Thursday, December 07, 2006

Brain correlates of visual consciousness

Lau and Passingham report interesting experiments using "relative blindsight" that try to distinguish brain regions essential to visual performance from those involved also in subjective visual consciousness. Here are some edited clips from that work:

Blindsight refers to the phenomenon that, after a lesion to the primary visual cortex, a subject can exhibit above-chance performance in detecting or discriminating visual stimuli in a forced-choice setting, despite the lack of acknowledged consciousness of the stimuli. In some instances, blindsight subjects can perform at an impressively high level of accuracy (>80%) in the forced-choice task, even when the subjects believe that they are guessing. This potentially high level of performance makes blindsight an interesting case study for visual consciousness, because it indicates that it is consciousness, but not the basic capacity to process information, that is completely abolished.

Attempts to unequivocally demonstrate blindsight in normal observers have proved to be controversial, so instead of looking for a complete dissociation of performance and visual consciousness as in the case of blindsight, the authors set out to look for a relative difference ("relative blindsight") in the level of visual consciousness in two conditions in which performance levels are matched. These conditions can be created by using a psychophysical paradigm based on metacontrast masking, shown in Figure 1. In metacontrast masking, a figure that overlaps with the contour of the target is presented after the target. Discrimination performance for a target stimulus decreases and then increases as the temporal distance between the target and a metacontrast mask increases gradually; this distance is referred to as stimulus onset asynchrony (SOA).

Fig. 1. Visual discrimination task with metacontrast masking measured to demonstrate the phenomenon of relative blindsight. The stimuli were presented on a black background. The mask overlaps with part of the contour of the target without leaving gaps or overlapping with the target spatially. After the presentation of the target and the mask, the participants were first asked to decide whether a diamond or a square was presented. Then they had to indicate whether they actually saw the target or whether they simply guessed the answer. Shown in the brackets are the durations of each stimulus.

For the SOA points at 33 and 100 ms, the performance levels (i.e., accuracy rates for the square vs. diamond discrimination) were very similar. However, the subjective judgment of consciousness differed in that at the earlier SOA point volunteers were more likely to claim to have just guessed the answers. Thus the subjective level of consciousness can differ in the absence of a difference of performance levels.

Fig. 2 Activity in the mid-DLPFC reflects visual consciousness (long SOA > short SOA). The activity in this area is higher in the long SOA condition than in the short SOA condition, despite the fact that the two conditions did not yield different discrimination accuracy. There were, however, more trials during which the stimuli were classified by the participants as consciously seen in the long SOA condition than in the short SOA condition. This area was the only one found to be activated in this comparison; parietal activity did not differ significantly even at liberal thresholds.

To identify the brain areas where activity reflected discrimination performance in general, they also analyzed the fMRI results by comparing correct and incorrect trials, combining short SOA and long SOA trials. They found widespread activations in the ventral prefrontal, premotor, parietal, and temporal cortices. The pattern of these activations resembled that in a so-called frontal–parietal network, typically reported in previous studies of visual consciousness. VLPFC, ventrolateral prefrontal cortex; IPS, intraparietal sulcus; MTG, middle temporal gyrus.

Fig. 3 Activations reflecting performance in general (correct trials > incorrect trials). Activations were found in these areas when correct trials were compared against incorrect trials, combining all trials of both SOA conditions.

Although activity in the parietal cortex was found to be related to performance in general, it did not significantly differ between the conditions in which the subjective criteria for conscious perception differed. ...This seems to differ from the results of many previous NCC studies, which have found parietal activity to be as significant as the prefrontal activity, if not more so. The authors argue that it is possible that some of those previously reported activations in the parietal cortex may reflect performance, given that a difference in consciousness level is typically associated with a difference in performance. The potential role of the prefrontal cortex might be further clarified by giving the task used in the present study to patients with lesions that include the mid-DPLFC. Such a study would help contribute further to our understanding of the role of the prefrontal cortex in subjective conscious perception.

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