Changes in event-related potentials in a three-stimulus auditory oddball task after mild head injury

Previous research has demonstrated changes in event related potentials in a variety of cognitive tasks after severe closed head injury. We sought to establish if similar changes were present in patients who had sustained only apparently mild head injury by recording event related potentials in a group of 25 patients following mild head injury and 25 control subjects during a three-stimulus auditory 'oddball' task. For this task subjects were required to respond to rare target tones by pressing a button and to ignore frequent non target tones and rare novel sounds. Analysis of behavioural performance showed no difference in reaction times or error rates between the two groups. There were, however, changes in event related potentials which appear to indicate that subjects in the mild head injury group were deploying increased attentional resources in order to achieve the same level of performance. There was a significant increase in negativity in the orienting or 'O' wave following the P3a elicited by novel sounds in a distribution consistent with increased activation of frontal and parietal attentional systems. There was also a non-significant trend towards an increase in the fronto-central N2 amplitude in the mild head injured subjects consistent with other findings in the same subject group. There were, however, no significant changes in P3b latency or amplitude. In conclusion it appears that even mild head injury can lead to measurable changes in the electrophysiological correlates of attentional mechanisms. The nature of the changes would seem to lend support to the hypothesis that apparently mild head injury can cause subtle cognitive impairments which require a compensatory increase in attentional effort to maintain normal performance.