Behavior-decrementing effects of low doses of haloperidol result from disruptions in response force and duration.

By using a factorial experimental design, the joint effects of two levels of sucrose reward, two levels of required force, and four levels of haloperidol dose (0. 0.04, 0.08, 0.16mg/kg) were examined for three measures of operant response: peak force, duration, and interresponse time. Even though a 24% sucrose reward led to more rapid acquisition of the operant than an 8% concentration during the drug-free response shaping period, neither the reward nor the required-force manipulations interacted with haloperidol dose during subsequent testing. Haloperidol had significant elevating effects on peak force and duration of response, while lengthening interresponse time. A within-session analysis revealed drug-related slowing of both response duration and interresponse time as the operant session progressed. Finally, dose effects on peak force and duration were apparent from the beginning of the session, but effects on interresponse time reached significance later in the session. Taken together the results downplay the importance of stimulus efficacy, anhedonia and required effort in accounting for haloperidol's behavior-decrementing effects. Instead, the results raise the possibility that the haloperidol-treated rats experienced difficulty in sensorimotor control of the operant response.