Choice biases in delayed matching-to-sample duration with pigeons: Manipulations of ITI and delay illumination.

Two experiments investigated the effects of similarity between intertrial interval (ITI) and delay illumation on the choose-short effect. Different groups of pigeons learned to match "short" (2 s) and "long" (6 or 8 s) food samples to green and red test stimuli in a matching-to-sample procedure with a 5-s training delay. Subsequent 10- and 20-s delay tests revealed choose-short effects if the ITI and delay were both illuminated (i.e., group ON-ON), if the ITI and delay were both dark (i.e., group OFF-OFF), and if the ITI was illuminated and the delay was dark (i.e., group ON-OFF). In addition, either a choose-short effect or a choose-long effect was observed if the ITI was dark and the delay was illuminated (i.e., group OFF-ON). Results are incompatible with the confusion/instructional failure view of the choose-short effect.