Localisation of function in the brain and hemispheric lateralisation

localisation of function

• The theory of localisation of function posits that specific areas of the brain are responsible for particular cognitive functions. This is in contrast to the holistic view of the brain, where all areas are involved in all functions.

• The brain consists of two hemispheres connected by a bunch of fibres (called the corpus callosum). This bridge allows the two hemispheres to communicate with each other. 

motor cortex (posterior frontal lobe)

• at the posterior of the frontal lobe

• involved in the planning and execution of skilled and voluntary movements

• divided into the primary motor cortex and the non-primary motor cortex, each responsible for different parts of planned movement

• The top of the cortex stimulates movements of the leg, whereas the movements of the face are stimulated by the lowest part of the motor cortex

• both hemispheres of the brain have a motor cortex - it is located in the frontal lobe

  • role - motor cortex is responsible for the generation of voluntary motor movements - it sends neural messages to muscles via the CNS

  • Organized - the process of motor movements is contralateral, which means the right motor cortex controls the voluntary movements of the left side of the body and vice versa, meaning that any damage to one side of the brain in this area ( through a stroke) will affect the control of movements on the opposite side of the body

  • soma topically organized - areas which are finely controlled ( the hands have larger positions of the cortex, whereas coarsely controlled areas have smaller portions.

the somatosensory cortex (inferior parietal lobe)

• sits at the anterior end of the parietal lobe and is responsible for interpreting incoming sensory information

• mainly tactile information that is received here

•  Different parts of the somatosensory area receive messages from different locations of the body. 

• Both hemispheres of the brain have a somatosensory cortex in the parietal lobe

• role is to process information from the body, including touch, temperature, pain, and precipitation

• also contributes to higher-order functions like tactile attention and sensorimotor and emotional processing

• organized contralateral- the left somatosensory cortex detects sensory information from the right side of the body and vice versa

• also somatotopically organized - areas which have more sensory receptors (the hands) have larger portions of the cortex, whereas other areas with fewer sensory receptors have smaller portions,

visual cortex (occipital lobe)

• Responsible for processing visual information. Information from the right-hand side visual field is processed in the left hemisphere, and information from the left-hand side visual field is processed in the right hemisphere.

• The visual area contains different parts that process different types of information, including colour, shape or movement.

• located in the visual cortex, in both hemispheres, within the occipital lobe of the brain

• role - visual processing - begins in the retina at the back of the eye

• organization - right hemisphere receives its input from the left visual field and vice versa so contralateral but slightly different from motor and somatosensory 

• The visual cortex contains several different areas with each of these areas processing different types of visual information, such as colour, shape, or movement

auditory cortex (temporal lobe)

• The auditory area is located in the temporal lobe and is responsible for analyzing  and processing acoustic information

• Information from the left ear goes primarily to the right hemisphere, and information from the right ear goes primarily to the left hemisphere

• right ear goes primarily to the left hemisphere.  The auditory area contains different parts, and the primary auditory area is involved in processing simple features of sound, including volume, tempo and pitch.

• located in the auditory cortex within the temporal lobes in both hemispheres in the brain

• The role is concerned with hearing

• inner ear - sound waves are converted to nerve impulses, which travel to the brain stem to be decoded

• auditory cortex - sound is recognized and interpreted

• organization - contralateral with information from the right ear traveling primarily to the left auditory cortex

  
  

evidence of localisation of function - 🧠 Phineas Gage (1848) – Case Study

• Background:
  Phineas Gage was a 25-year-old railroad construction foreman who survived a severe brain injury when an iron rod was accidentally blasted through his skull. The rod entered through his left cheek and exited through the top of his head, destroying much of his frontal lobe.

• ⚡ Effects of the Injury:

  • Before the accident: Gage was responsible, hard-working, and sociable.

  • After the accident: He became rude, impulsive, aggressive, and unreliable.

  • His intelligence, memory, and language abilities remained largely intact.

  • His personality and behaviour changed dramatically.

🔍 What This Shows:

• Provides evidence for localisation of function in the brain.

• Suggests the frontal lobe is responsible for:

  • Personality

  • Impulse control

  • Decision-making

  • Social behaviour

• When this area is damaged, these functions can be impaired.

🧩 Use in Biopsychology:

Phineas Gage’s case supports the theory that different areas of the brain are responsible for different functions — known as localisation of function.
It also shows that brain damage can affect psychological processes like emotion and personality.

evaluation for localisation of function

• weakness:

• - Point: One weakness of the theory of localisation of function is that it relies too heavily on evidence from case studies.

  Evidence: For example, much of the supporting evidence comes from case studies such as Broca’s patient "Tan" and Wernicke’s patient, who both showed specific language impairments (aphasias) following damage to certain brain areas. These individual cases were then used to make broad generalisations about how language is localised in the brain.

  Elaboration: However, this is problematic because case studies are based on unique individuals with potentially unusual brain structures or damage patterns. The findings from these cases may not apply to everyone, meaning that generalising from them to the wider population is unreliable. To make stronger claims, research should use larger samples and more systematic brain imaging studies to ensure findings are representative.

  Link: Therefore, while case studies like Broca’s and Wernicke’s provide valuable insights, the over-reliance on them reduces the credibility of the localisation theory, as the findings may not be generalisable to all people.

Point: Another weakness of the theory of localisation of function is that it suffers from beta bias, as it ignores potential differences between males and females in how brain functions are localised.

Evidence: The theory assumes that brain functions are organised in the same way for everyone, regardless of sex. However, research has shown that there are gender differences, particularly in areas such as language. For example, studies have found that women tend to use language functions more bilaterally across both hemispheres, whereas men often show stronger localization to the left hemisphere.

Elaboration: By failing to account for these differences, the theory may present an oversimplified and potentially inaccurate view of how brain functions are organized. This means the theory lacks population validity, as it does not accurately represent both sexes, and may lead to misleading conclusions about how localization works in real-world populations.

Link: Therefore, the theory’s beta bias reduces its overall credibility, as it ignores important biological differences between males and females that influence how functions like language are localized in the brain.

Point: A further weakness of the theory of localization of function is that it is biologically reductionist.

Evidence: The theory attempts to explain complex human behaviours and cognitive processes by attributing them to specific brain regions. For example, it suggests that areas like Broca’s area are solely responsible for speech production, and the motor cortex is solely responsible for movement.

Elaboration: However, this approach oversimplifies how the brain actually works. The brain is highly interconnected, with many regions working together through neural networks to produce behavior. Reducing complex processes such as language, memory, or emotion down to one specific area ignores the holistic and dynamic nature of brain functioning. As a result, the theory fails to capture the true complexity of neural communication and coordination.

Link: Therefore, the theory’s reductionist approach limits its explanatory power, as it neglects the broader networked interactions that underpin human cognition and behaviour.

Answer/Point: Another weakness of the localisation of function theory is that Lashley’s theory of equipotentiality offers a more convincing alternative explanation.

Evidence: Lashley (1930) argued that while basic motor and sensory functions may be localised, higher mental functions such as problem-solving and learning are not confined to specific brain areas. Instead, these complex processes are distributed across multiple regions of the cortex, and if one area is damaged, other areas can often compensate for the loss.

Elaboration: This equipotentiality model suggests that the brain is more flexible and interactive than the localisation theory proposes. It recognises the plasticity and interconnectedness of brain functions, giving it greater face validity, as it aligns more closely with research showing that people can recover abilities following brain injury through functional compensation by other areas.

Link: Therefore, Lashley’s equipotentiality challenges the strict localisation view, suggesting that the theory may oversimplify how complex cognitive functions are organised in the brain

strengths:

Point: A strength of the theory of localization of function is that it is supported by research evidence from case studies, such as HM.

Evidence: HM underwent surgery to treat his epilepsy, during which his hippocampus was damaged. Following the operation, he developed severe anterograde amnesia, meaning he was unable to form new long-term memories, although his short-term memory and procedural memory remained intact.

Elaboration: This provides strong support for localization because it shows that specific areas of the brain, like the hippocampus, have specialised roles—in this case, the formation of long-term declarative memories. The clear link between the damaged area and the resulting impairment suggests that memory functions are localized rather than distributed across the whole brain.

Link: Therefore, HM’s case strengthens the credibility of the localization theory by demonstrating that damage to a specific brain region can lead to specific behavioural and cognitive deficits.

Point: A key strength of the localisation of function theory is its practical applications in healthcare, particularly in predicting and managing symptoms following brain injury or stroke.

Evidence: For example, localisation allows clinicians to predict likely deficits based on the area of damage. A stroke affecting Broca’s area (in the left frontal lobe) is often associated with Broca’s aphasia, leading to difficulties in speech production, whereas damage to Wernicke’s area (in the left temporal lobe) can cause Wernicke’s aphasia, resulting in problems understanding language.

Elaboration: This knowledge enables medical professionals, such as doctors and nurses, to anticipate a patient’s specific needs and plan targeted rehabilitation. For instance, patients with aphasia can be referred for speech therapy, while those with motor impairments may benefit from occupational therapy. This improves the quality of care and recovery outcomes, demonstrating the usefulness of the theory beyond research.

Link: Therefore, the localisation theory has strong real-world value, as it helps healthcare professionals deliver tailored and effective treatment, increasing its overall credibility and practical significance.

Answer/Point: Another strength of the localisation of function theory is that it has valuable practical applications in understanding and managing brain damage, such as after a stroke.

Evidence: Knowledge of localisation helps healthcare professionals interpret brain imaging scans like CT or MRI by identifying which areas of the brain have been affected. This allows them to understand the extent and location of the damage and predict its impact on the patient’s functioning.

Elaboration: For example, if scans show damage to the motor cortex, clinicians can anticipate motor impairments on the opposite side of the body. Understanding which regions are affected also helps nurses explain the patient’s condition to family members, set realistic recovery goals, and plan effective rehabilitation strategies. This supports better patient care and clearer communication about prognosis.

Link: Therefore, the localisation theory has strong real-world usefulness, as it enables healthcare professionals to interpret brain scans accurately and deliver more informed, individualised treatment and support.

Point: A further strength of the localisation of function theory is its practical value in improving communication and education within healthcare settings.

Evidence: Understanding which areas of the brain control specific functions allows nurses and clinicians to clearly explain the effects of a stroke or brain injury to patients and their families. For example, if a patient has language difficulties due to damage in Broca’s area, the nurse can explain why these symptoms have occurred and what therapies may help.

Elaboration: This not only helps reduce anxiety and confusion but also encourages patients and families to engage more fully with treatment and rehabilitation. By using localisation knowledge, nurses can also educate patients about the purpose of different therapies and the importance of active participation in recovery, leading to better outcomes and greater patient motivation.

Link: Therefore, the localisation theory has strong real-world applications as it supports effective communication, patient education, and emotional support — all of which are essential for successful recovery and care

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Hemispheric lateralisation

• Lateralisation of function suggests that functions are limited to only one hemisphere. For example, speech and language is lateralised to the left hemisphere. Each hemisphere has specialised functions.

• Hemispheric lateralisation refers to the idea that certain mental processes and behaviours are primarily controlled by one hemisphere of the brain rather than being shared equally across both.

  • The left hemisphere is generally associated with language, logic, and analytical tasks.

  • The right hemisphere is linked to spatial awareness, creativity, music, and emotional expression.

  • The brain works contralaterally, meaning the left hemisphere controls the right side of the body, and vice versa.

• left hemisphere

• The left hemisphere focuses on detail and will be more active on tasks where an individual is asked to identify small details

• Language - for most people, their language processing is done in the left hemisphere 

• Analytical / Logical processing

  • Maths, reasoning, and scientific thinking

• Sequential processing

  • Step-by-step, ordered thinking

• Controls the right side of the body (motor + sensory)

right hemisphere

• appears to process overall patterns and shows more activity when individuals are asked to make sense of the whole picture

• spatial relationships - the right hemisphere appears to be dominant for spatial functions like finding your way, and people with damage to the right hemisphere may have difficulties with spatial tasks

• recognition of emotions - the right hemisphere seems to be particularly dominant for recognizing emotions in others

• Research - Heller and Levy - found that participants shown a split photo ( half smiling and half neutral) would recognize the emotion shown on the left side

• Spatial awareness – understanding space, distance, and depth

• Visual-motor tasks – drawing, map reading, recognising patterns

• Face recognition

• Music and creativity – appreciation of art and melody

• Emotion processing – recognising emotional tone in language and facial expressions

• Holistic processing – sees the “big picture” rather than details

• Controls left side of body (motor + sensory)

language centres

• Language is lateralized in the left hemisphere - 2 main language processing areas are Broca’s and Wernicke’s

• Broca’s area - discovered by a neurosurgeon - found patients with deficits in their speech production but had no issues with speech comprehension - studied these deficits and then performed post-mortems on them after their deaths - famous case with patient Tan

• Broca’s area is located in the posterior (back) of the left frontal lobe 

• Broca’s area helps coordinate speech production

• What happens if damaged - lesions in this area can lead to Broca’s aphasia, causing difficulty with speech production - interestingly, not all words are equally affected - nouns and verbs seem to be fine - conjunctions and prepositions have major problems ( with Broca’s aphasia you couldn’t read to “to be” or “not to be” but could read “two bee oar knot two bee”

Wernicke’s area was discovered by German neurologist Carl Wernicke, who observed people with damaged posterior temporal lobes and noticed how a stroke patient could hear and speak but could not understand written words. This led to the discovery of a lesion in the rear parietal/temporal region of the left hemisphere near the auditory region.

• located in the posterior part of the left temporal lobes, with a lesion, could not understand but could speak

• and hear - called Wernicke’s aphasia - language needs to separate motor and sensory regions in cortical regions  - sensory transfers information to Wernicke’s area

• Role - area is important for speech comprehension and processing

• What happens if it’s damaged? They could speak, but were unable to understand language and caused difficulty speaking in coherent sentences or understanding others' speech.

neural loop - arcuate fasciculus

• Broca’s area and Wernicke’s area work together to process and produce language

• There is a neural loop running from Broca’s area ( responsible for the production of language) and Wernicke’s area (responsible for the processing of spoken language)

• Damage to both Broca’s and Wernicke’s areas may lead to global aphasia, which is an inability to understand or to produce speech

brain works contralaterally or soma topically

• left hemisphere controls the right side of the body, and vice versa. 

• So, if an individual had a stroke in their right hemisphere, it would likely affect their left side of the body or face. 

• soma topically - the organized mapping of sensory and motor information from different parts of the body to specific areas within the body 

Evaluation of lateralisation of function

weakness:

Point: A weakness of localisation of function is that brain functions are not always fixed to one hemisphere or area.

Evidence: For example, Szaflarski et al. (2006) found that most cognitive tasks become less lateralised in healthy adults, suggesting that the distribution of brain functions can change with age and experience.

Elaboration: This challenges strict localisation because it shows that brain functions are flexible rather than permanently confined to specific regions or hemispheres. Age and experience can influence which areas of the brain are involved in particular tasks, reducing the generalisability of localisation claims.

Link: Therefore, the theory may oversimplify brain organisation, as it does not fully account for how functions can shift across different regions over time.

Answer/Point: Another weakness is that some cognitive functions, like language, are too complex to be localised to a single region.

Evidence: Language processing involves multiple brain areas and networks, including Broca’s area, Wernicke’s area, the auditory cortex, and the angular gyrus.

Elaboration: This means localisation oversimplifies cognitive processes, ignoring the interconnectedness and collaboration between regions that allow complex behaviours such as understanding, producing, and interpreting language.

Link: Therefore, assigning complex functions to single brain areas reduces the accuracy and explanatory power of the localisation theory.

Answer/Point: A further weakness is that much supporting evidence comes from unique or clinical cases, which may not represent the general population.

Evidence: For example, participants in split-brain research or patients with brain injuries were used to support claims about specific brain areas controlling particular functions.

Elaboration: These individuals often have unusual or abnormal brain structures and damage patterns, so findings from them may not generalize to healthy people. Relying heavily on these cases reduces the external validity of the theory.

Link: Therefore, the theory of localization may lack general applicability, as it is based on a small, atypical subset of brains rather than broader populations.

Strengths:

Point: A strength of localisation and hemispheric lateralisation is the research evidence provided by Sperry’s split-brain studies.

Evidence: Sperry (1968) studied patients who had their corpus callosum severed to reduce epileptic seizures. He found that when information was presented to the right visual field (processed by the left hemisphere), patients could verbalise what they saw, but when presented to the left visual field (processed by the right hemisphere), they could not name the object but could draw or select it.

Elaboration: This demonstrates that certain functions, like language, are lateralised to one hemisphere (the left), whereas other functions, such as spatial processing, are more associated with the right hemisphere. The controlled experimental design strengthens the causal conclusions about brain lateralisation.

Link: Therefore, Sperry’s research provides strong empirical support for both localisation of function and hemispheric lateralisation, increasing the credibility of the theory.

Answer/Point: Another strength is the case study evidence provided by Broca, which supports the idea of localisation of function.

Evidence: Broca studied a patient known as “Tan”, who could only produce the word “Tan” but otherwise understood language. Post-mortem examination revealed a large lesion in the left frontal lobe (Broca’s area), which is responsible for speech production.

Elaboration: This case demonstrates a clear link between damage to a specific brain region and a specific deficit in function, providing strong support for the idea that certain cognitive abilities are localised.

Link: Therefore, Broca’s research strengthens the localisation theory by showing that damage to a particular brain area produces predictable impairments in speech.

Answer/Point: A further strength comes from Wernicke’s research, which supports the localisation of language functions.

Evidence: Wernicke studied patients who could speak fluently but could not understand language. Post-mortem analysis showed lesions in the left temporal lobe (Wernicke’s area), which is responsible for language comprehension.

Elaboration: This complements Broca’s findings and shows that different aspects of language are controlled by different specialised brain regions, reinforcing the concept of localisation.

Link: Therefore, Wernicke’s research provides additional empirical support for localisation by demonstrating that different cognitive functions can be attributed to distinct brain areas.

• Point: Localisation of function has clear real-world applications in healthcare.

• Evidence: Knowing which brain areas control certain functions allows doctors and nurses to predict deficits following brain injury or stroke. For example, damage to Broca’s area can indicate speech production problems, while damage to the motor cortex predicts motor impairments.

• Elaboration: This enables targeted rehabilitation such as speech therapy or physiotherapy, improving recovery outcomes. It also allows clinicians to plan individualised care, supporting patients’ functional needs effectively.

• Link: Therefore, localisation is valuable in practice, not just theory, as it guides treatment and rehabilitation.

• Point: Localisation underpins modern neuroimaging research.

• Evidence: Techniques like fMRI and PET scans rely on the assumption that specific mental processes are localised to particular regions. Researchers can observe which areas “light up” during language, memory, or motor tasks.

• Elaboration: This has led to advances in cognitive neuroscience and allows scientists to map functional areas, test predictions from localisation theory, and improve understanding of normal and abnormal brain function.

• Link: Therefore, localisation has scientific and technological value, guiding research and improving our understanding of brain-behaviour relationships.

• Point: Knowledge of localisation can help in education and skill development.

• Evidence: Understanding which areas of the brain are involved in memory, attention, or language can inform teaching methods and strategies to support learning difficulties.

• Elaboration: For example, interventions for dyslexia or speech delays can be tailored to stimulate specific brain regions or compensate for deficits.

• Link: Therefore, localisation has applications beyond medicine, influencing education and developmental support.

split brain research

• 🧠 Sperry’s Split-Brain Research (1968) Aim:

  To investigate the extent of hemispheric lateralization by studying patients who had undergone corpus callosotomy (severing the corpus callosum) to treat severe epilepsy.

• Procedure:

  • Participants: 11 split-brain patients.

  • Each hemisphere was isolated by presenting information to one visual field at a time:

    • Right visual field (RVF) → processed by left hemisphere.

    • Left visual field (LVF) → processed by right hemisphere.

  • Stimuli (e.g. words, objects, pictures) were flashed quickly to one visual field.

  • Findings:

    • Language (Left hemisphere):

      • RVF → could verbally describe the object or word.

      • LVF → couldn’t name the object but could draw or select it by touch.

    • Non-verbal tasks (Right hemisphere):

      • Could identify or match objects using spatial and visual processing, but could not verbalize what they saw.

    • Emotional responses:

      • The right hemisphere could recognize emotions in faces, even though it couldn’t describe them.

Conclusion:

• The left hemisphere is dominant for language and verbal tasks.

• The right hemisphere is dominant for spatial and visual tasks.

• Supports hemispheric lateralization and shows that each hemisphere has specialized functions.