Human evoked potentials to shifts in the lateralization of a noise.

A continuous noise was generated by running a sequence of random numbers through a digital-analog converter and connecting the output through an amplifier and filter to an earphone. Two channels were programmed to generate identical noise stimuli with one channel delayed relative to the other. When these stimuli were presented through earphones, the subject lateralized the noise to the side receiving the leading stimulus. Changes in the relative timing of the two stimuli caused the noise to shift its lateralization. Since these shifts occurred without any detectable change in the ongoing monaural noise, any potentials they evoked were specifically related to binaural interaction. The response recorded from the vertex contained a positive-negative-positive complex with peak latencies of 75, 136 and 220 ms. This response was similar to that evoked by the onset of a monaural stimulus although it was slightly smaller and significantly later. Despite several attempts, we were unable to record any definite earlier evoked potentials.