The 40-Hz event-related potential in brain damage : effects of lesion site, distraction and stimulus rate

The utility of the 40-HZ ERP procedure as a clinical dissection tool for identifying brainstem and frontal lobe injury was investigated in this study. Subjects with brainstem and/or frontal lobe damage as well as normal controls were assessed with the 40-Hz procedure in the auditory modality. Rates of stimulation used were 40/ clicks/sec and 20/clicks/sec. Subjects were tested with and without distraction presented to the ear contralateral to the test stimulus. Amplitude, 40-Hz power and phase were analyzed. The frontal lobe group produced ERP's with higher amplitude and power than the normal or brainstem groups. The brainstem group produced ERP's with lower amplitude and power than the normal or frontal lobe group. Reducing the rate of stimulation from 40/sec to 20/sec resulted in a significant reduction in amplitude and power for all groups. With 20/sec stimulation the majority of brainstem subjects were unable to produce a 40-Hz ERP. The effect of distraction in the normal and frontal lobe groups was to reduce amplitude and power, but distraction increased amplitude and power in the brainstem group. Phase values were not significantly affected by lesion site, stimulus rate, or distraction.