Candidate codes in the gustatory system of caterpillars.

Abstract

Larvae of tobacco hornworms offer unique opportunities to relate the electrophysiological output of identified chemosensory neurons to specific behavioral responses . Larvae can discriminate among three preferred plants with only eight functioning gustatory receptors . They can be induced to prefer any one of the plants, and these preferences can be reversed . All eight neurons respond to each plant sap . Two fire too infrequently to permit detailed analysis . Analyses ofthe remaining six show that all electrophysiological responses consist ofphasic and tonic components. Only the "salt best" cell fires during the phasic period . Temporal analysis of the spike train during this period shows that tomato and tobacco could be distinguished from Jerusalem cherry but not from each other by a rate code. Measurements of behavioral response times together with the nonspecificity of this with respect to food plants, unacceptable plants, and sodium chloride eliminate a phasic period rate code as a probable mechanism for complex discrimination . Events occurring in the tonic period, when all cells are firing, suggest a major role for this period . Analyses of variance in the interval frequencies of the large and medium spikes suggest that a variance code could allow discrimination among the three plants as long as both cells were firing at the same time . Evidence has been found for temporal patterning in the tonic response of the "salt best" cell to Jerusalem cherry but is absent elsewhere . The most likely basis for coding the difference between each of the three plants is across-fiber patterning in which the relative rates of firing and the variances of all the sensory neurons in the tonic phase are critical .Although the choice of plants by phytophagous insects involves integration of information provided by many sense organs (visual, tactile, olfactory, and gustatory), it is well documented that the gustatory system plays a leading role in feeding and that a high level of discrimination by caterpillars is still possible when all chemosensory input except that from gustatory receptors is blocked (Hanson and Dethier, 1973) . Furthermore, it has been postulated that the preferential selection and acceptance of food is based upon a capacity to discriminate plants qualitatively (Dethier, 1976a(Dethier, and b, 1977(Dethier, , 1978 . In other words, plants of various degrees of acceptability are not discriminated because

Temporal Coding Mediates Discrimination of “Bitter” Taste Stimuli by an Insect

“…Despite numerous reports of taste-specific discharge patterns in peripheral gustatory afferents (Ogawa et al, 1973(Ogawa et al, , 1974Nagai and Ueda, 1981;Dethier and Crnjar, 1982;Varkevisser et al, 2001) and central gustatory circuits (Di Lorenzo and Schwartzbaum, 1982;Katz et al, 2002b;Di Lorenzo and Victor, 2003;Verhagen and Scott, 2004), there have been comparatively few attempts to link discharge patterns to taste-guided behavioral responses. One series of experiments by Di Lorenzo and Hecht (1993) and asked whether taste-specific discharge patterns from neurons in the nucleus of the solitary tract (NST; the first synaptic relay for taste in vertebrates) modulate taste-guided licking responses of rats.…”

“…We limited our data analysis to the initial 5 s of biting. This approach was made possible by the observation that the repetitive biting sequence of M. sexta is disrupted almost immediately by aversive gustatory input, resulting in longer and more variable interbite intervals (IBIs) (Dethier and Crnjar, 1982;Frazier, 1986).…”

“…Proponents of labeled-line coding contend that tastant identity is represented by activity in a subset of afferent gustatory neurons (Hellekant et al, 1997;Zhang et al, 2003;Marella et al, 2006), whereas proponents of ensemble coding contend that tastant identity is represented by activity across large populations of afferent gustatory neurons (Dethier and Crnjar, 1982;Erickson et al, 1995;Caicedo et al, 2002). We believe that the persistence of this debate reflects the fact that these coding frameworks fail to capture the full complexity of taste processing.…”

Modeling the insect feeding decision system: A new approach utilizing fuzzy system theory

“…Candidate Code Module. The output of the Demultiplexer Module is an estimate of the spike activity in each neuron (Figure 1, estimated XMI-4)-This simple spike frequency is effectively a code representing the sensed stimuli; however, to distinguish it from the actual (unknown) code in the insect's central nervous system (CNS), it is called the 'candidate code' (Dethier & Crnjar, 1982). The actual biological code is probably more complex: the CNS may subtract spontaneous activity, or standardize to weight more heavily responses of those sensory neurons which have intrinsically low response rates (Schoonhoven, 1987).…”

“…The actual biological code is probably more complex: the CNS may subtract spontaneous activity, or standardize to weight more heavily responses of those sensory neurons which have intrinsically low response rates (Schoonhoven, 1987). In addition, temporal patterning of a sensory cell's activity may affect the weighting of its information by the CNS (Dethier & Crnjar, 1982). Accordingly, in the model the sensory data pass through the Candidate Code Module that could be programmed with transfer functions that more closely approximate biological realities, and transform the data accordingly (Figure 1, estimated YM1-4).…”