Reduced hippocampal volume in association with P50 nonsuppression following traumatic brain injury

Traumatic brain injury (TBI) may produce persistently impaired auditory gat ing. This cholinergic-dependent, hippocampally mediated preattentive cognit ive function that facilitates filtering of auditory stimuli may be indexed by the P50 evoked waveform to paired auditory stimuli. Abnormal P50 suppres sion post TBI is believed to result from injury to the hippocampus and/or i ts afferent cholinergic projections. This hypothesis was tested by comparin g hippocampal and total brain volumes on MRI between ten P50-nonsuppressing TBI patients and ten normal control subjects matched for age, gender, and education. TBI subjects had highly significant bilateral hippocampal volume reductions, even when covaried for reductions in total brain volume. Degre e of volume loss was not correlated with initial TBI severity. Findings sup port the hypothesis that hippocampal injury underlies P50 nonsuppression po st TBI and suggest that such structural abnormalities may be observed even in "mildly" injured persons.