A double dissociation in the affective modulation of startle in humans: effects of unilateral temporal lobectomy.

In the present study we report a double dissociation between right and left medial temporal lobe damage in the modulation of fear responses to different types of stimuli. We found that right unilateral temporal lobectomy (RTL) patients, in contrast to control subjects and left temporal lobectomy (LTL) patients, failed to show potentiated startle while viewing negative pictures. However, the opposite pattern of impairment was observed during a stimulus that patients had been told signaled the possibility of shock. Control subjects and RTL patients showed potentiated startle while LTL patients failed to show potentiated startle. We hypothesize that the right medial temporal lobe modulates fear responses while viewing emotional pictures, which involves exposure to (emotional) visual information and is consistent with the emotional processing traditionally ascribed to the right hemisphere. In contrast, the left medial temporal lobe modulates fear responses when those responses are the result of a linguistic/cognitive representation acquired through language, which, like other verbally mediated material, generally involves the left hemisphere. Additional evidence from case studies suggests that, within the medial temporal lobe, the amygdala is responsible for this modulation.

Performance on indirect measures of race evaluation predicts amygdala activation.

We used fMRI to explore the neural substrates involved in the unconscious evaluation of Black and White social groups. Specifically, we focused on the amygdala, a subcortical structure known to play a role in emotional learning and evaluation. In Experiment 1, White American subjects observed faces of unfamiliar Black and White males. The strength of amygdala activation to Black-versus-White faces was correlated with two indirect (unconscious) measures of race evaluation (Implicit Association Test [IAT] and potentiated startle), but not with the direct (conscious) expression of race attitudes. In Experiment 2, these patterns were not obtained when the stimulus faces belonged to familiar and positively regarded Black and White individuals. Together, these results suggest that amygdala and behavioral responses to Black-versus-White faces in White subjects reflect cultural evaluations of social groups modified by individual experience.