Spatial Attention Enhances the Neural Representation of Invisible Signals Embedded in Noise

Recent evidence suggests that voluntary spatial attention can modulate neural representations of visual stimuli that do not enter conscious awareness (i.e. invisible stimuli), supporting the notion that attention and awareness are dissociable processes (Watanabe et al., 2011; Wyart et al., 2012a). It remains unclear, however, whether spatial attention can modulate neural representations of invisible stimuli that are in direct competition with highly salient and visible stimuli. Here we developed a novel electroencephalography (EEG) frequency-tagging paradigm to obtain a continuous readout of neural activity associated with visible and invisible signals embedded in dynamic noise. Participants (N = 23) detected occasional contrast changes in one of two flickering image streams on either side of fixation. Each image stream contained a visible or invisible signal embedded in every second noise image, the visibility of which was titrated and checked using a two-interval forced-choice detection task. Steady-state visual-evoked potentials (SSVEPs) were computed from EEG data at the signal and noise frequencies of interest. Cluster-based permutation analyses revealed significant neural responses to both visible and invisible signals across posterior scalp electrodes. In line with previous findings, spatial attention increased the neural representation of visible signals. Crucially, spatial attention also increased the neural representation of invisible signals. As such, the present results replicate and extend previous studies by demonstrating that attention can modulate the neural representation of invisible signals that are in direct competition with highly salient masking stimuli. Significance Statement There has been much debate about the extent to which attention can effect neural representations of stimuli that do not enter conscious awareness. It remains unclear, however, whether spatial attention can modulate representations of invisible stimuli that are in direct spatial and temporal competition with salient masking stimuli. We developed a novel paradigm that for the first time allowed us to measure weak neural representations of invisible stimuli embedded in spatially coincident noise, and tested the effect of spatial attention on these representations. We found that spatial attention enhanced the neural representation of invisible stimuli, demonstrating that competition with highly salient stimuli does not suppress the effects of spatial attention on weak neural representations of invisible stimuli.