Properties of 'two-tone inhibition' in primary auditory neurones.

1. Properties of two-tone inhibition in primary auditory neurones of cats were studied with phase-locked sound stimuli. One sound was a continuous tone at the best frequency of a given neurone, and the other, a tone burst which was changed in amplitude, frequency, and phase relative to the continuous tone.2. The tone burst which caused two-tone inhibition had either an excitatory or no effect when it was delivered alone. Inhibitory areas commonly appeared on both sides of the excitatory area when the best frequency was higher than a few kc/s.3. Two-tone inhibition began and ceased within a few milliseconds of the onset and termination of the excitation caused by a tone burst. The degree of inhibition was greatest at the beginning of the tone burst and reached a plateau within 500 msec. The discharge rate during inhibition could be lower than the rate for either tone alone or for spontaneous activity. At the termination of inhibition, prominent rebound in the discharge rate was found.4. With an increase in amplitude of a tone burst, for either a fixed or equally increased continuous tone, the discharge rate during inhibition decreased to a minimum and then began to increase. That is, the degree of inhibition was non-monotonically related to the sound level.5. Compound period histograms of discharges during inhibition showed that single neurones usually carried information about the combined wave form of the two tones. The information about each tone was, however, modified by the inhibitory phenomenon in both amplitude and phase from that indicated by the compound period histograms for the individual tones.6. Possible mechanisms and functional significance of two-tone inhibition are discussed.