It is now more than forty years since Bykov (1924-1925) reported that transection of the corpus callosum in dogs prevents the irradiation of the conditioned response between corresponding points on the left and right sides of the body. The subsequent work falls into two periods. First, there was what might be called the “negative” period. A large number of investigations were carried out during this period, for the most part on patients with surgical section or disease of the corpus callosum, notably by Akelaitis (1944) and his colleagues and also by others. The bulk of the findings were interpreted as “negative,” that is, as tending 31to show no behavioral alterations that could be specifically ascribed to the commissural damage. However, even during this so-called “negative” period certain subtle deficits of the kind that might be expected to follow commissural damage, for example, the inability to read letters in the left half-field of vision, came to light (cf. Maspes, 1948; Smith, 1951; Trescher & Ford, 1937). The second or “positive” period took its origin from the animal experiments of Myers (1956), Myers and Sperry (1953, 1956), and Sperry, Stamm, and Miner (1956). These and the many subsequent experimental investigations have demonstrated beyond doubt that the forebrain commissures can participate actively in the exchange of information between the cerebral hemispheres. The application of new and more sensitive testing methods soon gave evidence also in man of manifold behavioral defects consequent upon commissural damage (cf. Gazzaniga, Bogen, & Sperry, 1962; Geschwind & Kaplan, 1962).