ACOUSTIC FEATURES AND ACOUSTIC CHANGE ARE REPRESENTED BY DIFFERENT CENTRAL PATHWAYS

The central processing of acoustic stimulus changes can be observed ne urophysiologically in the mismatch negativity auditory evoked potentia l (MMN). Stimuli differing in interaural phase were used to investigat e the contributions of the primary and non-primary auditory pathways t o the encoding of binaural stimuli and to investigate passively elicit ed measures of binaural processing in experimental animals. In guinea pigs, the MMN was obtained in response to 1000 Hz tones embedded in wh ite noise (S:N = 2 dB). Using a modified oddball paradigm (that is, tw o stimuli presented in a series, each with a different probability of occurrence), stimuli were presented binaurally with both the tone and noise in-phase to the two ears (S0N0) as the standard stimulus and the tone 180 degrees out-of-phase (SPiN0) as the deviant stimulus. The MM N, by definition, should occur only in response to a change, or 'misma tch,' between the standard and deviant stimuli. The response to the de viant stimulus in the oddball paradigm was compared to the response to the same stimulus when presented in a series alone. The responses to S0N0 and SPiN0 collected in a series alone, termed the intrinsic respo nses, were also compared. Responses were recorded from two surface epi dural electrodes - one at the posterior midline and one over the left temporal lobe. AEPs from these locations have been shown to reflect th e activity of primary and non-primary thalamo-cortical pathways respec tively. A significant MMN was observed at the midline electrode, but n o MMN was observed over the temporal lobe. However, there was a signif icant difference in the intrinsic responses to the two stimuli over th e temporal lobe while no difference in the intrinsic responses was see n over the midline. The results suggest that the primary and non-prima ry auditory pathways appear to provide distinctly different contributi ons to the encoding of changes in binaural phase. Additionally, the MM N to stimuli differing in interaural phase can be obtained in anesthet ized animals and may provide a useful measure of binaural processing.