The selective effects of central masking on brain stem potentials.

The effect of contralateral white noise stimulation on ipsilaterally presented click evoked brain stem potentials was examined. Continuous contralateral white noise does not influence the brain stem response components but pulsed white noise simultaneously presented with the clicks produced a central masking effect which was observed as an amplitude reduction confined to Wave V. Our findings confirm those of Zwislocki (1971) that pulsed noise is a more effective central masker than continuous noise. A computer addition of the responses to pulsed white noise and clicks recorded independently revealed no reduction in Wave V amplitude but the response to the two stimuli presented simultaneously showed a specific reduction in Wave V amplitude. No change in the latency of Wave V was observed to suggest any 'cross-over' masking of the click stimulus. Patients with total unilateral deafness did not exhibit this phenomenon. The effect of a time delay of the masker to one ear in relation to the click presented to the other ear reduced the amplitude of Wave V less than when the two stimuli were presented simultaneously. The reduction in the amplitude, being specific to Wave V, suggests that the effect is central and that the particular locus for this aspect of central masking is at the level of the inferior colliculus.