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 differe nt types of stimuli. We found that right unilateral temporal lobectomy (RTL ) patients, in contrast to control subjects and left temporal lobectomy (LT L) patients, failed to show potentiated startle while viewing negative pict ures. However, the opposite pattern of impairment was observed during a sti mulus that patients had been told signaled the possibility of shock, Contro l subjects and RTL patients showed potentiated startle while LTL patients f ailed to show potentiated startle. We hypothesize that the right medial tem poral 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. I n contrast, the left medial temporal lobe modulates fear responses when tho se responses are the result of a linguistic/cognitive representation acquir ed through language. which, like other verbally mediated material, generall y involves the left hemisphere. Additional evidence from case studies sugge sts that. within the medial temporal lobe, the amygdala is responsible for this modulation.