Neural Correlates of Emotion Word Processing in Autism Spectrum Disorders

Background: Emotion words are a special category of words. They are remembered better, automatically attract attention, and activate a network of brain regions including the limbic system, producing a different pattern in the brain activation and in event-related potentials (ERPs). One of such emotion-specific ERP components is the Late Positive Component (LPC): a long-lasting positive-going wave most prominent over centroparietal regions.  For Autism Spectrum disorders (ASD), understanding emotions in others and conveying their own emotional state may be problematic. However, little is known about neural mechanisms of emotion processing of ASD. Furthermore, most of research has been done on recognizing emotions in faces, but less is known about processing of emotion in other types of stimuli, such as language.

Objectives: To investigate processing of neutral and emotion words in individuals with ASD by using reaction times (RTs) and EEG.

Methods: We tested 21 ASD and 20 healthy controls. Participants performed a lexical decision task while their EEG and RTs were recorded. We analyzed the effects of lexicality (words vs nonwords), frequency (high vs low frequency words), and valence (positive vs neutral vs negative words).

Results: In the RT analysis, we found that both ASD and control groups provided faster responses for emotion words compared to neutral. This effect was modulated by verbal IQ scores in the ASD group but not in the control group: in patients with higher scores, the difference in RT between emotional and neutral words was much smaller. In the EEG data, lexicality and frequency had a significant effect on the ERP with similar timing and distribution in both groups. With regard to emotional valence, there was a significant group by valence interaction in the negative vs neutral valence comparison: the effect identified as the LPC was present in the control group, but not in the ASD. For the positive vs neutral valence contrast, the interaction did not reach significance, even though within-group comparisons demonstrated presence of a significant LPC effect in the control group and absence of an effect in the ASD group.

Conclusions: Even simple emotional stimuli like single words are processed differently in ASD patients. The processing of emotion is modulated by verbal IQ, and patients with high and low IQ may in fact constitute two different subtypes of ASD. The EEG data suggest that the neural mechanisms of emotion processing are different in ASD and controls. In the control group the LPC was significantly modulated by valence, with timing and distribution being consistent with other studies. The LPC is hypothesized to reflect allocation of additional cognitive resources towards emotional words, because of their greater salience and motivational importance. Because this effect was absent in the ASD group, we surmise that in individuals with ASD emotion words are in general less salient.