task 8 - lesion studies Flashcards
Lesion method assumptions (also limitations)
fractionation assumption: damage to the brain can produce selective cognitive lesions
Transparency assumption: lesions affect one or more components within the preexisting cognitive system, but they do not result in a completely new cognitive system
Universality assumption: lesions affect one or more components within the preexisting cognitive system, but they do not result in a completely new cognitive system
Lesion method limitations
- Identifying the site of a lesion caused by a tumor is particularly problematic: certain tumors may infiltrate surrounding tissue and so have no clear boundary
- diaschisis: very discrete brain lesion can disrupt the functioning of distant brain regions that are structurally intact
- challenge of differential vulnerability: some areas of the cortex are particularly likely to be damaged by stroke > then locations of brain damage are not randomly distributed in the brain
- to understand how different regions function, we need to have a good idea of the temporal sequence of information processing > lesion method does not allow this
Single dissociation
-patient is impaired on a particular task (task A) but relatively spared on another task (task B)
Classical: patient performs entirely normally on task B compared with a control group
Strong: patient performs entirely normally on task B compared with a control group
Single dissociation interpretation
- two functions are separate to some degree but one of them is necessary for the other
- This suggests a kind of hierarchical relationship between the two
-leaves open the possibility that the critical lesion produced its effect, not because the area it disrupted is specifically involved in the particular function in question, but because that region of the brain is simply more important in some gen¬eral sense
Double dissociation
-Two single dissociations that have a complementary profile of abilities
-task A doesn’t function, task B does
And
-task B doesn’t function, task A does
Double dissociation interpretation
-provide evidence that 2 functions are relatively separate / independent
Task – resource artifact
If two tasks share the same neural/cognitive resource but one task uses it more, then damage to this resource will affect one task more than the other.
Task-demand artifact
One task is performed worse than another because the task is performed sub-optimally (but not because some aspect of the task is compromised).
Syndromes
-cluster of different symptoms that are believed to be related in some meaningful way.
Single case studies principles
-In cognitive neuropsychology, the data from different patients are not combined.
Single case studies disadvantages
- one cannot create a theory based on observations from only a single case (counterargument: nobody is trying to construct whole new theories of cognition)
- not possible to generalize
Single case studies advantages
-helpful for establishing how cognitive processes might be subdivided.
Group case studies
-In neuropsychology, the performance of different patients is combined to yield a group average.
Ways of grouping studies
by syndromes: Patients are assigned to a particular group on the basis of possessing a cluster of different symptoms
> appropriate for understanding the neural correlates of a given disease pathology rather than developing theories concerning the neural basis of cognition
by cognitive symptom: possessing one particular symptom
> advantage of working forward from a symptom to a lesion location: potentially reveals more than one region
as being critically involved
Group case studies advantages
- more appropriate for establishing lesion-deficit associations
- lesions of patients typically large, rarely restricted to the region of interest
- > to localize which region is critical for a given task, several patients may need to be considered
Group case studies disadvantages
- each patient may have a different lesion to the cognitive system (L1 to Ln) and so difference in observed performance may be attributable to differences in lesion rather than between-patient noise and, as such, averaging across patients is not possible.
- As individual brains differ in brain shape, size and structure, brains from different individuals need to be transformed to a standard stereotaxic space to make group comparisons
Interpretation of data from lesion method:
necessity
tells us only that the lesioned area is necessary for the function in question and that the other lesioned area, which did not produce the impairment, is not.
Interpretation of data from lesion method:
localization
-dissociation does not tell us that the impaired function is localized in the area of the lesion
Interpretation of data from fMRI
- can show every part of a neural network that is involved in a task or a behavior
- evidence that a particular task correlates with activation in a particular brain region
- BUT: not clear whether this region is necessary to perform this task
- We cannot conclude that an area that does not show changes in bloodflow during a task is not involved with that task
Control patients in lesion studies
-simple overlay plots for patients who have a disorder can be misleading
> regions that they highlight might reflect increased vulnerability of certain regions to injury rather than any direct involvement with the disorder of interest
Example control patients lesion studies - speech
Example:
- compare patients who show impaired speech production with those who do not. These control patients should have brain lesions in the same hemisphere
- regions of the brain that are simply vulnerable to brain damage should be commonly damaged in both groups
- regions that are specifically involved with speech production should be selectively damaged in those patients who have problems in generating speech
Example lesion method – spatial neglect
-brain’s attention network is damaged in one hemisphere
-typically occurs from a stroke
-reduced arousal and
processing speeds, as well as an attention bias in the direction of their lesion (ipsilesional)
- One way to observe this phenomenon is to look at the patterns of eye movements in patients with neglect
- eye movements are compared to those of patients with right hemisphere strokes who showed no signs of neglect
- neglect can also affect the imagination and memory
Extinction
- visual field testing can show that neglect patients detect stimuli normally when those stimuli are salient and presented in isolation
- patient does not have a primary visual deficit
- presence of the competing stimulus in the ipsilateral hemifield prevents the patient from detecting the contralesional stimulus
Balint’s syndrome
three main deficits:
Simultanagnosia: difficulty perceiving the visual field as a whole scene
ocular apraxia: deficit in making eye movements (saccades) to scan the visual field, resulting in the inability to guide eye movements voluntarily
optic ataxia: problem in making visually guided hand movements
Comparing neglect and Balint’s syndrome
Dissociation:
- neglect can be based on spatial coordinates either with respect to the patient (egocentric reference frame) or with respect to an object in space (allocentric reference frame)
- the brain mechanisms involved with attending objects can be affected even when no spatial biases are seen
- Bálint’s syndrome: relatively normal visual fields but cannot attend to more than one or a few objects at a time, even when the objects overlap in space
Sodium amobarbital test
-creates a transient and reversible inactivation of one cerebral hemisphere
-sodium amobarbital test
thus serves the critical role of providing the surgeon with information about whether the intended removal is in the language hemisphere in patient
