Conclusions

12.5.1 Behavioral Dissociation of Attention Sharing and Time Accumulation

The experiments reviewed here were designed to dissociate the switch, decay, and attention-sharing hypotheses by manipulating temporal and nontemporal parameters of the gap procedure. In accord with the attention-sharing hypothesis, data reviewed here show that increasing the salience of the interrupting event by (a) manipulating the content (illuminated vs. dark) of the event, (b) increasing the similarity between the gap and the ITI, or (c) increasing the intensity of the stimuli prompted rats to further delay their response functions after the gap. These results are in accord with the interpretation that more processing resources are allocated to the (temporal or general) processing of salient stimuli. On one hand, bright lights are judged to be longer than dim lights by pigeons (Kraemer et al., 1997a) and rats (Kraemer et al., 1995), possibly due to allocation of more resources to the timer. Conversely, data reviewed here show that high-intensity gaps displace the response function more than low-intensity gaps in both rats (Figure 12.5) (Buhusi et al., 2002) and pigeons (Figure 12.4) (Buhusi et al., 2002), possibly due to allocation of more resources to the general processor. Similarly, filled intervals are judged to be longer than empty intervals by pigeons (Mantanus, 1981), possibly due to allocation of more resources to timing of filled intervals than of empty intervals (Grondin et al., 1998; Thomas and Weaver, 1975). Conversely, filled gaps were found to displace the response function more than empty gaps (Figure 12.4) (Buhusi and Meck, 2000), possibly due to allocation of more resources to the general processing of filled gaps than of empty gaps. Moreover, few resources might be dedicated to timing during the ITI, thus, during a gap similar to the ITI, resulting in a larger effect of the gap in the similar condition than in the dissimilar condition (Figure 12.5) (Buhusi and Meck,

2002). These results complement previous findings suggesting that automatic or controlled variations in the allocation of attentional resources influence interval timing (e.g., Casini and Macar, 1997, 1999; Macar et al., 1994; Penney, this volume; Zakay, 2000). The present findings suggest that besides temporal parameters of the procedure, nontemporal features of the events influence the timing of the response in the gap procedure in both rats (Buhusi and Meck, 2000, 2002; Buhusi et al., 2002) and pigeons (Buhusi et al., 2002). Because in each experiment the temporal parameters were identical among groups and conditions, these results cannot be easily accounted for by either the switch or decay hypothesis. These findings suggest that the timer shares attentional resources with other concurrent processes, and that this sharing of resources controls the response of rats and pigeons in the gap procedure (Figure 12.7A).

Taken together, the data presented here support the proposal that there might be a continuum of outcomes in the gap procedure, dependent on the salience of the stimuli involved. Both the decay and attention-sharing hypotheses allow for a continuum of possibilities in between the stop and reset extremes. However, while the decay hypothesis predicts that only variations in temporal parameters would yield such a continuum, the attention-sharing hypothesis suggests that the continuum is also related to nontemporal characteristics of the interrupting event. Therefore, in relation to temporal manipulations of the gap procedure, the present data reject the basic form of the switch hypothesis, but cannot fully differentiate the decay and attention-sharing hypotheses. Importantly, such a differentiation might not be required. Attention sharing is a mechanism concurrent with interval timing, and the decay of accumulated time might represent the effect of reallocation of attention on interval timing. As such, rather than decaying passively, the representation of accumulated time might decay at a variable rate, dependent on the attention paid to the gap (left lower panel of Figure 12.7). Consequently, within the framework of the information-processing model of interval timing (Gibbon et al., 1984), an active decay mechanism whose rate of decay is controlled by an attention-sharing mechanism seems most adept at addressing the manipulations of both temporal and nontemporal characteristics of the gap.

12.5.2 Dissociation of Clock and Attentional Effects of Methamphetamine and Haloperidol

Another way to differentiate the attention-sharing process from the pulse generator of the timer is to evaluate the effect of pharmacological manipulations on the gap procedure. Buhusi and Meck (2002) showed that MAP, a drug known to increase distractibility (Crider et al., 1982; Gray et al., 1992; Iwanami et al., 1995; McKetin and Solowij, 1999; McKetin et al., 1999), resets timing, while HAL, a neuroleptic drug known to increase selective attention (reviewed by Gray et al., 1997), stops timing (see Figure 12.3). The attentional resetting effect of MAP was eliminated in rats trained with a filled ITI and tested with empty gaps (Buhusi and Meck, 2002) (see Figure 12.6), suggesting that the behavioral and pharmacological manipulations affect the stop-reset mechanism of interval timing by acting on the same processes. Most importantly, Buhusi and Meck (2002) found that MAP and HAL shift the peak a)

General Processor

Other task

Dopamine-

Attention Sharing

Time Processor

Active Decay

Pacemaker

Drug Addiction

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