Introduction to Brain Hemispheres
The Human brain is considered as unitary being, yet it has two distant parts called as cerebral/ brain hemispheres. Dividing the brain sagittal section gives us Left and right brain hemispheres. The cerebral hemispheres are the most rostral part of the CNS, and are characterized by extensive folding (gyri & sulci) of the cerebral cortex (Shoykhet & Clark, 2011). It consists of cerebral cortex, basal nuclei (ganglia), hippocampus, and amygdala. Two hemispheres are connected by thick fibre of neurons called as corpus callosum.
Understanding brain hemisphere is important to understand the origin, progression & treatment of disorders like aphasia or apraxia, which are often caused by damage of ONLY ONE hemisphere. It is also helpful to learn about Lateralization of cerebral functions and split-brain phenomenon.

Sagital section of human brain
Hemispheric Lateralization
Definition of Lateralization
According to American Psychological Association (APA), Hemispheric lateralization is the processes whereby some functions, such as manual control (handedness) or speech production, are controlled or influenced more by one cerebral hemisphere than the other. Also known as cerebral lateralization or hemispheric specialization.
History
- Marc Dax’s Observations (1836): Marc Dax noted that none of his 40 brain-damaged patients with speech impairments had right hemisphere damage. His findings were largely ignored because of the belief that the brain acted as a whole, with no localization of functions.
- Paul Broca’s Findings (1861-1864): Broca examined two patients with aphasia and found lesions in the left hemisphere, specifically in the frontal cortex near the motor cortex. By 1864, after examining seven more patients, Broca confirmed that damage to the left inferior prefrontal cortex (later named Broca’s area) consistently caused language deficits.
- Hugo-Karl Liepmann’s Work on Apraxia (Early 1900s): Liepmann linked apraxia (difficulty performing intentional movements out of context) to left-hemisphere damage, despite its symptoms affecting both sides of the body.
- Theory of Cerebral Dominance: Evidence of the left hemisphere’s role in language and voluntary movement led to the concept of cerebral dominance. The left hemisphere was termed the dominant hemisphere due to its control over complex cognitive and behavioral processes. The right hemisphere was referred to as the minor hemisphere.
Search for Other Lateralized Functions: The discoveries of language and motor lateralization triggered investigations into additional lateralized functions. This marked the establishment of lateralization as a significant area of neuroscientific research.
The early findings challenged the holistic view of brain function. They provided foundational insights into hemispheric specialization, influencing future research into the lateralization of cognitive and behavioral processes.
Evolutionary Perspectives to Lateralization
- Analytic–Synthetic Theory : It Suggests two basic modes of thinking:
- Analytic Mode (Left Hemisphere): Logical, analytical, sequential processing, and abstraction; attaches verbal labels.
- Synthetic Mode (Right Hemisphere): Processes information holistically as gestalts or wholes.
- Evolutionary Basis: These modes became segregated over time in the left and right hemispheres.
- Criticism: (1) Lacks empirical validation due to the difficulty of quantifying tasks as analytic or synthetic. (2) Considered vague and overly generalized.
2. Motor Theory : It proposes the left hemisphere is specialized for controlling fine motor movements, with speech being one such movement.
- Support: Lesions causing aphasia also frequently lead to motor deficits (Kimura, 2011).
- Limitation: Does not explain why motor functions became lateralized during evolution.
3. Linguistic Theory of cerebral asymmetry : It argues that the primary specialization of the left hemisphere is for language, and is not a secondary outcome as said in analytic-synthetic theory or motor theory.
- Evidence:
- Studies on deaf individuals using American Sign Language (ASL): Left hemisphere damage disrupts ASL but not pantomime gestures.
- Highlights that left hemisphere’s specialization is specific to linguistic functions.
- Key Insight: Language is the fundamental role of the left hemisphere, independent of analytic or motor functions.
Tests for Cerebral Lateralization
1. Sodium Amytal Test (Wada Test, 1949)
- Purpose: Conducted prior to neurosurgery to avoid damaging language-related cortical areas.
- Procedure:
- A small dose of sodium amytal is injected into the carotid artery on one side of the neck, anesthetizing one hemisphere for a few minutes.
- The patient’s abilities are tested by asking them to recite known series (e.g., alphabet) and name common objects.
- The procedure is repeated on the other hemisphere.
- Findings:
- When the left hemisphere (typically specialized for speech) is anesthetized:
- Patients become mute for a short time, followed by errors in naming and serial tasks.
- When the right hemisphere is anesthetized:
- Mutism is rare, and errors are minimal.
- When the left hemisphere (typically specialized for speech) is anesthetized:
2. Dichotic Listening Test
- Purpose: A noninvasive method to assess language lateralization, applicable to healthy individuals.
- Procedure:
- Participants listen to three pairs of spoken digits presented simultaneously to both ears via headphones (e.g., “3, 9, 2” to the right ear and “1, 6, 4” to the left ear).
- They report as many digits as possible.
- Findings:
- Most individuals report more digits from the right ear, reflecting left-hemisphere dominance for language.
- Patients with right-hemisphere specialization for language recall more digits from the left ear.
- Sounds are transmitted to both hemispheres, but contralateral connections dominate during competing auditory input.
3. Functional Brain Imaging
- Purpose: Study lateralization using advanced imaging techniques like PET or fMRI.
- Procedure: Volunteers perform activities such as reading while brain activity is monitored.
- Findings: Greater activation is observed in the left hemisphere during language tasks, consistent with its specialization for language.
- Applications: Provides visual evidence of hemispheric differences in language processing and helps confirm findings from behavioral methods.
Split Brain Research Study
The ground-breaking experiment was conducted by Myers and Sperry (1953) on split brain cats.
- Purpose and Theoretical Insights : The experiment demonstrated two major points:
- The corpus callosum transfers learned information between hemispheres.
- When the corpus callosum is severed, each hemisphere functions independently, effectively creating “two brains.”
- Methodology in Cats:
- Cats were trained to perform a visual discrimination task involving two panels (circle and square) to receive a food reward.
- Visual input to one hemisphere was isolated by cutting both the corpus callosum and optic chiasm and covering one eye with a patch.
- This ensured all visual information went only to the hemisphere ipsilateral to the uncovered eye.
- Phase 1 Results:
- Experimental cats (with both the corpus callosum and optic chiasm transected) learned the task as quickly as control cats.
- This suggested that one hemisphere alone can learn simple tasks as efficiently as two hemispheres working together.
- Phase 2 Results: When the eye patch was transferred to the other eye:
- Control cats performed the task with no change in accuracy.
- Experimental cats’ performance dropped to baseline (50% accuracy) as if they were learning the task for the first time.
- This revealed that the untrained hemisphere had no knowledge of the task learned by the other hemisphere.
- Conclusions:
- The cat brain can function as two separate brains when the corpus callosum is severed.
- The primary function of the corpus callosum is to facilitate communication between the two hemispheres.
- Implications:
- The research confirmed the duality of brain function and the critical role of the corpus callosum in information transfer across hemispheres.
- It paved the way for further studies on split-brain humans, deepening the understanding of hemispheric specialization.
Anatomical Asymmetries in Brain
The discovery of cerebral lateralization of function has driven efforts to identify anatomical differences between the left and right cerebral hemispheres, particularly to understand their functional implications.
Historical Observations
- Pierre Gratiolet (1860s): Observed that the left hemisphere’s cortical convolutions mature faster than the right.
- Geschwind and Levitsky (1960s): Identified significant asymmetry in the planum temporale (posterior to Heschl’s gyrus).
- Left hemisphere’s planum temporale is larger in ~65-90% of studied brains.
- Right hemisphere typically has two Heschl’s gyri, compared to one in the left.
Key Anatomical Differences
- Hemispheric Size and Weight: Right hemisphere slightly larger/heavier; left has more gray matter relative to white matter.
- Temporal Lobes: Structural asymmetry aligns with language specialization in the left and music functions in the right.
- Thalamus: Asymmetry supports language dominance in the left thalamus.
- Lateral Fissure: Gentler slope on the left, making the temporoparietal cortex larger on the right.
- Broca’s Area:
- Surface area larger on the right.
- Cortex within sulci larger on the left, correlating with grammar (left) and tone of voice (right).
- Neurotransmitter Distribution: Asymmetrical distribution of acetylcholine, GABA, norepinephrine, and dopamine across regions.
- Gross Structural Differences:
- Right hemisphere extends farther anteriorly; left extends farther posteriorly.
- Occipital horns of lateral ventricles are longer on the right.
- Sex and Handedness: Both influence anatomical asymmetry. Asymmetry (left larger than right) in the planum temporale is seen more often in men than in women. In fact, an MRI study by Jennifer Kulynych and her colleagues found a large asymmetry in males (left 38% larger) but no asymmetry in females.

Anatomical Asymmetry in cerebral hemisphere
Much attention has focused on three cortical areas associated with language, one of the most lateralized cognitive abilities:
- Frontal Operculum: Located in the frontal lobe, this area corresponds to Broca’s area in the left hemisphere, critical for speech production.
- Planum Temporale: Situated in the posterior lateral fissure, this area is thought to support language comprehension and is linked to Wernicke’s area.
- Heschl’s Gyrus: Found in the temporal lobe anterior to the planum temporale, this region is the primary auditory cortex.
While research consistently demonstrates anatomical differences between the hemispheres, such as the left hemisphere’s often larger planum temporale, the functional implications of these differences remain unclear. Studies on healthy individuals reveal substantial variability in the size and boundaries of these language-related areas, with reports of left-hemisphere anatomical biases often not exceeding 65%, despite 90% of people being left-hemisphere dominant for language (Amunts & Zilles, 2012; Bishop, 2013).
Functional Specialization of Brain Hemispheres
The brain’s hemispheres exhibit functional specialization, where specific cognitive abilities are more prominent in one hemisphere than the other. However, contrary to popular media portrayals, functional lateralization is statistical rather than absolute—both hemispheres contribute to most functions, with one showing a slight dominance. Misconceptions, such as the left hemisphere being solely responsible for language or the right hemisphere for creativity, oversimplify this complex phenomenon.
Language as a Case Study
Language, the most lateralized cognitive ability, exemplifies the statistical nature of hemispheric dominance. While the left hemisphere predominantly processes language, substantial activity occurs in the right hemisphere, particularly in understanding emotional tone, context, and figurative speech. For instance, in dichotic listening tests, participants with left-hemisphere language dominance demonstrate only a modest 55% right-ear advantage, highlighting the contributions of both hemispheres (Gazzaniga, 2013).
Examples of Cerebral Lateralization
- Motor Control : The left hemisphere demonstrates greater ipsilateral activation during complex, cognitively driven movements, leading to left-hemisphere lesions more frequently impairing left-hand motor tasks than right-hemisphere lesions impair right-hand tasks (Hervé et al., 2013).
- Spatial Abilities : The right hemisphere excels in spatial tasks. In a study by Levy (1969), split-brain patients demonstrated superior performance using their left hand (controlled by the right hemisphere) for tasks involving mental manipulation of 3D shapes.
- Emotional Processing : The right hemisphere is generally more adept at recognizing facial expressions of emotion and processing emotional tone, although functional brain-imaging studies reveal overlapping contributions from both hemispheres (Mitchell & Phillips, 2015).
- Musical Abilities : The right hemisphere exhibits superior melody perception. For example, in dichotic listening tests involving melodies, participants showed a left-ear (right-hemisphere) advantage (Kimura, 1964).
- Memory : Both hemispheres contribute to memory but specialize in different domains. The left hemisphere is better at encoding verbal material, while the right hemisphere excels in nonverbal and spatial memory tasks (Willment & Golby, 2013).

Cerebral lateralization
The Left-Hemisphere Interpreter :
A unique aspect of left-hemisphere specialization is its interpreter mechanism, which seeks patterns and meaning in events. Split-brain studies provide compelling evidence:
- In an experiment, split-brain patients were tasked with predicting which of two lights would illuminate next. The top light appeared 80% of the time, but participants weren’t informed of this.
- The left hemisphere performed like intact brains, attempting to discern a nonexistent pattern and achieving suboptimal performance. In contrast, the right hemisphere consistently selected the more frequent option, maximizing success (Gazzaniga, 2013).
Conclusion
The human brain exhibits functional asymmetry, with each hemisphere specialized for distinct cognitive functions. The left hemisphere handles language, logic, and analytical tasks, while the right hemisphere processes spatial awareness, emotions, and holistic thinking. This division of labor, known as lateralization, enhances cognitive efficiency.
Research methods like the sodium amytal test and functional brain imaging show that specific functions, particularly language, are predominantly lateralized to the left hemisphere. However, effective brain function depends on communication between hemispheres, such as through the corpus callosum. Disruptions to this communication, as seen in split-brain patients, highlight the independent functions of each hemisphere.
Hence, lateralization reveals how the brain is organized for specialized tasks, while also emphasizing the importance of inter-hemispheric cooperation in complex cognitive processes.
Read more about Neuropsychology
References
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