Aled L. Evans

Technical aspects of multifocal ERG recording

There are a wide range of variables which can influence the quality of the multifocal response. It is possible to place these variables into one of four categories. First, the method of stimulus delivery will determine the field of view, interference levels and the duration of on-state stimulation. Second, data acquisition variables such as electrode type and placement, amplifier specifications and filter bandwidth settings will have a direct impact on waveform shape and on the topographic distribution of signal amplitudes. Third, patient variables such as fixation, pupil dilation and refractive error will also contribute to the multifocal response. Fourth, there are many measurements that can be taken from multifocal recordings. In addition to standard amplitude and implicit time measures (the implicit time measure in the multifocal response is becoming increasingly important particularly in early stages of disease processes), the scalar product measure provides information on waveform shape. The conventional impulse and higher order responses will be different for different modes of stimulation such as Cathode Ray Tube (CRT) and Liquid Crystal Display (LCD) systems and latency shifts will be introduced if not corrected in software. Procedures which could lead to misleading interpretation include artefact rejection, averaging with neighbours and summing of responses. These procedures should be handled with caution.

Comparison of repeatability of the multifocal electroretinogram and Humphrey perimeter.

Functional mapping of the retina by multifocal electroretinographic recordings is now possible. We compared the normal range, repeatability and response topography of this new technique with conventional static Humphrey perimetry to assess its suitability in clinical practice. The multifocal technique was performed on 60 age-matched controls. Measures of repeatability and reproducibility were obtained. Results were then compared with those obtained from a customized perimetry test. In both tests the coefficients of repeatability were found to decrease with eccentricity. The inherent measurement variation between techniques was comparable. Overall system variation indicates that the technique could be a useful tool at the clinical level.

The effect of filter bandwidth on the multifocal electroretinogram

The effect of filter bandwith on the multifocal electroretinogram was assessed by means of theoretical calculation, electronic simulation and real multifocal electroretinogram recordings. Waveform distortion by high-pass filtering on simulated square waves, normal electroretinogram waveforms and negative electroretinogram waveforms was demonstrated. The theory of the effect of differentiation on electroretinographic waveform shape by electronic filtering indicates that little effect would be observed by changing the input filter cut-off for normal electroretinographic waveform shapes. However, negative electroretinogram waveforms are differentiated when the high-pass filter setting is increased. The differentiation effect artificially recreates a positive component that could be mistaken as a b-wave component. To eliminate this effect when recording multifocal electroretinograms, a high-pass filter setting of less than 1 Hz should be used to preserve the true electroretinographic waveform shape.

The multifocal ERG: unmasked by selective cross-correlation

The purpose of this paper is to provide the reader with a better insight into the mechanisms of multifocal ERG (mfERG) recording. The construction of the first and second order mfERG responses were examined by recovering the response to specific pulse trains embedded in the m-sequence.A custom built pc based multifocal system driving a LED stimulator was used to record a 61 element mfERG and a global ERG. The global ERG recording was used to enable the recovery of different pulse trains embedded in the m-sequence. Summation of these individual pulse trains was performed and the results compared with the standard full cross-correlation. An isolated pulse response is defined as a flash of light that has no other flashes within two m-sequence base periods before or after the flash. This isolated pulse response was recovered from the raw data and this response input into a simple superposition model to predict the waveform shape for specific pulse trains. The superposition model was compared with the actual selective cross-correlation for a particular pulse train. The summations of the selective cross-correlation components give identical responses to the full cross-correlation. The superposition model also predicts the waveform shapes recovered by the selective cross-correlation procedure. The mfERG response is a complex composite response from a number of different pulse trains. Examination of the individual waveform shapes provides some insight into the origin of the mfERG waveform. The main contributions to the P1 component are the same as for an isolated response and as with the standard ERG this component is likely to be dominated by the mid retina. The N1 component is also likely to have similar origins to that of the isolated response but the amplitude is dominated by contributions from pulse trains where there is no change of state and therefore includes a component from the interaction between two consecutive stimuli. The N2 component is a composite response dominated by the interaction between two successive stimuli two base periods apart and the P1 component of a second stimulus delayed one frame from the first stimulus.

A comparison of CRT and Digital stimulus delivery methods in the multifocal ERG

The purpose of this paper is to compare and evaluate the multifocal ERG response from raster based CRT and Digital Projection (LCD) stimulus delivery systems. A custom built p.c. based multifocal system was used to generate a 61 hexagonal element stimulus array. The stimulus was presented on a high luminance CRT display and on a back projected screen using a Digital polysilicon projection system. A fast response photodiode was used to analyse the stimulus pulse characteristics of both systems. A number of recordings were performed to assess the effect of stimulus delivery on a standard m-sequence response, inserted full-field filler response and on separation of onset and offset components. The pulse width for a CRT system is dependant on the type of phosphor and is typically 2 msec whereas the Digital Projection system produces a 13.3 msec pulse equivalent to the frame rate for the system. Slowing down the m-sequence by a factor of eight results in a pulse width of 106 msec which should enable the recovery of true offset responses. The CRT stimulus consists of a series of eight pulses of 2 msec duration each separated by 11.3 msec. First order responses are larger from the CRT system and second order responses larger from the Digital system. In conclusion, there are fundamental differences in the two delivery systems. The CRT system may have more potential in examining non-linear aspects of the multifocal response. Although both systems may be able to record offset responses, the Digital system will generate true offset responses whereas the CRT system may not allow true separation of these components.

The impact of fixation on the multifocal electroretinogram

There are a number of variables which can influence the quality of multifocal ERG waveforms. In common with visual field measurements, fixation quality may be an important parameter on the integrity of the acquired data. A low cost, fixation-monitoring device was used to assess fixation quality on a group of normal volunteers. Data was successfully acquired while five subjects viewed a fixation target for a period of time equal to that of a single multifocal recording segment. The target was presented on a stationary grey background and as the central fixation mark on a 61-element multifocal flicker stimulus. The results show no significant difference with or without the flickering pattern. The percentage of samples falling within 1.2° of the point of fixation was 51%. This suggests that fixation quality is adequate for scaled stimuli where the central element subtends 2.4°. High resolution stimuli of less than 2.4° may be more susceptible to fixation fluctuations during the recording process.

An instrument to investigate temporal processing mechanisms with the multifocal ERG

The multifocal ERG technique is a powerful method of studying the function of different areas of the retina. Display systems such as the CRT, which are commonly used for stimulation, are subject to limitations such as those imposed by the raster method of scanning. This work describes a novel stimulating display using LEDs that retains the established hexagonal areas but overcomes some of the limitations of the CRT display systems. The design and construction of the instrument is described together with some preliminary results.

Three-dimensional electromagnetic model of the human eye: advances towards the optimisation of electroretinographic signal detection.

Classical electromagnetic theory is used to examine the topographical variation in electrical potentials over the corneal surface resulting from specific retinal stimuli. Results from a three-dimensional mathematical model show that over 97% of calculated electromagnetic field potentials lie within 3% of previous analytical model data for an axially symmetric case. Maps of corneal potentials are produced that are shown to be characteristic of specific retinal stimuli and location. The maximum variation in corneal potential for a full field global stimulus is found to be approximately 1%. This is considered encouraging, as current electrophysiology techniques measure ocular potentials from a single corneal or scleral site, the position of which is often difficult to localise and reproduce. The model is used to simulate both central and peripheral stimuli and scotoma conditions. A 20° central scotoma simulation shows an overall reduction in central corneal potential of only 3%, whereas peripheral stimuli are found to cause up to 10% variations in this potential. There is therefore a possibility that a single recording site for multifocal retinal stimulation is not ideal. These data may be used to suggest more appropriate electrode recording positions for maximum signal recovery, not least in optimising signal detection for multi-focal electroretinography stimulation.

The effect of motion on pattern-onset visual evoked potentials in adults and children

Visual evoked potentials can be elicited by a variety of visual stimuli, including pattern-onset and motion-onset. It may be desirable to combine pattern-onset with motion-onset stimuli, for example, to make a direct comparison between optokinetic nystagmus and visual evoked potential acuity thresholds. Both procedures employ grating stimuli; however, the gratings must be moving to produce optokinetic nystagmus. We compared pattern-onset visual evoked potentials with both a static and a moving pattern to investigate the effect of motion on the pattern-onset visual evoked potential waveform. Visual evoked potential recordings were made from 10 adults (aged 20–37 years) and 10 children (aged 5–7 years) with the active electrode at Oz. Stimuli consisted of onset of high-contrast vertical bars of three sizes (12′, 30′ and 60′) both with and without motion (3 cycles/s). In a subgroup of subjects, visual evoked potentials were recorded to motion onset of constantly present gratings. Motion of the pattern had no significant effect on any of the latency components of the visual evoked potential waveform in adults or children. The amplitude of the C2–C3 component was significantly increased (p < 0.001) in adults. The motion appears to add a late negative component to the visual evoked potential similar to that produced by the motion-only stimulus. The latency of the early components of the pattern-onset visual evoked potential was unaffected by the presence of motion. Therefore, pattern-onset visual evoked potentials with moving gratings could be used to estimate visual acuity, and direct comparisons could be made between visual evoked potential and optokinetic nystagmus acuity thresholds with the use of the same stimulus parameters.

The Pocket Speech Aid

A rapid-access but limited vocabulary communication aid for the nonvocal and speech impaired is useful in some well defined situations. The design and construction of such a device is described. Three distinct situations have been identified where an aid is essential and an initial evaluation has shown the Pocket Speech Aid to be very successful.