What is the difference between the left and right brain?

The left-brain right-brain myth: an overview of hemispheral specialization in the brain.

When we refer to someone being “Left-Brained” or “right-brained,” we are generally talking about the popular notion that our brains are segregated into two distinct personality profiles. The left side of the brain is said to represent logic, language and analysis, whereas the right side represents emotion, intuition and creativity. Is this a valid representation of hemispherical function? Or is this simply another example of a widely accepted neuro-myth? In order to explore the reality behind the popular notion of hemispherical specialization in the brain, let’s examine the history of this theory and then compare it to modern-day findings based upon advances in neuroscience using fmri technology.

The left-brain right-brain theory: an historical perspective

Roger Sperry (Nobel laureate) did much to promote the idea of left-brain vs. Right-brain thinking in the 1960s. His research on individuals with severed corpus callosa indicated that each hemisphere may possess unique functional capabilities. Individuals undergoing corpus callostomy procedures to eliminate seizures experienced reduced seizure frequency. Corpus callostomy is a surgical method used to separate the left and right hemispheres of the brain, eliminating neural pathways between them. Sperry demonstrated through his work that each hemisphere possessed differing levels of functionality. For example, when objects were held in the right hand (the side controlled by the left hemisphere), verbal descriptions were possible. However, when objects were held in the left hand (controlled by the right hemisphere), verbal identification was not possible despite pointing to the object. Since the right hemisphere cannot communicate with the verbal processing centers in the left hemisphere, sperry concluded that each hemisphere specializes in unique functions.

More functionality than segregation

Although it is true that there are distinctions between brain hemispheres and although the majority of functions are lateralized (i.e., primarily processed by one hemisphere), this is not always an absolute split between hemispheres. Many researchers agree that lateralization of function is a more nuanced phenomenon than simple separation. Functions are generally more concentrated in one hemisphere than the other, but the degree of concentration varies among individuals and functions.

Language processing

Most right-handed individuals exhibit strong left-hemisphere dominance for language processing. Specifically, Broca’s area, which is responsible for speech production and Wernicke’s area, which is critical for language comprehension, reside in the left hemisphere. Damage to either of these regions will result in varying degrees of aphasia (speech disorders). Although the left hemisphere appears to dominate language processing, the right hemisphere continues to play a role in understanding emotional tone in speech, interpreting metaphors, and identifying humor. Approximately 10% to 15% of left-handed individuals display language function that is bilaterally represented or primarily resides in the right hemisphere.

Visual processing & spatial awareness

In general, the right hemisphere of the brain is better suited to handle spatial awareness tasks such as navigation, map reading, and solving geometric problems. Additionally, it helps recognize patterns in visual stimuli and aids in artistic expression.

Motor output & sensory input

one important aspect of understanding hemispherical function involves understanding motor output and sensory input. one primary characteristic of motor control is contralateral control. In essence, the left hemisphere controls movement on the right side of the body and vice versa. Furthermore, each hemisphere receives sensory input from corresponding portions of the body (e.g., right hand -> left hemisphere). This cross-over is a key reason why strokes that affect the left hemisphere can cause paralysis on the opposite side of the body (right side).

The mythology surrounding personality profiles

While there is evidence of hemispherical specialization, there is little support for a rigid classification system stating that an individual is primarily “Left-Brained” or “right-brained”. The fact remains that both hemispheres interact in virtually all cognitive activities, and thus provide a base line for all higher-order cognition. Studies utilizing fmri imaging have failed to demonstrate any consistent difference between subjects in terms of unilateral hemispheric activation during most cognitive tasks.

Facts about hemispheres:

most cognitive functions involve bilateral hemispherical participation.
all cognitive functions are not lateralized (localized) within one hemisphere.
functional localization occurs within specific hemispheres for many cognitive functions.
some cognitive functions (such as spatial processing) appear to be more lateralized within the right hemisphere.
there are no known examples of complete hemispheric lateralization within humans.
all hemispheres participate in nearly all cognitive functions.

The mythology behind creative people

Many believe that creative people are “right-brained” while logical thinkers are “Left-Brained.” This too is a myth. While there is evidence to suggest that the right hemisphere contributes strongly to emotional and creative processes and the left hemisphere to logical and linguistic ones, both hemispheres are required to produce a truly creative product. There is no scientific evidence indicating that creative or logical thinking is predominantly supported by one hemisphere of the brain over another.

Corpus callosum

The corpus callosum is a mass of interconnecting nerve fibers between the left and right hemispheres of the brain. It serves as a conduit for rapid exchange of information between the two hemispheres. Communication between hemispheres is necessary for coordinated cognitive functioning.

Functional compensation

In addition to providing structural connectivity between hemispheres, the corpus callosum facilitates compensatory mechanisms throughout adulthood and early childhood. When one hemisphere sustains damage, there is evidence suggesting that the remaining undamaged hemisphere can assume some of those lost functions. The likelihood of successful compensation depends on several factors, including age of onset, type and severity of injury and whether or not cognitive training occurred prior to sustaining an injury.

Hemispheric lateralization in left-handed individuals

There is growing evidence supporting that a subset of left-handed individuals demonstrate differential lateralization compared to their right handed counterparts. In particular, approximately 10% to 15% of left-handed individuals display increased bilateral representation of language functions compared to right-handed individuals. Thus, while both groups of individuals utilize both hemispheres for language processing, left-handed individuals tend to rely equally on both hemispheres. However, in many cases, both groups utilize their brains similarly regarding language processing.

Overall,

This article has explored what science currently knows about how our two brain hemispheres operate independently yet together in unison as well as how they process information.