Visonal Awareness

JOURNAL CLUB: Crossing the threshold to consciousness from Neuroscience forum on Nature Network

Crossing the threshold to consciousnessThe target stimulus consisted of a single digit (a number) projected for 16 ms (a brief, but supraliminar stimulus). The mask was a group of numbers projected soon after, for 250 ms, at the same visual location. The authors varied the time interval (called SOA, “target-mask stimulus onset asynchrony”) between stimulus onset and the presentation of the mask, from 16 to 100 ms. Shorter intervals were predicted to cause backward masking of the stimulus (basically, relegating the stimulus below the threshold of consciousness), by means of a perturbation of the sequence of brain events necessary to generate the corresponding conscious state. Longer intervals would not perturb the brain processing of the stimulus, allowing the propagation of excitation to higher cortical areas. High-density event-related potential (ERP) recordings were collected to determine whether a change (or a “transition”) in brain activity occured between the lower and higher SOA values. The temporal location of this transition was defined behaviorally, using two measures: a forced-choice comparison of the presented digit with another one (checking for both sub- and supraliminar perception), and a scale of visibility, which roughly assesses the conscious access the subject had to the stimulus. This methodology produced several behavioral and physiological (ERP) results. Behaviorally, a “significant nonlinearity” was found for the SOA interval from 33 to 66 ms. Below 16 ms, the performance was at chance level, suggesting that the presented digit was completely masked by the second. In the 16-33 ms interval, performance in the forced-choice task was above chance, while in the 33-66 ms interval both the forced-choice and the (conscious) visibility ratings increased non-linearly. Above 66 ms there was not a significant change in visibility and the subjects consistently had conscious access to the presented stimulus. The authors concluded that “a major transition in processing occurs around SOA = 50 ms”.The authors then looked for ERP components temporally correlated with the transition in visibility elicited by the 50 ms SOA. Using statistical testing across subjects, they found activity within a fronto-parieto-temporal network (called the P3 component in the paper) to be strongly correlated with the non-linear increase in visibility. The change in P3 that correlates with the transition occurs about 270 to 300 ms after target onset. This finding is consistent with the hypothesis that conscious access to a visual stimulus involves the sequential activation of several cortical areas. Thus, we now have some fairly strong evidence suggesting that the conscious process of visual stimulus sensation involves more than the simple activation of occipital visual areas. So, in order to cross the threshold to consciousness, we need network activity that is likely greater than the sum of its parts.