Until recently it has been possible only to measure the behavioral products of cognitive processing, e.g. reaction time. However, this is a rather indirect way of studying brain substrates of cognition. Psychophysiological techniques can be used to study the neural mechanisms of cognition. In this experiment brain electrical activity was measured and a subcomponent of this electrical activity, the P300 wave, was identified in a population of normal and brain-injured adults while they were engaged in a visual discrimination task. The averaged amplitude of the P300 wave was significantly decreased and the averaged latency of the P300 wave was significantly increased in the brain-injured group. However, a trial-by-trial analysis revealed that near normal waveforms could be produced by the brain-injured subjects in a small percentage of trials. Since the P300 waveform is related to mechanisms of selective attention and short term memory, the appearance of normal P300 waves in the brain-injured group indicates that the stability of these cognitive processes, and not the processes themselves, were affected by the neurological insult to these patients. (Author)