Hemispheric asymmetry for a visuo-spatial task in monkeys.

The hypothesis that hemispheric asymmetries for visuo-spatial ability exist in rhesus monkeys was tested using a task for which the right hemisphere is specialized in man. The task required discrimination between two squares, one containing a central dot and the other a dot displaced upwards from centre. The animals' thresholds were determined before and after left-sided (LH-lesion) or right-sided (RH-lesion) unilateral occipital lobectomy combined with splenial transection, and the percentage change in threshold calculated. The LH-lesion monkeys were all worse than the RH-lesion monkeys on this measure, indicating the existence of a functional asymmetry.