Brain‐Stem Neuronal Coding of Binaural and Monaural Tones Free

Within the auditory medulla, there exists a class of neurons that, when stimulated by binaural tones, demonstrates a large latency shift. The range of this shift beyond the monaural latency varies from several milliseconds to 60 msec, depending upon the intensity of sound in the ear opposite the driving ear. Also, units in this part of the brain are responsive to changes in the binaural phase difference of low‐frequency sinusoids. These units are inhibited by in‐phase binaural tones; however, as the phase angle is increased in small steps to 180°, this inhibition is decreased. Beyond 180°, where now the opposite ear leads in phase, inhibition is effective again, and at 330° the response grouping of the unit is identical to its response grouping at 30°. Monitoring of the cochlear‐microphonic responses at the two ears throughout these phase studies shows that no peripheral cancellations or distortions are taking place. Evidence for introfiber volleying is shown by medullary units. These units discharge, during stimulation by monaural tones, with interspike intervals that are equal to and multiples of the period of the stimulating tone up to approximately 2.0 kc/sec. This intra‐fiber volleying is maintained down to the unit's threshold at each of the effective frequencies.