The Sequential and the Parallel
Language, and especially speech, is sequential. You cannot say six words at the same time. They unfold one after another in time.
Vision is parallel. Show someone a picture with six animals on it and they can take in all six at once.
Davidson uses this distinction to frame the deeper point about what each hemisphere may actually be specialized for.
Some tasks are better served by parallel processing. Others require sequential activity. Creative work in any serious sense draws on both kinds of processing at once.
Every human being has the potential to be creative. When humans are most creative, they are harnessing the full capabilities of the brain. Not just the right hemisphere. Not just the left. Both.
A massive fiber bundle called the corpus callosum. It contains roughly 200 million neurons running from the front of the brain to the back, allowing the two hemispheres to communicate continuously.
It is the largest white matter fiber bundle in the brain. Whatever specialization exists in either hemisphere, the two are almost always working in an integrated way.
The Pop-Science Era
Books in this era, including titles like Drawing on the Right Side of Your Brain, popularized the idea that the right hemisphere is the creative hemisphere and the left is the analytic, logical, boring hemisphere. The framing implied you could intentionally shift into your "right brain" to access creativity.
Davidson calls these generalizations way overdone. There was a grain of truth, but it was applied far past what the science supports.
More than a hundred published papers on brain asymmetry in one form or another. He also edited two major compendiums on the topic, published by MIT Press, with the Norwegian scientist Kenneth Hugdahl.
This was a primary line of research for him through the late 70s and 80s.
The Handedness Caveat
The broad statements about the left and right hemispheres are roughly true only for right-handed people. Roughly 85% of the population is right-handed. For people who write with their left hand, the pattern is not necessarily the same.
Davidson brackets the full discussion of left-handed brain organization as a separate topic worthy of its own conversation.
The lateralization for language, particularly speaking, is more definitive than the lateralization for spatial tasks. For right-handed people, the left hemisphere can speak and the right hemisphere essentially cannot.
For visual and spatial skills, the right hemisphere is typically a little better than the left. Not absolutely. Just better at the margin.
Typically a little bit better. Not absolutely better. It is not the case that the right hemisphere can do something and the left hemisphere cannot, or vice versa, for most cognitive tasks.
The one exception is speech production, where the lateralization is unusually clean.
What the Hemispheres Actually Do
It collapses a complex, distributed system into a tidy story. Every human has the potential for creativity, and when creativity actually happens, the brain is harnessing both hemispheres together, not switching one off.
Davidson describes the popular generalizations as having been "taken way too far."
A lot. The anterior portions of the cerebral cortex, like the prefrontal region, are doing something quite different from the back of the brain.
The visual areas, the parietal area, and the temporal regions all have distinct roles. Within-hemisphere specialization may matter as much as or more than between-hemisphere specialization for many functions.
The parietal area is a region of the brain where visual information, auditory information, and kinesthetic information are all integrated.
It is the part of the brain where different sensory streams come together into a unified experience.
The Bridge Between Hemispheres
By using fixation tricks. If a subject focuses straight ahead and you present information on the left of that fixation point, it projects initially to the right hemisphere because the visual pathways are crossed. Information on the right of the fixation point goes initially to the left hemisphere.
In a normal, intact brain, the information then transfers across the corpus callosum to the other hemisphere almost immediately, so both hemispheres receive it.
Split-brain patients are people whose corpus callosum has been surgically severed. This was done historically to treat extremely severe epilepsy that originated in one hemisphere and spread to the other, producing grand mal seizures. Cutting the connecting fiber bundle dramatically reduces seizure severity.
The procedure is rarely performed today, but earlier patients walking around with two relatively independent hemispheres provided foundational evidence about hemispheric specialization.
The Accidental Discovery
EEG. Non-invasive scalp electrodes that record electrical patterns from the brain. Functional MRI was not yet available. EEG allowed inferences about which brain regions were more or less active during different tasks.
The resting-state baseline. Before giving subjects any task, the lab would always record eight minutes of resting EEG, alternating one minute of eyes open and one minute of eyes closed. The point was just to confirm that the electrodes were working properly.
The actual data of interest, they assumed, was what happened when subjects did tasks. The resting data was discarded.
That subjects showed clear individual differences in their asymmetry at rest. Some people had greater left-sided activation. Others had greater right-sided activation. The pattern differed based on whether the electrodes were in the back or front of the head.
At first the team assumed it had to be random shifting that would wash out over time. So they brought subjects back a week later, then a month later, then three months later. The pattern was strikingly stable.
A stable, trait-like pattern of brain activation that persists across testing sessions, days of the week, and experimenters. The asymmetry Davidson found at rest behaved like a characteristic of the person, not a fluctuating state.
If somebody showed left-sided activation today, they would almost certainly show a similar pattern three months later.
Years later, the default mode network was discovered through almost the same kind of accident. Researchers analyzing what was happening during "resting state" periods in fMRI studies, which had been treated as uninteresting background, found a coherent and important network.
Sometimes what scientists first dismiss as noise turns out to be the most interesting part of the experiment.
From Asymmetry to Affective Style
Enduring ways in which people respond differently to emotional challenges. Davidson's lab found that the stable resting asymmetry pattern was linked to these enduring differences in how people process and react to emotion.
This finding launched the broader research program into what is now called affective neuroscience.
Walle J. H. Nauta, a famous neuroanatomist at MIT, who Davidson studied with. Nauta wrote a paper in the Journal of Psychiatric Research that was, for its time, the first serious mainstream-scientist speculation about the role of the prefrontal cortex in emotion.
That paper is what got Davidson started on the path of studying the prefrontal cortex and emotion specifically.
The notion of asymmetries in the front of the brain related to emotion.
Earlier work on hemispheric asymmetry, including the work behind the pop-science books of the 90s, was focused on the back of the brain, on language and visual-spatial functions. Davidson's group was the first to focus on frontal asymmetries and link them to emotional processing.
Can You Train Brain Asymmetry?
The lab gave subjects real-time feedback based on the difference in activation between their two hemispheres, not on either hemisphere alone. Subjects were asked to use this feedback to shift their asymmetry.
Davidson described it as a very early form of neurofeedback.
To some extent, yes. With very little training, subjects were able to shift the relative activation between their hemispheres using the biofeedback signal.
This is not the same as turning a hemisphere "on" or "off" or shifting a specific function like language to a different side. It is more a modest, trainable nudge in the balance of activation.
Real asymmetries do exist. But the pop-science framing referred mostly to specialization in the back of the brain, the parietal and temporal regions, in tasks like language, visual, and spatial processing. It was not really about creativity at all in any direct sense.
The framing then got overgeneralized into a whole-person typology ("you are a right-brain person") that the science never supported.
Creativity and Inter-Hemispheric Coordination
That what may really stimulate creativity is engaging in tasks that demand a lot of inter-hemispheric coordination. Not training the right hemisphere in isolation, but training the two sides to work together fluidly.
He describes this as speculative. There is not yet much hard science on it.
The right hand is controlled mostly by the left motor cortex. The left hand is controlled mostly by the right. When you juggle, the two hands have to coordinate in real time, which forces the two hemispheres to do the same.
It is not just hand-eye coordination. It is coordination between the hands, which means coordination between the hemispheres that drive them.
Mudras are intricate hand gestures used in Tibetan Buddhist practice. They often involve very different motions in each hand performed simultaneously, with the practitioner also holding elaborate visualizations in mind at the same time.
Davidson speculates that ancient yogis may have intuited something experientially about the value of inter-hemispheric coordination that future science could eventually formalize.
The popular "right brain vs. left brain" story got the geography roughly right and the meaning almost entirely wrong. Real differences between the hemispheres exist, but they are modest, statistical, and constantly bridged by 200 million neurons firing across the corpus callosum. The most interesting finding in this body of work came not from where the lab was looking, but from data the lab was throwing away. And the most provocative idea about creativity now is not that you should favor one side, but that you should practice using both at once.