D12 Neuropsychological evaluation

Topic

Cognitive performance is quantified during neuropsychological assessment using a battery of standardized tests, and the result for each cognitive domain evaluated is expressed as a normalized score relative to a reference sample adjusted for age and educational level. The further a person's cognitive performance deviates from this reference sample, the more extreme the normalized score received in that cognitive domain.

Based on the normalized scores from the various cognitive domains evaluated, a composite cognitive index can be constructed as a weighted average of those domains, extending the same general composite-score formalism already used in clinical scales.

Two additional measures, sensitive to different aspects of cognitive performance, are reaction time—reflecting the person's processing speed—and intra-individual reaction-time variability—reflecting the integrity of the person's sustained attention: the greater this intra-individual variability, the less intact the sustained attention. This intra-individual variability directly extends the same neural-response variability index already used to describe autism spectrum disorder.

In episodic memory tests, a person's cognitive performance across repeated trials follows an exponential saturation learning curve: cognitive performance improves rapidly during the initial repeated trials, but this improvement slows progressively as more trials accumulate, eventually approaching a maximum performance level for that individual.

By comparing a person's current cognitive performance with their estimated premorbid level—that is, the level of cognitive performance expected prior to any illness, based on demographic and educational variables—it is possible to quantify the degree of acquired cognitive impairment: the greater the difference between the estimated premorbid level and current cognitive performance, the greater the acquired cognitive impairment attributed to the individual. Ultimately, the ecological validity of this entire cognitive performance assessment is established by correlating the performance observed in the tests with the individual's actual functioning in activities of daily living: the stronger the correlation between test-measured cognitive performance and daily life functioning, the greater the ecological validity of the neuropsychological assessment. Taken together, it is this very cognitive performance—quantified through standardized scores, composite indices, measures of speed and variability, learning curves, and comparisons with premorbid levels—that must ultimately demonstrate its ecological validity in relation to the individual's actual functioning in daily life.

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