In a bipolar EEG montage, what signifies a localizing sign?

In a bipolar EEG montage, a phase reversal between adjacent channels that share a common electrode serves as a localizing sign. This phase reversal indicates a significant change in the electrical activity occurring at a specific location within the brain. When adjacent electrodes are positioned over areas with distinct electrical activity, the waves recorded can exhibit opposite polarities due to the underlying pathology or brain function, which makes it easier to pinpoint the origin of the abnormal activity. This is particularly important in intraoperative settings where identifying localized brain regions affected by injury or other conditions can guide surgical decisions and interventions.

In contrast, uniform waveforms across all channels suggest that the electrical activity is similar across those brain regions, indicating a more generalized activity rather than a local event. Decreased amplitude of waves could reflect diffuse brain effects or overall reduced cortical activity but does not specifically localize an event. Increased frequency of waves might indicate a change in brain state, such as heightened arousal or some pathological activity but does not provide specific localization of abnormal function. Therefore, the phenomenon of phase reversal is essential for determining localized brain functions or lesions during monitoring.