The effects of scopolamine on memory for time in rats and pigeons

Abstract

In Experiment 1, pigeons were trained to match short (2 s) and long (8 s) sample durations to red and green comparison stimuli and red and green samples to vertical and horizontal line comparisons. They received injections of scopolamine hydrobromide (0.02 mg/kg), scopolamine methylbromide (0.02 mg/kg), or saline, and the delay interval was manipulated (0, 1, 3, and 9 s). In Experiment 2, rats were trained to discriminate short (2 s) or long (12 s) durations of house-light illumination using a choice procedure. During the test phase of each trial, the left and right levers were presented with the cue light on above one of them (cued lever) while the other was off (uncued lever). For half of the rats, the correct response following the short sample was to press the cued lever, whereas following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. The rats received injections of scopolamine hydrobromide (0.15 mg/kg), scopolamine methylbromide (0.15 mg/kg), or saline, and the delay interval was manipulated (0, 1, 3, and 9 s). In pigeons, scopolamine equivalently disrupted both temporal and nontemporal memory. Memory for time, in both rats and pigeons, was significantly poorer following scopolamine injections than following methylscopolamine or saline injections. No choose-short effect was observed in either rats or pigeons during saline test sessions. The data indicate that central cholinergic blockade in both pigeons and rats disrupts the accuracy of delayed temporal discriminations. However, scopolamine does not appear to accelerate the rate at which memory for temporal events is foreshortened.