Figure 1.
Contingent and free reward in a tactile discrimination task. A, Schematic diagram of the behavioral apparatus used in the tactile discrimination task. As a trial begins, the rat enters the stimulus chamber and uses its whiskers to touch two bars placed around the NP, which determine a narrow or wide aperture. The animal then returns to the reward chamber and makes a choice of left or right window to receive a water droplet as reward. Reward contingency was manipulated to vary stimulus relevance. In the CR condition, reward was only delivered when animals poked the correct reward window given the stimulus presented (narrow aperture for left reward window and wide aperture for right reward window). In CR sessions, correct stimulus discrimination was highly relevant for obtaining reward. In the FR condition, reward was delivered regardless of the stimulus presented, and therefore the stimuli were irrelevant for the execution of the task. B, The time course of one single trial is shown, with the approximated discrimination period indicated by the gray shade. RTI is the time interval between the opening of the CD, at the beginning of the trial, and the nose poke at stimulus chamber; RTII is the time difference between NP and RW; RTIII is the time difference between discrimination beam (photobeam between the bars) and nose poke. C, The performance in CR and FR sessions is shown for each subject for all animals recorded in both conditions. D, A behavioral bias in the choice of reward window was present in FR sessions but not in CR sessions and was used as an independent criterion for the inclusion of FR trials in the dataset analyzed. Behavioral bias was calculated as the absolute value of (X − Y)/(X + Y), where X is the number of left choices, and Y is the number of right choices. After switching from the CR to the FR condition, animals usually took <10 trials to display the bias. Unbiased trial blocks in FR sessions were discarded from the experiment. The behavioral bias in FR sessions was reminiscent of the biases detected during the early phases of task acquisition (data not shown). E, Reaction times within the trials (RTI, RTII, and RTIII) did not show significant differences between the performance of CR and FR sessions (ANOVA, RTI, F(1,7) = 1.822, p = 0.22; RTII, F(1,7) = 0.24, p = 0.63; RTIII, F(1,7) = 3.26 × 10−4, p = 0.98). Error bars represent SEM.