Figure 2: Single-unit recordings of odour-responsive units.

(a,b) Shown are recording locations and baseline firing rate distributions in the sham (a) and OFC-lesioned groups (b). The width of each box (1 mm) represents the approximate spread of recording wires, and the height represents the estimated range through which the bundle was driven during recording sessions. Each box represents one rat. Scale bar, 1 mm. AC, anterior commissure; CC, corpus callosum. Baseline firing rates were not significantly different in the two groups, for either the whole population (t₅₉₀=−0.15, P=0.88) or in the odour-responsive population, (t₉₀=−1.5, P=0.13). (c,d) Pie charts represent all odour-responsive units (defined as those with a significant (sig.) increase over baseline during the odour epoch). Coloured sections represent units with significant effects of number (black), flavour (blue) or both (cross-hatched), based on an analysis of variance (ANOVA) on firing rate in that unit’s preferred direction during the odour epoch. There was no significant difference in the proportion of odour-responsive units in the two groups (by chi-square, P=0.25). However, with OFC lesions, units were significantly more likely to be flavour-selective and significantly less likely to be number-selective with no effect of flavour (chi-square on distribution of units across these categories, P<0.0001). Indeed, by chi-square, in the sham group, the number of purely flavour-selective neurons (with no effect of number) was not significantly different from chance (P=0.74), while the number of purely number-selective neurons was significantly greater than chance (P<0.00001); in the lesion group, the opposite was true (purely flavour-selective, P<0.00001; purely number-selective, P=0.90). (e–h) Single-unit examples from the sham (e,g) and lesion groups (f,h). Black and grey bars indicate bins with a significant effect of number or flavour, respectively, by ANOVA on a sliding average of five bins. Significant effects were found mostly on the preferred side (left side of each panel). Units in the sham group tended to prefer the odours predicting large outcomes, in some cases with no effect of flavour (e) and in other cases with an additional effect of flavour (g). Units in the lesion group were more likely to be flavour-selective without a strong effect of number. choc., chocolate; init., initiation; van., vanilla.