A lasting impairment in circle-ellipse discrimination after inferotemporal lesions in monkeys

Monkeys with bilateral lesions in the inferotemporal cortex learn and relearn visual discrimination problems slowly, but previous studies have not settled the question of whether they, in addition, have a lasting deficit. I have trained monkeys to discriminate a circle from ellipses on a titration schedule and they produced the same thresholds as previously found in young humans. After incomplete inferotemporal lesions the monkeys were retrained in 10,000 trials during one year. Thresholds were impaired in 3 of 5 monkeys with no tendency to improvement during the last 6 months. Neither the transient nor the lasting deficit occurred after control lesions in the superior temporal gyrus. The small lasting deficit was not owing to geniculo-striate damage; in addition to histological evidence only striate lesions increased choice reaction times. The small lasting deficit is compatible with the hypothesis, that the visual system contains a hierarchy of sequential feature detectors.