Task 8 Lesion studies Flashcards
Lesion methods (def)
study of the effect of damage to a particular area
Classical neuropsychology
- infer function of a brain region by taking patients with lesions to that region and examine pattern of impaired/spaired abilities
- favours group studies
Cognitive neuropsychology
- pattern of spared/impaired abilities used to infer building blocks of cognition (irrespective of location)
- favours single cast studies
Limits to interpretation: Modularity
assumption that discrete anatomical modules deal with different cognitive functions (homunculus)
–> brain functions are carried out in distributed manner: most brain damage is not limited by boundaries of underlying functional modules
Limits to interpretation: Universality
assumption that functional modules are in the same locations in different individuals
–> brain shows great anatomical differences & plasticity
Limits to interpretation: Differential vulnerability
- locations of brain damage are not randomly distributed –> difficulties in interpreting lesion overlap plots
Limits to interpretation: Functional compensation
- brain regions can be disabled but intact
Limits to interpretation: Temporal resolution
- not possible to assess time course of information processing
- testing patient in accute stage of illness: can’t accurately identify all impaired brain areas
- waiting for initial problems (e.g., swelling) to resolve: brain plasticity
Single case study
data from different patients are not combined
Assumptions of Single case studies
- Fractionation assumption: damage to brain produces selective lesions (unlikely due to functional distribution)
- Transparency assumption: lesions affect components within preexisting cognitive system but don’t result in new system being created : needed because we study abnormal brain to understand normal brain
- Universality assumption: generalizability of all cognitive systems
Strong arguments for single-case study
- in non-brain damaged population: okay to average observations of group because the only existing difference between participants is noise
- in brain-damaged population: each patient has different lesion –> difference in performance may be due to lesion rather than noise
Group studies
performance of different patients is combined to yield group average
Basic concept
- to localize which region is critical for performing a given task –> in fMRI: activity does not imply critical involvement!
- lesions are rarely restricted to the area of interest –> several patients need to be considered
Grouping in group studies
- Grouping by syndrome: patients are assigned based on cluster of different symptoms (schizophrenia)
- Grouping by cognitive symptom: assigned based on one particular symptom
- Grouping by anatomical lesion: based on having lesion to particular anatomical region
Disadvantages of group studies
- indivdiual brains differ –> standardization required
Localization of function: single dissociation
- if patient is impaired on task A but not on task B:
1. Tasks rely on different cognitive processes
2. Tasks rely on same cognitive process but Task A requires more neural resources than B
3. Hierarchical relationship: functions are separate but one function is necessary for the other
Localization of function: double dissociation
- two single dissociations in two individuals with complementary profile of abilities:
–> provides stronger evidence for functional specialization and that two functions are independent (speech and comprehension)
Possible artifacts:
1. Task-resource artifact: brain damage depletes important resource that leads to impairment of task
2. Task-demand artfifact: patient performs one of the tasks sub-optimally
Advantages of double dissociation
- tells which areas are involved in particular function
- lesion is not due to nonspecific brain dysfunction
- tells which areas are not involved (unimpaired task B)
Limitations of double dissociations
- dissociations don’t tell us where the function is localized, only necessary areas
- lesion may exert disruptive effects on neighbouring tissue (diaschisis)
Association
- consistent co-occurrence of 2+ impairments suggests one underlying process
- can be due to proximity of regions
- disruption of single underlying process is responsible for both impairments
Application lesion method (Broca’s area)
Claim: third convolution of inferior frontal gyrus is involved with speech production
Support:
- patient who was only able to produce one syllable –> studied correlation between behavioural disorder & location of brain injury
Hemineglect
- unilateral damage to parietal, temporal, and frontal cortices
- Consequence: difficulties in attending to and processing information from one side of body or environment
Neglect as disorder of attention
- neglect patients show activation of visual cortex
- able to detect objects if cued or externally motivated (salient items, emotionally charged)
Line cancellation task
- patients are asked to bisect lines in middle –> only bisect unaffected side
Vision vs. visual imagery: single case study
Patient C.K.:
- car accident and suffered head injury: damaged ventral stream of visual processing pathway –> cannot recognize objects anymore but has intact imagery
Study:
- 2 opposing single dissociations form double dissociation
- previous study: perception intact, imagery impaired
Patient C.K.: Experiments
Experiment 1: to find out whether there is impairment:
- impaired object recognition but semantic unimpaired –> appropriate semantic information cannot be accessed for items presented in visual modality
Experiment 2: What is C.K. able to recognize?
- able to identify letters and objects when presented by touch or auditory input
Experiment 3: Perceptual processing: Is C.K. impairment bsed on interference with other visula impairments?
-> funcitonal deficits concern intermediate stages of visula processing because
- not for all types of input
- not for all types of objects (letters but not faces)
- not for based visual input
Experiment 4: Imagery of objects, size, colour, form
–> C.K. retained knowledge about phyiscal characteristics of objects & could use it for imagery
–> proof of single dissociation between imagery (intact) and perception (impaired)
