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Implications for Psychological Models of Interval Timing
Several models have been previously applied to interpret data obtained in interval-timing experiments involving gaps (retention-intervals). A switch model, assuming that during the gap time fails to accumulate due to the opening of a stimulus-controlled switch, predicts that, irrespective of gap and criterion durations, subjects should use a stop rule [14], [15], [30], [31]. In contrast, the present data indicate that rats flexibly change their behavior with both gap and criterion duration. An instructional-ambiguity model proposing that subjects perceive the gap as an ambiguous, ITI-like event, predicts that manipulations that render the gap similar to the ITI should reset the clock, while manipulations that render the gap dissimilar from the ITI should stop the clock [37]. Because in our experiment the gap and ITI were identical (dark) this hypothesis predicts the use of the same rule (reset) at all gap and criteria durations, in contrast to the current data showing the use of different rules for different temporal criteria. To account for the flexible use of the run/stop/reset rules by rats and pigeons, Cabeza de Vaca et al. [38] proposed a passive memory-decay model which assumes that subjective time--stored in WM--decays passively during the gap. This model predicts that the effect of a gap should depend solely on absolute gap duration, but not on criterion duration, as found here (Fig. 1B-D). These models can not account for the differential effects of gaps on multiple intervals.
In contrast to these alternatives, the present resource allocation model assumes that during the gaps resources are re-allocated (diverted away from timing), each clock is unable to maintain its current subjective time in WM, and the response is delayed [16], [17], [27] in proportion to the perceived salience of the distracter relative to the times signal [34], [35]. Indeed, the effect of a gap is affected by the contrast in intensity between the gap and the timed signal [27], [34], [39] and by the perceptual acuity of the subjects [27]. Importantly, the predictions of this resource re-allocation model are not restricted to retention-intervals (i.e., gaps), but extend to distracters presented during the continuous presentation of a timed signal. Previous data were quantitatively accounted for by assuming that during a gap and/or distracter the accumulated time decays at a rate that varies with the relative salience of the distracter to the timed signal [25], [26], [35]. Here we found that the comparison between the gap and the (multiple) timed signal(s) extends to the temporal domain. Simulations indicate that both the differential effect of the gap on multiple durations, and the gradual hierarchical reset due to increasing gap duration can be quantitatively accounted for assuming that the three clocks have separate resources re-allocated independently during a gap at a rate proportional to the ratio g/Tk between the duration of the gap and each criterion duration (Fig. 2A:D-Model).
