Pattern electroretinogram and visual evoked potential amplitudes are influenced by different stimulus field sizes and scotomata

The pattern electroretinogram and the visual evoked potential were recorded simultaneously with various stimulus fields and artificial scotomata of increasing sizes. In contrast to an earlier study, a smaller check size (20) and two stimulus field sizes (20° × 20° and 10° × 10°) for the scotomata were used. With a concentric decreasing stimulus field, a reduction of both the pattern electroretinogram and visual evoked potential was found. Both showed a simultaneous reduction of amplitudes, but, compared with the amplitude in the full field, the reduction was more extensive for the pattern electroretinogram at each test field size. This implies a greater contribution to the pattern electroretinogram from more eccentric retinal parts. An artificial central scotoma of increasing size in the 20° × 20° field had less influence on the pattern electroretinogram than on the visual evoked potential. The percentage amplitude loss of the visual evoked potential was more pronounced. The visual evoked potential was eventually abolished by a scotoma size from 10° × 10° upward, while the pattern electroretinogram was still registrable. When scotomata of similar size were introduced in a smaller (10° × 10°) field, percentage pattern electroretinogram and visual evoked potential amplitude losses were less separated than in a larger (20° × 20°) test field.     

Effect of stimulus field size and localization on the binocular pattern reversal visual evoked response

The role of the central and peripheral stimulus fields on monocular and binocular amplitude and binocular summation of the pattern reversal visual evoked response were investigated. When the central stimulus field size was smaller than 2.4 °, there was no significant difference between the amplitude of the monocular and the binocular responses, but when it was equal to or larger than 3.2 ° × 3.2 °, the binocular amplitude was significantly larger than the monocular. The value of binocular summation was highest at the central stimulus field of 4.0 ° × 4.0 °; at larger sizes, there were no significant changes in the value. Use of a central stimulus field size larger than 3.2 ° × 3.2 ° was therefore considered a prerequisite for the effective assessment of visual function, especially binocular function, by means of the pattern reversal visual evoked response.With regard to the role of peripheral stimulus field on pattern reversal response, both the monocular and binocular responses, but particularly the latter, were found to be sensitive to a scotoma produced by covering the center of a full-field stimulus. The value of the binocular summation showed a significant reduction with a small central scotoma. We concluded that the pattern reversal visual evoked response is very sensitive to a central scotoma and that binocular function is mediated mainly through the central stimulus field.     

Monocular contribution to the peak time of the binocular pattern visual evoked potential

The contribution of each monocular pathway to the timing of the binocular pattern visual evoked potential was assessed in situations where a significant interocular timing discrepancy was observed. Monocular and binocular pattern visual evoked potentials to 0.5° checks were recorded from normal subjects, normal subjects in whom one eye was blurred, patients with monocular amblyopia, and patients with resolved unilateral optic neuritis. Normal subjects showed facilitation, while suppression was evidenced in subjects with monocular blurring. In patients with amblyopia, the affected pathway had no effect on binocular pattern visual evoked potential latency, suggesting that the amblyopic eye was suppressed. In contrast, all patients with optic neuritis showed binocular averaging. Our results show that different forms of binocular interaction are evidenced in normal subjects, in amblyopia and in optic neuritis, and suggest that a comparative analysis of monocular and binocular pattern visual evoked potential peak times brings valuable information to the clinical evaluation that could be used to distinguish disease processes further.Abbreviation BPVEP binocular pattern visual evoked potential     

Pattern electroretinogram,visual evoked potential and psychophysical functions in maculopathy

To compare pattern electroretinograms and visual evoked potentials with psychophysical examinations, such as visual acuity, static (automated) perimetry and color vision in unilateral maculopathies of various origins, 20 patients with unilateral retinal diseases within the macula and the posterior pole were tested. Pattern electroretinography, visual evoked potential testing and static perimetry (Octopus program M1) were performed with three different test field sizes (20° × 20°, 10° × 10° and 6° × 6°). The best correlation in all three test field sizes was found between visual acuity, static perimetry and visual evoked potential. This result is surprising, since central area defined functions (visual evoked potentials, visual acuity) correlated well with a total area integrating function (mean defect in static perimetry). The pattern electroretinogram, which seems to reflect an area-related function as well, showed a correlation to static perimetry only in the smaller 10° × 10° and 6° × 6° fields and not a significant correlation in the 20° × 20° field. Smaller stimulation fields may therefore produce sharper results in pattern electroretinographic testing. There was no correlation between pattern electroretinograms and visual evoked potentials or visual acuity. The pattern electroretinogram was recorded under monocular and binocular viewing conditions. In 60% of the patients, the amplitude of the affected eye was more reduced in the monocular than the binocular viewing condition; the healthy fellow eye controlled stable fixation of the affected eye more readily during binocular pattern electroretinogram registration. The degree of the color vision disturbance (C-index, desaturated panel D-15 test) did not correlate to any of the other examinations.Abbreviations C-index confusion index - MAR minimal angle of resolution - RCT retino-cortical time     

Luminance-unbalanced pattern onset-offset electroretinogram and visual evoked cortical potential

The effects of a luminance-unbalanced pattern onset-offset mode of stimulation on electroretinograms and visual evoked cortical potentials were investigated. With the use of originally devised software, only offset luminance was varied from 14.7 to 62.3 cd/m². A vertical grating pattern (1.5c/deg, 38.8 cd/m² mean luminance, 0.95 contrast) was presented for 260 ms and was absent for 260 ms to normal subjects. With an increase in the luminance level of the offset pattern from the lowest level, the amplitude of the onset electroretinogram increased by degrees, while that of the offset electroretinogram gradually decreased. Conversely, onset visual evoked cortical potential responses decreased gradually, and offset visual evoked cortical potentials increased correspondingly. Furthermore, the spatial tuning of the onset pattern electroretinogram was already ambiguous, even when there was only a 4-cd/m² difference between onset and offset pattern luminances. Thus, luminance control is indispensable for pattern onset-offset stimulation.     

On the search for an appropriate metric for reaction time to suprathreshold increments and decrements

Weber contrast, ΔL/L, is a widely used contrast metric for aperiodic stimuli. Zele, Cao & Pokorny [Zele, A. J., Cao, D., & Pokorny, J. (2007). Threshold units: A correct metric for reaction time? Vision Research, 47, 608-611] found that neither Weber contrast nor its transform to detection-threshold units equates human reaction times in response to luminance increments and decrements under selective rod stimulation. Here we show that their rod reaction times are equated when plotted against the spatial luminance ratio between the stimulus and its background (L_max/L_min, the larger and smaller of background and stimulus luminances). Similarly, reaction times to parafoveal S-cone selective increments and decrements from our previous studies [Murzac, A. (2004). A comparative study of the temporal characteristics of processing of S-cone incremental and decremental signals. PhD thesis, New Bulgarian University, Sofia, Murzac, A., & Vassilev, A. (2004). Reaction time to S-cone increments and decrements. In: 7th European conference on visual perception, Budapest, August 22-26. Perception, 33, 180 (Abstract).], are better described by the spatial luminance ratio than by Weber contrast. We assume that the type of stimulus detection by temporal (successive) luminance discrimination, by spatial (simultaneous) luminance discrimination or by both [Sperling, G., & Sondhi, M. M. (1968). Model for visual luminance discrimination and flicker detection. Journal of the Optical Society of America, 58, 1133-1145.] determines the appropriateness of one or other contrast metric for reaction time.     

Properties of visual evoked potentials to onset of movement on a television screen

In 80 subjects the dependence of movement-onset visual evoked potentials on some measures of stimulation was examined, and these responses were compared with pattern-reversal visual evoked potentials to verify the effectiveness of pattern movement application for visual evoked potential acquisition. Horizontally moving vertical gratings were generated on a television screen. The typical movement-onset reactions were characterized by one marked negative peak only, with a peak time between 140 and 200ms. In all subjects the sufficient stimulus duration for acquisition of movement-onset-related visual evoked potentials was 100ms; in some cases it was only 20ms. Higher velocity (5.6°/s) produced higher amplitudes of movement-onset visual evoked potentials than did the lower velocity (2.8°/s). In 80% of subjects, the more distinct reactions were found in the leads from lateral occipital areas (in 60% from the right hemisphere), with no correlation to handedness of subjects. Unlike pattern-reversal visual evoked potentials, the movement-onset responses tended to be larger to extramacular stimulation (annular target of 5°–9°) than to macular stimulation (circular target of 5° diameter).Abbreviation PREP pattern-reversal visual evoked potentials     

Motion adaptation in chromatic motion-onset visual evoked potentials

The aim of this study was to investigate the influence of motion adaptation on visual evoked potentials (VEPs) elicited by the onset of isoluminant chromatic motion. VEPs were recorded from the occipital cortex of human subjects using a sinusoidal grating stimulus of one cycle per degree which moved at either a velocity of 2 or 10°/s and subtended a field of 7° with a mean luminance of 30 cdm^–2. In the first experiment the effects of adaptation were investigated via the manipulation of the stimulus duty cycle which was varied between 11–90%. The results showed a significant (p < 0.001) reduction in the N2–P2 amplitude of the chromatic response. In contrast, P1–N2 amplitude was not significantly affected by motion adaptation. Subsequent experiments demonstrated that the chromatic motion onset VEP was attenuated not only following adaptation to isoluminant chromatic motion, but also to luminance motion as well. These results indicate that the chromatic motion onset VEP is just as susceptible as its luminance counterpart to motion after effects (MAEs) and as a result it is highly likely that it is a motion specific response. Furthermore, the fact that the VEP shows that there are cross-adaptation effects between motion defined by change in colour and by change in luminance, suggests that the two types of motion stimuli have inputs into a common motion mechanism.     

Color-coded pattern suppresses visual evoked cortical potentials and electroretinograms

We developed a new visual stimulating system for recording visual evoked cortical potentials and electroretinograms. The stimulus was a color checkerboard, in which each check kept its chromaticity but changed its luminance with its corresponding check. Color-coded pattern stimuli using red and green checks did not produce visual evoked cortical potentials, while yellow checks produced clear responses in a normal subject. Moreover, five color stairs from red and green to yellow showed only that the more colors are different, the smaller the visual evoked cortical potentials become. In addition electroretinogram recordings indicated that color-coded patterns behave in the same way as in visual evoked cortical potentials. The mechanism that causes the small color visual evoked cortical potentials may already be present in the retina. Color perception may be able to induce a suppression of responses for luminance contrast that appears to be formed already in the retina. Retinal responses were affected whether the stimulus field was color coded or not. Pattern electroretinograms appear to be more than the sum of local on and off responses.     

Induced intraocular light scatter and the sensitivity gradient of the normal visual field

The influence of intraocular light scatter on the perimetric sensitivity profile of the normal eye was investigated using a series of light-scattering cells containing 0.01%, 0.02% and 0.025% concentrations of 500 nm diameter latex beads. The degree of induced intraocular light scatter was quantified by measuring contrast sensitivity using the Nicolet CS2000 system in the presence and absence of both wide- and narrow-angle glare light. Perimetric sensitivity out to an eccentricity of 30° was assessed, using the Octopus 201 and the Dicon AP3000 automated perimeters, with the three light-scatter cells and in the cell-free control condition. The results for both functions were expressed as the difference between the control response and that recorded under the particular experimental condition. Perimetric attenuation increased with increase in intraocular light scatter; the extent of the attenuation varied with stimulus type, bowl luminance and eccentricityPresented in part at the 7th International Visual Field Symposium, Amsterdam, September 1986     

The influence of spatial frequency on the reaction times and evoked potentials recorded to grating pattern stimuli

The simple reaction times recorded to sine-wave and square-wave grating stimulus patterns of both constant physical contrast and of constant suprathreshold contrast were appreciably delayed by an increase in spatial frequency from 0.5 to 10 c/deg. There was no comparable increase, however, in the peak latency of the initial visual evoked potential component, C1, recorded to the same stimulus patterns. In view of the evidence that C1 has a striate cortical origin, these results suggest that the large spatial-frequency dependent variations in RT do not reflect delays of stimulus-induced neuronal responses in the primary visual pathway from retina to striate cortex.     

Colored focal visual evoked potentials by cathode ray tube versus scanning laser ophthalmosope

We compared the focal visual evoked potentials obtained in 52 young subjects with normal vision, evoked by means of three alternating black/color checkerboards generated by a trichromic cathode ray tube (dominant wavelength, 514 nm; colorimetric purity, 0.45) and by means of a scanning laser ophthalmoscope (argon laser beam, 514 nm; colorimetric purity, 1). These three checkerboards, with an area of 3.5° × 3.5° (stimulating the fovea), then with an area of 3.5° × 3.5° with a central exclusion of 1.5° × 1.5° (stimulating the perifoveola) and finally with an area of 1.5° × 1.5° (stimulating the foveola) were presented within a field (8° × 8°) of homogeneous luminance of 170 cd/m² and 1500 cd/m², respectively. Their check sizes were 30, with a reversal temporal frequency of 0.75 Hz. The transient focal visual evoked potentials recorded with these three stimuli generated by the two types of stimulators were clearly detected for at least 85% of subjects. Their characteristics (waveform, amplitude and culmination times of the different waves) were comparable, regardless of the stimulator used (cathode ray tube or scanning laser ophthalmoscope). These results suggest that, under these various conditions of luminance and colorimetric purity, the neurophysiologic circuits tested function in identical ways. The focal visual evoked potential signs, now clearly defined by means of stimuli generated by cathode ray tubes, therefore apparently can be applied to the focal visual evoked potential evoked by stimuli generated by the scanning laser ophthalmoscope.Abbreviations FVEP focal visual evoked potential - SLO scanning laser ophthalmoscope     

Effect of experimental scotoma size and shape on the binocular and monocular pattern visual evoked potential

A small experimental, central scotoma significantly attenuates the human pattern visual evoked potential. The steady-state pattern visual evoked potential was recorded from seven visually normal adults who viewed a reversing checkerboard with 24 checks and a central scotoma that varied in size and shape. We found that square scotomas had to be at least 3 × 3° to significantly (p < 0.05) attenuate the pattern visual evoked potential. Receptor density has been shown to be greater along the horizontal meridian than the vertical meridian. We hypothesized that this results in greater cortical representation of the horizontal meridian than the vertical meridian and, therefore, the pattern visual evoked potential might be significantly attenuated by a smaller rectangular scotoma oriented along the horizontal meridian than along the vertical meridian. One dimension of the rectangular scotoma was fixed at either 1° or 3°, while the other dimension was varied from 1° to 8°. The threshold scotoma size that significantly (p < 0.05) attenuated the pattern visual evoked potential was a horizontal scotoma subtending 1 × 4° and a vertical scotoma subtending 5 × 1° (vertical × horizontal). Meridional differences in cortical representation were not apparent to the larger scotoma series in which the fixed dimension subtended 3° (3 × 2° and 2 × 3°). Further analysis of the data revealed that the apparent meridional difference for the 1° scotoma series was a function of data variability. The determinant of the PVEP amplitude was scotoma area, not orientation. Monocular and binocular threshold scotoma sizes were the same, which could be due to the level of binocular summation demonstrated by our subjects.     

Low-frequency attenuation in the detection of gratings: Sorting out the artefacts

Human visual sensitivity was measured with sinusoidal gratings in which a constant number of periods were presented either (1) within a homogeneous surround, the luminance of which was the average luminance of the grating or. (2) within narrow dark borders or dark surrounds. In condition (1), sensitivity decreased steadily as spatial frequency was reduced from 8 c/deg, and was only affected slightly by the number of grating periods that were presented. Condition (2), which caused illusory luminance gradients across the stimulus, resulted in a substantial loss of sensitivity to intermediate spatial frequencies, which flattens the sensitivity vs frequency function. This effect should not be misinterpreted as an absence of low-frequency attenuation.     

Comparison of preoperative 10-Hz visual evoked potentials to contrast sensitivity and visual acuity after cataract extraction

Cataract patients whose surgical outcomes were in question were referred for testing by visual evoked potentials, elicited through closed eyelids by a luminance stimulus (flash) that appeared 10 times per second. Visual evoked potentials were rated as normal (predicted acuity of 20/50 or better) or abnormal (predicted acuity of 20/60 or worse). Postoperative Arden and Optronix contrast sensitivities and visual acuities were determined in 37 patients who had no intraoperative or early postoperative complications. Arden grating scores of less than 100 were rated as normal. The optimal and cutoff spatial frequency values were determined for the Optronix scores. Optimal and cutoff values of greater or equal to 1 c/deg and 12 c/deg, respectively, were rated as normal. Visual acuities were considered normal at 20/50 or better. Preoperative visual evoked potentials were quantitatively compared to the postoperative contrast sensitivities and visual acuities by 2 × 2 contingency tables. The accuracy of prediction was 79% for the visual acuities, 62% for the Optronix optimal values, 70% for the Optronix cutoff values and 62% for the Arden gratings.     

Simultaneously recorded retinal and cerebral potentials to windmill stimulation

Visual evoked retinal and cerebral potentials were recorded to onset rotation of an isoluminant sectored disc. While the retinal potentials recorded to onset rotation closely resembled the electroretinogram to a checkerboard or stripe pattern of fixed element size, the visual evoked potential changed interindividually and intraindividually from a fast positive wave at high contrasts, velocities and number of windmill segments to a later negative component at low contrasts, velocities and windmill segments. With change in luminance, contrast, speed and extent of rotation field size and number of disc segments, the visual evoked potential was generally less affected than the electroretinogram.     

Neural and psychophysical correlates of induced interocular transmission disparities

We investigated the influences of luminance and temporal asymmetries on the pattern visually evoked potential (pVEP) as a function of stimulus contrast. Monocular and binocular baseline steady-state (ss) and transient (t) pVEP's were recorded on 10 visually normal young adults using a reversing checkerboard pattern ranging in contrast from 4 to 65%. Neutral density (ND) filters were then placed before the right eye in 0.5 ND increments (maximum 3.5 ND), as monocular and binocular pVEP's were recorded. Visual acuity, brightness sense, and the Pulfrich effect were then measured to similar luminance and temporal asymmetries. Neural and psychophysical data were compared. Monocular visual acuity and pVEP results appeared to be luminance-dependent. Brightness sense, the Pulfrich effect, and binocular pVEP amplitudes appeared to be temporarlly dependent. The binocular pVEP amplitude seemed to be less sensitive to temporal asymmetries for lower contrast targets.     

Changes in reaction time and search time with background luminance in the mesopic range

Vision relies on both rod and cone signals over a large range of ambient illumination that encompasses a number of common situations. It is important, therefore, to understand how performance changes with light level in functional visual tasks. We measured reaction times and search times using achromatic targets to examine the relationship between latency and luminance contrast as a function of background luminance. Visual search was more robust to changes in luminance than reaction time; search performance could be made invariant by scaling the effects of contrast, but the range of reaction time changed significantly over the mesopic range. We also investigated the extent to which two mesopic visual performance models described the dependence of reaction time and search time on stimulus spectra, using coloured stimuli. The 'effective contrast' model that we examined described the spectral dependence of both reaction time and search time well. A model for mesopic luminous efficiency based on reaction times described the spectral dependence of each response only in conditions where there was little influence of chromatic signals.     

Contrast evoked responses in man

In man visually evoked responses (EP's) can be recorded that are specific to changes in spatial contrast and cannot be derived from luminance responses. No spatial contrast component could be demonstrated in the ERG. Contrast EP's depend on various parameters such as acuity, retinal location, size and configuration of spatial elements, time course of luminance change. They are affected by overlapping steady contrasts presented either monocularly or dichoptically. For a given condition the contrast EP is mainly determined by the instantaneous relative contrast irrespective whether this contrast is reached by an increase or decrease in luminance. The EP's to the appearance and disappearance of a pattern bear different relations to stimulus parameters and seem to originate from different populations of cortical cells. The responses to short appearances obey the contrast equivalent of Bloch's law and correlate with psychophysics.     

The diagnostic significance of the multifocal pattern visual evoked potential in glaucoma.

The concept of objective perimetry is an exciting one because it strives to assess glaucoma damage without relying on psychophysical testing. The recent introduction of multifocal stimulus recording has enhanced our ability to examine the human visual field using electrophysiology. A multifocal pattern visual evoked potential can now be recorded, testing up to 60 sites within the central 25 degrees. The test requires only that the subject fixate on a target, while a cortically scaled dartboard pattern stimulus undergoes pseudorandom alternation within each of the test segments. In its present configuration the test requires at least 8 minutes recording time per eye. Modified bipolar electrode positions are required to ensure that adequate signals are detected from all parts of the visual field. In glaucoma patients, pattern visual evoked potential amplitudes have been shown to reflect visual field loss with reduction of signal amplitude in the affected areas. This technique represents the first major step toward objective detection of visual field defects in glaucoma.