Activity in the lateral occipital cortex between 200 and 300 ms distinguishes between physically identical seen and unseen stimuli

Abstract

There is converging evidence that electrophysiological responses over posterior cortical regions in the 200–300 ms range distinguish between physically identical stimuli that reach consciousness or remain unseen. Here, we attempt at determining the sources of this awareness-related activity using magneto-encephalographic (MEG). Fourteen subjects were presented with faint colored gratings at threshold for contrast and reported on each trial whether the grating was seen or unseen. Subjects were primed with a color cue that could be congruent or incongruent with the color of the grating, to probe to what extent two co-localized features (color and orientation) would be bound in consciousness. The contrast between neural responses to seen and unseen physically identical gratings revealed a sustained posterior difference between 190 and 350 ms, thereby replicating prior studies. We further show that the main sources of the awareness-related activity were localized bilaterally on the lateral convexity of the occipito-temporal region, in the Lateral Occipital (LO) complex, as well as in the right posterior infero-temporal region. No activity differentiating seen and unseen trials could be observed in frontal or parietal regions in this latency range, even at lower threshold. Color congruency did not improve grating's detection, and the awareness-related activity was independent from color congruency. However, at the neural level, color congruency was processed differently in grating-present and grating-absent trials. The pattern of results suggests the existence of a neural process of color congruency engaging left parietal regions that is affected by the mere presence of another feature, whether this feature reaches consciousness or not. Altogether, our results reveal an occipital source of visual awareness insensitive to color congruency, and a simultaneous parietal source not engaged in visual awareness, but sensitive to the manipulation of co-localized features.

Cortical Neural Synchronization Underlies Primary Visual Consciousness of Qualia: Evidence from Event-Related Potentials

“…All those ERP studies showed an increased P3 amplitude in widespread scalp regions in relation to primary visual consciousness (Fernandez-Duque et al, 2003 ; Del Cul et al, 2007 ; Lamy et al, 2009 ; Chica et al, 2010 ), especially when experimental demands did not induce a stimulus expectancy (Melloni et al, 2011 ). In this context, the topographical specificity of our findings was confirmed by magnetoencephalographic (MEG) evidence (Liu et al, 2012 ). That MEG study reported cortical source activity in bilateral occipital-temporal (but not parietal) regions at about 200–350 ms post-stimulus in association with primary visual consciousness of non-spatial stimuli (Liu et al, 2012 ).…”

“…In this context, the topographical specificity of our findings was confirmed by magnetoencephalographic (MEG) evidence (Liu et al, 2012 ). That MEG study reported cortical source activity in bilateral occipital-temporal (but not parietal) regions at about 200–350 ms post-stimulus in association with primary visual consciousness of non-spatial stimuli (Liu et al, 2012 ). The findings of our “visuospatial” experiment corroborated the involvement of parietal and occipital cortex in the primary visuospatial consciousness (Babiloni et al, 2006 ), thus complementing the previous view on the functional role of parietal cortical areas in visuo-spatial attention and consciousness (Driver et al, 2001 ; Chica et al, 2010 ).…”

A model of subjective report and objective discrimination as categorical decisions in a vast representational space

“…In many studies, objective discrimination performance improves dramatically when the stimuli are reported as 'seen' compared with unseen, even when sensory stimulation is identical [23,25,27]. How does the model account for these findings?…”

Intracranial markers of conscious face perception in humans

“…In the last couple of decades, the relationships between brain activity and the contents of perceptual consciousness have been investigated using a variety of experimental techniques operating at different spatial and temporal scales, from single-unit, multi-unit and local field potential recordings in monkeys (Logothetis and Schall, 1989; Leopold and Logothetis, 1996; Wilke et al, 2006; Maier et al, 2007; Wilke et al, 2009), to non-invasive neuroimaging techniques such as EEG, MEG and fMRI in humans (e.g., (Tong et al, 1998; Grill-Spector et al, 2000; Dehaene et al, 2001; van Aalderen-Smeets et al, 2006; Liu et al, 2012; Schurger et al, 2015)) (see (Rees et al, 2002; Tononi and Koch, 2008; Dehaene and Changeux, 2011; Boly et al, 2013; Panagiotaropoulos et al, 2014) for reviews).…”