Previous research shows that the amygdala automatically responds to a face’s trustworthiness when a face is clearly visible…the human amygdala is sensitive to subliminal variation in facial trustworthiness. Regions in the amygdala tracked how untrustworthy a face appeared (i.e., negative-linear responses) as well as the overall strength of a face’s trustworthiness signal (i.e., nonlinear responses), despite faces not being subjectively seen.
The findings demonstrate that the amygdala can be influenced by even high-level facial information before that information is consciously perceived, suggesting that the amygdala’s processing of social cues in the absence of awareness may be more extensive than previously described.
With only a glance, humans instantly form impressions of another’s face. Such impressions occur spontaneously and are often beyond our conscious control (Zebrowitz and Montepare, 2008). They help us distinguish friend from foe, or those whom we should trust from those of whom we should be wary. Indeed, a mere 50 ms exposure to a face permits trait inferences that are highly correlated among multiple perceivers, indicating that facial cues provide reliable signals about another’s underlying disposition (Bar et al., 2006; Willis and Todorov, 2006).another’s face. Such impressions occur spontaneously and are often beyond our conscious control (Zebrowitz and Montepare, 2008).
Previous behavioral studies suggest that face-based evaluations are underpinned by two fundamental dimensions, trustworthiness and dominance. Facial trustworthiness in particular accounts for the bulk of variance in social evaluation, and recent behavioral studies have provided preliminary evidence that individuals might be sensitive to trustworthiness without perceptual awareness.
However, the neural basis of evaluating high-level social information such as trustworthiness from a face outside awareness remains unexplored. Evaluations of trustworthiness reflect more general face valence and show correlated activity in the amygdala, a subcortical region involved in processing the affective significance of social stimuli and important for a variety of social and emotional behaviors. Consistent with the sensitivity of the amygdala to negative, threat-related stimuli:
– initial studies reported linear effects, with amygdala activation increasing for faces appearing less trustworthy. This effect held true regardless of task demands , suggesting that the amygdala may code trustworthiness implicitly when a face is clearly visible.
– More recent studies have reported quadratic effects, with amygdala activation increasing for faces appearing either more or less trustworthy, relative to neutral, potentially reflecting the coding of the salience or motivational relevance of a stimulus derived from a face’s trustworthiness.
– A recent meta-analysis found that both linear and nonlinear responses to trustworthiness coexist in different amygdala subregions.
However, it is currently unknown whether the amygdala is sensitive to trustworthiness before a face can reach perceptual awareness…By facilitating the coding of another’s trustworthiness before awareness, the amygdala could modulate cortical processes and motivate appropriate behavioral responses.
Across two experiments, our findings demonstrate that the human amygdala is automatically responsive to a face’s trustworthiness
in the absence of perceptual awareness.
The present results are striking in that they raise a conundrum as to how the amygdala could evaluate such complex social information under constricted processing. Researchers have documented amygdala responses to subliminal fear expressions, which have been suggested by some to be related to an early threat-detection mechanism driven by a retino-collicular-pulvinar pathway that responds to salient, behaviorally relevant stimuli…it is the feedback loops between subcortical and cortical areas that are thought to be critical for awareness, rather than the involvement of particular brain regions.
Complex arrangements of such lower-level facial features together form this fundamental trustworthiness dimension, which is spontaneously perceived on encountering a face and accounts for the majority of variance in social evaluation. Here we show that this dimension can sensitively modulate amygdala responses before reaching awareness.
Work in nonhuman primates has also converged on the finding of both forms of coding in the amygdala, with one subregion showing linear responses to threatening faces (coding negative valence), and another showing nonlinear responses to both threatening and appeasing faces relative to neutral.
…both experiments provide clear support for our primary hypothesis that the amygdala is automatically responsive to facial trustworthiness without perceptual awareness. Regions in the amygdala track how untrustworthy an unseen face appears as well as the overall strength of the trustworthiness signal. Moreover, this tracking generalizes across real and computer-generated faces, where trustworthiness was both measured and manipulated, respectively.
Whereas the negative-linear effects found here are consistent with the amygdala’s role in vigilance for threats and tracking of valence, nonlinear effects are consistent with the amygdala’s processing of salience and motivational relevance. Faces with stronger cues for untrustworthiness or trustworthiness are more motivationally relevant, as these cues spontaneously elicit approach and avoidance behaviors. Both forms of coding are consistent with the amygdala’s importance for interpreting implicit social signals. Faces that appear more untrustworthy and likely to inflict harm, or faces with a stronger trustworthiness signal in general, would benefit more from automatic amygdala responsivity, which could adaptively modulate cortical processes and motivate appropriate social behavior.
In summary, we demonstrated in two experiments that the amygdala is sensitive to subliminal variation in facial trustworthiness. Thus, the amygdala can be influenced by even high-level facial information before that information is consciously perceived. These findings provide evidence that the amygdala’s processing of social cues in the absence of of awareness may be more extensive than previously described.