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Abstract: Research examining multisensory integration suggests that the correspondence of stimulus characteristics across modalities (cross-modal correspondence) can have a dramatic influence on both neurophysiological and perceptual responses to multimodal stimulation. The current study extends prior research by examining the cross-modal correspondence of amplitude modulation rate for simultaneous acoustic and vibrotactile stimulation using EEG and perceptual measures of sensitivity to amplitude modulation. To achieve this, psychophysical thresholds and steady-state responses (SSRs) were measured for acoustic and vibrotactile amplitude modulated (AM) stimulation for 21 and 40Hz AM rates as a function of the cross-modal correspondence. The study design included three primary conditions to determine whether the changes in the SSR and psychophysical thresholds were due to the cross-modal temporal correspondence of amplitude modulated stimuli: NONE (AM in one modality only), SAME (the same AM rate for each modality) and DIFF (different AM rates for each modality). The results of the psychophysical analysis showed that AM detection thresholds for the simultaneous AM conditions (i.e., SAME and DIFF) were significantly higher (i.e., lower sensitivity) than AM detection thresholds for the stimulation of a single modality (i.e., NONE). SSR results showed significant effects of SAME and DIFF conditions on SSR activity. The different pattern of results for perceptual and SSR measures of cross-modal correspondence of AM rate indicates a dissociation between entrained cortical activity (i.e., SSR) and perception. [Copyright &y& Elsevier]

Abstract: Recent research suggests that multisensory integration may occur at an early phase in sensory processing and within cortical regions traditionally though to be exclusively unisensory. Evidence from perceptual and electrophysiological studies indicate that the cross modal temporal correspondence of multisensory stimuli plays a fundamental role in the cortical integration of information across separate sensory modalities. Further, oscillatory neural activity in sensory cortices may provide the principle mechanism whereby sensory information from separate modalities is integrated. In the present study we aimed to extend this prior research by using the steady-state EEG response (SSR) to examine whether variations in the cross-modality temporal correspondence of amplitude modulated auditory and vibrotactile stimulation are apparent in SSR activity to multisensory stimulation. To achieve this we varied the cross-modal congruence of modulation rate for passively and simultaneously presented amplitude modulated auditory and vibrotactile stimuli. In order to maximise the SSR response in both modalities 21 and 40Hz modulation rates were selected. Consistent with prior SSR studies, the present results showed clear evidence of phase-locking for EEG frequencies corresponding to the modulation rate of auditory and vibrotactile stimulation. As also found previously, the optimal modulation rate for SSR activity differed according to the modality, being greater at 40Hz for auditory responses and greater at 21Hz for vibrotactile responses. Despite consistent and reliable changes in SSR activity with manipulations of modulation rate within modality, the present study failed to provide strong evidence of multisensory interactions in SSR activity for temporally congruent, relative to incongruent, cross modal conditions. The results are discussed in terms of the role of attention as a possible factor in reconciling inconsistencies in SSR studies of multisensory integration. [Copyright &y& Elsevier]