HEMIDECORTICATION AND REVERSAL LEARNING IN THE RAT: HEMISPHERIC ASYMMETRY FOR TENDENCY TO PERSEVERATE

Michael Noonan, Canisius College, Buffalo, New York and Seymour Axelrod, State University of New York at Buffalo

Supported in part by NSF IBN-9209551

Presented at the Society for Neuroscience annual meeting, San Diego, California, November, 1995.

INTRODUCTION

Noonan and Axelrod (1992) studied the effects of hemidecortication on rats' ability to acquire the second of two related water-escape tasks. They found, in a sample biased toward turning left, a consistent hemispheric asymmetry in which left hemidecorticates were impaired, and right hemidecorticates were facilitated, on acquisition of the second task. The present experiment was designed to test (a) whether those results could be attributed to an asymmetrical neural representation of a tendency to perseverate, and (b) whether the direction of the underlying neural asymmetry was opposite for left- and right-biased sub-samples.

PROCEDURES

Eighty-one naive subjects were trained on a left-right response differentiation (LRRD) in an M-shaped water maze (Fig.1). On each trial, both arms of the maze were either lit or unlit, this condition varying randomly from trial to trial. When the arms were lit, the escape ladder was found on the right; when the arms were unlit the ladder was on the left. Based on the turning bias shown during acquisition of this task, the rats were classified as biased either toward left-turning or toward right-turning. Half of each subgroup was then subjected to either left- or right-hemidecortication (Fig. 2). Six weeks after surgery, all subjects were tested for acquisition of a reversal of the LRRD, in which the escape ladder was now located in the right in the unlit condition and on the left in the lit condition.

RESULTS

1.Unlike the sample in the Noonan & Axelrod 1992 study, the present sample of rats had approximately equal numbers of left- (N=37) and right-biased (N=44) rats.

1.As predicted, an ANOVA on acquisition scores (trials-to-criterion) on the reversal task, with turning bias and side of lesion as independent variables, and trials-to-criterion on the original LRRD task as covariate, revealed a significant (p = .024) interaction between turning bias and side of lesion. The results are depicted on the right side of Figure 3: for each turning-bias group, decortication ipsilateral to the bias resulted in high scores (poor performance) on the reversal task, and contralateral decortication resulted in low scores. There were no significant main effects.

DISCUSSION

On the assumption that trials-to-criterion scores on a reversal task index a tendency to perseverate, our findings are compatible with the hypothesis that perseveration is asymmetrically represented in the rat's cortex, and that the direction of this asymmetry is predicted by the rat's turning bias. For rats with left-turning biases, the right hemisphere appears to be more perseverative; for right turners, the left hemisphere is more perseverative. Our current efforts are directed toward (a) testing this hypothesis using other tasks, and (b) testing it against alternative explanations.

Contact Info: Michael Noonan, PhD, Canisius College , 2001 Main St., Buffalo, NY 14208                                                                             noonan@canisius.edu