Caudate nucleus lesions impair spontaneous alternation.

Abstract

I t has been reported that rats with caudate nucleus lesions rarely display the exploratory responses or the reactivity to novel environmental stimuli generally observed in intact animals ( 5 ) . TO investigate further the effect of caudate nucleus lesions on exploratory behavior, we tested caudectomized and control rats on spontaneous alternation. Ss were 38 Sprague-Dawley rats. Fifteen rats received bilateral lesions to the caudate nucleus-putamen complex, 8 rats received bilateral lesions to the posterior cortex, and 1 5 rats served as unoperated controls. Surgical technique and histological evaluation have been outlined in an earlier paper ( 3 ) . Prior to the present experiment all Ss had been tested on a series of avoidance tasks ( 3 ) . A minimum of 25 days elapsed between the final avoidance trials and the commencement of alternation testing. In the T maze, described previously ( 4 ) , each S was given two trials daily for three days. A trial commenced when S was placed in the startbox. Three sec. later the guillotine door was raised and S allowed to enter the maze. A four-footed entry into either side of the T completed the trial. S was immediately returned to the startbox for the second trial. If an animal failed to respond within a 3-min. time limit the trial was terminated and that day's results discounted. The unoperated control group alternated significantly above chance (82.9%, x2 = 7.8, df = 1, P < .01, two-tailed); however, both the caudate lesioned group (44.2%) and the cortically lesioned group (72.0%) performed randomly. The caudate lesioned group alternated significantly less than either the unoperated control group (Xa = 13.6, df = 1, P < ,001, two-tailed) or the cortical control group ( X 3 = 3.9, df = 1, P < .O5, two-tailed). The performances of the control groups were not significantly differenr. O f the 1 5 caudate rats, 1 alternated on all three occasions, 6 on two occasions, 4 on one, and 4 not at all. Percentages of failures to leave the startbox within the 3-min. time limit (caudate, 4.5%; cortical control, 3.7%; unoperated control, 4.5%) were not significantly different. This suggests that the differences in alternation were not caused by differences in emotionality as reflected in the freezing response. Whether the spontaneous alternation deficits observed following bilateral lesioning of the caudate nucleus resulted from increased perseverative tendencies ( 2 ) , removal of inhibitory processes ( I ) , or disruption of the neural mechanisms involved in the acquisition and integration of sensory information (4 ) awaits further investigation.