Abstract
Evidence from human neuroimaging and animal electrophysiological studies suggests that signals from different sensory modalities interact early in cortical processing, including in primary sensory cortices. The present study aimed to test whether functional near-infrared spectroscopy (fNIRS), an emerging, non-invasive neuroimaging technique, is capable of measuring such multisensory interactions. Specifically, we tested for a modulatory influence of sounds on activity in visual cortex, while varying the temporal synchrony between trains of transient auditory and visual events. Related fMRI studies have consistently reported enhanced activation in response to synchronous compared to asynchronous audiovisual stimulation. Unexpectedly, we found that synchronous sounds significantly reduced the fNIRS response from visual cortex, compared both to asynchronous sounds and to a visual-only baseline. It is possible that this suppressive effect of synchronous sounds reflects the use of an efficacious visual stimulus, chosen for consistency with previous fNIRS studies. Discrepant results may also be explained by differences between studies in how attention was deployed to the auditory and visual modalities. The presence and relative timing of sounds did not significantly affect performance in a simultaneously conducted behavioral task, although the data were suggestive of a positive relationship between the strength of the fNIRS response from visual cortex and the accuracy of visual target detection. Overall, the present findings indicate that fNIRS is capable of measuring multisensory cortical interactions. In multisensory research, fNIRS can offer complementary information to the more established neuroimaging modalities, and may prove advantageous for testing in naturalistic environments and with infant and clinical populations.
Highlights
Perceptual judgments often reflect a combination of inputs from multiple senses [1]
Whilst the most appropriate statistical criterion for identifying multisensory interactions in neuroimaging data is a subject of ongoing debate [6,7,8], it is widely accepted that multisensory effects can modulate both the amplitude [9, 10] and temporal dynamics [11] of the blood oxygenation level-dependent (BOLD) fMRI signal in primary sensory cortices
In the A-ONLY condition, there was some evidence of positive activation in the region of interest (ROI), while an isolated channel located in the top-right corner of the array (Ch# 4) appeared to show deactivation compared to baseline
Summary
Perceptual judgments often reflect a combination of inputs from multiple senses [1]. Over recent years, converging evidence from human neuroimaging studies and single-unit recordings in animals has revealed that interactions among the senses arise early in cortical processing, including in primary sensory cortices, which were traditionally considered to be strictly unisensory (for reviews see [2,3,4,5]). Other studies have exploited the temporal resolution of electroencephalography (EEG) and magnetoencephalography (MEG) to confirm that multisensory interactions occur early in cortical processing [12,13,14,15], and, more recently, to study the putative role of synchronized oscillatory brain activity in mediating these effects [16, 17]
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