Understanding the Brain: Hemispheres or Levels?

Hemispheric differentiation is a popular theory that describes people or behaviors as either "left-brained" using a logical, impersonal, detail-oriented mind, or "right-brained" using a second mind that is more emotional, relational, and expansive. The cortex, which is the top layer of the human brain, does have left and right halves that are separated by a deep canyon called the central sulcus, and each half of the cortex can operate semi-independently. Hemispheric differentiation suggests that differences in behavior arise from different modes of functioning in the left versus right hemispheres of the cortex. TMT would agree in some ways with this analysis, in suggesting that people do have two minds with sometimes divergent goals and characteristically different ways of approaching the world. But TMT locates humans' two minds at different levels of the brain — neocortex versus cingulate cortex and deep-brain structures — rather than in a division between the brain’s left and right hemispheres. This blog post will examine the evidence for a two-hemispheres explanation and suggest ways in which differences between the inner versus the outer brain may more usefully account for classic “left- versus right-brain” differences.

Consider the following list of features typically identified as “left-brained” versus “right-brained”:

• Left Brain: language, linear or sequential thinking, formal logic, mathematics, reductive or analytic understanding, knowledge of facts or data
• Right Brain: music and art, rhythm, visual imagery, creative or nonlinear thinking, intuition, holistic understanding, imagination, feelings, spatial awareness, understanding nonverbal cues

These differences are then often extrapolated into an explanation of differences between people, the degree of fit between individuals and specific occupations, the origins of classical mythology, or even differences between social movements and schools of thought. The distinction between rational thinking and emotional thinking is appealing and seems to fit with both our own experience and the ideas of philosophers of mind dating back to Plato and Aristotle.

Unfortunately, these ideas about hemispheric differentiation do not hold up to empirical scrutiny, and the concept of people being left- or right-brained is so discredited that it is now characterized as a "neuromyth." Certain types of cognitive processing do involve areas specific to the left or right hemisphere -- for instance, Wernicke’s and Broca’s areas are in the left hemisphere for most right-handed people, and are essential in people's abilities to understand and produce spoken language. Similarly, judgments about the emotions shown on faces seem to involve greater activation in the right hemisphere. And some tasks activate different areas depending on how the question is worded: In studies of visual processing, the right hemisphere is stronger at representing coordinates in space while the left hemisphere is better at categorical judgments about a visual stimulus such as "is the cup on the table?" But the available evidence strongly contradicts the idea that individual people are characteristically left-brained or right-brained in the sense of generally having a high level of activation in one side of the cortex or the other.

Recognition of objects involves both hemispheres, not just the more "visual" areas on the right. Even the tasks most strongly associated with "right-brained" thinking, like visual processing of emotions, seem to involve both hemispheres and strongly involve subcortical areas like the amygdala. The cortex's spatial awareness may be involved in perception, but emotional meaning is attached by deep-brain structures that are part of the Intuitive System. Hemispheric differentiation may be simply a way to increase the efficiency of mental processing, which confers a survival advantage but does not translate to different personality types based on the "dominance" of one hemisphere over the other. Some contemporary reactions to these findings serve only to reinforce the neuromyth, for instance arguing that even though the hemispheres work together, one is generally "stronger" or "more preferred" than the other for individual people. As a whole, the data contradict this idea.

Even though current neurological evidence does not support the “left- versus right-brain” model of behavior, it remains a popular conceptualization even among people with formal training in education or neuroscience. This may be because it reveals a great truth: People do in fact see the world in different ways, and these ways are grounded in two different neurocognitive systems. Even the delineation of features between our two “brains” is not far off the mark. Where the idea of hemispheric differentiation is mistaken is in identifying these functions with different areas of the cortex specifically. Instead, TMT suggests that some of them depend on lower-brain areas:

• Narrative System (prefrontal cortex and associated areas): language, linear or sequential thinking, formal logic, mathematics, reductive or analytic understanding, declarative knowledge of facts or data (i.e., ability to talk about them). But also: visual imagery, spatial awareness, imagination in the sense of consciously envisioning alternate scenarios, ability to foresee the social consequences of ones actions.
• Intuitive System (subcortical areas): nonlinear thinking via massively parallel processing of information, understanding of emotional tone, intuition and imagination in the sense of knowledge outside of language, creativity in the sense of non-logical and associative responses.

This differentiation between "top-brained" versus "bottom-brained" thinking is not unique to TMT. The "BrainWise" curriculum used in some elementary schools teaches children a set of skills for engaging "wizard brain" (cortical, Narrative System) thinking in place of "lizard brain" (subcortical, Intuitive System) thinking. But subtle differences between the top/bottom distinction and the left/right distinction are worth noting. For example, the cortex is particularly good at social imagination, so efforts to promote Narrative thought should engage imagery as well as logic. And the subcortical areas are able to initiate creative verbal responses as well as nonverbal ones. Using the "whole brain" is a useful goal as described in our initial publication on TMT, as the Narrative and Intuitive system each have weaknesses and have potentially complementary strengths. But perpetuating the "neuromyth" of left- versus right-brained persons might get in the way of achieving this goal.