (O?Doherty et al. 2000; Gottfried, O?Doherty, and Dolan 2002; Gottfried, O?Doherty, and Dolan 2003; Gottfried and Dolan 2004; Gottfried 2007). To a lesser extent, similar profiles have been observed in piriform cortex (Gottfried, O?Doherty, and Dolan 2003).
One of the unusual characteristics of the mammalian olfactory system is the lack of an obligate thalamic relay between the periphery and the primary sensory neocortex. Anatomical and physiological data indicate that the major route of odor information into olfactory neocortex is a ?direct? pathway between piriform cortex and OFC (Johnson et al. 2000; Cohen et al. 2008). However, an ?indirect? trans-thalamic pathway does exist. The mediodorsal nucleus of the thalamus receives direct projections from the piriform cortex (Kuroda et al. 1992) and in turn projects to the OFC and to other prefrontal regions. Interestingly, the termination of direct piriform inputs into OFC appears to overlap with those arriving indirectly via mediodorsal thalamus, suggesting a convergent triangulation between these regions (Ray and Price 1992). The mediodorsal thalamus also receives input from the cortical and basal nuclei of the amygdala, potentially enriching the olfactory input to OFC with emotional context and hedonic valence (Pickens et al. 2003). The specific function of thalamocortical olfactory pathways in odor perception is not entirely known. Lesions of the mediodorsal thalamus in rats impair learning and memory of discriminative odor cues (Slotnick and Kaneko 1981), although this impairment appears to not be related to odor discrimination per se (Staubli, Schottler, and Nejat-Bina 1987b; Zhang, Schottler, and Nejat-Bina 1998). Recent work combining human olfactory fMRI and effective connectivity analysis (Plailly et al. 2008) shows that the transthalamic pathway is selectively recruited during olfactory attentional processes, perhaps helping to optimize conscious analysis of a smell. Other recent studies suggest involvement of human mediodorsal thalamus in both olfactory hedonic processing and associative learning (Zelano et al. 2007; Small et al. 2008; Sela et al. 2009).
It is worth noting that neurons within all central olfactory regions express some aspect of multimodal coding. Thus, the activity of even second-order neurons reflects not only receptor input, but also aspects of internal state (Pager et al. 1972), behavioral arousal (Gervais and Pager 1979; Kay and Laurent 1999) and past experience (Wilson and Leon 1987; Kendrick, Levy, and Keverne 1992; Fletcher and Wilson 2003). For example, mitral cell responses to food odors are enhanced in hungry animals (Pager et al. 1972). In awake rats performing an odor discrimination task, only 10%?20% of mitral cell activity differentially encodes odor information; most neurons respond to other aspects of the task (Kay and Laurent 1999; Rinberg, Koulakov, and Gelperin 2006). Similarly, piriform cortical neuron activity reflects not only odor stimulation but also a variety of non-olfactory task-related stimuli (Schoenbaum and Eichenbaum 1995), arousal (Murakami et al. 2005), predictive reward value (Li et al. 2008), and memory and past experience (Schoenbaum and Eichenbaum 1995; Dade et al. 1998; Kadohisa and Wilson 2006a; Li et al. 2006; Moriceau et al. 2006). Single neurons within the olfactory tubercle show bimodal activation, with activity driven by both odors and sounds (Wesson and Wilson 2010). Finally, the OFC is strongly multimodal, with activity reflecting olfactory, gustatory, somatosensory, and visual aspects of stimuli (Rolls and Baylis 1994; De Araujo et al. 2003; Gottfried and Dolan 2003; Kadohisa, Rolls, and Verhagen 2004; Small et al. 2004; Gottfried 2007).
Finally, the olfactory pathway is heavily innervated by neuromodulatory systems known to regulate cell excitability and plasticity in the setting of attention, arousal, and internal state. Both the olfactory bulb and cortex receive a strong cholinergic input from the horizontal limb of the diagonal band of Broca (Shipley and Ennis 1996), which itself is responsive to olfactory input (Linster and Hasselmo 2000). This creates an interesting feedback loop where cholinergic modulation of olfactory processing is itself partially under olfactory control. Norepinephrine from the locus coeruleus in the brainstem is another key neuromodulator (Shipley and Ennis 1996). Its release in the olfactory bulb is dependent on behavioral state and multimodal stimulus novelty, and it can modulate both mitral/ tufted and piriform cortical neuron responses to odors (Jiang et al. 1996; Bouret and Sara 2002) as well as short-term and long-term olfactory plasticity (Gray, Freeman, and Skinner 1986; Sullivan, Wilson, and Leon 1989; Brennan, Kaba, and Keverne 1990; Shea, Katz, and Mooney 2008).
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Old January 10th, 2014, 01:31 AM   #3
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5.4 ODOR OBJECT SYNTHESIS AND PLASTICITY

Earlier sections of this chapter illustrate how ecological constraints and behavioral necessities have combined to guide the development of an olfactory brain system that can optimally extract meaningful information from odorous stimuli. As described above, the first step in the neural assembly of an odor object takes place with the progressive convergence and synthesis of molecular features present within an odorant (and across mixtures of odorants). This combinatorial scheme begins with receptor neuron axons projecting into olfactory bulb glomeruli and reaches its full expression in olfactory cortex, especially piriform cortex. Odor decorrelation (discrimination) improves as information moves from bulb to paleocortex to neocortex, though information flow to limbic regions such as amygdala and hypothalamus may be less specific.
The combinatorial processing of odors within a foreshortened sensory pathway (two synapses from sensory neuron to cortical neuron) and the strong odor-evoked activity within limbic brain areas suggest that cortical representations of odor are a strong echo of their stimulus inputs. However, the rapid and repeated convergence of non-olfactory information from higher-order areas into multiple levels of the information stream creates considerable opportunities for perceptual modulation and plasticity in the brain, such that odor representations become irreducibly connected to their meanings and associations. The goal of this section is to highlight some of the basic ecological pressures and physiological mechanisms driving sensory plasticity in the olfactory system.
Sensory perception and central processing of an odor are both strongly affected by past experience, current context, and behavioral state. The olfactory system shows remarkable plasticity in its response to odors, with rapid adjustments in sensitivity and acuity. These neural changes contribute to odor habituation, perception of odors against odorous backgrounds (background segmentation), and learning-induced changes in acuity as a result of perceptual and associative experience.
5.4.1 Reduced Input

The olfactory system regulates its sensitivity to odors through a variety of mechanisms and in reaction to different environmental settings. For example, a sustained period of reduced input, e.g., nasal obstruction due to rhinitis or (in the lab) due to nostril occlusion, results in an increase in odor sensitivity at the olfactory bulb (Wilson and Sullivan 1995). Such a mechanism of gain control helps to maintain bulb responsiveness in the face of diminished afferent information. However, this increase in gain comes at the cost of discriminability. Thus, spatial patterns of odor-evoked olfactory activity become blurred in the deprived hemisphere (Guthrie, Wilson, and Leon 1990), with increasing expansion and overlap of mitral cell receptive fields (Wilson and Sullivan 1995). A major component of the change in gain and loss of discrimination is an activity-dependent regulation of dopamine expression by juxtaglomerular interneurons (Baker et al. 1983; Baker 1990). Decreased sensory neuron input depresses synthesis and release of dopamine from juxtaglomerular neurons. Dopamine has a number of local circuit effects, but one is a D2 receptor-mediated pre-synaptic inhibition of glutamate release from sensory neuron terminals (Koster et al. 1999). In fact, the D2 receptor antagonist spiperone mimics the effects of sensory deprivation on mitral cell odor responses (Wilson and Sullivan 1995).

5.4.2 Prolonged Input

In contrast to the effects of reduced input, prolonged or repeated odor stimulation induces habituation and a decrease in odor sensitivity (Dalton and Wysocki 1996). Although OSNs (Kurahashi and Menini 1997; Zufall and Leinders-Zufall 2000) and mitral cells (Scott 1977; Wilson 1998b) both adapt to odors, piriform cortex undergoes the most robust adaptation, often exhibiting marked response suppression within one minute of stimulation (Wilson 1998b; Sobel et al. 2000; Poellinger et al. 2001; Li et al. 2006). As noted above, olfactory cortical adaptation is highly odor specific, allowing maintained responsiveness to novel odors (Wilson 2000a). Short-term cortical adaptation is mediated by activity-dependent depression of mitral/tufted cell cortical afferent synapses. These are glutamatergic synapses that express a pre-synaptic metabotropic glutamate receptor (group III). Following tens of seconds of normal odor exposure, these synapses are depressed (release less glutamate), reducing piriform pyramidal cell responses to afferent input and to odor stimuli (Wilson 1998a; Best et al. 2005). Blockade of these receptors prevents both cortical adaptation and behavioral odor habituation in animals (Best and Wilson 2004; Yadon and Wilson 2005). Both the synaptic depression and cortical odor adaptation recover within a few minutes (Best et al. 2005).
One important functional outcome of odor-specific cortical adaptation is its contribution to odor background segmentation (or “figure-ground” separation in the parlance of vision) (Kadohisa and Wilson 2006a; McNamara et al. 2008). Reduced piriform responsiveness to a steady background odor leaves intact the responses to novel odors appearing against that background olfactory landscape (Kadohisa and Wilson 2006a). This cortical mechanism allows for a separation of unvarying background information from dynamic inputs to optimize feature extraction of behaviorally relevant odor objects.

5.4.3 Perceptual (Non-Associative) Learning

It is important to bear in mind that cortical adaptation is more than just a high-pass filter of static information. To the extent that prolonged odor exposure implies an opportunity to acquire odor experience and familiarity, cortical adaptation is a critical component of olfactory perceptual learning. The consequence is that as an odor becomes more familiar, it becomes more distinct from other, similar odorants (Fletcher and Wilson 2002).
Olfactory perceptual learning is associated with changes in odor coding at the olfactory bulb (Fletcher and Wilson 2003; Moreno et al. 2009), piriform cortex (Wilson 2003; Kadohisa and Wilson 2006b; Li et al. 2006), and OFC (Li et al. 2006). In the olfactory bulb, mitral and tufted cells express narrowed receptive fields to familiar odorants (Fletcher and Wilson 2003), perhaps due to fine-tuning of local inhibitory circuits (Moreno et al. 2009). These more acute receptive fields presumably encode the stimulus features more accurately. In the anterior piriform cortex, co-occurring features become synthesized into an odor object by experience, providing information about the identity of the stimulus (e.g., amyl acetate; Gottfried, Winston, and Dolan 2006b; Kadohisa and Wilson 2006b), whereas posterior piriform cortex encodes aspects of odor quality or category (e.g., fruity or banana; Gottfried, Winston, and Dolan 2006b; Kadohisa and Wilson 2006b; Howard et al. 2009), reflecting the influence of experience.
These experience-dependent changes can occur after simple exposure and familiarization (Fletcher and Wilson 2003; Wilson 2003; Moreno et al. 2009), even in the absence of specific training or associative learning, and thus represent an implicit perceptual learning process. In fact, this mode of learning may reflect the primary mechanism by which organisms compile their “vocabulary” of smells, with ever-increasing perceptual refinement and differentiation of odor objects. Based on the high-molecular dimensionality of odor stimuli and the roughly N-factorial combinations of N odorants that can be mixed together (Gottfried 2009), the number of unique discriminable smells is nearly limitless, highlighting a profound perceptual acuity that may largely be driven by mechanisms of implicit learning.

5.4.4 Associative Learning

Explicit training and associative experience can also modify receptive fields and odor processing, allowing learned important odors to be more discretely encoded and perceived (Rabin 1988; Cleland et al. 2002; Fletcher and Wilson 2002; Kadohisa and Wilson 2006a; Li et al. 2008). Thus, an organism can direct its behavior toward particular stimuli that have come to signal specific consequences. For example, in Pavlovian conditioning paradigms, learning that an odor predicts delivery of an aversive electric shock modifies mitral and tufted cell responses selectively to that odor (Sullivan and Wilson 1991), modulates piriform cortical ensemble responses selectively to that odor (Li et al. 2008), enhances odor-evoked activity within the basolateral amygdala (Sullivan et al. 2000; Rosenkranz and Grace 2002), and enhances behavioral discrimination of that odor from other very similar odors that previously could not be discriminated (Fletcher and Wilson 2002; Li et al. 2008). In rodents, the learned changes in behavior and cortical activity are acetylcholine-dependent (Wilson 2001; Fletcher and Wilson 2002).
In addition to changes in local circuit function and neural ensembles within specific brain regions, olfactory associative learning also modifies functional connectivity between different components of the olfactory and limbic systems. For example, associative conditioning enhances coherence of electrical activity between the olfactory bulb and piriform cortex (Martin et al. 2006), as well as between the olfactory bulb and hippocampus (Martin, Beshel, and Kay 2007). Associative conditioning can enhance synaptic strength of both afferent input to the piriform cortex (Truchet et al. 2002) and intracortical association fiber synapses (Saar, Grossman, and Barkai 2002), as well as strengthen connectivity between the OFC and the piriform cortex (Cohen et al. 2008). Through such changes, neurons in the piriform cortex (especially posterior regions) come to encode not only odor identity or quality, but also hedonic valence (e.g., Calu et al. 2007).

5.4.5 A Critical Interface in the Olfactory Orbitofrontal Cortex

The olfactory OFC appears to be a critical locus linking odor sensation, perception, and experience. Human olfactory fMRI studies increasingly show that OFC activity is highly plastic and can be updated by contextual, emotional, and cognitive factors. For example, odor-evoked activation in OFC is enhanced when a smell (e.g., scent of a rose) is presented with a semantically congruent picture (e.g., image of a flower), compared to a semantically incongruent picture (e.g., image of a bus), demonstrating the response sensitivity of OFC to familiar semantic contexts (Gottfried and Dolan 2003). Similar contextual findings in OFC have been observed with combinations of odors and tastes (Small et al. 2004) and odors and words (De Araujo et al. 2003). The idea that OFC helps integrate prior (learned) information about an odor receives further support from the perceptual learning study described above (Li et al. 2006). In this experiment, the magnitude of experience-induced response enhancement in OFC closely correlated with the degree of olfactory perceptual improvement, on a subject-by-subject basis, suggesting that OFC is instrumental in mediating behavioral changes in odor expertise.
The OFC also shows striking plasticity to manipulations of odor reward value. Human imaging studies of sensory-specific satiety (O’Doherty et al. 2000; Small et al. 2001; Gottfried, O’Doherty, and Dolan 2003; Kringelbach et al. 2003) indicate that when subjects consume a food until it is no longer palatable, the odor or flavor corresponding to that sated food elicits reduced activity in OFC, suggesting that this brain region provides a dynamic index of food motivational state. These latter findings closely accord with neurophysiological work in monkeys (Critchley and Rolls 1996b), which reveals similar orbitofrontal response profiles as a function of current olfactory reward value. It is interesting to speculate that the tendency of food rewards, and their olfactory cues, to lose their “rewarding-ness” upon consumption (i.e., satiety) might perhaps be based on the biological design of olfactory sensory systems that already had a tendency to adapt with prolonged and/or intense stimulation. The net result of odor-specific neuronal adaptation in a sensory-motor interface area like OFC would be to dampen consummatory behavior and perhaps to encourage a new search for food items containing different nutritional constituents.
Finally, a recent lesion study provides new evidence to suggest that the materialization of olfactory conscious awareness relies on an intact OFC (Li et al. 2010). A previously healthy 33-year-old man without any prior history of smell or taste problems developed anosmia (complete smell loss) following focal traumatic brain injury to the right OFC. Despite a total inability to perceive smells presented to either nostril, the patient nevertheless demonstrated preserved odor-evoked autonomic (skin conductance) responses to unpleasant vs. neutral smells, with concomitant odor-evoked fMRI activity in piriform cortex bilaterally as well as in left OFC. These findings implicate a central role of the right OFC in facilitating the transformation of an upstream olfactory message into a conscious percept, and at the same time suggest that the left olfactory pathway is not sufficient to sustain conscious olfaction.

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5.5 FROM SENSATION TO REWARD: ARE ODORS REWARDING?

The main function of sensory systems is to extract behaviorally relevant information (meaning) from sensory signals in the environment. For an olfactory system, the natural world abounds with meaningful odor information: these are the smells of mother, child, and kin; home and territory; predator and prey; suitable and unsuitable mates. That survival throughout much of the animal kingdom hinges so substantially on odor sensations begs important questions. Is odor meaning an intrinsic or acquired feature of the physical stimulus? Are there innately pleasant (rewarding) and unpleasant (aversive) smells? Like food, water, and sex, do smells qualify as primary unconditioned reinforcers of behavior, an end unto themselves? That is, will an animal work to obtain more smell?
For aquatic microscopic species the answer is probably yes, because in the water there may be very little functional difference between olfaction and gustation. Inasmuch as a “smell” signifies a distant beacon or cue for an upstream reward (such as a source of food), a unicellular bacterium like E. coli, or a protozoan like the Amoeba (Figure 5.3), can ingest the cue—diverse soups of amino acids, peptides, disaccharides, algae particles—every bit as easily as it can the source. Thus the “smell” cue itself represents a consummatory object, a primary reinforcer. The fact that these water-bound organisms utilize a common chemosensory system for smell (i.e., food search, localization, detection) and taste (i.e., food reception, ingestion, phagocytosis) underscores the idea that there was little selective pressure for distinguishing cue from source in the earliest stages of animal evolution.
FIGURE 5.3

The functional boundaries between smell and taste are obscured in an aquatic protozoan like this amoeba. Standard bright-field microscopy depicts the sequence of events surrounding amoeboid food search and consumption. (a) An amoeba, moving slowly towards (more...)


Further along the evolutionary time-line, a direct link between odor stimulus and innate value becomes slightly more tenuous. In multicellular organisms, and particularly in terrestrial species, the progressive increase in body size, metabolic requirements, and biological complexity meant that food smells alone could no longer satisfy the alimentary want. The scent of ripe fruit or young rabbit may be a potent food cue for locating fruits or rabbits, but the act of inhaling these aromas has no positive impact on nutritional status. Interestingly, the idea that smells do not a meal make (what would today appear to be an indisputable fact) was lost on some of history’s eminent philosophers and clergymen. In his seventeenth-century quarto on traveling to the moon (Figure 5.4), Dr. John Wilkins (1614–1672), scientist, bishop, and visionary, proposed that the smell of food alone might be sufficient to sustain the nutritional needs of lunar voyagers (Wilkins 1684), with the following plausible rationale:
FIGURE 5.4

Frontispieces of the first (1638) and fourth (1684) editions of a book by John Wilkins, the late Lord Bishop of Chester, in which he surmised that the smell of food might provide sufficient nourishment to ensure a healthy, comfortable sojourn and nutritionally (more...)

Or, if we must needs feed upon something else, why may not smells nourish us? Plutarch and Pliny and divers other ancients, tell us of a nation in India that lived only upon pleasing odors. And ’tis the common opinion of physicians, that these do strangely both strengthen and repair the spirits. Hence was it that Democritus was able for divers days together to feed himself with the meer smel of hot bread.
The above viewpoint aside, it is clear that certain odor stimuli provoke hard-wired, unlearned behavioral responses. The predator smell of cat urine (Apfelbach et al. 2005; Takahashi et al. 2005), the moth sex pheromone bombykol (Carde et al. 1997), the alarm and recruitment pheromones of fire ants (Vander Meer, Slowik, and Thorvilson 2002), and the ink-opaline chemorepellent of the sea hare (Kicklighter et al. 2005) are just a few of the many examples of odor signals endowed with innate biological significance. In fact in rodents, cat odor can serve as an unconditioned stimulus in conditioning paradigms (Blanchard et al. 2001). Through natural selection, olfactory systems evolved to optimize detection and processing of these vital chemical messages in a species-specific manner. In invertebrates and vertebrates (with the exception of humans), specialized systems including vomeronasal organs and accessory olfactory bulbs may have evolved to handle these intrinsically meaningful odors—though to what extent the main olfactory system may also support such functions remains unclear.
Is there any reason to believe that the reinforcing properties of an odor might lie within the odor itself? A sensory ecology approach to the chemistry of pheromones (discussed in Dusenbery 1992) suggests that molecular size of the physical stimulus is a key determinant of behavior. Small molecules (100–200 daltons; 6–15 carbons) make good alarm signals and repellants, because they diffuse rapidly into the air and dissipate as soon as the danger has passed. If the receiver has the appropriate receptors for that molecule and if those receptors are connected to the appropriate central motor control circuits, information eliciting escape can be quickly transmitted and acted upon. In turn, large molecules (200– 300 daltons; 15–20 carbons) make good territorial markers because they stay in place for a long period of time due to their lower volatility. Large molecules also make good attractants, because their structural complexity provides numerous opportunities for chemical substitutions to the odorant molecule, helping to ensure species specificity (for example, a female silk moth would be most dismayed to find a male gypsy moth following her plume).
It is therefore plausible that basic appetitive or aversive features of a smell are methodically linked to the physical composition of an odorant (Yeshurun and Sobel 2010). Theories propounding a molecular basis for odor pleasantness have been present since antiquity (Cain 1978; Finger 1994). Democritus and Epicurus, and later Lucretius, championed the idea that the atoms of pleasant smells were smooth and round, whereas those of unpleasant smells were hooked, rending the nasal membrane full of nasty holes. Of the many odor classification schemes dating back to Linnaeus (1707–1748), most of them contain at least one category for unpleasant smells, and even as recently as the 1960s “putrid” smells formed one of seven “primary odors” with its own unique electrochemical configuration (Amoore 1952, 1970). Although empirical evidence in support of these ideas has historically been in short supply, multidimensional scaling studies indicate that valence is the primary dimension along which humans categorize odors (Berglund et al. 1973; Schiffman 1974), and a recent study using principal component analysis suggests that the subjective pleasantness of a smell can be roughly estimated from a set of >1500 chemical and molecular properties describing a given odorant stimulus (Khan et al. 2007).
Finally, it is worth noting that the meaning of a smell (its reinforcing property) depends importantly on the receiver. The fragrance of a red fox will simultaneously evoke dread in a rabbit and drool in a hound dog. In humans the earthy scent of ?poisses cheese, or the brackish scent of sea urchin roe, may evoke strongly contrasting emotional responses even in two members of the same family. Both animal and human data (Critchley and Rolls 1996b; Rolls and Rolls 1997; O’Doherty et al. 2000) make it abundantly clear that through the mechanism of sensory-specific satiety, the pleasurable smell and flavor of a delectable food become unbearable after a gluttonous surfeit of said food. These examples underscore the hedonic relativity of odors, between species, within species, and even within individuals, suggesting that a “universal grammar” of chemical determinants to define the affective meaning of a given smell is unlikely to be identified.
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Old January 10th, 2014, 01:32 AM   #4
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5.6 THE NEUROBIOLOGY OF SENSATION AND REWARD: CONVERGENCE AND CODA

Faced with the natural complexities of an airborne odor, different organisms (and sometimes even the same organism) may adopt either analytical (elemental) or configural (synthetic) strategies for contending with an olfactory stimulus (Staubli et al. 1987a; Livermore et al. 1997; Kay, Lowry, and Jacobs 2003; Wiltrout, Dogra, and Linster 2003; Kay, Crk, and Thorngate 2005). In analytical odor processing, the whole stimulus (odor blend) is perceived to smell the same as the sum of the parts (odor elements)—or, in non-human animals, to evoke the same behavioral response, or to carry the same meaning. For example: [A + B + C] = [A] + [B] + [C]. Such mechanisms would promote stimulus generalization, by optimizing sensitivity to a biologically salient odor component, regardless of whether it appears alone or in a mixture (see Derby et al. 1996). Analytical processing may also help prevent signal corruption of the original input, preserving stimulus fidelity of the chemical message. However, these gains come at the potential expense of response over-generalization and a loss of odor specificity. An organism may respond indiscriminately to any compound stimulus that contains one particular “salient” odor X (e.g., [A + B + X] and [C + D + X]), though that response may be impulsive or inappropriate to the present context.
In configural odor processing, the whole stimulus is perceived to smell different from the sum of the parts, such that a novel percept is generated, with a different meaning from the components themselves. In this instance, [A + B + C] ≠ [A] + [B] + [C]. The capacity to generate new perceptual configurations adds substantially to stimulus discrimination (Livermore et al. 1997), with the possibility of emergent interactions and non-linearities among odor inputs. Synthetic processing may also be a more efficient mechanism of odor information storage (i.e., memory), because it reduces the computational burden that an olfactory brain would otherwise require to encode each and every element of a complex odor stimulus. At the same time, the integration of odor inputs is a useful way for assigning a unique perceptual stamp or signature to every odor-emanating object in the environment. In such cases, odor meaning is extracted from identifying the gestalt of the mixture, which is merged into a unique odor object. The main drawback of a synthetic system is the inevitable loss of signal input integrity, which could become problematic if detection of salient odor signal X decreases as a consequence of being incorporated into a new perceptual entity (where X is being suppressed) (Staubli et al. 1987a).
Biologically it appears that these two mechanisms can be engaged simultaneously. As discussed in Section 5.3, electrophysiological work by Wilson and colleagues (Wilson 2000b; Barnes et al. 2008) has demonstrated a functional double dissociation in rodents, whereby odor mixtures are encoded analytically in mitral/tufted cells of the olfactory bulb, and synthetically in the pyramidal cells of anterior piriform cortex. Complementary behavioral studies actually indicate that the ratio of components, or their perceived similarity, in a binary mixture can determine whether a rat will react to the odor blend analytically or synthetically (Kay, Lowry, and Jacobs 2003; Wiltrout, Dogra, and Linster 2003). That different levels of information processing in the rodent olfactory brain are organized to handle odor blends as elements or wholes is compatible with the notion that the rat can adopt different olfactory behavior strategies depending on environmental contingencies and task necessities.
Traditionally, the dichotomy between elemental and configural representations has strongly informed conceptual models of learning and memory (Rescorla 1972; Pearce 1987; Rudy and Sutherland 1992; Pearce and Bouton 2001). Since Pavlov it has been evident that an animal conditioned to a complex stimulus may not respond when the constituent stimuli are presented separately. Recognizing a behavioral distinction between the parts and the whole, Pavlov wrote that “a definite interaction takes place between the different cells of the cortex, resulting in a fusion or synthesis of their physiological activities on simultaneous excitation” (Pavlov 1927, 144). Woodbury (1943), evaluating these ideas more systematically in a reinforcement learning paradigm, found that when dogs were trained to respond to a combination of a high-pitched buzzer and a low-pitched buzzer (HL), they failed to react to the sound of either buzzer alone (Figure 5.5). Such observations led to the idea that a compound conditioned cue (AB) could gain access to a representation of an unconditioned stimulus (US) in either of two ways: elements A and B might each form a separate associative link with the US, such that either A or B would elicit a conditioned response; or, A and B might be conjoined into a unique configuration (AB), which itself becomes linked with the US. In this latter instance, only AB would elicit a response.
FIGURE 5.5

An early example of configural learning. In an instrumental conditioning task, dogs were trained to lift a wooden bar with the nose in order to receive positive reinforcement in the form of food pellets (Woodbury 1943). Each trial began with sounding (more...)


In all likelihood, elemental and configural mechanisms operate in tandem, though there is reason to believe that configural learning should confer much greater behavioral flexibility upon an organism. Rudy and Sutherland (1992) elegantly illustrated this point in a review article in which they considered two retrieval cues (A and B) that gain access to different reinforcers depending on stimulus context (Figure 5.6). In context 1 (C1), A activates a representation of the US, and B activates a representation of the absent US (“no-US”). In context 2 (C2), these contingencies are reversed: A activates a representation of the no-US, and B activates a representation of the US.
FIGURE 5.6

A schematic diagram contrasting the different internal representations that might arise during the formation of elemental (left) or configural (right) associations. A richer, more complex layer of associations can be generated during configural learning, (more...)


Using a simple model Rudy and Sutherland show that elemental representations cannot easily sustain context-dependent associative switching (the so-called trans-switching discrimination problem). With the formation of elemental associations (Figure 5.6, left), representational cues a and b and contexts c1 and c2 are by definition fully connected both to US and to no-US. As a result, there are no unique combinations of retrieval cues and contextual states that can selectively activate the full range of representational outcomes. However, with the formation of configural associations (Figure 5.6, right), unique conjunctions of stimulus and context information (c1a, c1b, c2a, c2b) ensure associative flexibility between a given retrieval cue and a behavioral reinforcer. The specificity of such an arrangement will also help to disambiguate potentially conflicting associations.
This marks a perfect example of systems convergence between sensation and reward, in keeping with a central theme of this book. For many animals the sensitivity of odor discrimination and the efficacy of reward learning both rely on an ability to forge novel associations between physically distinct stimuli. It is tempting to speculate that the neurobiology of (olfactory) sensation and the neurobiology of reward must have co-evolved, to the extent that many of the same anatomical circuits and physiological mechanisms are employed to achieve the same basic end. The potential co-dependence of these systems would have important implications for how sensory systems and reward systems operate. More complex organisms with a greater capacity for configural learning (odor-to-odor in the case of olfaction; cue-to-context in the case of reward) will be better equipped to adapt their behavior to changing environmental contingencies and homeostatic states.
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Footnotes

*The ?building-blocks? comparison of odorant molecular components to visual light wavelength is commonly made but is really only appropriate at the receptor input stage of sensory information processing. Wavelength tells us about color per se, whereas it is the combination (and context) of many colors assembled into complex patterns that tell us what visual objects we see?just as it is the combination and context of many odorants assembled into complex patterns that tells us what odor objects we smell.
*Again, echoing the prior footnote, once we move from wavelengths to objects (faces, voices) and their cortical representations, the integrity of these fundamental dimensions breaks down both perceptually and topographically within sensory pathways. In area IT, for example, a neuron responsive to a garden gnome may be located adjacent to a neuron responsive to Greta Garbo, the objects of which share very little in the way of color, contrast, or curves.

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