Left Brain, Right Brain: One More Time
"Right brain versus left brain" is one of those popular ideas that will not die. Speculations about the educational significance of brain laterality have been circulating in the education literature for 30 years. Although repeatedly criticized and dismissed by psychologists and brain scientists, the speculation continues.6 David Sousa devotes a chapter of How the Brain Learns to explaining brain laterality and presents classroom strategies that teachers might use to ensure that both hemispheres are involved in learning.7 Following the standard line, the left hemisphere is the logical hemisphere, involved in speech, reading, and writing. It is the analytical hemisphere that evaluates factual material in a rational way and that understands the literal interpretation of words. It is a serial processor that tracks time and sequences and that recognizes words, letters, and numbers. The right hemisphere is the intuitive, creative hemisphere. It gathers information more from images than from words. It is a parallel processor well suited for pattern recognition and spatial reasoning. It is the hemisphere that recognizes faces, places, and objects.
According to this traditional view of laterality, left-hemisphere-dominant individuals tend to be more verbal, more analytical, and better problem solvers. Females, we are told, are more likely than males to be left-hemisphere dominant. Right-hemisphere-dominant individuals, more typically males, paint and draw well, are good at math, and deal with the visual world more easily than with the verbal. Schools, Sousa points out, are overwhelmingly left-hemisphere places in which left-hemisphere-dominant individuals, mostly girls, feel more comfortable than right-hemisphere-dominant individuals, mostly boys. Hemispheric dominance also explains why girls are superior to boys in arithmetic -- it is linear and logical, and there is only one correct answer to each problem -- while girls suffer math anxiety when it comes to the right-hemisphere activities of algebra and geometry. These latter disciplines, unlike arithmetic, are holistic, relational, and spatial and also allow multiple solutions to problems.
Before we consider how, or whether, brain science supports this traditional view, educators should be wary of claims about the educational significance of gender differences in brain laterality. There are tasks that psychologists have used in their studies that reveal gender-based differences in performance. Often, however, these differences are specific to a task. Although males are superior to females at mentally rotating objects, this seems to be the only spatial task for which psychologists have found such a difference.8 Moreover, when they do find gender differences, these differences tend to be very small. If they were measured on an I.Q.-like scale with a mean of 100 and a standard deviation of 15, these gender differences amount to around five points. Furthermore, the range of difference within genders is broad. Many males have better language skills than most females; many females have better spatial and mathematical skills than most males. The scientific consensus among psychologists and neuroscientists who conduct these studies is that whatever gender differences exist may have interesting consequences for the scientific study of the brain, but they have no practical or instructional consequences.9
Now let's consider the brain sciences and how or whether they offer support for some of the particular teaching strategies Sousa recommends. To involve the right hemisphere in learning, Sousa writes, teachers should encourage students to generate and use mental imagery: "For most people, the left hemisphere specializes in coding information verbally while the right hemisphere codes information visually. Although teachers spend much time talking (and sometimes have their students talk) about the learning objective, little time is given to developing visual cues." To ensure that the left hemisphere gets equal time, teachers should let students "read, write, and compute often."10
What brain scientists currently know about spatial reasoning and mental imagery provides counterexamples to such simplistic claims as these. Such claims arise out of a folk theory about brain laterality, not a neuroscientific one.
Here are two simple spatial tasks: 1) determine whether one object is above or below another, and 2) determine whether two objects are more or less than one foot apart. Based on our folk theory of the brain, as spatial tasks both of these should be right-hemisphere tasks. However, if we delve a little deeper, as psychologists and neuroscientists tend to do, we see that the information-processing or computational demands of the two tasks are different.11 The first task requires that we place objects or parts of objects into broad categories -- up/down or left/right -- but we do not have to determine how far up or down (or left or right) one object is from the other. Psychologists call this categorical spatial reasoning. In contrast, the second task is a spatial coordinate task, in which we must compute and retain precise distance relations between the objects.
Research over the last decade has shown that categorical and coordinate spatial reasoning are performed by distinct subsystems in the brain.12 A subsystem in the brain's left hemisphere performs categorical spatial reasoning. A subsystem in the brain's right hemisphere processes coordinate spatial relationships. Although the research does point to differences in the information-processing abilities and biases of the brain hemispheres, those differences are found at a finer level of analysis than "spatial reasoning." It makes no sense to claim that spatial reasoning is a right-hemisphere task.
Based on research like this, Christopher Chabris and Stephen Kosslyn, leading researchers in the field of spatial reasoning and visual imagery, claim that any model of brain lateralization that assigns conglomerations of complex mental abilities, such as spatial reasoning, to one hemisphere or the other, as our folk theory does, is simply too crude to be scientifically or practically useful. Our folk theory can neither explain what the brain is doing nor generate useful predictions about where novel tasks might be computed in the brain.13 Unfortunately, it is just such a crude folk theory that brain-based educators rely on when framing their recommendations.
Visual imagery is another example. From the traditional, folk-theoretic perspective, generating and using visual imagery is a right-hemisphere function. Generating and using visual imagery is a complex operation that involves, even at a crude level of analysis, at least five distinct mental subcomponents: 1) to create a visual image of a dog, you must transfer long-term visual memories into a temporary visual memory store; 2) to determine if your imagined dog has a tail, you must zoom in and identify details of the image; 3) to put a blue collar on the dog requires that you add a new element to your previously generated image; 4) to make the dog look the other way demands that you rotate your image of the dog; and 5) to draw or describe the imagined dog, you must scan the visual image with your mind's eye.
There is an abundance of neuroscientific evidence that this complex task is not confined to the right hemisphere. There are patients with brain damage who can recognize visual objects and draw or describe visible objects normally, yet these patients cannot answer questions that require them to generate a mental image. ("Think of a dog. Does it have a long tail?") These patients have long-term visual memories, but they cannot use those memories to generate mental images. All these patients have damage to the rear portion of the left hemisphere.14
Studies on split-brain patients, people who have had their two hemispheres surgically disconnected to treat severe epilepsy, allow scientists to present visual stimuli to one hemisphere but not the other. Michael Gazzaniga and Kosslyn showed split-brain patients a lower-case letter and then asked the patients whether the corresponding capital letter had any curved lines.15 The task required that the patients generate a mental image of the capital letter based on the lower-case letter they had seen. When the stimuli were presented to the patients' left hemispheres, they performed perfectly on the task. However, the patients made many mistakes when the letter stimuli were presented to the right hemisphere. Likewise, brain-imaging studies of normal adult subjects performing imagery tasks show that both hemispheres are active in these tasks.16 Based on all these data, brain scientists have concluded that the ability to generate visual imagery depends on the left hemisphere.
One of the most accessible presentations of this research appears in Images of Mind, by Michael Posner and Mark Raichle, in which they conclude, "The common belief that creating mental imagery is a function of the right hemisphere is clearly false."17 Again, different brain areas are specialized for different tasks, but that specialization occurs at a finer level of analysis than "using visual imagery." Using visual imagery may be a useful learning strategy, but if it is useful it is not because it involves an otherwise underutilized right hemisphere in learning.
The same problem also subverts claims that one hemisphere or the other is the site of number recognition or reading skills. Here is a simple number task, expressed in two apparently equivalent ways: What is bigger, two or five? What is bigger, 2 or 5? It involves recognizing number symbols and understanding what those symbols mean. According to our folk theory, this should be a left-hemisphere task. But once again our folk theory is too crude.
Numerical comparison involves at least two mental subskills: identifying the number names and then comparing the numerical magnitudes that they designate. Although we seldom think of it, we are "bilingual" when it comes to numbers. We have number words -- e.g., one, two -- to name numbers, and we also have special written symbols, Arabic numerals -- e.g., 1, 2. Our numerical bilingualism means that the two comparison questions above place different computational demands on the mind/brain. Using brain-recording techniques, Stanislaus Dehaene found that we identify number words using a system in the brain's left hemisphere, but we identify Arabic numerals using brain areas in both the right and left hemispheres. Once we identify either the number words or the Arabic digits as symbols for numerical quantities, a distinct neural subsystem in the brain's right hemisphere compares magnitudes named by the two number symbols.18
Even for such a simple number task as comparison, both hemispheres are involved. Thus it makes no neuroscientific sense to claim that the left hemisphere recognizes numbers. Brain areas are specialized, but at a much finer level than "recognizing numbers." This simple task is already too complex for our folk theory to handle. Forget about algebra and geometry.
Similar research that analyzes speech and reading skills into their component processes also shows that reading is not simply a left-hemisphere task, as our folk theory suggests. Recognizing speech sounds, decoding written words, finding the meanings of words, constructing the gist of a written text, and making inferences as we read all rely on subsystems in both brain hemispheres.19
There is another different, but equally misleading, interpretation of brain laterality that occurs in the literature of brain-based education. In Making Connections, Renate Caine and Geoffrey Caine are critical of traditional "brain dichotomizers" and warn that the brain does not lend itself to such simple explanations. In their view, the results of research on split brains and hemispheric specialization are inconclusive -- "both hemispheres are involved in all activities" -- a conclusion that would seem to be consistent with what we have seen in our brief review of spatial reasoning, visual imagery, number skills, and reading.
However, following the folk theory, they do maintain that the left hemisphere processes parts and the right hemisphere processes wholes. In their interpretation, the educational significance of laterality research is that it shows that, within the brain, parts and wholes always interact. Laterality research thus provides scientific support for one of their principles of brain-based education: the brain processes parts and wholes simultaneously. Rather than number comparison or categorical spatial reasoning, the Caines provide a more global example: "Consider a poem, a play, a great novel, or a great work of philosophy. They all involve a sense of the 'wholeness' of things and a capacity to work with patterns, often in timeless ways. In other words, the 'left brain' processes are enriched and supported by 'right brain' processes."20
For educators, the Caines see the two-brain doctrine as a "valuable metaphor that helps educators acknowledge two separate but simultaneous tendencies in the brain for organizing information. One is to reduce information to parts; the other is to perceive and work with it as a whole or a series of wholes."21 Effective brain-based educational strategies overlook neither parts nor wholes, but constantly attempt to provide opportunities in which students can make connections and integrate parts and wholes. Thus the Caines number among their examples of brain-based approaches whole-language instruction,22 integrated curricula, thematic teaching, and cooperative learning.23 Similarly, because we make connections best when new information is embedded in meaningful life events and in socially interactive situations, Lev Vygotsky's theory of social learning should also be highly brain compatible.24
To the extent that one would want to view this as a metaphor, all I can say is that some of us find some metaphors more appealing than others. To the extent that this is supposed to be an attempt to ground educational principles in brain science, the aliens have just landed in Egypt.
Where did things go awry? Although they claim that laterality research in the sense of hemispheric localization is inconclusive, the Caines do maintain the piece of our folk theory that attributes "whole" processing to the right hemisphere and "part" processing to the left hemisphere. Because the two hemispheres are connected in normal healthy brains, they conclude that the brain processes parts and wholes simultaneously. It certainly does -- although it probably is not the case that wholes and parts can be so neatly dichotomized. For example, in visual word decoding, the right hemisphere seems to read words letter by letter -- by looking at the parts -- while the left hemisphere recognizes entire words -- the visual word forms.25
But again, the parts and wholes to which the brain is sensitive appear to occur at quite a fine-grained level of analysis -- categories versus coordinates, generating versus scanning visual images, identifying number words versus Arabic digits. The Caines' example of part/whole interactions -- the left-hemisphere comprehension of a text and the right-hemisphere appreciation of wholeness -- relates to such a highly complex task that involves so many parts and wholes at different levels of analysis that it is trivially true that the whole brain is involved. Thus their appeal to brain science suffers from the same problem Kosslyn identified in the attempts to use crude theories to understand the brain. The only brain categories that the Caines appeal to are parts and wholes. Then they attempt to understand learning and exceedingly complex tasks in terms of parts and wholes. This approach bothers neither to analyze the brain nor to analyze behaviors.
The danger here is that one might think that there are brain-based reasons to adopt whole-language instruction, integrated curricula, or Vygotskian social learning. There are none. Whether or not these educational practices should be adopted must be determined on the basis of the impact they have on student learning. The evidence we now have on whole-language instruction is at best inconclusive, and the efficacy of social learning theory remains an open question. Brain science contributes no evidence, pro or con, for the brain-based strategies that the Caines espouse.
The fundamental problem with the right-brain versus left-brain claims that one finds in the education literature is that they rely on our intuitions and folk theories about the brain, rather than on what brain science is actually able to tell us. Our folk theories are too crude and imprecise to have any scientific, predictive, or instructional value. What modern brain science is telling us -- and what brain-based educators fail to appreciate -- is that it makes no scientific sense to map gross, unanalyzed behaviors and skills -- reading, arithmetic, spatial reasoning -- onto one brain hemisphere or another.