MASKED PRESENTATIONS OF EMOTIONAL FACIAL EXPRESSIONS MODULATE AMYGDALA ACTIVITY WITHOUT EXPLICIT KNOWLEDGE

Functional magnetic resonance imaging (fMRI) of the human brain was us ed to study whether the amygdala is activated in response to emotional stimuli, even in the absence of explicit knowledge that such stimuli were presented. Pictures of human faces bearing fearful or happy expre ssions were presented to 10 normal, healthy subjects by using a backwa rd masking procedure that resulted in 8 of 10 subjects reporting that they had not seen these facial expressions. The backward masking proce dure consisted of 33 msec presentations of fearful or happy facial exp ressions, their offset coincident with the onset of 167 msec presentat ions of neutral facial expressions. Although subjects reported seeing only neutral faces, blood oxygen level-dependent (BOLD) fMRI signal in the amygdala was significantly higher during viewing of masked fearfu l faces than during the viewing of masked happy faces. This difference was composed of significant signal increases in the amygdala to maske d fearful faces as well as significant signal decreases to masked happ y faces, consistent with the notion that the level of amygdala activat ion is affected differentially by the emotional valence of external st imuli. In addition, these facial expressions activated the sublenticul ar substantia innominata (SI), where signal increases were observed to both fearful and happy faces - suggesting a spatial dissociation of t erritories that respond to emotional valence versus salience or arousa l value. This study, using fMRI in conjunction with masked stimulus pr esentations, represents an initial step toward determining the role of the amygdala in nonconscious processing.