Severe traumatic brain injury (TBI) patients in a vegetative state were
exposed to passive P300 (PP300) evoked potential tests under two
conditions: two auditory tones (unimodality condition), and a flash and
auditory tone (bimodality condition). A third non-P300 condition using a
single repetitive auditory tone was also presented. Patients produced PP300
responses under all three conditions, even though the severity of their
clinical condition did not allow them to respond to even simple commands.
No peak latency differences were found. PP300 amplitude was significantly
larger under the bimodality stimulus condition than either the unimodality
or non-P300 condition. The PP300 amplitude under the unimodality condition,
in turn, was larger than the P300-like response in the non-P300 condition.
This replicates earlier findings with normal subjects. PP300 responses
appear to be a tool that might find utility in evaluating TBI patients.
Results raise questions about the neuropsychological/neurophysiological
nature of the PP300 response.