The present thesis deals with behavioral and neurophysiological aspects of interhemispheric processing. Studies and models on interhemispheric processing suggest various sometimes controversial mechanisms how the two hemispheres might process information. One aspect of interhemispheric processing is examined within tasks with bilateral redundant stimulus presentation. Previous behavioral studies revealed superior performance after bilateral redundant stimulation compared to unilateral stimulation. This effect has been called bilateral advantage and was obtained for meaningful stimuli (e.g. words, familiar faces) but not for meaningless stimuli (e.g. pseudowords, unfamiliar faces).Within this thesis the bilateral advantage was examined with different experimental designs. The first experiment replicated earlier findings regarding the bilateral advantage, which was present for words but absent for pseudowords. Furthermore, evoked potential and minimum norm data indicated that bilaterally presented words were accompanied by more negative amplitudes and higher source activation than pseudowords and unilaterally presented words. The second experiment revealed a bilateral advantage for objects compared with non-objects, which supported the assumption that only meaningful stimuli having neural representations might elicit a bilateral advantage. With the third experiment the question was addressed, whether a processing advantage in bilateral conditions depended on bi-hemispheric stimulation or on redundant stimulus presentation. The comparison of the two effects indicated a larger bilateral than unilateral redundancy advantage. Bi-hemispheric stimulation caused a wider distributed activation and interhemispheric summation might therefore outrange intrahemispheric summation regarding behavioral performance. The fourth experiment examined bilateral processing with auditory stimulation a passive oddball paradigm. Binaurally presented words were found to elicit a larger mismatch negativity (MMN) than monaurally presented words and binaurally presented pseudowords. This neurophysiological manifestation of a word-specific bilateral redundancy gain is interpreted as evidence for interhemispheric cooperation in the automatic access to memory traces for spoken words. Accordingly, word-related cortical networks distributed over both hemispheres may allow summation of neural activity between and within hemispheres, thereby potentiating the word-related MMN.In summary, the neurophysiological results demonstrate that a behavioral bilateral processing advantage might be explained with summation of neuronal activation. Moreover, a neurophysiological correlate of the bilateral advantage was revealed during visual as well as auditory stimulus presentation and independently of subjects focused attention. These findings indicate that cooperation between the two cerebral hemispheres might be influenced by certain aspects of processing. Interhemispheric cooperation in terms of superior behavioral performance or higher cortical activation was observed for different kinds stimuli (verbal and non-verbal) and for different perceptual channels (visual and auditory). However, interhemispheric cooperation might depend on task requirements. Tasks requiring full stimulus processing generate only a bilateral advantage when previously learned meaningful stimuli are processed as opposed to meaningless stimuli. In contrast, tasks requiring only stimulus detection, generate a bilateral advantage independently of stimulus type, due to a summation of perceptual activation
