| Although the stimulant-induced leftward shift in peak time is considered a "classical" finding in the timing literature, the commonly accepted "clock speed hypothesis," (which attributes the shift to an increase in the speed of an internal clock), is incompatible with findings from the delay discounting literature showing reductions in impulsive behavior following stimulant administration (implying a stimulant-induced rightward shift in peak time). The current study proposes and tests an alternative mechanism by which stimulants (amphetamine) bias memory sampling, such that only relatively "short" durations are selected to be timed. The "memory sampling bias" hypothesis is able to address both bodies of data, and was tested by manipulating two distinct variable-interval reinforcement schedules. For a variety of proposed reasons (see discussion) however, the current study failed to find support this hypothesis, and it still remains unclear as to whether stimulants influence clock speed or the way in which memories are sampled. |
