Brain asymmetry means the two cerebral hemispheres are not perfect mirror images of one another. This non-symmetrical organization involves both structural differences and functional specialization, known as lateralization. Lateralization means one hemisphere is typically dominant for certain cognitive processes. This organization is a defining feature of our species, allowing for the complex specialization of human thought and behavior.
Structural Variations in the Brain
Structural differences begin with a large-scale twisting of the brain known as the Yakovlevian torque. This rotational asymmetry causes the right hemisphere to project slightly further forward in the frontal lobe, while the left hemisphere extends further back in the occipital lobe.
A prominent example of structural asymmetry is the planum temporale, a cortical area generally larger in the left hemisphere. This leftward asymmetry is linked to the left hemisphere’s role in language processing. Beyond these macro-level differences, microstructural asymmetries also exist, such as variations in cortical thickness or the density of neurons in homologous regions. For instance, some posterior temporal and occipital areas can exhibit greater cortical thickness in the right hemisphere.
How Hemispheres Specialize for Function
Functional asymmetry refers to the specialization of each hemisphere for different cognitive and behavioral tasks. The most widely known example is the dominance of the left hemisphere for language in approximately 95% of right-handed people. This specialization involves specific areas like Broca’s area, which is responsible for speech production, and Wernicke’s area, which plays a major role in language comprehension. The left hemisphere manages the complex sequential and analytic aspects of language.
The right hemisphere specializes in processing global, holistic information and is dominant for many spatial and non-verbal functions. This includes the ability to navigate through space, recognize faces, and process complex visual patterns. For example, damage to specific areas in the right hemisphere can impair a person’s ability to recognize familiar faces, a condition called prosopagnosia.
Emotional processing also shows a degree of lateralization, although it is more nuanced than a simple division. A prominent theory suggests a valence-based asymmetry: the left hemisphere is more engaged in processing positive emotions and motivational approach behaviors. Conversely, the right hemisphere demonstrates greater involvement in processing negative emotions, such as fear and sadness, and motivational withdrawal behaviors. The right hemisphere is also dominant for interpreting the emotional tone, or prosody, of a speaker’s voice.
Asymmetry and Dominant Hand Preference
Handedness is a visible behavioral manifestation of brain asymmetry, linking motor control to hemispheric specialization. Roughly 90% of the population is right-handed, a preference strongly correlated with left-hemisphere dominance for language. Since the left hemisphere controls the right side of the body, it is typically responsible for both the dominant hand and speech functions.
Left-handed individuals, making up the remaining 10%, exhibit greater variability in lateralization patterns. While about 70% of left-handers still show typical left-hemisphere dominance for language, a larger proportion show either right-hemisphere dominance or bilateral representation. This suggests that cognitive functions are less strictly lateralized in the left-handed population.
Genetic factors contribute to handedness and its correlation with brain asymmetry. Handedness is associated with measurable differences in structural asymmetry across brain regions related to motor function and language. For instance, left-handers often show a slight rightward shift in structural asymmetry in motor cortex areas, aligning with the right hemisphere’s control over the preferred left hand.
What Happens When Asymmetry is Atypical
Atypical asymmetry refers to patterns that deviate from the population norm, such as reduced, absent, or reversed lateralization for specific functions. Deviations from typical hemispheric specialization have been observed across various neurodevelopmental and psychiatric conditions. A lack of the expected anatomical asymmetry, particularly in the planum temporale, is frequently reported in several clinical populations.
Individuals with developmental dyslexia often show a reduction in the typical leftward functional lateralization for language tasks. Altered structural asymmetry patterns have also been reported in groups affected by schizophrenia, sometimes showing reduced or reversed asymmetry in the superior temporal gyrus. More symmetrical organization has also been observed in the brains of some individuals with autism spectrum disorders.