Attentional Load Modulates Responses of Human Primary Visual Cortex to Invisible Stimuli

Abstract

SummaryVisual neuroscience has long sought to determine the extent to which stimulus-evoked activity in visual cortex depends on attention and awareness. Some influential theories of consciousness maintain that the allocation of attention is restricted to conscious representations [1, 2]. However, in the load theory of attention [3], competition between task-relevant and task-irrelevant stimuli for limited-capacity attention does not depend on conscious perception of the irrelevant stimuli. The critical test is whether the level of attentional load in a relevant task would determine unconscious neural processing of invisible stimuli. Human participants were scanned with high-field fMRI while they performed a foveal task of low or high attentional load. Irrelevant, invisible monocular stimuli were simultaneously presented peripherally and were continuously suppressed by a flashing mask in the other eye [4]. Attentional load in the foveal task strongly modulated retinotopic activity evoked in primary visual cortex (V1) by the invisible stimuli. Contrary to traditional views [1, 2, 5, 6], we found that availability of attentional capacity determines neural representations related to unconscious processing of continuously suppressed stimuli in human primary visual cortex. Spillover of attention to cortical representations of invisible stimuli (under low load) cannot be a sufficient condition for their awareness.

Can binocular rivalry reveal neural correlates of consciousness?

“…This study found that directing attention toward the spatial location of a grating enhanced fMRI responses relative to attending elsewhere, and that this was true regardless of whether the grating was perceptually suppressed (replicating e.g. [52,57,58]). More surprisingly, the study also reported that perceptual suppression left V1 responses altogether unaffected, which contradicts earlier findings (see review by Tong et al [59]).…”

“…Apparently, inattention contributes to the reduction in effectiveness that accompanies suppression in the absence of such an attention instruction. In a similar vein, Bahrami et al [52] measured the fMRI responses in primary visual cortex (V1) to a stimulus suppressed in rivalry and reported that the smaller responses commonly found under suppression can be either compounded or partially counteracted, depending on the difficulty of a concurrent target detection task that presumably impacts the availability of residual attention that can be allocated to the suppressed stimulus.…”

The hidden persuaders break into the tired brain

“…In a similar vein, the present study demonstrates that fairly complex stimuli can elicit subliminal manipulation of behavior (which of course requires that participants' visual system can differentiate between stimuli, cf. Bahrami, Lavie, & Rees, 2007). This sheds further light on the question of how complex stimuli can actually be for subliminal processing (e.g., Greenwald, 1992;Cooper & Cooper, 2002).…”

Neural Dissociation between Visual Awareness and Spatial Attention

“…This highfrequency oscillatory activity is therefore likely to correspond to an attention-related process (Womelsdorf and Fries, 2007). Moreover, the fact that this attentional modulation was present whether stimuli were consciously seen or not adds to the growing evidence that attentional processes can influence the processing of unseen stimuli (Naccache et al, 2002;Woodman and Luck, 2003;Kentridge et al, 2004;Sumner et al, 2006;Bahrami et al, 2007).…”