John B. Siegfried

Spectral sensitivity of human visual evoked cortical potentials: a new method and a comparison with psychophysical data.

Abstract VECP amplitude vs. stimulus intensity functions, upon which spectral sensitivity data is based, are generally quite variable and often non-linear. Sometimes, the amplitude-intensity functions are, in fact, non-monotonic. Under these conditions, choice of criterion amplitude can affect the resulting spectral sensitivity function. This study utilizes a technique of stimulus presentation and data analysis designed to reduce variability in the amplitude-intensity functions. This study is also concerned with the degree of correspondence between VECP data and psychophysical data. Thus, I have obtained both classes of measurement upon the same observers during the same experimental sessions. Results indicate low variability associated with data collected by this method. Clearly photopic functions were obtained with VECP data, which compare closely with psychophysical spectral sensitivity functions obtained under identical conditions.

Effects of meridional variation on steady-state visual evoked potentials.

Abstract Two experiments were carried out to assess the influence of meridional variations on the visually evoked potentials (VEPs) elicited by pattern alternation. The first experiment involved a comparison of the oblique effect obtained with grating and checkerboard stimuli. Greater amplitudes resulted using stimuli which contained vertically, as opposed to obliquely, oriented fundamental Fourier components. The second experiment revealed that the peak delay * of the VEP was markedly shorter when subjects wore cylindrical lenses oriented such that they emphasized a fundamental Fourier component of the checkerboard stimulus. These results underline the importance of describing complex patterned stimuli in terms of Fourier analysis when evaluating meridional aspects of visual function.

Spectral sensitivity of the human VER obtained with an alternating barred pattern.

Abstract Monopolarly recorded VERs from the occiput were elicited with an alternating barred pattern composed of chromatic lights selected from across the spectrum. The amplitudes and latencies of averaged VERs were related to a series of different radiance levels and spectral sensitivity curves were computed from criterion amplitude and latency measures. The resulting curves were in general agreement with psychophysically derived measures recorded in the same sessions and compared well to previously reported VER spectral sensitivity data. Local irregularities in the red were noted and discussed.

Effects of check size on visual cortex activation studied by functional magnetic resonance imaging.

We studied functional magnetic resonance imaging (fMRI) of visual cortex during checkerboard visual stimulation with three standard check sizes to examine whether activation in the visual cortex varied among these sizes. We acquired fMRI at 1.5 T in 8 normal subjects, each receiving the best refractive correction. Each subject underwent an experiment consisting of four conditions: black and white checkerboards with three check sizes (0.25-, 0.5-, and 1.0-degree) flickering at 8 Hz, and a black screen. SPM96 was used for a group data analysis with a random effects model after each of the subject’s data was motion-corrected and spatially normalized to a standard brain. The activation in the visual cortex showed the greatest signal changes with the 0.5-degree check among the three check sizes. When standard check sizes are used to stimulate visual cortex in fMRI experiments, our results suggest that 0.5-degree checks flickering at 8 Hz produce the most vigorous activation in visual cortex.

Early Potentials Evoked by Macular Stimulation: Optic Nerve Potentials?

Electrical potentials were recorded from the region of the human “temple”, and compared with simultaneously recorded ERG and VECP records. The properties of the “temple” recordings are such that they are not explained by volume conduction of ERG or VECP potentials, in these initial experiments.

Sensitivity and Specificity of Short-Duration Transient Visual Evoked Potentials (SD-tVEP) in Discriminating Normal From Glaucomatous Eyes

PURPOSE To evaluate the ability of the short-duration transient visual evoked potential (SD-tVEP) to discriminate between healthy eyes and eyes with early to advanced glaucomatous visual field loss. METHODS We tested 30 eyes of 30 healthy controls and 45 eyes of 35 glaucoma patients. Normal eyes had 20/30 or better visual acuity and normal 24-2 Swedish interactive thresholding algorithm (SITA) Standard visual fields. Glaucoma was staged as mild (mean deviation, MD > -6.0 dB), moderate (MD between -6.0 and -12.0 dB), and severe (MD < -12.0 dB). There were 15 eyes in each group. SD-tVEPs were recorded using the Diopsys NOVA-LX System. Each eye was stimulated with a low (Lc) and a high (Hc) Michelson contrast checkerboard pattern. Each test resulted in an Lc and an Hc SD-tVEP response. Each response was evaluated for overall waveform quality, P100 latency, and P100 amplitude referenced to the N75. The sensitivity, specificity, negative predictor value (NPV), and positive predictor value (PPV) were calculated. RESULTS Lc latency showed the highest accuracy for discrimination using receiver operating characteristic curves for high and low contrast parameters. The analysis for all subjects resulted in a 91.1% sensitivity, 93.3% specificity, 95.3% PPV, and an 87.5% NPV. Evaluating the mean Lc latency of the mild, moderate, and severe glaucoma patients against controls showed discrimination consistent with the glaucoma severity. CONCLUSIONS Short-duration transient VEP objectively identified decreased visual function and discriminated between healthy and glaucomatous eyes, and also showed good differentiation between healthy eyes and those with early visual field loss. VEP may be useful for early diagnosis of glaucoma.

High-frequency VEP wavelets in early infancy: A preliminary study☆

Abstract The flash-evoked visual evoked potential (VEP) was recorded in 12 infants 0–12 weeks of age. Along with traditional low-frequency waves (1–35 Hz), high-frequency (50–100 Hz) wavelets were examined. No wavelets were apparent in records obtained from infants below 4 weeks of age, although the low-frequency waves were clearly present at all ages tested. Wavelets showed a steady increase in amplitude and a small decrease in implicit time across the time period studied. The changes in wavelet development can be interpreted as a reflection of the significant postnatal maturation of the primary visual system, particularly the maculo and visual cortex.

The effects of checkerboard pattern check size on the VECP

Studies have shown that there is an optimal size of element in a checkerboard pattern, used to stimulate the visual system, for eliciting the visual evoked cortical potential (VECP). This size of element is between 8 and 25 min of visual angle. Since most studies have not reported fixation or location of fixation on the pattern, this study was performed in which fixation was specified at a location which ensures a constant illumination of the fovea regardless of check size. The resulting function, VECP amplitude vs. check size, resembles previously obtained functions, indicating either that previous subjects were fixating or that fixation is not important for these results.

Recording monocular VEPs under binocular conditions.

This report describes a technique for recording the visual evoked potential (VEP) arising from 1 eye under conditions of binocular pattern stimulation. The subject views a sinusoidally modulated vectographic checkerboard with orthogonally oriented polarizing filters before the eyes. This technique results in waveforms that contain monocular VEPs distributed in time. The technique is sensitive to interocular suppression and controls for the influence of various extraneous sources of variability. It is useful in clinical VEP testing.