Differential effects of unilateral and bilateral caudate lesions on side preferences and timing behavior in rats.

Rats were trained to bar press on a differential-reinformcement-of-low-rate 16-sec (DRL 16) schedule for water reinforcement. On alternate days, rats were tested in the presence or absence of a light that signaled the availability of reinforcement. Rats were allowed to bar press on either of two levers (left and right). All rats showed consistent side preferences. For the non-signaled condition but not for the signaled condition, normal baseline rates were related to the strength of side preferences; lower rates and better timing performance were significantly correlated with greater preferences. Unilateral lesions in the caudate nucleus ipsilateral to side preferences facilitated timing performance during nonsignaled test sessions and increased side preferences during both nonsignaled and signaled sessions. Unilateral caudate lesions contralateral to side preferences impaired timing performance during nonsignaled test sessions and decreased side preferences during all sessions. Bilateral caudate lesions transiently depressed response rates without significantly affecting timing performance or side preferences. It is suggested that side preferences are intimately involved in the control of behavior by internal stimuli and that an inherent asymmetry in nigrostriatal function underlies side preferences; the effect of a unilateral striatal lesion will depend upon whether the lesion is placed in the more or less active striatum.