Lesion Methods Flashcards
Agnosia
Loss of ability to recognise objects, people, sounds, shapes, or smells.
Inability to attach appropriate meaning to object sense-data
Many different subtypes eg finger agnosia : inability to distinguish the fingers on the hand. Result of lesions of the parietal lobe
Aphasia
General term relating to a loss of language ability
Apraxia
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 and strength and direction of limb movement
What lesions is visual agnosia associated with
Lesions to the left occipital and temporal lobes
What do selective deficits tell us
Something about the way function is organised in the brain
What is behavioural testing used for
To relate brain anatomy to behaviour and to investigate mental processes
Should tell us what functions are compromised and what functions are spared
What causes dissociations in cognitive functions and functions of brain regions
Selective impairment
Describe dissociation studies
Require minimum of two groups and two tasks
Comparison between patient/control groups shows deficit
Two tasks needed to determine whether a deficit is specific to a particular function, or reflects a more general impairment
What is a single dissociation study
Two groups are tested on two tasks and a difference is only apparent in one task
What are limitations of single dissociation studies
Differences in performance in the two tasks could be the result of differences in task difficulty
What are double dissociations
One patient shows impairment in task A but not task B
Other patient shows impairment in task B but not A
What is good about double dissociations
Remove the variable of task difficulty being an influence - provide evidence that there observed differences in performance reflect functional difficulties between the groups and not unequal sensitivity of the two tasks
Provide strong evidence that there are cognitive processes critical for task A that aren’t critical for task B
Limitations of patient studies
Assumption of modularity
Lesions extensive and varied
Lesions anatomy inaccurate, connections not considered
Individual differences in functional anatomy
Poor temporal resolution of
Explain assumption of modularity limitation of patient studies
Assumption that mental processes occur with a high degree of isolation from other mental processes
And when one area is damaged other regions don’t adapt to their function
But actually brain plasticity: brain reorganises quickly, intact regions change their behaviour. So difficult to infer function from damage region
Explain lesions extensive and varied limitation
Most work done with patients who have large lesions
Lesions often damage several functional centres, so few patients with ‘pure’ deficits
Lesion size and location variable, hard to find group of similar patients. Inferences from single patients are weak
Explain lesions anatomy inaccurate and connections not considered limitation
Anatomical scans show regions that are destroyed, but intact regions may not be functioning
Regions may be disconnected from other regions that provide input
Explain individual differences in functional anatomy limitation
We assume that an anatomical region of the brain does the same function in all individuals
Clearly violated assumption - Wada test indicates left hemisphere predominates in language processing in most individuals
Variability of function across individuals reduces the power of group studies
Explain poor temporal resolution and experimental control limitation
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 experiment ablation can provide this
Other methods over come these limitations
Benefits of patient studies
Show which areas are necessary for a particular function (double dissociation)
Show cognitive, emotional, social consequences of a deficit
Cost and time effective, single case studies are possible
Can be done right to limit criticism
What is the importance of a control group
Colour-coded overlays of lesions in patients with left visual field deficits show all patients have lesions in temporal-parietal junction
This region is susceptible to damage because it lies near to the blood supply
Need to include a control group of patients with right hemisphere brain damage but without VFDs
In this way we control for the effect of right - hemisphere damage and contrast only across the factor of VFD presence
