When a brief visual stimulus is accompanied by two brief tactile stimuli, subjects often perceive a second illusory visual stimulus (“double-flash illusion�, DFI). We investigated the neural mechanisms of this illusion with whole-head 151-channel MEG-recordings. Twenty-two subjects received visuo-tactile stimulations and reported the number of perceived visual stimuli. We sorted trials with identical physical stimulation according to the subjects’ percept and assessed differences in spectral power.
In DFI trials, occipital sensors displayed a bilateral de-synchronization in the alpha-band (7.5-15 Hz) before stimulus onset (-400 to -200 ms) and a contralateral enhancement of oscillatory activity in the gamma-band (70-130 Hz) in response to stimulation. This enhancement was similar in time- and frequency extent to the somatosensory gamma-band response to tactile stimulation. In somatosensory sensors, the DFI was associated with an increase of spectral power for low frequencies (5-15 Hz) around stimulation and a decrease of spectral power in the 25-30 Hz range between 400-850 ms post-stimulation.
Several of the observed components have been frequently related to increased attention or excitability in visual and somatosensory areas. The DFI might therefore occur when the somatosensory gamma-response spreads to visual cortex. This spreading might be supported by the observed modulations in low-frequencies.