The typical visual evoked potential (VEP) elicited by a pattern-reversal stimulus is a negative-positive-negative complex that is recorded maximally in the midoccipital region with reference to either the midfrontal region or the linked ears. y The positivity is the most conspicuous and consistent of these potentials and has a latency to its peak of approximately 100 msec (.Fig 24-12 ). It is therefore called the P100 response, and it is to this component that attention is directed.
In analyzing the VEP, the latency of the P100 response is measured after stimulation of each eye and then the interocular difference in latency is determined. These values are compared with normal values obtained using an identical stimulation and recording technique. This is important because certain physical attributes of the stimulus (such as its contrast or the check size) normally influence the response latency. The amplitude of the response and the latency of the preceding and succeeding negativities (the N75 and N145, respectively) are of uncertain clinical utility, but absence of the P100 is abnormal.
Figure 24-12 Visual evoked potential elicited by 50-minute checks and recorded between the midoccipital and midfrontal monocular stimulation with regiiA, a normal subject, andB, a patient with a past history of optic neuritis. Two trials are superimposed to show the replicability of the findings. The P100 response B i s prolonged to 146 msec.
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