The impact of affect and frequency on lexical decision: the role of the amygdala and inferior frontal cortex.

The current study used event-related fMRI to examine BOLD responses associated with two factors that behaviorally determine speed of lexical decision: frequency and emotion. Thirteen healthy adults performed a visual lexical decision task, discriminating between words and orthographically and phonologically legal nonwords. The study involved a 2 (Frequency: high and low) x 3 (Emotional arousal: highly negative, mildly negative, and neutral words) design with word categories matched for number of letters and concreteness. There were significant main effects for both frequency and emotion in lexical decision reaction times but no significant interaction. Negative word lexical decisions were associated with increased activation in bilateral amygdala and middle temporal cortex as well as rostral anterior and posterior cingulate cortex. Low-frequency word lexical decisions, relative to high-frequency word lexical decisions, were associated with increased bilateral activity in inferior frontal cortex. Inferior frontal cortex activation was particularly low during lexical decision for high-frequency emotional words but significant for high-frequency neutral emotional words. We suggest that this is because the semantic representation of high-frequency emotional words may receive sufficient additional augmentation via the reciprocal activation from the amygdala such that selective augmentation by inferior frontal cortex to achieve lexical decision is unnecessary.