Affective priming following unilateral temporal lobectomy : the role of the amygdala
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The way that emotions are processed in the brain has been widely debated. The two leading hypotheses are the cognitive appraisal viewpoint (Lazarus, 1982) and the affective primacy hypothesis (Zajonc, 1980). The former argues that higher cortical structures are needed to evaluate affective stimuli whereas the latter asserts that humans can use information only processed at the subcortical level to influence behavior. The current study tested the presence of this subcortical pathway by using an affective priming task developed by Murphy and Zajonc (1993). Happy and angry faces were presented for 4 ms before the presentation of a neutral stimulus (Chinese Ideograph) that participants were asked to rate based on how much they liked each one. Individuals do not report conscious awareness of primes presented at this suboptimal speed. In a young adult sample, participants rated ideographs preceded by happy primes significantly higher than those preceded by angry primes. Also, the priming effect was only observed in participants who reported a high positive mood. Next, when primes were presented in the left or right hemifield priming was only found in the right hemifield, and was driven by increased ratings for ideographs preceded by happy primes. Patients with epilepsy who have undergone a temporal lobectomy provide a unique opportunity to study emotional processing. In this procedure, not only is the seizure focus (typically the hippocampus) removed, but the amygdala and surrounding areas of the mesial temporal lobe are removed as well. Nine patients post right temporal lobectomy and three patients post left temporal lobectomy completed the study and did not show an effect of priming. However, 21 pre-surgical epilepsy patients were found to give higher liking ratings to ideographs preceded by angry primes as compared to those preceded by happy primes. Overall, these results support the affective primacy hypothesis however they also suggest that patients with temporal lobe dysfunction may process emotional stimuli differentially from controls. In this population, ideographs preceded by angry primes were rated as more liked than those preceded by happy primes. Directions for future studies to clarify the role of the amygdala in emotional processing are discussed.