Evoked Potential: Vision

A visual evoked potential (VEP) refers to a systematic change in the electrical activity of someone's brain in response to a change in what the person is viewing. It is revealed by averaging over many trials the raw electrical activity, the electroencephalogram (EEG), recorded from 2 to 256 electrodes at standard positions on the person's head from each individual trial. For example, if a person is looking at a set of lines with one eye and at an oppositely oriented set of lines with the other eye, and then one set of lines changes to be identical to the other, the EEG from electrodes on the back of the head changes in a typical way over about 400 milliseconds (ms): a VEP (color insert, Figure 23a). The VEP is an event-related potential (ERP), because it is linked to a specific stimulus event—in this case, a visual stimulus. VEPs are used for clinical purposes, such as for locating problems to particular parts of the visual system, and for research. This entry describes VEPs, their advantages and disadvantages, and their clinical and research uses.

Figure 33 Visual Processing: Subcortical Mechanisms for Gaze Control—Brain Stem Pathways for Gaze Control

Sources: (a) V. Henn, J. A. Buttner-Ennever, and K. Hepp, 1982. (b) Author.

Note: (a) Midsagittal view of the primate brain stem. Burst neurons are shown in blue, neural integrators in yellow, and motoneurons in green. The superior colliculus (SC), thalamus, and cerebellum are also shown. Inset shows the location of the brain stem. (b) A schematic of gaze shift commands traveling through the brain stem. Color scheme same as in (a). Horizontal components are indicated by (H), while vertical/torsional components are indicated by (V/T). SC = superior colliculus; BN = burst neurons; NI = neural integrator; MN = motoneurons; III = oculomotor nucleus; IV = trochlear nucleus; VI = abducens nucleus. See the Visual Processing: Subcortical Mechanisms for Gaze Control entry for additional information (pp. 1104–1107).

From the EEG to the VEP

In 1929, the German psychiatrist and neurologist Hans Berger placed an electrode on a person's scalp, recording the first EEG. He discovered that when the person closed his or her eyes, the EEG altered reliably even on a single trial, anticipating the VEP. In 1939, Pauline Davis reported that a unique, reproducible deflection of EEG activity could be evoked by an auditory stimulus. This was the first evoked potential, visible on a single trial. In 1960, Bill Cobb and George Dawson recorded the first, unambiguous VEP, averaged over many trials.

The fluctuations in electrical activity of a VEP depend on the location of the electrodes and on the sort of visual change. The VEP in the color insert, Figure 32(a), shows its first major component as a positive wave around 100 ms after the change, the P100 or P1. After that is a negative wave around 175 ms (N175, N2).

Figure 32 Visual Processing: Retinal—Neuroanatomy of the Retina

Source: Wassle, H. (2004). Parallel processing in the mammalian retina. Neuroscience Nature Reviews, 5, 747–757.

Note: Cell types: Cones (#1), rods (#2), horizontal cells (#3), bipolar cells (#4), amacrine cells (#5), ganglion cells (#6). Abbreviations: OS/IS = outer segments and inner segments; ONL = outer nuclear layer; OPL = outer plexiform layer; INL = inner nuclear layer; GCL = ganglion cells; NFL nerve fibers. See the Visual Processing: Retinal entry for additional information. (pp. 1101–1104).

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