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Abstract:
To form a unified percept of our environment, the human brain integrates information within
and across the senses. This MEG study investigated interactions within and between sensory
modalities using a frequency analysis of steady-Ââstate responses (SSR) to periodic auditory
and/or visual inputs. The 3x3 factorial design, manipulated (1) modality (auditory only, visual
only and audiovisual) and (2) temporal dynamics (static, dynamic1 and dynamic2). In
the static conditions, subjects were presented with (1) visual gratings, luminance modulated
at 6Hz and/or (2) pure tones, frequency modulated at 40 Hz. To manipulate perceptual synchrony,
we imposed additional slow modulations on the auditory and visual stimuli either at
same (0.2 Hz = synchronous) or different frequencies (0.2 Hz vs. 0.7 Hz = asynchronous).
This also enabled us to investigate the integration of two dynamic features within one sensory
modality (e.g. a pure tone frequency modulated at 40Hz amplitude modulated at 0.2Hz)
in the dynamic conditions. We reliably identified crossmodulation frequencies when these
two stimulus features were modulated at different frequencies. In contrast, no crossmodulation
frequencies were identified when information needed to be combined from auditory
and visual modalities. The absence of audiovisual crossmodulation frequencies suggests that
the previously reported audiovisual interactions in primary sensory areas may mediate low
level spatiotemporal coincidence detection that is prominent for stimulus transients but less
relevant for sustained SSR responses. In conclusion, our results indicate that information in
SSRs is integrated over multiple time scales within but not across sensory modalities at the
primary cortical level.
