Distinct and independent correlates of attention and awareness in a hemianopic patient

Attention and awareness are intimately related concepts. Nevertheless, the two phenomena are empirically dissociable: visuo-spatial attention can act in the absence of visual awareness. We used magnetoencephalography (MEG) to record cortical neural activity from hemianopic patient GY while he performed a peripheral orientation-discrimination task in the context of an attention-cueing paradigm. The luminance contrast of target stimuli was set at GY's threshold for reports of awareness (a feeling that something happened in his blind visual field). GY's accuracy was significantly greater than chance and comparable, with or without awareness. GY was significantly faster to respond correctly on valid-cue versus invalid-cue trials, even in the absence of awareness, confirming the action of visuo-spatial attention in the absence of awareness. Time-frequency analysis of spectral power in the gamma frequency range (30-90 Hz), averaged over left parieto-occipital sensors, revealed effects of cue-validity independent of reported awareness, and effects of awareness independent of cue-validity. GY's reports of awareness were not, however, independent of his pre-target attentional state (gamma-band response to the central cue), consistent with a one-way, but not a two-way, dissociation between attention and awareness. Our evidence suggests that the observed cue-validity effect is an awareness-independent involuntary re-orienting response, and that the neurodynamics underlying the exogenous capture of attention are similar with or without awareness. The finding of a significant awareness-independent effect in the area of 40 Hz implies that a stimulus-induced modulation of power in the canonical gamma band is not a sufficient condition for sensory awareness. (C) 2008 Elsevier Ltd. All rights reserved.