Size constancy in rhesus monkeys: effects of pulvinar, prestriate, and inferotemporal lesions

The present study tested the theory that inferotemporal cortex integrates 1) distance information transmitted via superior colliculus-pulvinar afferents, with 2) form information transmitted via striate-prestriate cortex afferents (Gross, 1973a, 1973b). Monkeys were trained to choose the larger of two objects, independent of distance, to obtain a reward. Based on the integration theory, the following predictions concerning this size constancy discrimination were made: 1) monkeys with pulvinar lesions, unable to code distance, should be impaired and adopt strategies based on retinal image size; and 2) monkeys with prestriate lesions, unable to code retinal image size, should be impaired and adopt strategies based on distance. Contrary to these predictions, pulvinar lesions produced no deficit; and although prestriate lesions did produce an impairment, it was due to a failure to code distance in assessing the true size of the object. Thus, monkeys with prestriate lesions consistently responded to retinal image size instead of object size. Replicating an earlier report (Humphrey and Weiskrantz, 1969), inferotemporal lesions also produced an impairment; however, errors made by monkeys with inferotemporal lesions were random and could not be attributed to any consistent strategy. All monkeys reacquired the discrimmination postoperatively, indicating that there are multiple mechanisms available to the brain-damaged animal for the perception of size constancy.