Physiology & Behavior, Voi. 16, pp. 529-536. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A. Effects of Hyperstriatal Lesions on Within-Day Serial Reversal Performance in Pigeons EUAN M. MACPHAIL Laboratory of Experimental Psychology, University of Sussex, Falmer, Brighton BN1 9QG, England (Received 7 April 1975) MACPHAIL, E. M. Effects of hyperstriatal lesions on within-day serial reversal performance in pigeons. PHYSIOL. BEHAV. 16(5) 529-536, 1976. - In Experiment 1, 12 pigeons performed each day for 17 days 3 serial reversals of a simultaneous color discrimination; 6 birds were then given hyperstriatal lesions, and 6 birds were sham operated. Analysis of the effects of lesions on postoperative reversal performance supported 2 hypotheses: first, that the hyperstriatum contains a mechanism involved in the generation of inhibition, and second, that minimal inhibition is generated (in intact birds) in the first of each day's reversals. Experiment 2 investigated the effects of hyperstriatal lesions on serial position reversal; results again supported the notion that the hyperstriatum is involved in response inhibition. Contrasts between the effects obtained in the 2 experiments suggested that inhibition plays a more central role in position, as opposed to color, reversals. Hyperstriatal lesions Reversal learning Inhibition THIS paper reports 2 experiments which investigated the effects of hyperstriatal lesions in pigeons on within-day reversals of a simultaneous discrimination. The particular lesion and behavioral situation were chosen because theoretical accounts for each of them have introduced the notion of inhibition in such a way that certain lesion effects may be predicted. Should these effects be obtained, then further evidence will have been provided, not only for the interpretation of the lesion, but also for the theoretical analysis of reversal learning. This study, in other words, hopes to use the outcome of a physiological investigation to contribute to behavioral theory. The avian hyperstriatal complex consists of three major components, the hyperstriatum ventrale, the hyperstriatum dorsale, and the hyperstriatum accessorium, the latter two structures forming the so-called Wulst [1]. Lesions within this complex disrupt both acquisition and retention of difficult successive discriminations [8, 13, 17], and impair reversals of simultaneous discriminations [ 8,15], but gener- ally have no effect on the acquisition or retention of simultaneous discriminations ([5, 8, 151; Macphail [10] reports an exception to this generalisation). Macphail [8] noted the parallel between these effects and those obtained by certain limbic lesions in mammals and proposed that there was in the avian hyperstriatum a mechanism for response inhibition; this hypothesis is of particular rele- vance to this study. In a recent review, Macphail [11] described 3 further hypotheses concerning avian hyperstriatal function. One of these, that hyperstriatal birds suffer from a perceptual deficit, [ 13] was rejected on the grounds that it could not accommodate the finding of selective impairment of reversal, as opposed to acquisition, of simultaneous dis- criminations. A second hypothesis is that hyperstriatal birds are less likely than normals to shift their mode of response as a consequence of nonreinforcement, a notion derived from Olton's [ 12] account of hippocampal function in the rat; as evidence for this hypothesis, it has been shown that hyperstriatal pigeons are more likely than normals to halt responding altogether in the course of reversal learning [101, a fact that clearly poses problems for the response suppression account. The third hypothesis is that hyper- striatal birds suffer an impairment in selective attention that causes them great difficulty in paying attention to the relevant stimulus in the initial stages of reversals [5,14]; consonant with this view is Stettner's [ 14] report that hyperstriatal quail performing reversals show little more perseverative responding to the former positive stimulus than do normals, but greatly increased position habits. The experiments to be reported here bear on all the hypotheses discussed, and so may help rule out one or more of them. The relevant behavioral situation was described by Macphail [7], in a study of pigeons that performed each day for 17 days 3 reversals of a simultaneous (red-green) 1 This research was supported by a grant from the U. K. Medical Research Council; computing facilities were made available by a grant from the U. K. Science Research Council. I am grateful to June Atherton for assistance in preparation of the figures, to Colin Atherton for photographic assistance, and to N. J. Mackintosh and G. Hall for their critical comments on an earlier draft of this manuscript. 529