Figure 1. Discrete timing of sensory responses in the ventral midbrain during auditory trace conditioning.

(a) Schematic of the auditory trace conditioning task. Freely behaving mice (N=5) were trained to obtain a water reward from a small port on the wall of the behavior box following a conditoned stimulus (CS; 500 ms tone). Licks at the reward port were measured using an infrared beam break. Water rewards were delivered by gating a solenoid valve. (b) Time course of the speaker control voltage, valve control, and average rate of licks (black, s.e.m shaded area) aligned to the CS for all acquisition blocks (N=86). A baseline period of 2 s prior to CS onset was used to estimate the background rate of licks (solid cyan line; dashed lines are 0.95 confidence intervals). (c) Average anticipatory licking (licks in trace interval – baseline licks) in 5 trial blocks plotted as a function of position within acquisition sessions (left) and extinction sessions (right). ‘Stable’ response is the first block above 30 trials. (d-f) Units with significant phasic responses to tones were divided into 3 groups: dopamine (’DA’, d); GABAergic, medium latency (’GABA2’, e); GABAergic, short latency (’GABA1’, f) using criteria defined in the main text including response pattern (d-f) and latency (g) . Left two columns show example raster plots and Z-scored peristimulus time histograms (PSTH) from each group. Right two columns show population means for units with significant phasic responses during acquisition blocks. (g) Histogram of the latency to the peak of the phasic response for all units (grey bars). Peaks corresponding to the GABA1 and GABA2 groups are indicated. Superimposed are cumulative histograms of peak latencies for GABA1+2 (cyan) and DA (red) units (right axis).