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Figure 3.19. Schematic diagram of an OCT scanner

Figure 3.20. OCT scanner

interference data is converted to colour coded maps. A representative schematic diagram of a typical OCT scanner is shown in Figure 3.19.

A commercial OCT scanner is shown in Figure 3.20.

The technique is non-contact and non-invasive and is generally well tolerated by patients. Some sample images taken by an OCT scanner are shown in Figure 3.21.

3.8 Ocular Electrophysiology

Ocular electrophysiology comprises of a range of procedures that enable the visual pathway to be probed in an objective manner. The same equipment can be used for all of the procedures and this is described in the next section.

There are many commercial electrophysiology systems available. The key components of an electrophysiology system are incorporated in the schematic diagram of Figure 3.22. Analog signals are acquired from the subject from electrodes which are attached to the skin using conductive gels. For tests specific to the retina, an additional electrode will be inserted under the eyelid to touch the sclera or cornea. A pre-amplifier will be mounted close to the patient. The purpose of this

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Figure 3.23. Electrophysiology system in use

differential amplifier is to amplify the signals close to the generator source and this amplifier is usually d.c. coupled, of low gain, typically ×10 and with high common mode rejection ratio (CMMR) to reject common signals to both inputs such as extraneous noise. Following initial amplification, the filter bandwidth will be set to remove d.c. drifts and limit high frequency noise contributions. These filter settings will vary depending on the frequency components of the particular physiological signals to be recovered. The main amplifier is likely to include the filters and this amplifier will have high gain of around ×200,000. Once amplified the signals are digitised by an analog to digital convertor (ADC) which will be at least 12 bit resolution. The digital signals are processed by a digital signal processing (DSP) card. Many commercial systems will incorporate the ADC and DSP cards in the personal computer. The DSP card may perform signal conditioning, signal averaging and digital filtering. The personal computer is responsible for controlling the stimulus drivers and for ensuring synchronisation of the analog data with the stimulator. The PC will also display the data and perform the data analysis under user control. A variety of stimulators are available to present flash light or pattern stimulation of the visual system. These stimulators include standard CRT devices, LCD digital projection systems, light emitting diode (LED) displays or organic light emitting diode (OLED) displays).

A commercial electrophysiology system is shown in Figure 3.23.

3.8.1 The Electrooculogram (EOG)

A standing potential exists between the cornea and the retina and this potential is dependent on the level of light or dark adaptation. This potential gives an objective measure of the integrity of the retinal pigment epithelium layer of the retina. It is not possible to obtain a direct measure of this potential but indirect methods can be employed to recover this information.

We can consider this potential to act like a battery or a single dipole in the eye with the cornea being positive with respect to the retina. If we place electrodes either side of the eyes at the outer canthus (temple area) and at the inner canthus then we can measure a potential which is dependent on the direction of gaze.

Figure 3.24. Ganzfeld stimulator

Description of test

In order to enable repeatability, reproducibility and comparisons between different clinical and laboratory centres, the EOG procedure has been standardised by the International Society for Clinical Electrophysiology of Vision (ISCEV). The standard sets strict periods of dark and light adaptation and states fixed adaptation light levels.

The standard states that the test should be performed using a Ganzfeld bowl (Figure 3.24).

This ensures an even distribution of illumination across the retina and removes any hotspots that can occur if a standard light source is used. Commercial Ganfeld bowls have red fixation LEDs built in at fixation angles of ±30◦. These fixation lights are specifically for the EOG procedure and they can be turned on and off under computer control. At specific intervals (typically every minute) the subject will be asked to perform a series of alternating saccadic eye movements which usually consists of around eight cycles. These saccadic eye movements produce a trace as shown in Figure 3.25.

This trace is an idealised trace which assumes d.c. amplification, perfect eye movements and no impedance drift of the electrodes. This procedure is repeated every 1 min during a period of dark adaptation with the amplitude of the square wave pulse train dependent on the time of dark adaptation. At the end of a 10 min period, the trace amplitude will have fallen to a minimum value. The background illumination is then switched on and this light level is defined as 1.5–3.0 Cd m−2 in the international standard and the same procedure is followed for a further 10 min period. By the end of the light adaptation time the square wave pulse train will have shown a significant increase in amplitude.

The Arden ratio

If the amplitude of the square wave is measured and plotted on a graph as a function of time then a graph such as that shown in Figure 3.26 is obtained. This is a graph