Forebrain Commissurotomy and Conscious Awareness

The left and right cerebral lobes of the mammalian brain in the natural state are largely separate anatomically except for cables of cross-connecting fibers, the cerebral commissures, most prominent of which is the enormous corpus callosum, the largest fiber system of the brain, estimated to contain in man over 200 million fibers. Experimental investigation of the functional role of the cerebral commissures was stimulated during the early 1940s by a series of clinical reports in which complete surgical section as well as congenital absence of the corpus callosum had seemingly failed to produce any consistent or distinct behavioral symptoms detectable in extensive neurological and psychological testing (Akelaitis 1943; Bremer 1958; Bremer, Brihaye, and Andre-Balisaux 1956). Animal studies started in the early 1950s, mostly on cats and primates (Myers and Sperry 1953; Sperry, Stamm, and Miner 1956; Sperry 1961; Downer 1962; Myers 1962), showed consistently, however, that each hemisphere after surgical separation functions independently to a very large extent in most higher activities, including perception, learning, and memory. In objective behavioral tests involving sensory discrimination learning, each surgically disconnected hemisphere was found to sense, perceive, learn, and remember independently of the other