Good Looking
Human brains steadily enlarged beginning 7 million years ago when species Homo split from the other primates (although the size has actually gone down a bit since peaking with Neanderthals). The interesting part is that all animals evolved two brains that are almost mirror images of one another. For the most part, the two halves of the brain enlarged in tandem although markings on the inside of ancient skulls suggests that there was some right left asymmetry as far back as Australopithecus 3.5 million years ago. That asymmetry goes through the evolutionary scale all the way from fruit flies to modern humans in whom dominant temporal lobes are somewhat larger than the non-dominant ones. That said, the differences are small. Since handedness and speech overwhelmingly reside in the left hemisphere, why has the right side continued to take up so much space and consume so much energy even though there has never been a human society in which right brain dominance was common? Surely a parsimonious evolution would not have kept so much metabolically expensive real estate around if it did nothing important.
At almost the same time Paul Broca identified the left hemispheric speech area, British neurologist John Hughlings Jackson suggested a unique function for the right side. “There are two modes of expression, one emotional and the other intellectual. By one we show what we feel, and by the other we tell what we think.”[1] Jackson saw emotional expression in patients who had lost all ability to speak. Although completely aphasic, his patients still had emotional outbursts and were especially adept at swearing.
As it turns out the right brain is essential not just for emotional reactions but for social interaction in general, and that is not unique to humans. We need to do a quick bit of clinical neurology to go farther into that. Look straight ahead and extend your right arm with the index finger sticking out. Now move the hand back even with your right ear. You won’t see the finger. Now gradually move your hand in an arc until you see the finger. It has entered your right visual field. Input from your eyes is arranged so that messages from the part of the left eye next to the nose merges with that from the outer side of the right eye, and they go together to the left occipital cortex to give you a coordinated view of the right side of your world. Same on the left, and the same all the way down the evolutionary ladder. Now you understand why animals encountering others of the same species have a strong preference for keeping them on the left. They want them in the visual field that goes to the “social” hemisphere.
That is not the only indication that the right brain is involved in social interaction. Nursing animals prefer to have infants on their left and human mothers have a strong tendency to hold infants in their left arm so the baby’s face is in their left visual field. Interestingly, that “cradling bias” is present in women who have never had children and in girls as young as six years old, but it is much less common in autistic mothers who are socially disconnected.
Muscle movements involved with emotion—smiling or sneering for example—are more pronounced on the left side than the right, and people with right hemisphere lesions have less ability to read the facial emotions of others. Studies of patients with brain injuries repeatedly confirm that human production and recognition of social stimuli are a right brain function.
The right hemisphere may have a role in species preservation as well. Response to infants is not the only thing lateralized to the right. Reaction to other emotional input—erotica or threat for example—is more rapid when the stimulus comes from the left visual field. That probably accounts for the fact that animal mothers in novel or threatening situations keep their infants on the right so their left visual field is unobstructed.
For the two brains, there seems to be a ladder of skills. On the left, the ability to repeat patterns of motor activity evolves into the ability to manipulate and use tools which culminates in the ability to communicate. On the right, predator avoidance evolves into recognition of emotions and culminates in social skills. The left side lets you learn how to eat; the right keeps you from being eaten.
Social interaction is not the right brain’s only skill. One way to control intractable epilepsy that originates in the left brain is to remove the entire hemisphere. If that is done before the patient is six years old, the child still learns to speak. In that limited sense, the right brain functions as a backup system. The right brain is also involved in spatial and perceptual relations. Another epilepsy operation involves cutting all the connections between the two hemispheres so out of control electrical activity cannot spread through the whole brain. The result (besides seizure control) is creation of two independent brains. In those patients, a right hander can still write with the dominant hand but cannot draw with it. People with right brain lesions also have a great deal of difficulty mentally rotating complicated figures. Musically, the left brain does a pretty good job with rhythm, but the right brain seems to be necessary for recognition of tone and pitch.
At least since Gutenberg, we have placed an inordinate value on what our left brains do. We concentrate on training the speech hemisphere and we judge intelligence on how well it performs. If we are entering a time when much of that work will be outsourced to machines, perhaps we will need to take a harder look at the other side. What would we look like if we spent as much time and effort training the right brain as we have the left?
References:
Coren, Stanley, The Left-Hander Syndrome: The Causes & Consequences of Left-Handedness. New York: The Free Press, 1992.
Forrester, Gillian S., William D. Hopkins, Kristelle Hudry, and Annukka Lindell, eds., Cerebral Lateralization and Cognition: Evolutionary and Developmental Investigations of Behavioral Biases. Cambridge, MA: Academic Press, 2018.
[1] John Hughlings Jackson in Medical Times and Gazette” 1866. cited in Taylor, James, ed., Selected Writings of John Hughlings Jackson, Volume Two: Evolution and Dissolution of the Nervous System, Speech, Various Papers, Addresses, and Lectures. London: Hodder and Stoughton Limited, 1932.