The Elicitation of Audiovisual Steady-State Responses: Multi-Sensory Signal Congruity and Phase Effects
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Most ecologically natural sensory inputs are not limited to a single modality. While it is possible to use real ecological materials as experimental stimuli to investigate the neural basis of multi-sensory experience, parametric control of such tokens is limited. By using artificial bimodal stimuli composed of approximations to ecological signals, we aim to observe the interactions between putatively relevant stimulus attributes. Here we use MEG as an electrophysiological tool and employ as a measure the steady-state response (SSR), an experimental paradigm typically applied to unimodal signals. In this experiment we quantify the responses to a bimodal audio-visual signal with different degrees of temporal (phase) congruity, focusing on stimulus properties critical to audiovisual speech. An amplitude modulated auditory signal (‘pseudo-speech’) is paired with a radius-modulated ellipse (‘pseudo-mouth’), with the envelope of low-frequency modulations occurring in phase or at offset phase values across modalities. We observe (i) that it is possible to elicit an SSR to bimodal signals; (ii) that bimodal signals exhibit greater response power than unimodal signals; and (iii) that the SSR power at specific harmonics and sensors differentially reflects the congruity between signal components. Importantly, we argue that effects found at the modulation frequency and second harmonic reflect differential aspects of neural coding of multisensory signals. The experimental paradigm facilitates a quantitative characterization of properties of multi-sensory speech and other bimodal computations.
KeywordsAudio-visual Cross-modal Magnetoencephalography Speech Multi-sensory
This project originated with a series of important discussions with Ken W. Grant (Auditory-Visual Speech Recognition Laboratory, Army Audiology and Speech Center, Walter Reed Army Medical Center). The authors would like to thank him for his extensive contributions to the conception of this work. The authors would like to thank Mary F. Howard and Philip J. Monahan for critical reviews of this manuscript. We would also like to thank Jeff Walker for technical assistance in data collection and Pedro Alcocer and Diogo Almeida for assistance with various R packages. This work was supported by the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health: 2R01DC05660 to DP, JZS, and WJI and Training Grant DC-00046 support to JJIII and AER. Parts of this manuscript comprise portions of the first two authors’ doctoral theses.
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