Memory-dependent auditory evoked potentials to change in the binaural interaction of noise signals

When non-identical binaural noise signals suddenly become coherent in the two ears, or coherent noise suddenly becomes incoherent, long latency binaurally evoked potentials (BINEP) are elicited which consist of P70, N130 and P220 components. Responses of similar morphology and latency were recorded to a change in the frequency of monaural click trains. The responses to onset or offset of the click trains were 20-50 msec shorter in latency. BINEP are also evoked when the sound image suddenly shifts due to the introduction of a short inter-aural delay in coherent noise signals. Responses to "isolated" shifts occurring once every 7 sec were 2-3 times (N130) or 3-5 times (P220) larger than responses to "frequent" shifts (6/7 sec) of the same magnitude in the same direction. Responses to "infrequent" shifts (1/7 sec) interspersed with frequent shifts in the opposite direction were of intermediate size, significantly larger than frequent responses. The BINEP could reflect the activity of location-specific neurones in the auditory cortex, but it seems more likely that they are due to a common neuronal pool responsive to any shift in the location of the sound image. Similar neuronal pools may be concerned with the detection of change in other auditory dimensions such as pitch. The difference between isolated, infrequent and frequent responses suggests that the BINEP amplitude is dependent on a memory of the shifts which have occurred in the preceding few seconds. The underlying process may be similar or identical to that which governs generation of the "mismatch negativity."