Behavioral And Pharmacological Evidence For Attention Sharing In The Gap Procedure

A possible interpretation of the findings discussed above in the gap procedure is that during the gap attentional resources are reallocated between the timer and the general processor. According to this view, the greater the extent that attention is paid to extraneous events — such as the gap — the more processing resources are diverted away from the timing component of the task, and the more pulses are lost, resulting in a larger delay of the peak time (peak time shift). We tested this hypothesis both at the behavioral and pharmacological levels.

At the behavioral level, the basic switch hypothesis predicts that the delay in peak time is always equal to the duration of the gap, while the basic decay hypothesis predicts that the effect of the gap depends solely on its duration. In contrast, attention sharing predicts that manipulations of nontemporal aspects of the gap procedure would affect the timing of the response. In particular, attention sharing might depend on the content of the gap, on the degree to which the gap can be discriminated from the intertrial interval (ITI), and on the intensity of stimulus events.

At the pharmacological level, assuming that MAP and HAL affect solely the speed of an internal clock, one would expect that, relative to vehicle administration, the peak of the response rate would occur proportionally sooner under MAP and proportionally later under HAL in PI trials (Maricq et al., 1981; Meck, 1983, 1986), but that the shift in peak time between PI and gap trials would not be affected by these manipulations. On the other hand, assuming that dopaminergic agonists increase the perceived salience of events (Gray et al., 1997), one might expect the gap to shift the peak time more than the PI trials under MAP, so that the response rate would peak later (reset rule). Similarly, HAL might reduce the effect of the gap on allocation of attentional resources and might cause the response rate to peak sooner after the gap (stop rule). Critically, should the effects of these behavioral and pharmacological manipulations be expected to act on the same chain of processes, related to attention sharing, they should counteract one another.

12.4.1 Comparative Studies in Pigeons and Rats

Previous data suggest that in the gap procedure rats use a stop strategy (Meck et al., 1984; Roberts, 1981; Roberts and Church, 1978) while pigeons are resetters (Cabeza de Vaca et al., 1994; Roberts et al., 1989). The upper left panel of Figure 12.4 shows that a 5-sec gap determines a rightward shift of the response function with about 5 sec in rats, suggesting that rats stop timing during the gap (Roberts, 1981). In contrast, the upper right panel shows that a 6-sec gap delays the response function with about 15 sec in pigeons, supporting the proposal that after the gap pigeons restart timing from the beginning of the interval (Cabeza de Vaca et al., 1994).

In contrast to the species-specific strategy interpretation of the results, recent findings from our laboratory suggest that nontemporal parameters of the to-be-timed event influence the response rule adopted by both rats (Buhusi and Meck, 2000) and pigeons (Buhusi et al., 2002) in the gap procedure. For example, Buhusi and Meck (2000) examined time perception and memory for timing the presence and absence of a visual stimulus. In accord with previous results, Buhusi and Meck (2000) found that when timing for the presence of a visual stimulus, a 5-sec standard (dark) gap prompts rats to stop timing, i.e., to delay their response function with about 5 sec in gap trials relative to PI trials (middle left panel of Figure 12.4). However, when timing for the absence of the stimulus, a 5-sec reversed gap during which the stimulus was turned on, prompted rats to reset the entire timing process after the gap (middle right panel of Figure 12.4). This reset result occurred at gap durations as short as 1 sec (Buhusi and Meck, 2000).

Flexible Timing Strategies in Rats and Pigeons Stop Rule Reset Rule



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