Effects of grating spatial orientation on visual evoked potentials and contrast sensitivity in multiple sclerosis (original) (raw)
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Objective measurement of contrast sensitivity function using contrast sweep visual evoked responses
British Journal of Ophthalmology, 1998
Aim/background-The contrast sensitivity function (CSF) measurement provides information that is not accessible by standard visual acuity determinations. The contrast sweep pattern reversal visual evoked responses (CSVER) technique was used to objectively measure the CSF in clinical practice. Methods-The contrast thresholds were measured at five spatial frequencies in 10 normal subjects. The CSVER were recorded with sinusoidally modulated vertical gratings at 10 contrast levels (96, 64, 48, 32, 16, 8, 4, 2, 1, and 0.5%) presented in five spatial frequencies (0.5, 1.0, 2.0, 4.0, and 8.0 cycles per degree). Each of 10 contrast levels was displayed for 2 seconds at the desired spatial frequency. The CSVER amplitudes at the second harmonic were calculated by discrete Fourier transform. The results were compared with those obtained using a psychophysical method. Results-An inverted U-shaped CSF which peaked at 2.0 cycles per degree with a contrast sensitivity of 34.5 (contrast, 2.9%) was observed. The CSF assessed electrophysiologically was 0.62 to 0.79 log units lower than the sensitivity measured using the psychophysical method. However, the overall shapes were highly correlated. Conclusion-One can objectively measure CSF with CSVER and this may be useful in patients in whom the psychophysical method is limited. (Br J Ophthalmol 1998;82:168-173)
Retinal and cortical evoked responses to chromatic contrast stimuli
Brain, 1996
// is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits (Ishihara). Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/ (red+green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and greenblack). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg~') modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However, the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast, suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour, suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.
Spatio-temporal Contrast Sensitivity in the Cardinal Directions of the Colour Space. A Review
Journal of Optometry, 2010
We review the psychophysics of the spatio-temporal contrast sensitivity in the cardinal directions of the colour space and their correlation with those neural characteristics of the visual system that limit the ability to perform contrast detection or pattern-resolution tasks. We focus our attention particularly on the influence of luminance level, spatial extent and spatial location of the stimuli-factors that determine the characteristics of the physiological mechanisms underlying detection. Optical factors do obviously play a role, but we will refer to them only briefly. Contrast sensitivity measurements are often used in clinical practice as a method to detect, at their early stages, a variety of pathologies affecting the visual system, but their usefulness is very limited due to several reasons. We suggest some considerations about stimuli characteristics that should be taken into account in order to improve the performance of this kind of measurement.
PURPOSE. We investigated the effects of image contrast, stimulus density, and viewing distance upon a multifocal steady-state visual evoked potential (MSV) method. METHODS. Fourteen adults with normal vision (mean age ¼ 27.0 6 6.6 years; 6 males) participated in the study. Each of the stimulus regions of the multifocal ensembles presented a contrast modulated grating, displaying spatial and temporal frequencies that evoke the spatial frequency doubling illusion. All subjects were tested at five contrasts: 0.06, 0.11, 0.22, 0.45, and 0.89; viewed at 16, 32, and 128 cm. A multivariate linear model estimated factors for each stimulus region, recording channel, number of stimuli (9 or 17 regions), and sex; and covariates for contrast, and octaves of viewing distance, and age. RESULTS. The responses per unit area for the 17-region display were significantly larger than for the 9-region display (P < 10 À12). The contrast-response function could be described by a power law with exponent 0.068 (P < 0.008). The effect of viewing distance was small but significant: response amplitude dropped by À0.17 6 0.03 dB per octave of viewing distance (P < 10 À6), or 10% over 8 octaves. CONCLUSIONS. The response per unit area indicated that cortical folding diminishes responses to larger stimuli. Viewing distance did not greatly affect response amplitude. This suggested that we can use similar, but scaled, stimuli to study central and peripheral disease. The rapidly saturating contrast responses imply that there would be nothing lost from testing at contrasts as low as 20% given that higher, saturating contrasts might mask visual field defects.
Measuring contrast sensitivity with inappropriate optical correction*
Ophthalmic and Physiological Optics, 2000
Spatial frequency-selective minima (notches) in the contrast sensitivity function (CSF) because of defocus can mimic those that occur with ocular disease. We examined the influence of measurement conditions on CSF shape in simulated clinical testing. CSF notches occurred with almost all levels of defocus for all subjects. Multiple notches were found under some conditions. Notches were found with defocus as small as 0.50 D. Effects of induced astigmatism depended on the orientation of the target. Notches were apparent in defocus conditions after stimulus size and room illuminance were modified and when subjects had insufficient accommodation to compensate for hypermetropic defocus. The equivalent of notches was not noted with the Pelli-Robson chart. As defocus-induced CSF notches may be mistaken for functional loss, careful refractive correction should be conducted prior to clinical or experimental CSF measurement, even at low spatial frequencies. ᭧