Unilateral removal of the posterior insula or of area SII: inconsistent effects on tactile, visual and auditory performance in the monkey.

Unilateral removals of the posterior insula or of the second somatic sensory projection area (SII) were made in respectively 8 and 6 monkeys. These animals were divided into a 'contralateral' group of 7 with removals from the hemisphere opposite to the preferred hand; and an 'ipsilateral' group of 7 animals with removals from the hemisphere on the same side as the preferred hand. In addition, 6 unoperated monkeys were trained comparably. Both the contralateral and ipsilateral animals were impaired in comparison with unoperated animals at threshold discriminations of graded roughness and/or sizes when using the hand opposite the removals. Animals with contralateral ablations were impaired in comparison with those having ipsilateral ablations only in a few of many instances: with the hand ipsilateral to the removals at re-learning a tactile discrimination and at learning an auditory discrimination; with the hand opposite to the removals when it was initially used on visual or auditory tasks that had already been mastered with the other hand. It is suggested that in addition to SII the insula is also organized asymmetrically in relation to hand preference in the monkey, but the insula of the predominant hemisphere may be involved as much in non-tactual as tactual performances, for example in tasks involving successive discrimination and when a familiar task has to be performed initially with the other hand.