Abstract
Initiating an eye movement towards a suddenly appearing visual target is faster when an accessory auditory stimulus occurs in close spatiotemporal vicinity. Such facilitation of saccadic reaction time (SRT) is well-documented, but the exact neural mechanisms underlying the crossmodal effect remain to be elucidated. From EEG/MEG studies it has been hypothesized that coupled oscillatory activity in primary sensory cortices regulates multisensory processing. Specifically, it is assumed that the phase of an ongoing neural oscillation is shifted due to the occurrence of a sensory stimulus so that, across trials, phase values become highly consistent (phase reset). If one can identify the phase an oscillation is reset to, it is possible to predict when temporal windows of high and low excitability will occur. However, in behavioral experiments the pre-stimulus phase will be different on successive repetitions of the experimental trial, and average performance over many trials will show no signs of the modulation. Here we circumvent this problem by repeatedly presenting an auditory accessory stimulus followed by a visual target stimulus with a temporal delay varied in steps of 2 ms. Performing a discrete time series analysis on SRT as a function of the delay, we provide statistical evidence for the existence of distinct peak spectral components in the power spectrum. These frequencies, although varying across participants, fall within the beta and gamma range (20 to 40 Hz) of neural oscillatory activity observed in neurophysiological studies of multisensory integration. Some evidence for high-theta/alpha activity was found as well. Our results are consistent with the phase reset hypothesis and demonstrate that it is amenable to testing by purely psychophysical methods. Thus, any theory of multisensory processes that connects specific brain states with patterns of saccadic responses should be able to account for traces of oscillatory activity in observable behavior.
Highlights
Neural oscillatory activity in the brain has been shown to play a key role in cognitive performance and attentional selection in both the visual and the auditory domain [1,2,3,4]
We presented a supra-threshold auditory accessory stimulus followed by a visual target stimulus at a specific stimulus onset asynchrony (SOA) that varied randomly between 0 and 200 ms in steps of 2 ms
Mean saccadic reaction time to the crossmodal stimulus exhibited a speedup of responses of up to 50 ms compared to responses to the unimodal visual stimuli
Summary
Neural oscillatory activity in the brain has been shown to play a key role in cognitive performance and attentional selection in both the visual and the auditory domain [1,2,3,4]. High-frequency gamma rhythms (30– 80 Hz) facilitate processing of stimuli in the locus of attention [6,7], whereas oscillatory activity in the beta frequency range (13– 30 Hz) has been associated with sensory-motor and integrative multisensory processing [8] (see [9] for a recent review). The amplitude of faster rhythms (beta, gamma) have been found to be a function of the phase of slower oscillations, i.e., delta (0–4 Hz) and theta (4–8 Hz) [10,11,12]. Probing of crossfrequency phase-amplitude coupling, as a mechanism to coordinate neural activity on multiple timescales and different levels of the sensory processing hierarchy, has become a major issue in many recent neurophysiological studies [12,13]
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