Recording sites and characterization of delay length and reward identity-selective activity. A, The colored dots indicate the approximate location of tips of electrode probes at the last recording session, with both hemispheres combined, and the same shade dots mean the recording sites from both hemispheres of the same rat. Two rats had four bundles of electrodes and thus have four dots shown, while the other two rats had eight bundles of electrodes and thus have eight dots shown. Bregma, +2.7 to +3.7 mm; scale bar, 1 mm. B, C, The colored sections represent the proportion of neurons with significant effects of delay length (B) or milk flavor (C) based on a t test performed on a firing rate in the 1,000 ms immediately after milk delivery across forced-choice trials (see Materials and Methods). The inset pie charts represent breakdowns for each category of neuron to illustrate the degree of overlap between neurons showing significant effects of flavor and significant effects of timing. D, Mean normalized population firing of short-responsive neurons (red line), long-responsive neurons (blue line), and general reward-responsive neurons (gold line) during short and long trials. Activity is aligned on well entry. The reward-responsive population responded phasically whenever reward occurred, including the trial-terminating drop of water (one-way ANOVA on a firing rate at the short reward epoch, 0.5–1.5 s: neuron classification, F_(2,798) = 50.2; p < 0.001; post hoc analysis: short-responsive vs reward-responsive cells, t₍₇₉₈₎ = 9.5; p < 0.001; long-responsive vs reward-responsive cells, t₍₇₉₈₎ = 5.8; p < 0.001; one-way ANOVA on activity at the long reward epoch, 3.0–4.0 s, neuron classification, F_(2,798) = 50.0; p < 0.001; post hoc analysis, short-responsive vs reward-responsive cells, t₍₇₉₈₎ = 9.3; p < 0.001; long-responsive vs reward-responsive cells, t₍₇₉₈₎ = 4.4; p < 0.001; one-way ANOVA on activity at the terminal reward epoch, 7.0–8.0 s, neuron classification, F_(2,798) = 35.4; p < 0.001; post hoc analysis, short-responsive vs reward-responsive cells, t₍₇₉₈₎ = 7.5; p < 0.001; long-responsive vs reward-responsive cells, t₍₇₉₈₎ = 5.8; p < 0.001; paired t test for comparing neuronal activities between at the terminal reward epoch, 7.0–8.0 s, and before terminal reward epoch 6.0–7.0, short-responsive neurons, t₍₂₀₃₎ = 3.4; p < 0.001; long-responsive neurons, t₍₁₅₂₎ = 1.6; p = 0.12; reward-responsive, t₍₄₄₃₎ = 7.1; p < 0.001). Shaded area represents SEM. E, Relation of firing dependent on delay length to firing dependent on identity of flavored milk for significantly short-responsive neurons (red dots), significantly long-responsive neurons (blue dots), and reward-responsive neurons (gold dots). Taken together, all task-responsive neurons showed no significant correlation between delay–response and flavor–response (n = 801; r = −0.03; p = 0.35; regression line not shown). However, specific populations of delay-responsive neurons showed strong and significant correlations between delay and flavor indices, such that the stronger the response to delay, in either direction, the stronger the response to one of the two flavors (short-responsive neurons, red dashed line, n = 204; r = 0.65; p < 0.001: long-responsive neurons, blue dashed line, n = 153; r = −0.71; p < 0.001).