Friday, June 29, 2007

Role of the amygdala in visual awareness.

There have been several reports that subliminal stimuli (such as flashing a picture of an angry face for 33 msec) can activate the amygdala even though the subject is unaware of the stimulus. Pessoa presented work at the recent ASSC meeting using more rigorous criteria for behavioral performance that suggests, to the contrary, that visibility or attention is required for the expression of the effect of valence on early visual processing (even as early as the 'automatic' parallel processing in V1.) A PDF of a recent Pessoa et al. article is here, and PDF of commentary on this work by Duncan and Barrett here). This work:
...shows that amygdala responses depend on visual awareness. Under conditions in which subjects were not aware of fearful faces flashed for 33 ms, no differential activation was observed in the amygdala. On the other hand, differential activation was observed for 67 ms fearful targets that the subjects could reliably detect. When trials were divided into hits, misses, correct rejects, and false alarms, we show that target visibility is an important factor in determining amygdala responses to fearful faces. Taken together, our results further challenge the view that amygdala responses occur automatically.
Duncan and Barrett, in their commentary, suggest
...that the amygdala is acting to increase neural activity in the fusiform gyrus, thereby increasing the likelihood that visual representations that have affective value reach awareness. The psychological consequence is that a person’s momentary affective state might help to select the contents of conscious experience.

Visual awareness is associated with amygdala activation. In the Pessoa et al. study, participants viewed backwardly masked images of faces that depicted fear, presented for either 33 ms or 67 ms. All participants showed greater amygdala activation when viewing fearful faces that were presented for 67 ms, compared with faces that depicted neutral expressions. (a) Pessoa et al. found an increase in amygdala activation (as well as fusiform gyrus activation, which is not shown in the figure) only among those participants who showed objective awareness of 33 ms presentations of faces that depicted fear. (b) Participants who did not show objective awareness did not have significant increases in amygdala activation. Given the excitatory projections from the amygdala to the ventral visual stream, this finding suggests that the amygdala enhances visual awareness for objectives with affective value.

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