Yasmine- Mattroser 1

Contact Information:

  • Email: matt.roser@plymouth.ac.uk
  • Office: PSQ B207
  • Office Appointment: Tuesday 10-11am, Thursday 10-11am
  • Check-in code: XX-XX-XX
  • Dr. Matt Roser

Recommended Reading:

  • Dissociation, lesion
  • Chapter 3 of Banich and Compton
  • Rorden, C., and Karnath, H. (2004). Using human brain lesions to infer function: a relic from a past era in the fMRI age? Nature Reviews Neuroscience, 5, 812-819.

Lecture Outline:

  • Introduction to patient studies and the lesion method.
  • Types of brain damage, example, terminology
  • Single and double dissociation (example)
  • Limits to the lesion method
  • What’s a control group?

Patient Studies:

  • A major source of knowledge about brain and mind.
  • Causes:
    • Trauma
    • Stroke / Vascular accidents
    • Tumor
    • Degenerative and Infectious Disease
    • Epilepsy, Neuropsychiatric Disorders
    • Neurosurgery

Neuropsychological Deficits:

  • Agnosia: Loss of ability to recognize objects, people, sounds, shapes, or smells; the inability to attach appropriate meaning to objective sense-data (“The man who mistook his wife for a hat”).
  • Aphasia: A general term relating to a loss of language ability.
  • Apraxia: A general term for disorders of action.
  • Amnesia: Lack of mnemonic abilities.
  • Ataxia: Poor coordination and unsteadiness due to failure to regulate the body's posture, strength, and direction of limb movements.

Many Subtypes:

  • Visual agnosia is associated with lesions of the left occipital and temporal lobes.
  • Form agnosia: Patients perceive only parts of details, not the whole object.
  • Finger agnosia: The inability to distinguish the fingers on the hand, present following lesions of the parietal lobe.
  • Simultanagnosia: Patients can recognize objects or details in their visual field, but only one at a time.
  • Associative agnosia: Patients can describe visual scenes and classes of objects but still fail to recognize them. For example, knowing a fork is something you eat with but mistaking it for a spoon.
  • Apperceptive agnosia: Patients are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli.
  • Prosopagnosia: Also known as faceblindness or facial agnosia.

Behavioural Testing:

  • The existence of selective deficits can tell us something about the way function is organised in the brain.
  • The goals are to relate brain anatomy to behaviour and to investigate mental processes.
  • Requires behavioural tasks.
  • Tasks should tell us about the patient’s deficits:
    • What functions are compromised?
    • What functions are spared?

Dissociating Cognitive Functions and Brain Regions:

  • Cognitive functions can be dissociated (separated to a degree) from each other through selective impairment.
  • The same is true for the function of brain regions.
  • Dissociation studies require a minimum of two groups and two tasks.
  • Comparison between patient/control groups shows deficit.
  • Two tasks are needed to determine whether a deficit is specific to a particular function or reflects a more general impairment.

Single Dissociation:

  • Demonstrates that a patient group performs poorly on one task (e.g., declarative memory) compared to a control group, while performance on another task (e.g., nondeclarative memory) is relatively spared.
  • Suggests the involvement of a specific brain region (e.g., temporal lobes) in the impaired function.
  • However, it does not definitively prove that the brain region is not involved in the spared function.

Limitations of Single Dissociation:

  • The conclusion that the temporal lobes are involved in declarative memory, and NOT in nondeclarative memory, does NOT follow.
  • Poor performance of patients might be caused by another factor, such as a deficit in concentration. The test of declarative memory might require more concentration than the test of nondeclarative memory.

Double Dissociation:

  • Provides strong evidence that there are cognitive processes critical for task X that are not critical for task Y, and vice versa, and that brain-area A is critical for task X but not for task Y etc.
  • Double dissociations provide evidence that the observed differences in performance reflect functional differences between the groups, rather than unequal sensitivity of the two tasks.
  • Participants don‘t have to be perfectly intact on either task, they just need to be significantly better at one task than the other.

Double Dissociations:

  • Involves two patient groups, each with damage to a different brain area, and two tasks.
  • Patient group 1 (e.g., temporal lobe lesion) is impaired on task X but not task Y.
  • Patient group 2 (e.g., cerebellum lesion) is impaired on task Y but not task X.
  • This reciprocal pattern of impairment provides strong evidence for the functional independence of the two cognitive processes and the selective involvement of the two brain areas.

Limitations of Patient Studies:

  1. Assumption of modularity
  2. Lesions extensive and varied
  3. Lesion anatomy inaccurate, connections not considered
  4. Individual differences in functional anatomy
  5. Poor temporal resolution

Modularity of Function:

  • Assumption that mental processes occur with a high degree of isolation from other mental processes, and when one area is damaged, other regions do not adapt their function.
  • Brain plasticity: In reality, the brain reorganizes quickly. Intact regions change their behaviour so it is difficult to infer function of damaged region
  • Processes/dynamics neglected: It is neurons, not black boxes, that perform the function - but how?

Lesions Extensive and Varied:

  • Most work is done with patients who have large lesions.
  • Lesions often damage several functional centres, so there are few patients with ‘pure’ deficits.
  • Lesion size and location are variable, making it hard to find a group of similar patients. Inferences from single patients are weak.
  • Individual differences in recuperative history.

Lesion Anatomy Inaccurate and Connections Not Considered:

  • Anatomical scans show regions that are destroyed, but intact regions may not be functioning.
  • Regions may be disconnected from other regions that provide input.

Individual Differences in Functional Anatomy:

  • We assume that an anatomical region of the brain does the same function in all individuals.
  • Clearly violated assumption – e.g. Wada test indicates the left hemisphere predominates in language processing in most, but not all, individuals.
  • Variability of function across individuals reduces the power of group studies.

Poor Temporal Resolution and Experimental Control:

  • Even if patient studies establish which regions are necessary for a task, and its inferred cognitive processes, it is not possible to infer the stages of processing.
  • A memory deficit may arise from a failure of encoding, retention, or recall.
  • There is no experimental control over lesion location, but animal studies using experimental ablation can provide this.
  • Other methods overcome these limitations.

Benefits of Patient Studies:

  • Show which areas are necessary for a particular cognitive function (double dissociation).
  • Show cognitive, emotional, social consequences of a deficit (example: Damasio‘s patient Elliot).
  • Cost- and time-effective, single-case studies are possible (e.g. H.M., no experimental design necessary, exploratory observations possible).
  • Can be done right (overlay plots and control groups) to limit criticism.

Why We Cannot Localize Speech Production in This Area:

  • Damage is not limited by functional boundaries.
  • The lesion might be smaller than the functional module.
  • Interindividual differences in brain organization.
  • The result might reflect the increased vulnerability of the region to injury (e.g., because of vasculature).
  • The area might just be interconnected with the actually relevant area (indirect disruption).

Importance of Control Group:

  • We need to include a CONTROL GROUP of PATIENTS with right-hemisphere brain damage but without VFDs (visual field deficits).
  • In this way, we CONTROL for the effect of right-hemisphere damage and contrast only across the factor of VFD presence.
  • VFDs are associated most strongly with damage to the occipital cortex (optic radiations and visual areas).

Summary:

  • Patient/lesion methods can reveal novel insights about brain function without any experimental design.
  • Done properly (with double dissociation groups of patients and an appropriate control group), some of the drawbacks can be prevented.