TOPOGRAPHIC DISPLAY OF EVOKED POTENTIALS: CLINICAL APPLICATIONS OF BRAIN ELECTRICAL ACTIVITY MAPPING (BEAM) *

It is widely agreed that long-latency sensory evoked potentials (EPs) are sensitive not only to stimulus parameters, but to variations of brain state. On this basis, one might predict that such EPs would achieve widespread clinical use paralleling that of the EEG. With a few exceptions, this has not been the case. We do not believe that this results from an inherent insensitivity of E P to cerebral pathophysiology. On the contrary, we propose that such measures of brain electrical activity represent not too little, but too much clinical information to be easily appreciated by unaided inspection or measurement. To assist clinical appraisal of such data, we have recently developed a system for the topographic mapping and computerized display of scalp-recorded signals referred to as brain electrical activity mapping or BEAM.’ As routinely used in our laboratory, BEAM images are constructed from data gathered from 20 scalp electrodes placed in the standard 10-20 EEG format (FIGURE 1). Resultant topographic images are displayed on a computer-driven color video monitor using a colored “grey” scale. For E P data, we visualize the dynamic change of electrical activity with time by sequential display of images; the display technique produces an animation effect highlighting the spread of E P activity over the scalp. These methods condense and summarize the spatiotemporal information obtained from multielectrode recordings to facilitate analysis by visual inspection. BEAM is now a routine clinical neurophysiological test in the Seizure Unit a t Childrens Hospital Medical Center, Boston. Clinical referrals over the last 12 months blanket a wide spectrum of neuropathology including epilepsy (21%), learning disability (22%), emotional disturbance and dementia (14%), headache (lo%), and unknown neurological disease (33%). BEAM appears to be most applicable to cases where the CT scan is either normal or noncontributory to the clinical question a t hand. Two refinements of the BEAM methodology have improved its applicability as a clinical instrument: (1) Significance probability mapping, a procedure for statistically delineating abnormal regions; and (2) Grid sector analysis, a procedure for quantifying the degree of overall and/or focal abnormality in BEAM images. In the following