11/29/2006

Traumatic Brain Injury: Tests and Theories

One of the principal techniques used by cognitive scientists is reverse engineering - for example, a classic experiment by Donders established the field of mental chronometry by subtracting the time for execution of a motor response from total response time.

Reverse engineering "normal" brains can only take you so far. Another approach is to look at the effects of brain damage on various cognitive functions, and use reverse engineering methodologies on those patients to determine what function the damages regions might subserve. While MRI technologies have vastly improved our ability to localize damage in the frontal cortex, behavioral techniques for assessing the type, extent, and potential outcomes of frontal damage are still relatively crude.

As discussed by authors Cicerone, Levin, Malex, Stuss and Whyte, traumatic brain injury typically causes two types of damage: diffuse axonal injury (DAI) and focal cortical contusions (FCCs). The former involves destruction of capillaries and axons in frontal white matter and may contribute to frontal hypometabolism, which is known to be related to executive function impairment. The latter involves abrasions to the cortex by the skull, which can damage cortex directly (through herniation) or indirectly (through lack of oxygen and/or blood flow as a result of inflammation or DAI).

To determine the effects of brain injury on executive functions, the authors distinguish between 4 basic types of EF: cognitive, behavioral/self-regulatory, activation regulation, and metacognitive operations. Each is covered in turn below:

Executive Cognitive Functions: The authors suggest that these functions are dependent on the "control and direction [...] of lower level, more modular, or automatic functions" and that they are mediated by working memory and inhibition. Such functions seem to depend on the archicortical trend, and dlPFC in particular.

Executive cognitive functions are most frequently assessed with the Wisconsin Card Sorting Test, the Trail Making test, and verbal fluency measures. The California Verbal Learning Test is sometimes used to investigate damage to the associative or strategic processes that dlPFC may contribute to memory.

Interestingly, frontal damage does not seem to impair all executive cognitive functions equally. For example, Cicerone et al. review a 1998 study of 30 brain-damaged patients who showed no more susceptibility to distractors and no more dual-task performance decrement than normal controls (though of course they made more errors in general, raising the question of whether there were floor effects here). Performance on this naturalistic task (involving packing a lunch) was not predictive of the degree of brain damage, but was "moderately correlated with a measure of functional outcome."

Behavioral self-regulatory functions: In contrast, orbitofrontal (aka ventromedial) regions of prefrontal cortex are closely connected with the limbic system and thus both reward and emotional processing. Tests of reversal learning are generally sensitive to damage in this region.

Activation Regulating Functions: Cicerone et al suggest that more limited medial damage can result in apathy or abulia, and that this maps onto Stuss's concept of drive. They link these functions to the anterior cingulate and superior frontal cortex, pointing to slowed RTs in veral fluency and Stroop tasks among patients with damage to this area.

Metacognitive Processes: Cicerone et al point towards the frontal poles as the origin of metacognitive processes like personality, consciousness, self-evalution and social cognition. Assessments of damage to the frontal poles involve "reactions to verbal and cartoon humor, visual perspective-taking tests, and comparison of performance on remember-know memory tasks."

All of the assessment techniques listed above have been criticized for a lack of specificity. It is also important to distinguish between the levels of analysis - impairment on laboratory tasks vs. impairment on real-world tasks involving those abilities. Such lack of specificity problematizes rehabilitation, which can involve pharmacological intervention, "direct remediation," or the use of compensatory technologies/devices.

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