FIGURE 12.3 Dopaminergic effects in the gap procedure. Rats were trained in a 30-sec PI procedure and subsequently tested under acute administration of vehicle (VEH), dopamine agonist methamphetamine, and dopamine antagonist haloperidol. Left panel: MAP produces a leftward shift of the response function, and HAL produces a rightward shift of the response function in PI trials. Right panel: Contrary to prediction, the shift in peak time between PI and gap trials is affected by dopaminergic drugs: MAP increases the shift, and HAL decreases the shift relative to the vehicle condition. (Adapted from Buhusi, C.V. and Meck, W.H., Behav. Neurosci, 116, 291-297, 2002.)

should not affect the shift in peak time between PI and gap trials. In contrast, in our experiment, besides determining a rightward displacement of the response functions in PI trials (left panel of Figure 12.3), HAL also decreased the shift in peak time in gap trials relative to PI trials (right panel of Figure 12.3). The different pattern of effects of MAP and HAL in trials with and without gaps suggests that, besides affecting the clock speed, dopaminergic drugs are involved in a separate process in the gap procedure.

To explore the nature of this second process, we pursued a series of behavioral and pharmacological manipulations of the gap procedure. Most of these studies, discussed below, involved two groups of subjects or two conditions that do not differ in terms of temporal parameters, but only in terms of nontemporal aspects (stimulus intensity, stimulus content, stimulus similarity, etc.). Yet we found that these manipulations affect the response function after the gap. Taken together, the studies reviewed below suggest that the gap procedure engages attention sharing, and that this is the process underlying the effect of dopaminergic drugs in the gap procedure.

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