Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

response is P2, and its amplitude is measured from the preceding negative peak N2 to the peak of P2. Latency of P2 is the time from stimulus onset to its peak and is near 100–120 msec in normal subjects. This nomenclature is recommended to automatically differentiate flash VEP components from pattern reversal and onset=offset components but is not universally followed. For example, P2 has been called P100 based on its approximate latency. Variations of size, timing, and shape of flash VEP components across normal subjects are commonly encountered.

Reporting VEP Results

Report of VEP results should specify which three standard responses were performed and whether the international standard was followed. Stimulus parameters such as check sizes should be stated. The polarity of the presentation of the waveform should be clearly labeled. Positive upward used commonly in ophthalmic literature is recommended rather than positive downward as used in neurologic literature. Amplitude and latency values should be available along with normative data.

SPECIALIZED VEP TECHNIQUES

Sweep VEP

The sweep VEP technique involves the recording of steadystate VEP activity to a high-frequency stimulus that lasts several seconds (13,14). The stimulus changes slowly with either increasing spatial or contrast frequency. In the sweep spatial frequency stimulus, the spatial frequency of a reversing stimulus is increased in steps while the rate of the reversal, that is, temporal frequency, remains unchanged. This type of stimulus usually consists of vertically oriented alternating black-and-white stripes whose width decreases in linear or logarithmic steps during recording. As the gratings go from coarse to fine, that is, increasing spatial frequency, a quasisinusoidal response is generated at the visual cortex, and the resultant VEP is recorded (Fig. 5.7). With this type of

Figure 5.7 Sweep VEP. The steady-state VEP response from a spatial linearly swept stimulus is shown. The stimulus consists of reversing vertically oriented alternating black-and-white stripes whose width decreases in linear steps during recording. As the gratings go from coarse to fine, that is, increasing spatial frequency, a quasi-sinusoidal steady-state VEP response is generated by the visual cortex. Three tracings are shown to demonstrate reproducibility. (From Ref. 14 with permission of Investigative Ophthalmology and Visual Science.)

rapid response retrieval, the steady VEP voltage is recorded without signal averaging. In the sweep contrast stimulus, the contrast of a reversing pattern is increased while the spatial and temporal frequencies remain unchanged. As the stimulus goes from faint to distinct, the VEP activity is recorded.

Sweep VEP is analyzed by measuring the VEP activity at the actual and harmonic temporal frequencies of the stimulus. In sweep spatial frequency VEP, the results are graphed by plotting the width of the gratings in cycles per degree against the corresponding VEP amplitudes to obtain the stimulus–response function (Fig. 5.7). Several studies have demonstrated that visual acuity can be estimated in infants and preverbal children based on extrapolating the graph

to obtain the highest spatial frequency that produces noise level amplitude (see Chapter 6) (15,16). Similarly, in sweep contrast frequency VEP, a stimulus–response function is generated by plotting the VEP amplitudes against the corresponding stimulus contrast.

Multifocal VEP

In multifocal VEP, VEP responses from multiple localized visual field areas are recorded simultaneously by using the same principle as the multifocal ERG (see Chapter 2). The result is a visual field map of VEP responses. The stimulus typically consists of an alternating black-and-white checkerboard pattern organized in sectors, which are scaled to account for cortical magnification (Fig. 5.8). During recording, the checkerboard elements of each visual field sector reverse in a pseudorandom maximum-length sequence (m-sequence) and have a probability of 0.5 of reversing on any frame change. To maintain overall isoluminance, at any given moment, about half of the total elements are white and half are black. The rate of frame change is in the order of 75 Hz. The VEP responses within each visual field sector are calculated and combined as the overall response for that visual field area. Although multifocal VEP may be recorded with one midline occipital electrode, recordings from multiple electrodes or channels reduce signal-to-noise ratio and improve the quality of multifocal VEP (17).

Summed multifocal VEPs are not equivalent to the conventional pattern-reversal VEP response (18). In contrast to conventional VEP, multifocal VEP from the upper visual field is reversed compared to multifocal VEP of the lower visual field (Fig. 5.8). This difference may be due to the fast multifocal VEP sequence that produces less contribution from the extrastriate cortex. When multifocal VEP responses from the lower visual field are summed, an initial negative component (C1) occurs at about 65 msec followed by a positive component (C2) at about 95 msec. The C1 and C2 components of the multifocal VEP are analogous to the N75 and P100 components of the conventional pattern VEP, but the C2

component is smaller and slightly faster than the conventional P100 component.

Multifocal VEP responses are variable among normal subjects due to anatomical differences of the cerebral cortex as well as the location of the external cranial bony landmarks that guide placement of the VEP electrodes (19). On the other hand, multifocal VEP responses from the two eyes of normal subjects are essentially identical except for small interocular difference in timing attributable to nasotemporal retinal differences so that interocular comparison of multifocal VEP may be helpful in identifying local monocular dysfunction (20).

Several authors have demonstrated a direct correlation between multifocal VEP responses and visual field defects such as those occurring in glaucoma (17,21–24). Whether multifocal VEP is more useful than conventional tests such as visual field remains to be determined. Because a majority of patients, who are poor visual field performers, are able to perform multifocal VEP, multifocal VEP may be helpful in patients who cannot perform reliable visual fields such as in those with non-organic visual loss. However, a few patients with normal automated visual fields cannot produce usable multifocal VEP recordings because of high noise level presumably due to a large EEG alpha contribution or scalp muscle tension or both.

Binocular Beat VEP

The binocular beat VEP technique assesses binocularity of the visual cortex by dichoptic luminance stimulation in which stimuli of different luminance frequencies are presented simultaneously to each eye (25). The stimulus consists of two uniform fields of equal average luminance but whose luminances are modulating sinusoidally at different temporal frequencies. During testing, the two eyes of the subject are stimulated simultaneously with each eye being stimulated by one of the two fields. As the two monocular stimulus fields fade in and out at two different rates, the two stimulus fields will come in and out of phase with each

other. The combined monocular signals at the normal binocular visual cortex will summate and cancel each other at a frequency known as the beat frequency, which is equal to the difference of the two luminance frequencies of the monocular stimuli (Fig. 5.9). The binocular response of the visual cortex is recordable as a VEP steady-state response at the selected binocular beat frequency. In subjects with impaired binocularity such as amblyopia, the cells of the visual cortex are predominantly monocular, and the binocular beat VEP is impaired (26–28).

Motion VEP

Several authors have reported VEP responses generated by a variety of motion stimuli consisting mostly of moving gratings or dot patterns. For example, the stimulus may alternate between moving in one direction and staying stationary to elicit a motion-onset response. Alternatively, a steady-state motion VEP response may be produced by a ran- dom-dot pattern which oscillates between phases of perceivable motion (coherent motion) and snowstorm (incoherent motion) (29). Motion VEP may also be generated by a stimulus which moves in alternating directions. Similar to conventional pattern reversal VEP, motion VEP typically has a positive peak (P1) at 100–120 msec followed by a negative peak (N2) at 160–200 msec, but the motion VEP waveform is more complex and variable. The motion VEP response is highly dependent on the characteristics of the stimulus such as stimulus type, contrast, and spatial frequency as well as the location of the recording and reference electrodes (30). In general, the motion response is represented by N2 while form processing is represented mostly by P1, and the motion response signals are located more laterally anatomically as compared to conventional VEP responses (31–35). Several authors have proposed that magnocellular pathway projecting to the cortical region V5 is involved (36); but other influences such as color sensitive motion mechanisms have also been implicated (37,38). The clinical utility of motion VEP still warrants further study. Motion VEP may be helpful

Figure 5.9 Principle of binocular beat VEP. Binocular beat VEP assesses binocularity of the visual cortex by dichoptic luminance stimulation in which stimuli of different luminance frequencies are presented simultaneously to each eye. The stimulus consists of two uniform fields of equal average luminance but whose luminances are modulating sinusoidally at different temporal frequencies. When two dichoptic uniform fields are sinusoidally modulated in luminance at frequencies fR in the right eye and fL in the left eye, the beat is manifested only after integration of the two separate monocular signals in binocular neural channels. This figure illustrates the simple linear combination of the two carrier frequencies, fR (18 Hz) and fL (20 Hz) to show a beat at the difference frequency, fR fL (2 Hz). Because the neural mechanisms evoking the actual beat perception are non-linear, the difference frequency (beat) is also accompanied in varying magnitudes by the sum of the two monocular frequencies (fR þ fL), as well as by harmonic combinations of the beat, sum and carrier frequencies. (From Ref. 25 with permission from Elsevier.)

in detecting defective binocularity in patients with infantile esotropia by demonstrating directional asymmetries of monocular VEP responses in which a nasalward vs. temporalward response bias occurs (39–41).

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