Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

The clinical VEP is dominated by activity from the central visual field because the topographical representation of visual field in the striate cortex is not evenly distributed (4). The fovea or central vision is represented disproportionately by a large cortical area occupying the posterior portion of the striate cortex, near the scalp where the VEP recording electrodes are placed (Figs. 5.1 and 5.2). This disproportionate representation of central vision reflects the high density of photoreceptors and the high number of retinal ganglion cell projections from the fovea. At least 50% of the cortical neurons of the striate cortex have receptive fields in the central 10 of the visual field. In contrast, the peripheral retina has a lower ganglion cell density with each ganglion cell receiving converging signals from several photoreceptors. The peripheral visual field is represented by smaller cortical area located more anteriorly in the striate cortex. This cortical area lies deep in the calcarine sulcus away from the VEP scalp electrodes and has a limited contribution to the clinical VEP.

Pattern stimulus consisting of alternating black-white checkerboard is commonly used in clinical VEP because it generates the most vigorous cortical response. While the cells of the retina and the lateral geniculate body respond well to a change in luminance in their receptive field, the cortical neurons of the striate cortex respond more actively to light–dark edges and orientation (5,6). Three physiologic categories of retinal ganglion cells are recognized, magnocellular, parvocellular, and koniocellular (7,8). Information derived from magnocellular ganglion cells is related to movement of objects in space, information from parvocellular ganglion cells is related to visual acuity and color perception, and information from koniocellular ganglion cells is related to form. These physiolo- gic-specific ganglion cells project to different layers of the lateral geniculate nucleus. This physiologic segregation continues in the striate cortex, V1, as well as in the projections from V1 to V2.

In addition to the striate cortex, the clinical VEP receives lesser contributions from other extrastriate visual processing areas in the occipital, parietal, and temporal lobes. These areas rely on signals from the striate cortex for their activation.

These areas may be organized into two broad pathways starting from the striate cortex forward. A ventral pathway to the temporal lobe is involved in object recognition, and a dorsal pathway to the parietal lobe is implicated in spatial vision. Visual processing areas such as V2, V3, V4, VP, MT, and MST are buried in deeper sulci, and each has specific response properties. For instance, V4 responds selectively to color of the stimulus while the middle temporal (MT) area responds selectively to direction. Specialized VEP techniques with different stimulus characteristics and electrode placements may be utilized to activate and record activities from more specific visual processing pathways.

Clinical VEP Recording

The noise-to-signal ratio of VEP recording is high, and computer averaging of multiple recordings or sweeps is necessary to isolate the VEP response. The VEP amplitude from a visual stimulus is usually less than 25 mV and significantly smaller than the continuous ongoing EEG that could be up to 100 mV. For a clinical VEP recording run, an average of at least 64 sweeps is recommended, and at least two similar results are required to confirm reproducibility.

The high noise-to-signal ratio of VEP explains why VEP amplitude has relatively high intra-subject as well as inter-subject variability (9,10). A variability of 25% in VEP amplitude is not too uncommon with repeat flash VEP testing during a single recording session in the same person. The placement of VEP electrodes is guided by surface bony

Figure 5.2 (Facing page) Placement of VEP electrodes based on the International 10=20 system. The electrode locations are obtained by percent distances with respect to bony landmarks. The anterior=posterior midline measurements for placement of the reference electrode at Fz and recording electrode Oz are determined by the distance between the nasion (the junction between the nose and the forehead) and the inion (the ridge at the back of the skull just above the neck). The circumferential distance from the frontal pole to Oz (occipital pole) is used to locate lateral electrode positions.

landmarks, therefore, the anatomical differences in folding patterns of gyri and sulci as well as in the relationship between visual cortex and surface landmarks contribute to VEP variability among individuals.

In contrast to VEP amplitude, VEP latency is the more consistent and useful clinical measure (9,10). The reproducibility of VEP latency in a normal subject is usually less than 5%, and variability among subjects is much less than VEP amplitude. Collection of age-matched normative data by each facility is recommended. The maturation and age-related changes of VEP are discussed in Chapter 6.

Pharmacologic dilation of the pupils is generally not needed for VEP testing. Pattern reversal VEP and pattern onset=offset VEP require best-corrected near visual acuity and consistent accurate fixation—both of which may be impaired by pupil dilation. Pupil dilation is typically not necessary for flash VEP. However, marked miosis can cause increased VEP latency, and a large anisocoria may produce falsely asymmetric VEP.

Monocular stimulated VEP should be performed for each eye to detect monocular visual pathway dysfunction. Interocular VEP asymmetry is relatively low in a normal subject. Binocular stimulated VEP is helpful when monocular responses are questionable to determine if any signals are reaching the visual cortex. Periodic calibration of the recording system should be performed as recommended by the manufacturer and based on the international calibration standard (11).

VEP Electrode Placement

Standard silver–silver chloride or gold disc EEG electrodes are placed on the scalp for recording VEP responses. The positions of the electrodes are determined by measurement from identifiable bony landmarks based on a method supported by anatomical studies known as the International 10=20 system (Fig. 5.2) (12). The designated electrode positions take into account brain size and underlying brain area but cannot account for interindividual variations in folding patterns of

gyri and sulci. With the reference electrode placed at Fz, VEP may be recorded by a single midline electrode at Oz. If more channels are available, additional electrodes may be placed laterally. Three channels using the midline and two lateral active electrodes are suggested if more specific detection of chiasmal and retrochiasmal dysfunction is desired. A ground electrode is typically placed on the forehead, vertex (position Cz), mastoid, or earlobe.

Three Standard VEP Responses

At least one of three recommended transient VEP responses should be included in clinical VEP testing—pattern reversal VEP, pattern onset=offset VEP, and flash VEP (Fig. 5.3). Transient VEP responses are produced when the stimulus rates are slow enough to allow the brain to recover to its resting state between stimuli. Steady-state VEP responses occur when stimulus rates are too fast to allow the brain to reach resting state between stimuli. Transient VEP responses are usually elicited when the stimulus rate is less than 5 Hz, which permits the identification of specific VEP waveform components and their amplitudes and latencies. Steady-state VEP provides higher frequency response components and is not ordinarily performed in the clinic except with specialized techniques.

The VEP tracing is presented often as positive upward in the ophthalmic literature or positive downward in the neurologic literature. Therefore, any VEP waveform should have its polarity clearly labeled. Positive upward is recommended by the ISCEV standard.

Pattern Reversal VEP

The pattern reversal VEP is elicited by a checkerboard-like stimulus of alternating black and white square checks that reverse in a regular phase frequency (Fig. 5.3). The pattern reversal VEP is the clinical VEP study of choice because it generates relatively consistent and vigorous responses from the visual cortex. The pattern reversal VEP has relatively high reproducibility within a subject and low variability of

Figure 5.3 Clinical VEP responses. At least one of the three standard VEP responses should be included in clinical VEP assessment. In most cases, the pattern reversal VEP is the study of choice because it generates relatively consistent and vigorous responses from the visual cortex.

waveform and peak latency across normal subjects. Steady fixation and best-corrected near visual acuity are required during testing.

The black-and-white checkerboard stimulus consists of equal number and size of alternating black and white squares. A fixation point is located at the center of the stimulus at the common corner of the central four checks. The luminance of the white squares should be at least 80 cd=m2 with a contrast of at least 75% compared to black squares. The

pattern stimulus is defined by the visual angle subtended by the side length of a single check. To calculate the visual angle, the check side length is divided by the distance from the stimulus center to the tested eye. The result is the tangent of the visual angle subtended by each check. Inverse tangent is then used to obtain the visual angle. The overall size of the stimulus should be greater than 15 at its narrowest dimension. The overall mean luminance should be uniform and remain stable during pattern reversal. The room is illuminated at a level approximately the same as the illuminance produced by the stimulus at the patient position.

60-min check stimulus will elicit more parafoveal response while the small 15-min check stimulus will elicit mostly foveal response. The reversal rate should be between 1 and 4 reversals per second, equivalent to a phase frequency of 0.5–2 Hz.

The pattern reversal VEP waveform consists of a series of negative (N) and positive (P) components designated by approximate latency (Fig. 5.4). Components N75, P100, and N135 are recognized. The amplitude of P100 is measured from the preceding negative peak N75 to the peak of P100. The latency is the time from stimulus onset to the peak of each component. The P100 peak has a latency of near 100 msec in normal subjects but its value depends on check size, check contrast, overall stimulus size, and pattern mean luminance.

Pattern Onset=Offset VEP

The pattern onset=offset VEP is elicited by a reversing checkerboard stimulus separated by regular periods of diffuse blank screen (Fig. 5.3). The checkerboard stimulus is the same as pattern reversal VEP, but the stimulus is periodically interrupted by a diffuse blank screen with the same luminance as the mean luminance of the checkerboard. For example, 200 msec of pattern reversal presentations are separated by 400 msec of isoluminant blank screen.

Pattern onset=offset VEP has greater variability across subjects than pattern reversal VEP and is less frequently

Figure 5.4 Normal standard pattern reversal VEP response. The pattern reversal VEP waveform consists of a series of negative (N) and positive (P) components designated by approximate latency. The amplitude of P100 is measured from the preceding negative peak N75 to the peak of P100. The latency is the time from stimulus onset to the peak of each component. The P100 peak has a latency of near 100 msec in normal subjects but its value depends on check size, check contrast, overall stimulus size, and pattern mean luminance.

performed. Pattern onset=offset VEP responses are less affected by poor fixation, and clinical applications include estimating potential visual acuity in preverbal children and VEP assessment in patients with nystagmus or poor fixation. Pattern onset=offset VEP is less susceptible than pattern reversal VEP to deliberate poor fixation, defocusing, or conscious suppression that may occur in patients with non-organic visual loss.

The pattern onset=offset VEP waveform consists of a series of positive and negative components designated by order of appearance (Fig. 5.5). Component C1, C2, and C3 are recognized. Positive peak C1 has a latency of approximately 75 msec, negative peak C2 has a latency of approximately 125 msec, and

Figure 5.5 Normal standard pattern onset=offset VEP. Positive C1, negative C2, and positive C3 have latencies of approximately 75, 125, and 150 msec, respectively, measured from stimulus onset. The amplitude of each component is measured from the preceding peak to the peak of the component. Only the pattern onset response is recorded with a recording time of 260 msec.

positive peak C3 has a latency of approximately 150 msec. The amplitude is measured from the preceding peak to the peak of the component, and latency is the time from stimulus onset to the peak of each component.

Flash VEP

The flash VEP is elicited by a flash stimulus as defined by the standard flash of the international ERG standard (see Chapter 1) (Fig. 5.3). The white flash stimulus can be delivered in a full-field dome in the presence of the light adapting (photopic) background from the ERG standard. The stimulus rate should not be high enough to elicit a steady-state response, and a typical rate of 2–3 Hz is commonly used to attain a transient response. Flash VEP is highly variable among subjects and are usually tested in persons who are unable to perform pattern VEP because of poor fixation due to poor visual acuity, nystagmus, or inability (Fig. 5.6).

The flash VEP waveform consists of a series of negative

(N) and positive (P) components designated by numerical order based on the timing sequence (Fig. 5.5). The most prominent