Frontal lobe neglect in monkeys.

Using a crossed-response task, monkeys with neglect induced by frontal lesions appear to have motor rather than sensory neglect. However, the crossed-response task may not reveal sensory neglect (inattention) if no perceptual discrimination is required. We therefore trained two monkeys in a perceptually complex crossed-response task. Following unilateral frontal (arcuate gyrus) ablation, we found no contralateral perceptual disorders or sensory neglect in either monkey but did find a failure to respond with the contralateral arm (motor neglect) in both monkeys. We also found that the monkeys made more incorrect responses with the arm ipsilateral to the lesion than they did with the contralateral arm. The errors made by the ipsilateral arm could be a compensatory strategy or a disinhibition phenomenon. Because these incorrect responses were not rewarded and became more frequent as motor neglect improved, the incorrect responses of the ipsilateral arm are not a compensatory strategy, but rather a defect we term "disinhibition hyperkinesia" or "allokinesia."