Electrophysiological assessment of cognitive disorder in closed head-injured outpatients

Severe closed head-injury results in a multitude of long-lasting cognitive deficits. ERPs can effectively complement the more traditional behavioural measures to provide information that is not available through any other means. It is now fairly clear that a late positive wave, P3, associated with contextual updating is attenuated and prolonged in a variety of conditions in the head-injured. It is also possible that measures of selective attention such as the processing negativity may be abnormal in this group. A more definitive statement will, however, have to wait the results of further investigations. A number of investigators have now indicated that cognitive processing is slowed in the head-injured. Again, ERPs have been instrumental in explaining why it is slowed. Decision-making time as measured by RT is generally longer than P3 latency. Because P3 latency is delayed in the head-injured, the time required for evaluation of the stimulus (recognition and classification) is slowed in the head-injured. The additional delay in RT must, however, be explained by other processes, most probably a response bias that perhaps emphasizes accuracy at a cost of speed. Such a strategy can be manipulated if the patient is provided with cues about their speed of responding. Finally, ERPs have been instrumental in explaining possible reasons for cognitive slowing. A powerful CNV technique may permit the categorization of the head-injured into at least 2 distinctive groups: those that tend to underprocess information (perhaps as a result of apathy or a lack of motivation) and those that tend to overprocess (perhaps as a result of an inefficient and needless processing of irrelevant information resulting in fatigue). The extent to which the different modes of information processing are related to the site of brain injury and possible personality change remains an issue of speculation.