A DIRECT PATHWAY FROM LATERAL GENICULATE BODY TO ASSOCIATION CORTEX

Abstract

Electrical stimulation of the optic nerve elicits a response in the medial lip of the middle part of the suprasylvian sulcus with a latency of 1.7-2.5 msec. The most prominent part of this response lasts about 10 msec. The response on the suprasylvian cortex varies in amplitude by the same amount and in the same direction as does the visual response recorded at the same time in the striate cortex. The response in suprasylvian cortex is not altered by isolating the ipsilateral LGB from post-tectal and medial thalamic structures, by isolating the ipsilateral gyri lateralis, postero-lateralis, splenialis and suprasplenialis or by destroying the contralateral LGB or optic nerve. The response is abolished by a nodal polarization of the ipsilateral LGB. The first small deflection at the foot of the response is attributed to the arrival in the suprasylvian cortex of activity propagated directly from the LGB by fibers with a conduction velocity greater than 10m/sec but less than 30 m/sec; the latter value has been extimated to be the conduction velocity in the group of fastest radiation fibers from LGB to the striate cortex. Responses to repetitive stimulation at the LGB recorded in the striate cortex revealed a spike following the primary radiation spike which could be attributed to geniculo-cortical fibers of the same slow conduction velocity. These slower conduction fibers have been called secondary radiation fibers. It is suggested that the striate cortex which receives the primary radiation fibers is part of a considerably larger cortical area innervated by the system of secondary fibers. The primary and secondary radiation fibers both originate in geniculate cells innervated by the group of fastest fibers in the optic tract (first tract fibers). It is suggested that the extra-striate portion of the geniculo-cortical system may mediate those forms of intensity and pattern discrimination which remain after removal of the striate cortex in cats.