Distilling the neural correlates of consciousness

Abstract

Solving the problem of consciousness remains one of the biggest challenges in modern science. One key step towards understanding consciousness is to empirically narrow down neural processes associated with the subjective experience of a particular content. To unravel these neural correlates of consciousness (NCC) a common scientific strategy is to compare perceptual conditions in which consciousness of a particular content is present with those in which it is absent, and to determine differences in measures of brain activity (the so called "contrastive analysis"). However, this comparison appears not to reveal exclusively the NCC, as the NCC proper can be confounded with prerequisites for and consequences of conscious processing of the particular content. This implies that previous results cannot be unequivocally interpreted as reflecting the neural correlates of conscious experience. Here we review evidence supporting this conjecture and suggest experimental strategies to untangle the NCC from the prerequisites and consequences of conscious experience in order to further develop the otherwise valid and valuable contrastive methodology.

Distilling Consciousness: Isolated the Underlying Brain Activity?

“…Finally, the observations of Li et al (2014) are difficult to reconcile with some previous studies regarding the meaning of gamma power. In fact, Li et al (2014) found that increased power in the gamma frequency correlated with OBJ but not with SUB, in line with Aru et al (2012) and, thus, arguing against the previous link with SUB found by Gaillard et al (2009). Further study of SCPs and consciousness should be encouraged to clarify these inconsistencies.…”

“…In line with the GNW, the authors found a consistent link between late activity and awareness, meaning a time window of 200 -600 ms after stimulus. In contrast, previous studies indicated a time window around 200 -300 ms as a "primary candidate for a neural correlate of awareness" and the interval around 400 -600 ms as activity related to further processes, such as working memory maintenance and, thus, part of co-NCC (Aru et al, 2012). Li et al (2014) opened a new discussion on the time duration of the tripartite distinction of NCCs and this highlights the potential overlap among these activities.…”

“…The lack of such discrimination is one of the weaknesses of the experimental paradigm classically used to study NCC, the "minimal contrast" approach (Baars, 1989;Dehaene and Changeux, 2011), in which stimulus conditions are held similar while conscious perception varies. Indeed, brain activity registered under the conscious condition may also reflect processes preceding or following conscious perception, but not responsible of it-the so-called "prerequisites" (NCC-pr) and "consequences" (NCC-co) of NCC (Aru et al, 2012). Li et al (2014) addressed this issue in a recent study published in The Journal of Neuroscience.…”

“…This behavioral paradigm, a combination of liminal stimulation, forced alternative choice, and trial-by-trial introspection (Lamy et al, 2009;Fleming et al, 2010;Hesselmann et al, 2011), is one of the main strengths of this study, because it gives a separate quantitative measure for each one of the studied neural processes. Considering that NCC contributes by definition only to SUB; that pr-NCC contributes both to SUB and OBJ; and that NCC-co affects SUB and CONF (Aru et al, 2012), a separate measure for each one of these processes permitted the authors to associate the brain activity registered by MEG to specific processes. Furthermore, the isolation of brain activity related to SUB permitted the authors to investigate the correlation between slow cortical potentials (SCPs) measured by MEG (Leistner et al, 2007) and SUB, a link hypothesized by He and Raichle (2009).…”

“…First, pr-NCC can be linked to a variety of processes, such as attention, decision bias, stimulus expectation, adaptation, working memory, and potentially other mechanisms still under investigation (Aru et al, 2012), which can influence both SUB and OBJ, and which are not directly measured by the used behavioral task. Furthermore, because sensor locations varied across MEG sessions, it was not possible to confirm the anatomical location of the studied processes in the retest.…”