Localization of Function
The concept that specific brain regions are responsible for distinct psychological functions.
This concept has evolved through decades of research and is understood across three primary frameworks:
Strict Localization
The idea that a clear, direct relationship exists between a specific brain region and a particular function.
Weak Localization
While certain brain areas are dominant in performing specific functions, other regions can contribute or even take over when necessary.
Distributed Functions
This perspective posits that certain complex processes, like memory or emotional responses, are not confined to one brain region but rather rely on the interaction of multiple areas.
Strict localization suggests that every function can be precisely mapped to a brain area. Broca’s area is associated with speech production, and damage to this region results in Broca’s aphasia, damage to speech production.
Common MistakeLocalization is not an absolute phenomenon but rather a relative and dynamic process. Evidence from neuroscience demonstrates that some functions, such as motor control and language, are localized to specific regions. Others, like memory or emotional processing, are more distributed, neuroplasticity illustrates the brain's remarkable adaptability, allowing regions to re-specialize when necessary.
Key Studies
Strict Localization: Broca’s Aphasia and Wernicke’s Aphasia
Case studyPaul Broca (1861)
Aim: To examine the role of specific brain areas in speech production.
Method: Post-mortem analysis of a patient ("Tan") who could only articulate the sound "tan" despite retaining comprehension and intelligence.
Findings: Damage was found in the posterior inferior frontal gyrus of the left hemisphere, now called Broca’s area.
Conclusion: Broca’s area is critical for the motor aspects of speech production.
Case studyCarl Wernicke (1874)
Aim: To identify brain regions involved in language comprehension.
Method: Post-mortem studies of patients with impaired understanding of language but intact articulate speech.
Findings: Damage to the left temporal lobe (Wernicke’s area) was observed.
Conclusion: Wernicke’s area is essential for processing written and spoken language.
Opposing Strict Localization: Karl Lashley
Case studyKarl Lashley (1929)
Aim: To explore whether memory is localized or distributed in the brain.
Method:
Rats were trained to navigate a maze. Varying portions of their cortex (10-50%) were surgically removed, and the rats’ ability to remember the maze was tested post-surgery.
Findings:
Memory loss correlated with the extent of damage rather than its location.
- Principle of Mass Action: The cortex works as a whole, with memory distributed across it.
- Equipotentiality: If one area is damaged, others can compensate.
Conclusion: Memory is widely distributed, challenging the idea of strict localization.
Weak Localization: Sperry (1968) and Gazzaniga (1967)
Case studySperry (1968)
Aim: To investigate how brain functions are divided between the two hemispheres in split-brain patients, where the connection between the brain's hemispheres (the corpus callosum) has been surgically severed.
Participants: Four individuals who underwent corpus callosum severance to treat epilepsy.
Method:
Visual stimuli were presented to one visual field at a time. Participants were asked to identify or manipulate unseen objects.
Findings:
- Objects shown to the right visual field (left hemisphere) could be named.
- Objects shown to the left visual field (right hemisphere) could be physically identified but not named.
- Language production is primarily localized to the left hemisphere, while the right can perform basic language tasks.
Conclusion: Language functions are weakly localized, with dominance in the left hemisphere but support from the right.
Case studyGazzaniga (1967)
Aim: To investigate how brain functions are divided between the two hemispheres in split-brain patients, where the connection between the brain's hemispheres (the corpus callosum) has been surgically severed.
Participants: Four individuals who underwent corpus callosum severance to treat epilepsy.
Method: Visual stimuli were presented to one visual field at a time. Participants were asked to identify or manipulate unseen objects.
Findings:
- Objects shown to the right visual field (left hemisphere) could be named.
- Objects shown to the left visual field (right hemisphere) could be physically identified but not named.
- Language production is primarily localized to the left hemisphere, while the right can perform basic language tasks.
Conclusion: Language functions are weakly localized, with dominance in the left hemisphere but support from the right.
Relative Localization: Penfield
Case studyWilder Penfield (1937)
Aim: To map sensory and motor functions of the brain.
Method: During surgeries, electrical stimulation was applied to cortical regions while patients were conscious, and their sensations or movements were recorded.
Findings: Created the cortical homunculus, a representation of body parts' relative cortical area.
Conclusion: Motor and sensory functions are strictly localized but organized hierarchically.
Critical Thinking Considerations
- Interconnectedness of Functions: Strict localization oversimplifies brain function. Most cognitive processes involve complex neural networks. Many researchers have argued that the brain works like a jigsaw and call for more holistic approaches.
- Dynamic Neuroplasticity: Evidence from stroke rehabilitation shows that undamaged areas can adapt to take over lost functions, challenging strict localization.
- This underscores the brain's ability to reorganize and reassign functions based on necessity.
- Population Validity: Many foundational studies relied on small or unique samples, such as individuals with brain damage or surgical interventions. These findings may not generalize to a broader population.
Memory relies on interactions between the hippocampus, prefrontal cortex, and amygdala.
Self reviewThink of one study from this section that may not generalize to a broader population!
