MICROELECTRODE STUDIES ON MEDIAL GENICULATE BODY OF CAT: III. RESPONSE TO PURE TONES

Publisher Website
Google Scholar
Abstract
Pure tones may evoke single unit responses from the medial geniculate, although a large number of units responsive to clicks or noise or both could not be excited by tones. The response evoked by pure tones typically exhibits 2 main features: (1) an early complex of slow waves and spikes (onset response) that closely resembles the response to a click, and (2) a later continuous discharge of the unit. The early complex cannot in all, if in any, cases be attributed to click transients accompanying the switching on of the tone, and the complex is absent when the tone is turned off. The band of frequencies that excites a unit is relatively wide even at threshold, and its width increases as the stimulus intensity increases. Attenuation of the tone is followed by diminution in amplitude of the slow components, an increase in latency of the onset of the response, and a drop in discharge rate of the single units. An occasional unit, not spontaneously active, is silent during the course of a pure-tone stimulus and discharges vigorously shortly after the tone is turned off. The electrical response of the medial geniculate that follows a click is suppressed when certain pure tones are presented simultaneously. Both stimuli must be presented to the same ear, and reasons are given for concluding that neural interactions important for suppression occur at levels below the medial geniculate. The effective suppressor frequencies may comprise a narrow or a wide band of tones. Low frequencies, particularly those below 2000 cps, were more frequently encountered as suppressors than were high ones. The band of effective frequencies increases in width as the intensity level of the tones is raised relative to that of the click. A slight change of frequency or intensity may convert a tone from one that suppresses into one that does not.