SURFACE MOVEMENTS OF CLICK RESPONSES FROM ACOUSTIC CEREBRAL CORTEX OF CAT: LEADING AND TRAILING EDGES OF A RESPONSE FIGURE

Abstract

By means of a recording technique which gives simultaneous samples of the electrical activity from 25 electrodes on the cerebral cortex, records of click responses are taken on the acoustically responsive cortex of cats. Results are presented of analysis of such responses. Previous papers gave evidence that responses to clicks consist of 3 dimensional shapes or forms changing with time and place on the cortical surface; these forms are called "figures". The parts of figures occurring in the responses which are analyzed in this paper are the leading and the trailing edges, which represent times near the beginning and near the ending of the cortical activity at each cortical point, respectively. The positions (a) of the leading and (b) of the trailing edges at 1-4 mse. intervals over about the first 100 msec. after the click, and (c) of lines of equal duration of cortical activity are shown on brain charts. It is found that the leading edge moves at a high velocity (about 1 m./sec.) in a region corresponding to the generally accepted projection areas (acoustic I and II) for the acoustic system. The leading edge suddenly slows (18 msec, after the click) at a definite boundary to about 1/10 of its previous velocity in a region corresponding to the posterior boundary of acoustic I and II and the anterior edge of the posterior ectosylvian region. In contrast, the trailing edge moves over the active region at velocities about those of the leading edge after it slows down posteriorly, and shows no unique changes in direction or speed in the vicinity of the anatomical boundary between acoustic I and II, and the posterior ectosylvian region. It is concluded (1) that the leading edge, in the projection area, reflects the magnitudes and the timing of parts of a pre-formed afferent figure exciting cortex at different places at different times, (2) that the leading edge in the posterior ectosylvian area represents activity of cortical cells only, and (3) that the trailing edge represents later activity of a cortical system which exists throughout the 3 anatomical areas; this postulated cortical system has no definite boundaries within the 3 areas, but probably has a gradient of structural change running predominantly from top to bottom of this region of the cat''s cortex. The resemblance of this latter system to that giving surface-positive responses to direct electrical stimulation of cortex (Adrian, Burns) is discussed.