Identifying mechanisms of time-place discrimination
Pizzo, Matthew James
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A series of experiments were designed to identify the mechanisms used to solve time-place discriminations. In each experiment, food was available at two or more locations depending on time of day (i.e., experimental stimuli did not signal the availability of food). The rats approached the temporally correct location before food was delivered, documenting time-place discrimination in each experiment. Time-place discrimination could have been based on (a) time of day, (b) interval timing, and/or (c) alternation. These confounded cues were dissociated by skipping an occasional meal and by phase shifting the time of testing. Rats were placed in a T-maze, separated by approximately 7 hr (Experiment 1), twice per day. The rats used an alternation strategy to solve the task. Rats were placed in an 8-arm radial maze for daily 56-min sessions that were divided into eight 7-min time zones, during each of which a different location provided food (Experiments 2, 3, and 4A-D). The rats timed with respect to the start of the session to get to the first location and timed successive 7-min intervals to get to subsequent locations. Once per day rats were placed in operant boxes to earn two meals separated by 45 min, 105 min, 3.5 hr or 7 hr (Experiments 5 and 6). When meals were separated by 3.5 or 7 hr, the rats used time of day and/or timed an interval with respect to the start of the session. When meals were separated by 45 or 105 min, the rats anticipated (a) the first meal by timing an interval with respect to the start of the session and (b) the second meal by timing an interval with respect to the first meal. Temporal discrimination developed earlier in training than spatial discrimination. A non-timing strategy was used when testing was interrupted by handling. The temporal resolution of a circadian system is estimated to be 1.75 – 3.5 hr. Interval timing was used for a wide range of durations (7, 45, and 105 min; 3.5 and 7 hr).