Physiological studies of central masking in man. I: The effects of noise on the 40-Hz steady-state response.
In a typical masking situation, two Békésy waves overlap on the basilar membrane, and each of them initiates a stream of nerve impulses that enters the brain via the auditory nerve. Much is known about the overlapping of the cochlear waves, but much less about where, how, and even if at all, the impulse streams interact once they get inside the brain. In these experiments the incoming impulses are measured electrophysiologically using the auditory brainstem response (ABR), and, simultaneously, using the 40-Hz auditory steady-state response (SSR) to monitor events at a probable site of their interaction, the auditory cortex. The principal finding is that, when progressively increasing levels of continuous noise are presented to the contralateral ear, the SSR to the signal drops to about half its control amplitude. Second, low levels of ipsilateral noise reliably enhance SSR amplitude. Third, moderate levels of ipsilateral noise reduce SSR latency. In none of these cases does the ABR show similar effects. These findings are interpreted to mean that, in each case, impulses excited by the signal interact with impulses excited by the noise, and regardless of ear of origin the interactions take place beyond the brainstem level where ABR wave V is generated, either before the impulses reach the cortex, or in the cortex itself.