The dependence of binaurally evoked auditory brain stem responses and the binaural difference potential on simultaneously presented interaural time and level differences is investigated in order to assess the representation of stimulus lateralization in the brain stem. Auditory brain stem responses to binaural click stimuli with all combinations of three interaural time and three interaural level differences were recorded from 12 subjects and 4 channels. The latency of Jewett wave V is shortest for zero interaural time difference and longest for the trading stimuli. The amplitude of wave V is largest for centrally perceived stimuli, i.e., the diotic and trading stimuli, and smallest for the most laterally perceived stimuli. The latency of the most prominent peak of the binaural difference potential DN1 mainly depends on the interaural time difference. The amplitude of the components of the binaural difference potential, DP13DN1, depends similarly on stimulus conditions as wave V amplitude in the case of the binaural stimuli : smallest amplitudes are found for the most lateral stimuli and largest amplitudes for central stimuli. The results demonstrate that interaural level and time differences are not processed independently. This supports the hypothesis that directional information in humans is already extracted and represented at the level of the brain stem. ß

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The precedence effect in subjective localization tasks reflects the dominance of directional information of a direct sound (lead) over the information provided by one or several reflections (lags) for short delays. By collecting data of both psychoacoustical measurements and auditory evoked potentials the current study aims at neurophysiological correlates for the precedence effect in humans. In order to investigate whether the stimulus features or the perception of the stimulus is reflected on the ascending stages of the human auditory pathway, auditory brainstem responses (ABRs) as well as cortical auditory evoked potentials (CAEPs) using double click-pairs were recorded. Potentials were related to the results of the psychoacoustical data.ABRs to double click-pairs with lead-lag delays from 0 to 20 ms and interaural time differences (ITDs) in the lag click of 0 and 300 ls show an emerging second wave V for lead-lag delays larger than 2 ms. The amplitudes of the first and second wave V are the same for a lead-lag delay of about 5 ms. For the lag-ITD stimuli the latency of the second wave V was prolonged by approximately ITD/2 compared to the stimuli without lag-ITD. As the amplitudes of the second wave V were not decreased for a lead-lag delay around 5 ms as could be expected from psychoacoustical measurements of the precedence effect, ABRs reflect stimulus features rather than the perceptive qualities of the stimulus.The mismatch negativity (MMN) component of the CAEP for double click-pairs was determined using a diotic standard and a deviant with an ITD of 800 ls in the lag click. The comparison between the MMN components and the psychoacoustical data shows that the MMN is related to the perception of the stimulus, i.e., to the precedence effect.Generally, the findings of the present study suggest that the precedence effect is not a result of a poor sensitivity of the peripheral bottom-up processing. Rather, the precedence effect seems to be reflected by the MMN, i.e., cognitive processes on higher stages of the auditory pathway.

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