Commissural connections between occipital lobes of the monkey

One of the prime concerns of the neurological sciences has been the relation of brain processes to processes of the mind. Despite this long term interest, insight into the physiological bases of the “higher mental functions” remains fragmentary. Further advance in this area requires among other things an expansion of our knowledge of the patterns of interconnection between and within the various regions of the cortex. In line with this need the present study of the commissural connections between the occipital lobes of Macaca mulatta was undertaken as a first effort toward defining in detail the anatomical organization of the visual cortical areas. A review of present information relating to the commissural connections of the occipital lobes in various mammalian species follows. It is hoped that such comparative anatomical knowledge may aid in the ultimate comprehension of the functional meaning of this interlinking tract. Interhemispheric projection between the occipital areas of the brain of the rat has received attention in a study using the Nauta silver technique (Nauta and Bucher, ’54). Small lesions restricted to the gray mantle of area 17 or of areas 17 and 18 of one hemisphere gave rise to degenerating fibers entering all layers of the lateral third of the opposite area 17 and the adjoining half of the opposite area 18“. A smaller number of degenerating fibers also entered to ramify in the deeper layers only of the intermediate third of the opposite area 17. No degenerating fibers were seen, however, to enter the most medial third of the opposite area 17 or in area 18 proper. Thus in the rat definite evidence was seen for heterotopy in commissural connections of the occipital lobe. Conflicting evidence exists regarding commissural connections between occipital areas of the cat’s brain. Polyak (’27) used the Marchi technique to follow degeneration resulting from discrete lesions of the cortex. He found that both the striate area (area 17) and the adjoining peri-parastriate area (field 18-19) associate through the corpus callosum with the opposite striate area and with the opposite peri-parastriate area. Beyond the indication that projection between homotopic regions was more dense than that between heterotopic, no finer analysis of the configuration of commissural connection between these areas was given. Curtis (’40) studied the interhemispheric transmission of nerve impulses using electrophysiological techniques. He stimulated electrically the exposed cortex of one hemisphere of the cat and recorded the electrical response of contralateral cortex through pick-up electrodes. He described projection of’ impulses between most of the homologous loci studied including the striate areas. The energy of projection between the striate areas was, however, much less than that between other areas of the exposed cortex. Conduction of maximal energy occurred between Polyak‘s pen-parastriate areas of the two sides. Curtis did not describe the heterotopic relations existing between the various fields of the occipital areas of the two hemispheres. Garol(‘42) investigated interhemispheric conduction of discharges using the technique of strychnine neuronography. He failed to demonstrate contralateral spiking after strychninization of area 17 of the cat. Strychninization of area 18 gave contra-