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Behavioral Specializations of SI and SII Cortex: A Comparative Examination of the Neural Logic of Touch in Rats, Cats, and Other Mammals

In what ways do passive touch and active touch require particular properties of cortical physiology? Because it is not easy to make predictions "bottom up" from neurophysiology and anatomy to behavior, there have been surprises in studying behavioral deficits following selective ablations of SI versus SII. For example, in earlier research on cats, unilateral ablation of SI unexpectedly had no effect on passive touch but did cause contralateral losses in posture and movement; it was ablation of SII plus subjacent cortex that led to contralateral losses in passive touch. In contrast, in a recent experiment with rats unilateral ablation of either SI or SII caused contralateral losses both in touch and postural reflexes. Some of the literature on dogs, monkeys, and humans suggests similarities with cats. To better understand these comparative findings, the following theoretical factors are considered: the degree of diffuseness versus specificity in brain input-output relations demanded or permitted by a behavior, the spatial and temporal scales of a behavior, the species' degree of encephalization, the need for stimulus generalization or functional equivalence of movements, and the relative sizes and sensitivities of different parts of the body. Hypotheses are also offered about why the evolution in rats of shortened forelimbs and increased vibrissal function may have entailed a peculiar compromise between active and passive touch and between functions of SI and SII.