Electrophysiological evidence for the role of extrastriate visual cortex in visual awareness

Abstract

Recent studies have suggested a critical role for cortico-cortical interactions in human visual awareness. However, to date there is no direct electrophysiological evidence for their involvement. To address this issue, Transcranial Magnetic Stimulation (TMS) was combined with Electroencephalography (EEG) while single pulses of TMS were delivered over right V1/V2 and V5/MT at phosphene threshold. Wavelet event-related power (ERPow) transformation, a measure of the regional oscillatory activity of neural assemblies, was used to identify electrophysiological correlates of conscious perception induced by TMS. The perception of phosphenes induced from the extrastriate area V5/MT was associated with a specific increase in oscillatory alpha-band activity (alpha-syncronization) but not beta-band activity in parieto-occipital regions across the two cerebral hemispheres relative to the pre-TMS power level. This findings demonstrate that synchronization of cortical oscillatory activity is a neural correlate of conscious perception. We also observed a network effect with a modulation of cortical oscillations that spread from the stimulated right parieto-occipital cortex to contralateral homologous regions, presumably via the corpus callosum. While our results clearly demonstrate a role for extrastriate visual cortical areas in visual awareness, it is likely that subcortical structures are also involved in the phenomenon. Indeed, it is possible that stimulation of the cortical cells generates feedback signals to the thalamus, as the thalamus is believed to play an important role in synchronising cortical oscillations. Our results demonstrate the value of the combined TMS/EEG approach when studying cortico-cortical interactions in the visual system. As interactions between cortical regions are central to most current theories of visual awareness, this approach will undoubtedly be pursued in future studies.

Footnotes

 G. Fuggetta was a member of the Marie Curie Research Training Network LAB (MRTN-CT-2004-512141). V. Walsh is supported by the Royal Society.