Friday, January 08, 2010

Distinguishing conscious from unconscious brain activity.

Schurger et al. nudge a bit further towards finding one holy grail of neuroscience - identifying the neuronal correlates of conscious awareness.

What qualifies a neural representation for a role in subjective experience? Previous evidence suggests that the duration and intensity of the neural response to a sensory stimulus are factors. We introduce another attribute—the reproducibility of a pattern of neural activity across different episodes—that predicts specific and measurable differences between conscious and nonconscious neural representations indepedently of duration and intensity. We found that conscious neural activation patterns are relatively reproducible when compared with nonconscious neural activation patterns corresponding to the same perceptual content. This is not adequately explained by a difference in signal-to-noise ratio.
Clips from their account:
Functional magnetic resonance imaging (fMRI)was used to measure brain activity while subjects performed a simple visual category-discrimination task. The stimuli were simple line drawings of faces and houses (12 of each), rendered in two opposing but isoluminant colors (see the figure and legend). Visibility of the stimuli was manipulated by using dichoptic color masking. Subjects were asked to identify the category of the stimulus (face or house) on each trial, guessing if necessary, and to wager ("high" or "low" for monetary rewards) on the accuracy of each of their perceptual decisions. Wagering was used as a collateral index of subjects’ awareness of the object.

Dichoptic-color masking. This method of manipulating awareness... relies on the phenomenon of dichoptic color fusion. The "same color" mode corresponds to the visible condition, and the "opposite color" mode corresponds to the invisible condition. In order to achieve disappearance of the image in the opposite color mode, the two colors must be approximately isoluminant and the object boundaries slightly blurred. Before the experiment, subjects were trained to maintain steady fixation and were cued to do so during each trial with the appearance of the fixation point (500 ms before stimulus onset). Stimuli were presented stereoscopically in the fMRI scanner by using a cardboard divider and prism lenses

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