Timing for the absence of a stimulus: The gap paradigm reversed

Contrary to data showing sensitivity to nontemporal properties of timed sig nals, current theories of interval timing assume that animals can use the p resence or absence of a signal as equally valid cues as long as duration is the most predictive feature. Consequently, the authors examined rats' beha vior when timing the absence of a visual or auditory stimulus in trace cond itioning and in a "reversed" gap procedure. Memory for timing was tested by presenting the stimulus as a reversed gap into its timed absence. Results suggest that in trace conditioning (Experiment 1), rats time for the absenc e of a stimulus by using its offset as a time marker. As in the standard ga p procedure, the insertion of a reversed gap was expected to "stop" rats' i nternal clock. In contrast, a reversed gap of 1-, 5-, or 15-s duration "res et" the timing process in both trace conditioning (Experiment 2) and the re versed gap procedure (Experiment 3). A direct comparison of the standard an d reversed gap procedures (Experiment 4) supported these findings. Results suggest that attentional mechanisms involving the salience or content of th e gap might contribute to the response rule adopted in a gap procedure.