![]() Thus, in the associated condition, discriminating stimuli in the cued modality could be facilitated by processing the item in the other modality, whereas in the non-associated condition, subjects could not reliably benefit from but rather be distracted by the other modality. In the “associated” condition, tones with a certain pitch were almost always paired with pictures of a bar with a certain tilt, whereas in the “non-associated” condition, tones and bars were combined at random. Although the overall sensory input across the two conditions was identical, the crucial manipulation concerned the overall statistical relationship between auditory and visual targets. Visual and auditory stimuli were presented simultaneously in each trial, but a preceding cue indicated which modality subjects should base their response on. We designed an experiment in which subjects had to differentiate either between two tones or between two visual objects. Here, we provide evidence that nature and sign of cross-modal neural interaction depend not only on congruence but also on whether sensory inputs are expected to convey associated or unrelated and thus potentially conflicting information, i.e., whether stimulus processing in one modality will be helped or disturbed by taking information in another modality into account. Hence, cross-modal interactions can be both mutually suppressive as well as facilitating. (2002) found that activity in visual cortex was reduced when subjects listened to sounds, whereas activity in auditory cortex was reduced during visual processing. (2000) demonstrated that tactile stimulation enhanced activity in the visual cortex, but only when it was administered to the same side as the visual target. The level at which these cross-modal interactions occur seems to depend on the categorical quality of the stimuli used, but it is far less clear which parameters control the sign of modulation, i.e., enhancement versus suppression. These behavioral observations have been complemented recently by neuroimaging studies showing that activity in one sensory system can be altered by input to another ( Macaluso et al., 2000 McDonald et al., 2000 Laurienti et al., 2002 Weissman et al., 2004 Johnson and Zatorre, 2005). Furthermore, it was shown that spatially nonpredictive peripheral cues (e.g., a sound) could attract covert visual attention to specific locations and that sounds were misallocated at their apparent visual source ( Driver and Spence, 1998). This view has been challenged by the demonstration of cross-modal effects on perception: a sound is misallocated toward the semantically matching visual stimulus, auditory speech perception can be modulated by lip reading, and effects of intermodal attention in visual and auditory event-related potentials showed that intermodal attention operates by a selective modulation of modality-specific stimulus-driven responses ( Eimer and Schröger, 1998). It has long been assumed that the early sensory processing chains in the brain are unimodal and operate primarily independently from each other with activity in one system exerting little if any influence on processing in another. Because thalamic structures were more active when the senses needed to operate separately, we propose them to serve gatekeeper functions in early cross-modal interactions. Our findings illustrate an ecologically optimal flexibility of the neural mechanisms that govern multisensory processing: facilitation occurs when integration is expected, and suppression occurs when distraction is expected. Conversely, when information in the two modalities is reliably associated, activity is enhanced in both systems regardless of which modality is task relevant. When concurrently presented auditory and visual stimuli are paired by chance, cue-induced preparatory neural activity is strongly enhanced in the task-relevant sensory system and suppressed in the irrelevant system. Here, we reconcile these apparently contradictory results by showing that the sign of cross-modal interactions depends on whether the content of two modalities is associated or not. Previous studies have shown that processing information in one sensory modality can either be enhanced or attenuated by concurrent stimulation of another modality.
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