Differential effects of unilateral and bilateral caudate lesions on side preference and passive avoidance behavior in rats.

The effects of unilateral and bilateral caudate lesions on side preferences, passive avoidance learning and locomotor activity were determined in rats. Unilateral lesions ipsilateral to preoperative side preferences increased side preference and facilitated passive avoidance learning. Contralateral lesions reversed side preferences and impaired passive avoidance learning. Bilateral lesions increased side preference in some rats and decreased or reversed side preference in other rats; the increase or decrease in side preference was related to a lesser or greater impairment in learning, respectively. All caudate lesions increased locomotor activity. It is suggested that an intrinsic nigro-striatal asymmetry has a modulatory role in learning and that the effects of bilateral caudate lesions are, in part, attributable to very slight asymmetries in lesion locations. The effect of a unilateral caudate lesion will depend upon whether the lesion is placed in the more or less active striatum.