Olfactory modality

Drosophila gustatory receptor (GR) genes a gene family that subserves both gustatory and olfactory modalities... 

The next point for consideration is whether pungency is a purely gustatory modality, or whether it is also an olfactory modality. There is a vast and growing literature on the interrelation of the senses, though the extent of their mutual influences are... 

A century ago, Fick proposed the concept of four primary tastes, namely, sweet, salty, sour, and bitter. It has since been found that taste sensations are not describable by a single collection of discrete primaries. Electrophysiological studies of afierent taste-units in the chorda tympani and glossophyrangeal nerves have revealed that a continuous spectrum of gustation may be based on these four taste elements. Furthermore, the intensities of the tastes that we commonly experience are due not only to gustatory sensations but also to tactile, hot and cold, and, above all, olfactory sensations. The complexities of taste studies are such that, unless one of the taste modalities is singled out for study, there is very little hope of success. 

Distel, H. and Hudson, R. (1985) The contribution of olfactory and tactile modalities to the performance of nipple-search behaviour in newborn rabbits. J. Comp. Physiol. A 157, 599-605. 

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure. 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

Second, the neural processing of this code defines a percept, called an odor . The first process takes place in a heterogeneous population of olfactory receptor neurons (ORNs) distributed in the epithelium. It determines which volatiles can be detected. The second process occurs in a series of neuropiles in the brain. It leads to some form of perception and can drive a behavioral response, depending on the animal s internal state and integration with other sensory modalities. How do individual neurons handle the transfer of information How do they connect to form a network in which this information is distributed, and what are the coding properties of this network ... 

Figure 15.1 The sequence of behaviors in heterosexual courtship of Drosophila melanogaster. The arrows represent the known sensory modalities by which flies communicate (+) for stimulatory and (-) for inhibitory signals. Olfactory and gustatory cues are represented respectively by white and black arrows auditory and visual cues by light and dark gray arrows. Several cuticular hydrocarbons exchanged during each sequence can affect precise behaviors 5-and 7-tricosene (respectively 5-T and 7-T), 7,11-hepta- and nonacosadiene (7,11-HD 7,11-ND) and 7- and 9-pentacosene (7-P 9-P see text). (Adapted from Ferveur and Sureau, 1996 Greenspan and Ferveur, 2000.)...

The ability to discriminate different molecules constitutes a criterion for olfaction. Because, as mentioned, anosmic persons can tell some pairs of odors apart based on nonolfactory cues, an experimenter must choose with care the compounds for study. 3-Phenethyl alcohol has an odor that many people find reminiscent of roses, and vapors from dilute solutions are widely accepted as an olfactory stimulus that does not interact with other chemosensory modalities in humans (Betcher Doty, 1998). Consider a human subject who can detect J3-phenethyl alcohol with the same sensitivity as nor-mosraics and can also detect -butanol (another alcohol often used for testing olfactory sensitivity (Hummel et al., 1997), which has an odor very different from that of P-phenethyl alcohol) with normal acuity. Suppose this subject cannot distinguish the two odors. How can an experimenter assess whether the subject exhibits the sense called olfaction ... 

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