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panel of Figure 12.4). However, a 10-sec gray (30% intensity) gap prompted pigeons to stop timing during the gap and to delay their response with about the duration of the gap (lower left panel of Figure 12.4). These findings support the proposal that in both rats and pigeons the effect of the gap depends to a large extent on nontemporal components. The next step in our exploration was to experimentally differentiate the switch, the passive decay, and the attention-sharing hypotheses.

12.4.2 Behavioral Manipulations of Attention Sharing in Rats

To differentiate the switch, decay, and attention-sharing hypotheses, we manipulated the content (illuminated vs. dark) of the to-be-timed visual signal, the similarity in auditory content between the gap and the ITI, and the intensity of the stimuli in rats. While attention sharing might depend on these features, the switch and decay hypotheses predict that only temporal aspects of the gap procedure are relevant. Therefore, to differentiate these hypotheses, in our experiments we maintained the same temporal parameters and manipulated only nontemporal aspects of the gap procedure. As shown in Figure 12.5, we found that all these nontemporal variables affect the peak time of responding in the gap procedure. For example, standard (dark) gaps prompt rats to stop timing, and reversed (illuminated) gaps prompt rats to reset the entire timing process after the gap (left panel of Figure 12.5) (Buhusi and Meck, 2000), possibly due to allocation of more resources to the general processing of filled gaps than of empty gaps.

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