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Figure 3.31. Pattern ERG response
3.8.4 The Visual Evoked Cortical Potential
This procedure is used to obtain objective information from the end point of the visual pathway. The stimulus is a reversing checkerboard as previously described and is shown in Figure 3.30. Electrodes are placed at the back of the head over the visual cortex. A reference electrode is placed at a frontal position as indicated in the diagram. Usually, 64 or more serial averages are performed to recover the evoked potential from the background brain activity. The response is shown in Figure 3.32 with the key measurement in this case the time from pattern reversal to the P100 peak.
This is a measure of the transmission time for a signal to be processed by the retina and transmitted down the optic nerve to the visual cortex. The response is highly dominated by central visual function.
3.8.5 Multifocal Electrophysiology
In the past decade or so, a technique of multifocal electrophysiology has revolutionised objective clinical testing of the visual system. Although the standard tests do provide complementary information on the integrity of the various stages of visual processing, they are limited mainly by their global nature. In particular, the ERG is a mass response from the retina which is proportional to the number of photoreceptors in the retina. There is a large variation in the receptor numbers in the normal population and this limits the sensitivity of the test. The test cannot be used to evaluate macular function and is limited to the assessment of generalised retinal dysfunction. Localised retinal dysfunction will therefore not be detected.
Figure 3.32. Visual evoked cortical potential
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Figure 3.33. Multifocal ERG stimulus
The pattern ERG can give some information on macular function but this test is difficult to perform and gives a single measure of central retinal function. The VECP gives information on central visual function including optic nerve function but gives no information from the peripheral visual field.
The multifocal ERG involves simultaneous stimulation of many retinal sites. A typical multifocal ERG stimulus is shown in Figure 3.33. The stimulus is scaled in this way to take account of the variation in photoreceptor density across the retina.
Focal electroretinography involves the projection of a spot stimulus onto the retina but conventional signal averaging coupled with extremely small signal amplitudes make the test impractical if more than a single area is to be tested. The multifocal technique utilises a special form of pseudo-random binary sequence called m-sequences to stimulate a particular retinal site. These sequences have many useful mathematical properties with the important property being that shifted versions of the same sequence can be run simultaneously and the sequences and therefore the evoked potentials will be orthogonal or truly independent. The sequences are binary and can therefore be in one of two states at any step in time, this is usually black or white. The sequence length is variable but in practice a sequence of length 215−1 is the common length for multifocal ERG. The stimulus patches will therefore flicker in a random manner dependent on the control sequence which will run at the stimulation frame rate of 75 Hz. This means that the full sequence for 103 areas for two eyes can be delivered in 32,767 steps or around 7.5 min. This is a considerable saving on the standard ERG recording protocol which can usually take around 1 h due to the light and dark adaptation times. The recording is made from a single active ERG electrode as with the standard ERG and a cross-correlation is performed between the analog data and the particular sequence associated with the area stimulated to obtain a response from that area. The resultant trace array showing 103 small multifocal ERG responses is shown in Figure 3.34.
As with the standard electrophysiology waveforms, there are a number of amplitude and implicit time measurements associated with each waveform. However, the number of parameters for a particular test together with normative data quickly
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Figure 3.34. Multifocal ERG trace array
Figure 3.35. Scalar product plot
becomes unmanageable and can be difficult for an ophthalmologist to interpret. Fortunately, another measure known as the scalar product measure is available. This is a measure of deviation from an ideal template waveform and can be used to detect changes in amplitude and implicit time. If the scalar product values are divided by the area stimulated then a response density or scalar product plot can be derived. This plot corresponds to the characteristic hill of vision plots which shows higher function at the central retina where photoreceptor density is greatest. A normal scalar product plot is shown in Figure 3.35. The multifocal ERG
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Figure 3.36. Multifocal ERG from a patient with Stargardt’s disease (central retinal dysfunction)
Figure 3.37. Scalar product plot of Stargardt’s disease
in a disorder which affects central retinal function such as Stargardt’s disease is shown in Figures 3.36 and 3.37.
If a modified version of this stimulus is used for visual evoked cortical potential (VECP) measurement and electrodes placed at the back of the head over the visual cortex it is possible to recover multifocal VECP responses. A typical stimulus scaled for cortical magnification is shown in Figure 3.38 and a multifocal VECP trace array is shown in Figure 3.39.
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Figure 3.38. Multifocal VECP stimulus
Figure 3.39. Multifocal VECP trace array
The multifocal VECP technique gives clinical electrophysiology the potential of providing objective perimetry. However the technique is less well developed than the multifocal ERG and has several additional difficulties associated with it. VECP responses are variable in shape between individuals and they also vary across the visual field. This is due to different cortical orientations and different dipole source generators. By examining Figure 3.39, it can be seen that responses are smaller in the periphery and there are significant waveform shape changes across the field. Many groups are currently working on addressing these issues and it is hoped that these obstacles will eventually be overcome.