Emotion and the Brain: Taking Sides

Brain Left and Right Sides
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You’ve heard this story: the left side of the brain is cold, calculating and language based, whereas the right side of the brain is artistic and emotional. But the brain is arguably the most complicated thing in the universe. It feels like any such dichotomy is bound to be too simplistic.

While there’s little debate that in the majority of people, language is predominantly managed by the left hemisphere, there’s quite a lot of debate about which side emotion comes down on, or if it’s simply equally divided between both sides.

Why should there be any division, after all? The so-called “epicenters of emotion,” the amygdalae, exists on both sides of the brain. The old limbic cortex, to which most emotional functioning has been historically attributed, is spread pretty evenly on both sides near the brain’s center.

When a brain function is managed more by one hemisphere than another, that function is said to be “lateralized” to that hemisphere. For example, for most of us, even left-handers, language is left-lateralized.

It turns out that there’s quite a lot of evidence suggesting that emotions do have some preference for one hemisphere over another. Just how they’re distributed, however, is another matter entirely, and the subject of a lot of scientific debate. There’s practically as many models as there are scientists who study emotion. Overall, though, emotion researchers who believe in the “lateralization of emotion” fall into two or three main camps.

Right Hemisphere Hypothesis

One of the biggest camps is called the “right hemisphere hypothesis.” This basically just states that all emotions are managed by the right side of the brain. This is particularly true of what researchers call "primary" emotions, usually meaning sorrow, joy, anger, disgust and fear.

Such emotions have been theorized to be shared not just across different cultures, but even across different species of the animal kingdom. Some data supports this hypothesis: the right amygdala also tends to be larger than the left amygdala, for example.

Other Theories

Not everyone believes in basic emotions, however. Some researchers believe instead that emotions are better understood by classifying them in simple dimensional terms such as arousal (how energized an emotion makes you) and valence (how positive or negative the emotion makes you feel). For example, anger might be considered a high arousal, low valence state.

Some people believe that emotions are lateralized into different hemispheres based off of their valence. The valence hypothesis suggests that the right hemisphere is involved in processing emotions characterized by withdrawal, such as fear, sadness and disgust, and the left hemisphere mediates processes related to approach such as happiness. Some have further suggested that it's even more complicated than that -- part of the hemisphere may actually be involved with inhibiting that behavior, while another part expresses it.

For example, while it has been suggested that left hemispheric damage may lead to more dysphoria due to its association with positive emotion under the valence hypothesis, damage to inhibitory circuitry in the left hemisphere may pathologically increase such positive emotion due to diminished suppression.

Some electrophysiological and imaging studies have suggested that positive emotional stimuli activate the left anterior and mid-insula, while negative emotional stimuli were more bilateral. EEG studies have shown that the right or left hemisphere is more active in the processing of withdrawal or approach emotions respsectively. These observations are complicated, however, as it is common for patients with lesions anywhere to suffer depression, and these lesions may also impact the ability of a patient to recognize and express their own feelings. Finally, there’s a line of evidence that suggests that the hemispheres differentially control autonomic functioning, with the left being regulatory of parasympathetic and the right for sympathetic functioning. For example, a body of evidence suggests that seizures associated with slowing of the heart rate more commonly originate from the right hemisphere. That said, other studies have found bilateral activation of orbitofrontal and other cortical regions during the processing of pleasurable emotions.

Yet another theory suggests that primary emotions and related displays are processed by the right hemisphere, whereas more complicated social emotions such as embarrassment are processed by the left. This theory is based largely off of observations during a Wada test, which temporarily shuts off half of the brain during an evaluation for surgery. The researchers noted that such patients were more able to describe complex than simple emotions when the right side of the brain was quieted. Others, however, have not found this distinction to be so clear.

Sources

Murphy GM, Jr., Inger P, Mark K, et al. Volumetric asymmetry in the human amygdaloid complex. Journal fur Hirnforschung 1987;28:281-289.

Ross ED, Monnot M. Affective prosody: what do comprehension errors tell us about hemispheric lateralization of emotions, sex and aging effects, and the role of cognitive appraisal. Neuropsychologia 2011;49:866-877.

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