Spatial Auditory Masking Affects the Interhemispheric Asymmetry of Evoked Responses

Abstract

Interhemispheric asymmetry of electrical brain activity was investigated in the conditions of spatial auditory masking. Moving test signals were presented either in silence or against the background of stationary maskers of various spatial positions. The spatial properties of the stimuli were defined by interaural level differences (ILD). Onset-energy responses (ON-responses), motion-onset responses (MOR) and OFF-responses were analyzed. To compute the topograms and to analyze asymmetry, the amplitudes of each component were averaged over the symmetric electrode clusters in the left and right hemispheres. The ON-responses showed a contralateral dominance of the N1 component in silence, and the degree of contralateral bias increased in masking conditions. Interhemispheric asymmetry of the P2 component was absent in silence. However, the P2 amplitude was higher in the right hemisphere in all combinations of masker and signal. The asymmetry of both deflections was maximal when the masker and the initial portion of the signal were separated by 180 degrees. On the contrary, the interhemispheric asymmetry of the motion-onset response was found only in silence: the cN1 deflection was biased to the side contralateral to the signal. The topography of the OFF-response was symmetrical under all experimental conditions.