Mismatch negativity (MMN) is often regarded as an objective electrophysiological index of sound stimuli discrimination. It is believed that MMN represents the auditory information proceccing at the preattentive and preconscious level. Discrimination thresholds resulting from psychophysical testing can be influenced by a number of subjective factors (attention, motivation, etc.), and represent, respectively, later stages of information processing.
White noise birsts (100 - 1300 Hz, 200 ms duration) were presented dichotically to 11 subjects. Gradual or abrupt displacement of the sound image from the head midline to different angular distances was modeled manipulating interaural time difference. In the psychophysical experiments the level of subjective discrimination between two patterns of the sound images motion for different distances was determined. According to the results of psychophysical testing two distances were selected. For the longer distance, the discrimination of the dynamic patterns reached the 75% threshold level, and for the shorter distance, it barely exceeded 50% level corresponding to the random choice. The selected stimuli were used as standards and deviants in the electrophysiological experiments on MMN recording.
It was clearly shown that subthreshold dynamic localization differences included into stimulation resulted in generation of reliable MMN. Thus, electrical brain activity could be detected in response to subjectively unrecognizable stimuli, which confirmed the ability to process auditory spatial information even in the absence of its conscious perception. With increasing the sound motion angular distance from 23 º to 90 º, a subjective discrimination of gradual and abrupt motion increased by 20% in average, and the MMN amplitude increased by 28% and 39% for the left-and rightward displacement. The responses to suprathreshold differences were greater then to the subthreshold ones in both objective and subjective measures taken as group average, but not at the individual level. MMN amplitude did not correlated significantly with the probability of the correct choice in psychophysical testing neither in shorter nor in longer distance of the sound motion.
The study is supported by the grant RFBR № 11-04-00008-а.