4.8.1 Electrodes and Electrode Positioning
In the recording o f a VEP it is necessary to ensure that electrodes are attached in the same position relative to the visual cortex in all subjects. All electrode placement systems rely on measurements taken from easily identifiable ‘bony landmarks’ on the skull i.e. the nasion (the bony ridge just above the nose), the inion (the ridge slightly above the base of the skull), and the preauricular notches (depressions located anterior to either ear). Some clinicians will position the electrodes at fixed measurements from these points, with the active electrode placed 5cm above the inion along the midline (Halliday et al, 1979). However, this does not allow for interindividual variability in head sizes. The more commonly used 10-20 system instead places electrodes according to a predetermined percentage of the distance between the bony landmarks (Harding, 1991). Different numbers and locations o f electrodes may be used to record different signals, but this system is consistent for all head sizes, and controls some
Chapter 4 Ocular Electrophysiology
of the intersubject variability inherent in VEP recording. Variability in responses can still be high, however, as a result of individual differences in cortical topography.
4.8.2 Pattern Stimuli
Pattern VEPs are generally recorded in response to a checkerboard, although sine- and square- wave stimuli may also be used. The checkerboard pattern VEP may be phase-reversed, such that white squares become black, and vice versa, at a rate determined by the stimulus frequency. The pattern may also be presented and then removed, to be replaced by a background of the same mean luminance as the pattern. In both cases the mean luminance of the display remains constant, ensuring that the VEP recorded is a visual response to the pattern itself, uncontaminated by luminance responses. Furthermore, since there is no change in the mean luminance o f the pattern stimulus, there is no excitation of the peripheral retina by time-locked stray light. Consequently, the pattern VEP response originates from the area of the retina stimulated by the checkerboard, with no scattered light response.
The VEP evoked to a pattern reversal checkerboard stimulus consists of three major components, the N75, P100, and N135 (See Figure 4.7). The initial letter denotes the polarity of the response (positive or negative), and the number indicates the approximate implicit time o f the peak. The P I00 is the most prominent, and most commonly assessed pattern VEP parameter.
Responses to patterned stimuli are strongly affected by optical factors, such as pupil diameter (the technique is performed undilated), refractive error and lens opacities.
N135 N75 +« P100 200 250 100 150 0 50 Time (ms)
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4.8,3 Flash Stimuli
Unlike the pattern VEP, the flash VEP is relatively unaffected by media opacities, as most opacities will scatter rather than absorb light. An amplitude reduction greater than 50%, or a response implicit time greater then 15 ms is, therefore, highly suggestive of central visual field dysfunction (Brigell, 2001). This makes the flash VEP a valuable tool in the assessment of retinal function in elderly patients with cataract.
The major disadvantage of the VEP recorded to a flash stimulus is the complexity o f the waveform. The flash VEP consists o f four negative, and three positive peaks, labelled N l, PI, N2, P2, N3, P3, N4, P4 (See Figure 4.8). As the flash VEP is often variable in waveform between individuals, it can be difficult to identify and analyse homologous waves.
Figure 4.8. The waveform o f the flash VEP (Brigell, 2001).
4.8,4 The Assessment o f Visual Evoked Potentials
Even when the components of the VEP are identified, assessment of the amplitude o f each response can be unreliable, with up to 25% intrasubject variability on different recording sessions (DeVoe, 1968). The implicit time o f the components, however, is found to be a more reliable indicator of dysfunction within the visual pathway. Normal intrasubject variability for implicit time of the pattern VEP P I00 component is between 2 and 5% (Oken et al, 1987). Although a prolonged implicit time of VEP components is generally associated with nerve demyelinating conditions of the visual pathway, such as multiple sclerosis, resulting in slowed signal transduction (Asselman et al, 1975), an increase is also reported in other conditions, such as central serous retinopathy (Sherman et al, 1986). In this case the increased implicit time is believed to be due to reduced efficacy o f cones in the absorption of photons of light, rather than in a slowed transmittance through the visual pathway (Smith et al, 1978).
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o 50 100 150 200 250
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4.8.5 Transient and Steady State Visual Evoked Potentials
The frequency o f stimulus presentation o f a VEP will also effect the waveform o f the response. As with the ERG, a response recorded to a high frequency stimulus (above 6-10 Hz) will be a steady state response. The transient VEP is recorded to lower stimulus frequencies, allowing analysis o f individual components.
4.8.6 The M ultifocal Visual Evoked Potential
This is a technique which has been developed as a means of assessing localised defects in the visual field (Sutter, 1992). A pseudorandom sequence o f stimuli (the m-sequence) is presented to record independent responses from multiple areas o f the visual field. These responses are then used to derive linear and non-linear components o f the VEP.
Betsuin et al (2001) recorded multifocal VEPs from 20 controls, and 6 subjects with AMD. When responses were analysed within the four quadrants o f the visual field tested, it was found that results corresponded to measured perimetric defects. A similar congruity between multifocal VEP and perimetry results has been reported previously (Klistomer et al, 1998). This suggests a possible value of the multifocal VEP in assessing localised defects of the central visual field.
4.8.7 The Dynamic Visual Evoked Potential
A further way in which the VEP has been used in the assessment o f macular integrity is in the monitoring of the recovery o f visual function following exposure to an intense light source. Parisi (2001) described an increased time to peak, and decreased amplitude of the major PI 00 component following a photopigment bleach. Both amplitude and implicit time can be recorded over time after the bleach until the VEP regains its baseline parameters. The recovery of VEP amplitude has been shown to take several times longer than the return of baseline visual acuity following a bleach. Both VEP and acuity recovery times have been shown to be age-dependent (Lovasik, 1983).
It has been claimed that the recovery time is dependent on the speed with which photopigment can regenerate. Any disease, therefore, which compromises the integrity of, or limits the
Chapter 4 Ocular Electrophysiology
Parisi {2001) described the dynamic VEP as being an objective, but not specific test of macular function as VEP recovery is found to be affected not only in diseases o f the outer retina, such as Stargardt’s disease {Parisi, 2002), and diabetes {Parisi, 1994), but also in conditions which have an effect on the inner retina or optic nerve head such as multiple sclerosis {Parisi et al, 1998), and primary open angle glaucoma {1992). The technique has not been used on subjects with age-related macular degeneration.