Effects of endbrain lesions on visual discrimination learning in pigeons.

Abstract

While there has been considerable study of the avain forebrain, we still have little understanding of its role in behavior. Zeigler (1963) found that lesions restricted to hyperstriatal areas produced deficits in pattern and intensity discrimination. Because this region is itself composed of two areas, a hyperstriatum and an accessory hyperstrian~m, it was not possible to determine to which area the deficits were attributable. The present study was designed to investigate the role of these two areas in visual discrimination learning by training 12 adult male pigeons to 90% criterion for 50 trials on two consecutive days to make pattern (triangle vs circle), intensity (high vs low), and color (red vs green) discriminations in key pecking, balancing stimulus and transfer effects. Ss were then divided into four surgical groups (unilateral and bilateral lesions for each area) and retested. Ablation of either accessory hyperstriatum produced deficits in visual discrimination confirming the findings of Zeigler (1963). While unilateral lesions were not accompanied by deficits, bilateral lesions of accessory hyperstriatum produced deficits in pattern discrimination (p < .08) and bilateral lesions of hyperstriatum produced deficits in color discrimination (p < .08). These results suggest that the deficit^ produced were more a function of the extent of the lesion rather than its loczls. Ss with lesions in accessory hyperstriatum showed no differences in color or intensity discrimination, and Ss with lesions in hyperstriatum showed no differences in pattern or intensity discrimination. Examination of the learning curves for these Ss indicated that the original learning on those tasks in which no deficit was produced was better, suggesting that the magnitz~de of the deficit was, in paTt, a function of difficulty in original learning and inherent problem difficulty rather than the specific cype of problem, i.e., pattern discrimination. It was also noted that positive and negative transfer effects in the sequential presentation of problems with similar elements conuibute to retention after ablation as well as original learning. It, therefore, seems methodologically important to minimize transfer effects when studying the effects of ablation on learning.