Two-Tone Inhibition in Neurons of the Cat's Medial Geniculate Body

In order to study carefully the configuration of response areas of auditory neurons in the medial geniculate body (MGB), we have examined an interaction between two tones known as "two-tone inhibition". This entails suppression of response activity to one tone by a second tone which is selected from a restricted range of frequencies and intensities. Single units were extracellulary recorded from non-paralyzed, lightly anaesthetized (pentobarbital, nitrous oxygen) adult cats. For each unit the discharge pattern, response latency, response area (RA), characteristic frequency (CF) and response-intensity functions were determined by binaural stimulation with pure sinusoids (0.1-25.6 kHz separated by Πoctave steps; stimulus duration 200 ms). During two-tone stimulation those tone bursts were delivered at fixed amplitude (usually 20 dB suprathreshold at CF) and a second tone of selected frequency and variable amplitude was presented simultaneously. Recording sites were defined electrophysiologically and localization was confirmed histologically.