Emotion, Cerebral Lateralization

The Right Hemisphere Hypothesis

The right hemisphere model posits that the right hemisphere plays a specialized role in the perception, identification, and expression of emotional stimuli. It dates back to observations made by neurologist Charles Mills as early as 1912 that brain-damaged patients with unilateral right-sided lesions demonstrate a decrease in emotional expression. The right hemisphere hypothesis was later elaborated by Guido Gainotti to explain the catastrophic reaction of patients with left-hemisphere (LH) lesions and the indifferent or euphoric reactions of patients who suffered right-hemisphere (RH) lesions. Gainotti believed that RH damage led to an inability to emotionally appreciate the damage that the patients had incurred, while LH lesions resulted in an emotionally intact understanding of the amount of damage—hence, a normal secondary catastrophic reaction to a catastrophic event.

Experimental evidence to support RH specialization for emotion perception and identification includes a left visual field advantage for emotional stimuli, in that stimuli presented to the left side of the visual field, which leads to predominantly RH processing, are responded to more quickly and accurately. Joan Borod demonstrated that patients with RH lesions are more likely to be impaired in identifying emotional stimuli such as faces, emotional tone of voice (prosody), and words or sentences with emotional content. In split-brain patients, individuals who have had the major pathway connecting their two hemispheres (the corpus callosum) severed, both hemispheres are found to be equally adept at recognizing facial emotions on standard tasks. However, there is a RH advantage for distinctly nonverbal emotion discrimination tasks, which suggests that RH specialization for perceptual discrimination of nonverbal stimuli may also contribute to emotion-processing advantages.

Regarding expression of emotion, the left side of the face has been found to be more dramatic and expressive than the right side across all major emotion categories (happy, sad, mad, angry, surprised, disgusted) although this finding is more robust for negative emotions. Patients with RH damage also demonstrate impairments in expressing emotional prosody in both spontaneous and posed (intentionally produced) conditions. Additionally, patients with RH lesions are impaired in their electrodermal response to emotional stimuli relative to LH damaged patients. Finally, greater right lateralized electrophysiological activity is found during self-reported recall of emotional reactions to emotionally evocative visual material and when individuals are asked to generate emotional imagery.

The Valence Hypothesis and Approach/Withdrawal

The valence hypothesis posits that hemispheric specialization in emotion processing depends on the valence (positive or negative) of the emotion. Positive emotions are associated with greater LH activity and negative emotions with greater RH activity. The valence model was developed to account for the finding that the expression of positive emotions is not as strongly right lateralized as that of negative emotions, as well as for evidence of an LH processing advantage when positive emotional states are elicited by video clips, facial expressions, or reward-punishment contingencies.

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