HUMAN LONG-LATENCY RESPONSES TO BRIEF INTERAURAL DISPARITIES OF INTENSITY

The electroencephalographic responses to abrupt changes in interaural differences of time (ITD) and intensity (IID) should provide important information on the dynamic characteristics and integrity of the binau ral mechanisms detecting the azimuthal shifts of a sound image. Howeve r, a change in either or both of these cues to sound lateralization wo uld stimulate not only the binaural mechanisms but also the monaural o nes. There are several reports evidencing that in the case of ITD chan ges this problem can be overcome by using time-shifted noise or repeti tive clicks. Any change in TID, however, will inevitably have a stimul ating effect also on purely monaural mechanisms. Therefore, the stimul ation techniques described in the literature so far for recording the long-latency responses related to IID mechanism cannot be regarded as being specific for binaural mechanisms. We used dichotically presented 100/s click trains which were amplitude modulated with a random seque nce of 50 or 100 ms square wave-intervals, so that the sound intensiti es at the two ears simultaneously alternated between 60 dB and 80 dB l evels except during brief periods of time (50 ms) in which the interau ral intensity balance was impaired, leading to an IID of 20 dB every 2 s. Owing to the fact that the cortical mechanisms remain unresponsive to repetitive stimuli presented with intervals shorter than a certain recovery period, this stimulus did not evoke any significant potentia l when it was presented monotically or diotically, yet it could produc e lateral sound image shifts and therefore evoke pronounced long laten cy responses when presented dichotically. The main components N1 and P 2 of these shift responses and those of the pip responses, also record ed from the same subjects, were compared with respect to their midline distributions and hemispheric or bilateral asymmetries. The significa nt differences found between the shift and pip responses indicated tha t those evoked by the IID stimulation we designed should not be consid ered simply as a non-specific vertex potential.