Pattern discrimination thresholds after partial inferior temporal or lateral striate lesions in monkeys

Ablation of inferior temporal (IT) cortex, particularly of the posterior region, produces severe impairment in pattern discrimination learning. The present study examined whether this impairment is associated with raised pattern discrimination thresholds. Groups of three monkeys each were given either anterior IT, posterior IT, or foveal striate lesions, or kept as controls. They were trained after surgery on a threshold task in which a 90 degrees white angle on a gray ground was the standard, and 15 angles ranging from 10 degrees through 88.5 degrees were the comparisons. As expected, monkeys with posterior IT lesions were the most severely impaired in learning the initial discrimination (90 degrees vs. 10 degrees). However, only the monkeys with foveal striate lesions showed significant impairment on the subsequent threshold determinations. The results indicate that raised pattern discrimination thresholds are not the cause of the pattern discrimination learning deficits produced by inferior temporal lesions. Data from additional visual discriminations presented after threshold testing was completed point, instead, to a loss of attention to stimulus features as the explanation for the learning deficit.