Figure 4.

LD neurons carry palatability-related information in the middle portion of their temporal codes. A, Photographs displaying the stereotyped orofacial reactions to the delivery of a palatable (top; licking to sucrose and NaCl) and unpalatable (bottom; gaping to citric acid and quinine) taste delivery. These responses reveal the perceived palatability of the taste qualities delivered. B, Breakdown of what tastes caused responses during time bins in which neurons responded to two to three tastes. S, Sucrose; N, NaCl; C, citric acid; Q, quinine. Sixty percent of the two to three taste responses were to either S/N (i.e., the pair of palatable tastes) or C/Q (i.e., the pair of unpalatable tastes). That is, with few exceptions (an occasional bin with responses to N/Q), LD neurons responded to tastes with similar palatabilities. C, This graph shows, for each epoch implied in A, the difference (in spikes per second; y-axis) between responses to tastes with similar palatabilities (dark green bars) and different palatabilities (light green bars). During the middle epoch of the response only (the epoch dominated by responses to pairs of tastes), there was much more difference between responses to tastes with different palatabilities (*p < 0.01). FR, Firing rate. D, Template-based classification of LD epoch 2 taste responses shows the actual taste delivered was identifiable (y-axis, percentage correct; x-axis, classification of taste trial) at twice chance (horizontal dashed line) rates. The greatest percentage of errors, consistent across all tastes, was the result of “palatability confusion” (sucrose and NaCl were most often confused for one another, as were citric acid and quinine). For ease of visualization, palatable tastes are in light and dark blue, whereas unpalatable tastes are in light and dark red.