Localisation of Function in the Brain and Hemispheric Lateralisation

This section explores the Localisation of Function in the Brain and Hemispheric Lateralisation with a focus on motor, somatosensory, visual, auditory, and language centres; Broca’s and Wernicke’s areas, split-brain research, and plasticity and functional recovery of the brain after trauma. Localisation of function refers to the theory that specific functions (such as movement, speech, and perception) are associated with specific areas of the brain. Hemispheric lateralisation is the concept that some mental processes and behaviours are controlled or more dominant in one hemisphere than the other. This is most prominently seen in language, where different aspects of language are located in distinct brain regions, usually within the left hemisphere.

Localisation of Function in the Brain and Hemispheric Lateralisation

Key Brain Areas and Their Functions

Motor Cortex

Located in the frontal lobe, specifically in the precentral gyrus.

Responsible for voluntary movement by sending signals to muscles across the body.

The motor cortex is arranged in a topographical map, with different regions controlling different parts of the body (the hands, face, and tongue have large representations due to fine motor skills).

Somatosensory Cortex

Located in the parietal lobe, specifically in the postcentral gyrus.

Processes sensory information from the skin, such as touch, pressure, pain, and temperature.

Like the motor cortex, it has a somatotopic organisation, where different areas correspond to sensations from different parts of the body.

Visual Cortex

Located in the occipital lobe at the back of the brain.

Processes visual information received from the eyes. Each hemisphere’s visual cortex processes information from the opposite visual field (i.e., the left visual cortex processes the right visual field).

Damage to the visual cortex in one hemisphere can lead to hemianopia, a loss of vision in one half of the visual field.

Auditory Cortex

Located in the temporal lobe.

Responsible for processing sound information from the ears, such as pitch, tone, and volume.

Information from both ears is processed in both auditory cortices, but predominantly in the hemisphere contralateral to each ear.

Language Centres: Broca’s Area and Wernicke’s Area

Language functions are usually lateralised to the left hemisphere in most people.

Broca’s Area:

Located in the left frontal lobe.

Responsible for speech production and grammatical structure.

Damage to Broca’s area can cause Broca’s aphasia, characterised by slow, laborious, and non-fluent speech while comprehension remains relatively intact.

Wernicke’s Area:

Located in the left temporal lobe.

Responsible for language comprehension.

Damage to Wernicke’s area can result in Wernicke’s aphasia, characterised by fluent but nonsensical speech and severe difficulty understanding language.

Split-Brain Research

Split-brain research has provided insights into hemispheric lateralisation and the independent functioning of the two hemispheres.

Roger Sperry’s Split-Brain Research:

Split-brain studies were conducted on patients who had undergone a corpus callosotomy, a surgical procedure severing the corpus callosum (the bundle of nerve fibres connecting the two hemispheres) to treat severe epilepsy.

Sperry found that when information was presented to only one hemisphere (due to split-brain), the two hemispheres could not communicate, leading to unique behavioural effects:

When an object was shown in the left visual field (processed by the right hemisphere), patients could not name it but could draw it with their left hand, as the right hemisphere processes visual information and controls the left hand.

When an object was shown in the right visual field (processed by the left hemisphere), patients could name it verbally, as the left hemisphere controls language.

Implications:

Sperry’s findings support hemispheric lateralisation, showing that certain functions (e.g., language) are localised in one hemisphere.

His research also suggested that the two hemispheres can operate independently when the corpus callosum is severed.

Plasticity and Functional Recovery of the Brain After Trauma

Brain plasticity refers to the brain's ability to change and adapt structurally and functionally in response to learning or damage. Functional recovery is a specific form of plasticity that occurs after trauma (such as brain injury), allowing other areas of the brain to compensate for lost functions.

Plasticity in the Brain

The brain remains plastic (flexible) throughout life, though it is particularly plastic during childhood.

New experiences and learning lead to synaptic pruning, where unused synaptic connections are eliminated, and frequently used ones are strengthened.

Studies, such as Maguire et al. (2000) on London taxi drivers, have shown that learning and experience can change the brain’s structure. The taxi drivers had a larger posterior hippocampus, an area associated with spatial navigation, than controls, illustrating adaptive brain plasticity.

Functional Recovery After Trauma

Functional recovery after trauma is the brain’s ability to reorganise and form new connections to compensate for damaged areas.

Mechanisms of Functional Recovery:

Axonal Sprouting: New nerve endings grow to reconnect neurons whose connections were severed by injury.

Recruitment of Homologous Areas: Similar areas on the opposite side of the brain may take over the functions of the damaged area.

Neural Reorganisation and Compensation: Other, undamaged areas of the brain adapt to perform the lost functions.

Factors Affecting Recovery:

Age: Younger brains tend to be more plastic and thus better able to recover from injury.

Rehabilitation: Physical therapy, cognitive therapy, and speech therapy can support recovery by promoting neuroplasticity.

Perseverance and Motivation: Patients who engage actively in recovery efforts often show better outcomes.

Evaluation of Plasticity and Functional Recovery:

Strengths:

• Understanding plasticity has led to therapeutic developments in rehabilitation for stroke and brain injury patients.
• Research evidence (e.g., from animal studies) strongly supports the capacity for functional recovery.

Limitations:

• Recovery is not always complete, and some functions may never fully return.
• Negative plasticity can occur, where maladaptive changes (e.g., phantom limb syndrome in amputees) lead to problematic outcomes.

Key Terms to Remember

Localisation of Function: The concept that specific functions are associated with specific areas of the brain.

Hemispheric Lateralisation: The tendency for some neural processes to be specialised to one hemisphere of the brain.

Plasticity: The brain’s ability to change and adapt in response to experience or injury.

Functional Recovery: The brain’s capacity to reassign functions to undamaged areas after injury.

Split-Brain Research: Studies conducted on individuals who have had their corpus callosum severed, providing insights into hemispheric lateralisation.

These notes cover localisation of function, lateralisation, split-brain research, plasticity, and functional recovery, key topics for understanding how different brain areas contribute to behaviour and adapt after trauma.