The Topography of the N70 Component of the Visual Evoked Potential in Humans

Abstract

The latency of the so-called PI00 or major positive wave of the pattern visual evoked potential (VEP) is deservedly used as a most reliable indicator of retinal and optic nerve neuropathy. The fact that it occurs with such a long latency has given rise to considerable interest in the possibility of utilizing earlier VEP components for clinical diagnosis, in the hope of establishing at which anatomical level of visual processing an abnormality may have occurred. The earliest reported components, which occur as a short “burst” of oscillations, have been studied using very bright, brief flash stimulation. In the single study using pattern stimulation evidence was reported that components near 30 ms show spatial tuning and do not arise from the retina. Unfortunately, the amplitude of these oscillatory potentials is small and therefore at this stage of technology their clinical utility is rather doubtful. On the other hand, pattern elicited VEPs, which have nearly 10 times the amplitude of oscillatory scalp potentials, are reported to contain “unreliable” components preceding the P100. Nevertheless, there are components of the VEP which precede the P100. In particular, a negative wave, which we shall label N70 for convenience, has some physiologically and clinically intriguing properties. In this paper we shall summarize the evidence for the propostion that one of the reasons why N70 is considered unreliable is the use of inappropriate stimulation.

The author and publisher gratefully acknowledge that the cost of colour prints was kindly subsidized by Dantec Elektronik