VEP assessment of visual function.
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Evoked Potentials↗
Biomedical subjects
D M Levi
Publications and source records attributed to D M Levi.
Flicker masking in spatial vision.
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Afterimage↗
Binocular interactions in normal and anomalous binocular vision.
Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.
Adult↗
Contrast sensitivity in amblyopia due to stimulus deprivation.
Contrast sensitivity functions for sinusoidal gratings of varying spatial frequency and stimulus duration were determined for both eyes of 2 patients with amblyopia due to early occlusion and lid closure. The amblyopic eyes showed reduced contrast sensitivity over a wide range of spatial frequencies and stimulus durations, and the temporal integration time of the amblyopic eye was increased by comparison with the non-amblyopic eye at high spatial frequencies. When the gratings were flickered at 10 Hz the sensitivity for both flicker and pattern detection was reduced in the amblyopic eye over the entire spatial frequency range. Abnormal flicker perception by the amblyopic eyes was also evident in the reduced photopic luminosity functions of these patients.
Adult↗
Disturbances of small-field horizontal and vertical optokinetic nystagmus in amblyopia.
Horizontal and vertical small-field optokinetic nystagmus (OKN) were examined in persons with strabismic or anisometropic amblyopia. Reduced velocity for the slow phase of OKN driven by temporalward and upward target motion presented monocularly was observed in both the amblyopic and nonamblyopic eyes of some subjects. Several experiments were conducted in search of a sensory disturbance of perceived motion sensitivity which could account for the abnormal OKN. Comparisons between the frequency response for OKN and the contrast sensitivity function for perceived motion revealed that amblyopes with asymmetric OKN had equal sensitivity to nasal and temporal target motion. Contrast thresholds for driving the temporal slow phase of OKN were elevated by over 1 log unit above contrast thresholds for perceived temporal target motion, whereas contrast thresholds for stimulating nasal movement and driving the nasalward slow phase of OKN were equal. Contrast sensitivity to nasal and temporal target motion was symmetrical at the fovea and parafovea of the amblyopic eye. These studies did not reveal a sensory a /mso observed in both eyes of persons with congenital strabismus without amblyopia and in the nondeprived eye in monocular congenital cataract. These observations suggest a relationship between directional asymmetries of OKN and the incomplete development of binocular vision.
Adult↗
Direction selectivity for perceived motion in strabismic and anisometropoic amblyopia.
Interactions between direction-selective mechanisms for motion were examined in strabismic and anisometropic amblyopia. Amblyopes were found to have a marked reduction in sensitivity to flicker or movement when the amblyopic eye was presented with a combination of two gratings drifting in opposite directions at the same velocity (i.e., a counterphase grating). Sensitivity to flicker of the counterphase grating was improved by preadapting the amblyopic eyes to a single grating which was drifting nasalward, but there was no improvement if the drift of the adapting grating was temporalward. Sensitivity to flicker was also improved by reducing field size from 10 degrees to 3 degrees. However, occluding the central 4 degrees of the 10 degrees field did not significantly alter the sensitivity to flicker. These data suggest that inhibition between motion detectors in the parafovea of amblyopic eyes reduces slow-motion perception in both the parafovea and the fovea. The results also demonstrated a reduction in amblyopic eyes of direction slectivity for motion detection.
Amblyopia↗
Humans deprived of normal binocular vision have binocular interactions tuned to size and orientation.
A suprathreshold grating presented to one eye elevated the threshold for the discrimination of gratings similar in size and orientation presented to the fellow eye. The magnitude and stimulus specificity of these binocular interactions in human observers with normal bioncular vision were similar to those in observers deprived of normal binocular visual experience; however, the latter showed a failure of binocular summation at threshold or subthreshold contract levels. Whereas strabismus or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections seem not to have been disrupted.
Amblyopia↗
Rapid assessment of visual function: an electronic sweep technique for the pattern visual evoked potential.
We have developed an electronic spatial frequency sweep technique for electrophysiological assessment of visual acuity and pattern vision. The technique allows an accurate and reliable measurement of VEPs to a full range of spatial frequencies in just 10 sec. Because the measurements are so rapid, the technique suggests several new improvements in the assessment of visual function. Sweeping spatial frequency linearly and extrapolating the high-frequency region of the VEP spatial-tuning function to zero voltage allows an estimate of acuity which correlates highly with psychophysical estimates of acuity. Variants of the procedure are appropriate for the assessment of refracture error, determination of equality of visual function for the two eyes and of binocular interactions, and for sequential assessment of therapeutic conditions.
Amblyopia↗
Suprathreshold spatial frequency detection and binocular interaction in strabismic and anisometropic amblyopia.
We have investigated suprathreshold contrast sensitivity and binocular interactions in strabismic and anisometropic amblyopes using a reaction time paradigm. For every spatial frequency, reaction time increased as the grating contrast decreased. At all spatial frequencies and contrast values the reaction times using the amblyopic eye were prolonged compared to the nonamblyopic eye, but most markedly at high spatial frequencies. In the middle range of spatial frequencies, the contrast vs. reaction time function for the nonamblyopic eyes was biphasic, suggesting that two separate mechanisms detect gratings at high and low contrast levels. These functions in deep amblyopia were monotonic, and in shallow amblyopia the break in the functions was present but shifted to lower contrast levels. Binocular interaction experiments showed that binocular summation was absent at all contrast levels, but binocular occlusion was evident at high contrast levels for amblyopic observers.
Adolescent↗
The blindspot syndrome.
Constant esotropia, with its accompanying sensorial anomalies, generally presents a poor prognosis for functional recovery of binocular vision. The blindspot syndrome presents an exception to this. The present paper suggests methods for the diagnosis and an approach to the management of this condition.
Child↗
Reaction time as a measure of suprathreshold grating detection.
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Form Perception↗
A sensory mechanism for amblyopia: psychophysical studies.
Psychophysical investigations of the effect of amblyopic process on the sustained and transient channels in the afferent visual pathway are reported. The experiments on photopic luminosity, increment-threshold spectral sensitivity, spatial contrast sensitivity, and reaction-time measures of suprathreshold grating detection provide converging evidence that in naturally occuring amblyopia of humans, the sustained neural channels are more severely affected than the transient channels. However, it appears that the sustained channels are not totally nonfunctional, but rather have a higher intensity requirement than normal.
Amblyopia↗
A sensory mechanism for amblyopia: electrophysiological studies.
Steady-state visual evoked potentials were elicited by the appearance/disappearance of sinusoidal gratings of varying spatial frequency for both eyes of amblyopic subjects at temporal frequences from 2--32 Hz. The amblyopic eyes showed reduced responses relative to those of the nonamblyopic eyes across a wide range of spatial frequencies, the difference between the 2 eyes being most marked for high spatial frequencies and low temporal frequencies. Both the nonamblyopic and the amblyopic eyes showed a linear relation between the evoked potential amplitude and the logarithm of the stimulus contrast; however, the slope of the line was flatter for the amblyopic eyes. Possible neural mechanisms for the amblyopic eyes' reduced responses to spatial contrast are considered.
Adolescent↗
Contrast evoked potentials in strabismic and anisometropic amblyopia.
Steady-state visual evoked potentials were elicited by the appearance/disappearance of sinusoidal gratings of varying spatial frequency for both eyes of amblyopic subjects. The amblyopic eyes showed reduced responses and a phase shift relative to the nonamblyopic eyes across a wide range of spatial frequencies. Both the nonamblyopic and amblyopic eyes showed a linear relationship between the evoked potential amplitude and the logarithm of the stimulus contrast; however, the slope of the line was flatter for the amblyopic eyes.
Adolescent↗
Increment threshold spectral sensitivity in anisometropic amblyopia.
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Adaptation, Ocular↗
Visual evoked responses in strabismic and anisometropic amblyopia: effects of check size and retinal locus.
Sinusoidally modulated checker board patterns were used to investigate the effects of check size and retinal location on the "steady-state" visual evoked response (VER) of strabismic and anisometropic amblyopes. Fourier analysis of the averaged VER suggests that the waveforms produced by these techniques are complex, and that they consist of a fundamental component plus 1 or more harmonics. In all of the subjects, the VERs elicited from the amblyopic eyes by small checks in a 3.5-deg disc were significantly smaller in amplitude than those from the nonamblyopic eyes, whereas the responses to larger checks were far more similar. Large annular targets were quite effective in eliciting VERs from the peripheries of both amblyopic and nonamblyopic eyes, and these responses were at times as large as, or larger than, those produced by the central targets. Statistically no significant differences were found between the responses of the amblyopic and the nonamblyopic eyes to the annular targets.
Adolescent↗
Spatio-temporal interactions in anisometropic and strabismic amblyopia.
Contrast sensitivity functions for sinusoidal gratings of different spatial frequencies and stimulus durations have been determined for both eyes of amblyopic subjects. For both long and short stimulus durations, the entire contrast sensitivity function of the amblyopic eyes is lower than that of the nonamblyopic eyes. When the gratings were flickered at 10 Hz, the sensitivity for pattern and flicker detection were similar for both eyes at low spatial frequencies. Possible neural mechanisms for the differences in contrast sensitivity are considered with respect to recent psychophysical and electrophysiological evidence for channels with different spatio-temporal properties in the mammalian visual system.
Adult↗
National health and optometry: a follow-up study.
A stratified probability sample of the Houston, Texas population was interviewed with a previously described questionnaire (Allen, D., and D. Levi, National Health and Optometry--a survey of public attitudes. Am. J. Optom., 52 (4): 291-300, (1975) to assess present opinion of national health care and public awareness of vision care and its role in a national health care system. Of the population surveyed, 72% favored a national health care bill, some 10% lower than reported 1 year previously. A large percentage of the population sampled favored inclusion of vision care. The results are described in terms of demographic trends for those who have received optometric and ophthalmological care.
Dental Care↗