hemispheric lateralisation

Hemispheric Lateralisation and Split-Brain Research

Overview of Hemispheric Lateralisation

  • Definition: Hemispheric lateralisation refers to the concept that the two halves of the brain (the left hemisphere [LH] and right hemisphere [RH]) have distinct functional differences. Specific mental processes and behaviors are predominantly controlled by one hemisphere rather than the other. For example, language is an area that is both localised (specific brain areas responsible for certain functions) and lateralised (predominantly processed by one hemisphere).

Key Concepts and Terms

  • Key Terms:
    • Hemispheric Lateralisation: The functional differences between the two hemispheres of the brain, where certain tasks and processes are primarily managed by either the LH or RH.
    • Split-Brain Research: A series of studies initiated in the 1960s involving individuals who have undergone a surgical procedure to sever the corpus callosum, which connects the two hemispheres, to alleviate severe epilepsy. This allows researchers to examine the functions of each hemisphere independently.

Experimental Methods in Split-Brain Research

  • Visual Presentation Method:
    • Involves presenting two words simultaneously to a participant, one in the left visual field (LVF) and one in the right visual field (RVF).
    • Example: If the word "KEY" is displayed in the LVF, the participant, being a split-brain individual, would respond by selecting a key with their left hand (linked to the RH), while stating the word “RING” shown in the RVF (linked to the LH).

Handedness and Lateralisation Results

  • In a study of 400 right-handed individuals, it was found that:
    • 380 of them exhibited LH dominance for language, yielding a percentage of:
      380400=0.95\frac{380}{400} = 0.95 or 95%
    • For a study involving 400 left-handed individuals, it was reported that:
    • 80 had RH dominance for language.
    • 80 displayed bilateral language processing across both hemispheres.
Questions:
  1. What percentage of right-handed individuals were LH dominant for language?
    • Answer: 95%
  2. Express the answer as a fraction:
    • Answer: 1920\frac{19}{20}
  3. What percentage of left-handed individuals exhibited bilateral language functions?
    • Answer: 40% (from the 80 of 200 left-handed individuals).
  4. Considerations regarding Sperry’s findings had left-handed individuals been included (3 marks worth):
    • The findings might have shown more variability in lateralisation, as lateralisation is not uniform across all individuals.
    • Participants could potentially involve different areas being dominant for language, thereby complicating the understanding of lateralisation effects.
    • This variance could illustrate a more biologically nuanced picture of lateralisation depending on handedness.

Localisation vs. Lateralisation

  • Localisation: This term refers to the specific locations within the brain that are responsible for particular functions. For instance, Broca's area in the left frontal lobe and Wernicke's area in the left temporal lobe are vital for language.
  • Lateralisation Complexity:
    • While some brain functions (such as language) are lateralised, others (like vision, motor functions) are distributed across both hemispheres.
    • Contralateral Wiring: The motor area operates under a contralateral model, meaning the RH controls the left side of the body and the LH controls the right side.
    • In terms of visual processing, both contralateral connections (where the left visual field connects to the RH) and ipsilateral connections (where the same-sided information is processed) exist, enhancing depth perception.

Split-Brain Surgery and Its Implications

  • Procedure Details:
    • Split-brain surgery involves severing the corpus callosum to limit excessive electrical activity during seizures.
    • This allows the researchers to understand how separate hemispheres process and respond to information in isolation.

Roger Sperry’s Research (1968)

  • Study Design:
    • Eleven individuals who underwent split-brain operations were examined using a specific setup where images are projected separately to each visual field.
    • The goal was to understand the individual processing capabilities of the LH and RH when segregated.
  • Findings:
    • If an object was shown to the RVF (LH), participants could verbally identify it. Conversely, objects shown to the LVF (RH) could not be named verbally but could be pointed to or selected with the left hand, indicating independent processing.

Evaluation of Lateralisation Findings

  • Research Strengths:

    • Evidence from functional imaging studies shows that even in connected brains, processing differs - the RH is better at global processing while the LH excels at details.
    • Research Reference (Fink et al. 1996): PET scans demonstrated distinct activations in hemispheres during visual tasks.

  • Limitations of the Dual Hemisphere Dominance Theory:

    • The simplistic model of LH as the analyser and RH as the synthesiser does not account for complexities in human processes.
    • Reference (Nielsen et al. 2013): Analysis of brain scans showed that, while individuals exhibit preference in certain tasks, no single hemisphere dominantly influences personality traits or capabilities (e.g., artist versus mathematician).

  • Neural Plasticity vs. Lateralisation:

    • Lateralisation enhances efficiency by allowing simultaneous task performance. Rogers et al. (2004) showed that lateralised chickens demonstrated better predator vigilance and foraging than their non-lateralized counterparts.
    • Neural plasticity allows for recovery and redistribution of functions, as shown in studies where language functions shifted across hemispheres following damage (Holland et al. 1996).

Recent Research and Generalisation Issues

  • Michael Gazzaniga’s Work:
    • Recent studies indicated that split-brain individuals performed certain tasks better than those with connected hemispheres, thereby supporting the notion of functional lateralisation.
  • Causal Relationship Limitations:
    • Comparison of Sperry’s patients with a control group lacking epilepsy raises confounding issues, as observed differences may stem from their epileptic condition rather than the split-brain state.

Ethical Considerations

  • Observations indicate that while participants showed unique abilities due to lateralisation, the consent process following their surgeries was complicated by long-term assessments, potentially affecting their understanding and the ethical implications of prolonged studies.
  • Conclusion of Sperry’s Findings:
    • The surgical interventions were not research-motivated, and informed consent was obtained, but the immediate impacts of brain trauma should also be factored into ethical evaluations.

Individual Case Study: Kim Peek

  • Case Overview:
    • Unlike Sperry's participants who underwent surgical intervention, Kim Peek was born without a corpus callosum, leading to natural split-brain characteristics.
    • Demonstrated extraordinary memory capabilities, recalling over 12,000 books and displaying abnormal reading speeds.
  • Differences from Sperry’s findings:
    • Peek likely had developed bilateral language centres, reflecting unique abilities not necessarily generalizable to the typical population from split-brain surgeries.

Reflection Questions

  1. Outline one procedure used in split-brain research.
    • Answer: Visual presentation method with images targeted at one hemisphere.
  2. Briefly evaluate split-brain research on hemispheric lateralisation (3 marks)
  3. Discuss implications of research on hemispheric lateralisation (4 marks)