Dopamine And The Internal Clock

12.2.1 An Information-Processing Model of Interval Timing

A representation of the mapping between the stopwatch metaphor (Church, 1978) and an information-processing model of interval timing (Gibbon et al., 1984) is given in Figure 12.1A. The information-processing model of interval timing continues the tradition of Treisman's (1963) internal clock in that pulses emitted by a pacemaker are temporarily stored in an accumulator, and that upon delivery of reinforcement, the number of pulses from the accumulator is stored in reference memory (Gibbon et al., 1984). The model implements the scalar expectancy theory (Gibbon, 1977) in that the response of the subjects is controlled by the ratio comparison between the current time (stored in the accumulator) and the criterion interval (stored in the reference memory). Figure 12.1B shows that according to the model, subjective time, stored in the accumulator, is linearly related to objective time, and that the error in time estimation increases with the to-be-timed interval. Therefore, two groups of subjects trained to time 20- and 40-sec criterion intervals will have similar response functions in terms of shape (shown in the left-hand side of Figure 12.1B), but the width of the response function will be twice as large in the 40-sec group as in the 20-sec group. However, the information-processing model (Gibbon et al., 1984) ignores the possible interactions between interval timing and other cognitive processes (see next section). The effects of manipulations of nontemporal aspects of the paradigms are attributed to changes in the speed of the clock or in delays in the activation of the various components of the model (see e.g., Church, 1978, 1984). For example, the model proposes that changes in clock speed account for data showing that bright lights are judged to be longer than dim lights by pigeons (Kraemer et al., 1997a) and rats (Kraemer et al., 1995), and filled intervals are judged to be longer than empty intervals by pigeons (Mantanus, 1981). Accordingly, Figure 12.1C shows that changes in clock speed are proposed to affect the slope of the linear relationship between subjective and objective time. The behavioral effects of such changes are predicted to be proportional to the criterion interval, a feature usually referred to as the clock pattern: a manipulation that supposedly changes the clock speed is predicted to shift the response function twice as much in a group of subjects trained to time a 40-sec interval as in the group trained with a 20-sec interval. Conversely, should the effects of a drug on time perception be proportional to the criterion interval, this would suggest that the drug affects interval timing by changing the speed of the clock. The important point here is that the model has specific predictions for manipulations of each of its components.

12.2.2 The Peak-Interval Procedure

How would one go about testing the above predictions experimentally? Data discussed in this chapter were collected using the peak-interval (PI) procedure. In the PI procedure subjects are exposed to two types of trials: fixed-interval (FI) trials and PI trials. In FI trials, subjects are presented with a signal for a fixed duration. The first response after the fixed duration terminates the to-be-timed signal and a)

Internal Clock Passive Decay

Internal Clock Passive Decay b) c)

Objective Time (s) Objective Time (s)

Drug Session

FIGURE 12.1 Dopamine and clock speed. (A) An information-processing model of interval timing. (Adapted from Gibbon, J. et al., Annals of the New York Academy of Sciences: Timing and Time Perception, New York Academy of Sciences, New York, 1984, pp. 52-77.) (B) The scalar property as implemented in the information-processing model: the peak time response function is twice as wide when subjects time a 40-sec interval as when they time a 20-sec interval. (C) Theoretical increase in speed of accumulation of temporal units by indirect dopamine agonist methamphetamine and theoretical decrease in speed of accumulation of temporal units by dopamine antagonist haloperidol. Effects on the peak time of responding are predicted to be proportional to criterion interval. (D) Effect of systemic administration of MAP and HAL on the peak time of responding. (Adapted from Meck, W.H., Cognit. Brain Res., 3, 227-242, 1996.) MAP produces a proportional leftward shift of the response function, and HAL produces a proportional rightward shift of the response function.

Drug Session

FIGURE 12.1 Dopamine and clock speed. (A) An information-processing model of interval timing. (Adapted from Gibbon, J. et al., Annals of the New York Academy of Sciences: Timing and Time Perception, New York Academy of Sciences, New York, 1984, pp. 52-77.) (B) The scalar property as implemented in the information-processing model: the peak time response function is twice as wide when subjects time a 40-sec interval as when they time a 20-sec interval. (C) Theoretical increase in speed of accumulation of temporal units by indirect dopamine agonist methamphetamine and theoretical decrease in speed of accumulation of temporal units by dopamine antagonist haloperidol. Effects on the peak time of responding are predicted to be proportional to criterion interval. (D) Effect of systemic administration of MAP and HAL on the peak time of responding. (Adapted from Meck, W.H., Cognit. Brain Res., 3, 227-242, 1996.) MAP produces a proportional leftward shift of the response function, and HAL produces a proportional rightward shift of the response function.

triggers the delivery of reinforcement. In PI trials, the to-be-timed signal is presented for about three times the FI, and subjects' responses are not reinforced. In these trials the mean response rate increases after the onset of the to-be-timed signal, reaches a peak about the time when subjects are (sometimes) reinforced, and gradually declines afterwards (Catania, 1970; Church, 1978). According to the information-processing model of interval timing (Gibbon et al., 1984), in a PI procedure the current value in the accumulator is compared to the reference memory and the response rate reaches a peak at the time when subjects' responses are (sometimes) reinforced. Therefore, according to the model, the experimentally estimated time at which the response rate reaches a peak in PI trials can be taken as an estimation of the criterion stored in reference memory. Should a manipulation change the clock speed after acquisition of a temporal criterion, the response function would shift leftward (if the speed of the clock increases) or rightward (if the speed of the clock decreases). Conversely, should a drug shift the peak function in a manner proportional to the intervals being timed, this would give a strong indication that the drug acts at the pacemaker or accumulator level of an internal clock.

12.2.3 Dopamine and the Clock Pattern

Psychopharmacological studies (Maricq and Church, 1983; Maricq et al., 1981; Meck, 1983, 1986) have provided considerable support for the neural basis of some of the parameters of the information-processing model of time perception (Gibbon et al., 1984), proposing that dopaminergic drugs selectively affect the subjective speed of an internal clock, while cholinergic drugs alter memory storage (e.g., Meck, 1983, 1996). An acceleration of the subjective clock speed is suggested by a variety of timing studies in rats and pigeons using indirect dopaminergic agonists such as methamphetamine (MAP) (e.g., Kraemer et al., 1997b; Maricq and Church, 1983; Maricq et al., 1981; Meck, 1983) and cocaine (e.g., Lau et al., 1999; Matell et al., 2002). On the other hand, dopaminergic antagonists such as haloperidol (HAL) have been shown to produce a deceleration of the subjective clock speed in proportion to their affinity to dopamine D2 receptor (Meck, 1986; see also Maricq and Church, 1983). The effects of MAP and HAL on the peak time of responding in a PI procedure (Meck, 1983, 1996) are shown in Figure 12.1D: MAP administration immediately shifts the response function leftward (decreases the mean peak time), and HAL administration immediately shifts the response function rightward (increases the mean peak time). Both effects are proportional to the criterion interval. Indeed, as shown in Figure 12.1D, the drugs shift the response peak time twice as much in rats trained to time a 40-sec interval as in rats trained to time a 20-sec interval. These results suggest that dopaminergic drugs selectively affect the speed of an internal clock (Gibbon et al., 1984).

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