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Biomedical subjects

D M Levi

Publications and source records attributed to D M Levi.

At least 145 records · Page 8Linked to original sources
PubMed1983

Suprathreshold contrast perception in functional amblyopia.

Three experiments were conducted to investigate suprathreshold contrast perception of a group of strabismic and anisometropic amblyopes. In the first experiment, simple reaction-time was measured as a function of the contrast of sinusoidal gratings. At all contrast levels, the amblyopic eyes showed prolonged reaction-times compared to the non-amblyopic eyes. In the second experiment, the perceived contrast of suprathreshold sinusoidal gratings was measured using an interocular successive matching paradigm. The third experiment compared perceived contrast of the amblyopic and non-amblyopic eye by contrast magnitude estimation. The results of the matching and magnitude estimation experiments showed that while amblyopic observers show large losses of contrast sensitivity at threshold, at suprathreshold contrast levels perceived contrast is essentially normal or near normal in the amblyopic eye. Taken together, these experiments suggest that: 1) the amblyopic eye has a higher contrast gain than the non-amblyopic eye, and 2) the amblyopic eye shows prolonged response latencies. Similar results can be obtained in normal observers under conditions of peripheral viewing and noise masking.

Adult↗
PubMed1983

Spatial localization in normal and amblyopic vision.

Spatial localization was investigated for each eye of amblyopic observers using a bisection paradigm. The stimuli were comprised of a grating composed of bright lines, and a test line. The test line was either placed above the grating (bisection-no overlap) or within the row of lines comprising the grating (bisection-with overlap) and thresholds for each bisection task were measured as a function of the fundamental spatial frequency of the grating. Vernier thresholds were also measured. For the nonamblyopic eyes at low spatial frequencies, bisection thresholds were a constant fraction ("Weber" fraction) of the space to be bisected, while at high spatial frequencies thresholds were approximately a constant retinal distance (a hyperacuity). However the spatial localization of an amblyopic eye depends upon both the type of amblyopia, and the stimulus configuration. Specifically, for anisometropic amblyopia, spatial localization (bisection-no overlap) and vernier, when scaled to the resolution losses, were normal. However, spatial adjacency (bisection with overlap), while enhancing the spatial localization of nonamblyopic eyes at high spatial frequencies, markedly elevated thresholds in the amblyopic eyes of anisometropic amblyopes. Strabismic amblyopes on the other hand show disturbances in both spatial localization tasks which can not be accounted for on the basis of reduced resolution. Their results are characterized by an absence of a constant Weber fraction at low spatial frequencies and "crowding" effects at high spatial frequencies. For strabismic amblyopes, the optimal localization thresholds were similar to the Snellen threshold, while for anisometropic amblyopes, the optimal localization thresholds were several times better than the Snellen threshold.

Adult↗
PubMed1983

Psychophysical studies on the binocular processes of amblyopes.

It has frequently been assumed that humans with abnormal binocular vision resulting from amblyopia or strabismus lack binocular cortical neurons. From this assumption, it would be predicted that amblyopes would fail to show binocular interactions on any psychophysical tasks. This paper describes some recent psychophysical experiments investigating the binocular processes of amblyopes. The results of these experiments have shown that most humans with abnormal binocular vision fail to show binocular interactions for psychophysical tasks such as stereopsis or binocular summation, but show partially normal binocular interactions for interocular transfer of visual aftereffects and show normal interactions for dichoptic masking with grating stimuli.

Amblyopia↗
PubMed1982

Color vision is altered during the suppression phase of binocular rivalry.

Increment-threshold spectral sensitivity functions were determined during the dominance and suppression phases of binocular rivalry. The shapes of the functions obtained during the dominance phase exhibited three maxima at approximately 440, 530, and 610 nanometers and resembled functions obtained for nonrivalrous control conditions. However, the functions measured during suppression had a single broad peak near 555 nanometers and were adequately described by functions measured with flicker methods during nonrivalrous conditions. The results indicate that binocular rivalry differentially attenuates opponent-color information relative to achromatic information.

Color Perception↗
PubMed1982

Hyperacuity and amblyopia.

The most frequent cause of visual loss in childhood is functional amblyopia, an abnormality of visual acuity usually associated with either anisometropia (unequal refractive errors) or strabismus (turned eye) during early development. The usual clinical investigation of the visual acuity of amblyopes involves discrimination of the high contrast letters of a Snellen chart; however, there are other aspects of acuity, for example, grating acuity (the high spatial frequency limit of vision) and Vernier acuity (the smaller perceptible misalignment). Because of the extreme precision of Vernier acuity compared with either grafting or Snellen acuity, it is considered to be a form of hyperacuity which requires very precise positional information. In an effort to understand the nature of the neural abnormalities which cause the reduced acuity of amblyopes, we have measured here the Vernier acuity of amblyopic observers using an extended Vernier grating stimulus, and compared these results with their Snellen acuity and grating acuity. The results showed that different acuity losses are associated wih anisometropic versus strabismic amblyopia. When scaled with respect to their grating acuity, anisometropic amblyopes, like normals, showed hyperacuity, even at high spatial frequencies, while strabismic amblyopes showed severe losses in Vernier acuity. Snellen letter acuity showed a similar deficit relative to grating acuity in strabismic but not in anisometropic amblyopes. Contrary to some previous theories which have considered that all forms of amblyopia share a common neural basis, these results strongly support the view that different neural losses are associated with amblyopias of different aetiologies.

Amblyopia↗
PubMed1982

The visually evoked potential in humans with amblyopia: pseudorandom modulation of uniform field and sine-wave gratings.

The visually evoked potential was recorded in response to pseudorandom modulation of a uniform field and sine-wave gratings in humans with naturally occurring amblyopia. Analysis in the frequency domain showed similar temporal tuning when the nonamblyopic and amblyopic eye were stimulated with a uniform field and with spatial stimuli. Although most observers showed some reduction in the cortical response to stimulation of the amblyopic eye with uniform field modulation, all observers showed reductions in the response obtained for stimulation of the amblyopic eye with spatial stimuli. This decrease in the cortical response to stimulation of the amblyopic eye for sine-wave gratings was present at either the low and middle temporal frequencies or over the range of temporal frequencies tested, and was greater than that observed in those amblyopes who also showed reductions with uniform field modulation. Latency measures in the time domain showed increases in the response of the early components when the amblyopic eye was stimulated with a uniform field and with sine-wave gratings above 2 c/deg. These electrophysiological results confirm the results obtained psychophysically which suggest that although amblyopia is primarily a spatial anomaly, the response is influenced by the temporal attributes of the stimulus as well.

Adolescent↗
PubMed1982

Contrast increment thresholds of rhesus monkeys.

Increment contrast thresholds ware determined behaviourally as a function of background contrast for sinusoidal gratings with spatial frequencies between 0.5 and 8.0 c/deg. As has been previously reported for humans, the contrast discrimination functions ere characterized by a facilitation effect for low background contrasts and a masking effect for high background contrasts. The shapes of the functions varied with spatial frequency, but for all spatial frequencies, portion of the functions were adequately described by a power law. The shapes of the reaction time distributions obtained for near threshold stimuli suggested that at least two mechanisms with different spatio-temporal sensitivities were involved in detecting the stimulus and that the exact shape of the contrast discrimination function was dependent on which mechanism dominated detection.

Animals↗
PubMed1982

The persistence of dichoptically presented grating patterns.

A dichoptic viewing device was used to present sinusoidal gratings with interocular asynchrony. The persistence of various spatial frequencies was then determined psychophysically by using blank detection, for near-threshold and suprathreshold contrast conditions and across the contrast range. Greater persistence was evident for higher spatial frequencies, and across all spatial frequencies the values were higher for near-threshold contrast conditions. Contrast specificity was evident, with the low spatial frequencies showing flat contrast functions while the higher spatial frequencies showed a steeper monotonic curve. Intermediate spatial frequencies displayed biphasic properties. In view of the different neural persistence for different spatial frequencies, we investigated the degree of binocular summation for different interocular asynchronies. Binocular summation was predominant for all spatial frequencies where the dichoptic asynchrony was within 25 msec. Binocular summation decreased as a function of asynchrony. The binocular/monocular ratio decreased to a greater extent and earlier for low than for high spatial frequencies.

Depth Perception↗
PubMed1982

Psychophysical mechanisms in humans with amblyopia.

This study shows that the loss of contrast sensitivity in the amblyopic eye does not result from abnormal spatial frequency selective channels. Knowledge of the sine-wave sensitivity of the amblyopic eye may allow accurate predictions of the detection threshold for more complex stimuli. More effective amblyopia treatment strategies may be designed when the amblyope's sensitivity to spatial contrast and relative phase are better understood.

Amblyopia↗
PubMed1982

Binocular interactions in normal and anomalous binocular vision: effects of flicker.

Temporal modulation thresholds were determined for monocular viewing and for binocular viewing of stimuli presented in phase or in counterphase to each eye of observers with normal binocular vision and those lacking stereopsis. The results showed that in individuals with normal binocular vision sensitivity was much greater for in-phase than for counterphase stimulation at low temporal frequencies, but that this superiority declined at higher temporal frequencies. Averaged across frequencies, binocular sensitivity for in-phase stimulation was 40-50% higher than monocular sensitivity. In contrast, in the observers lacking stereopsis the ratios of binocular in-phase/monocular sensitivity averaged 1.02, and there were no significant differences in sensitivity to in-phase and counterphase stimulation. This failure of binocular integration at threshold does not result from differences in transmission time between the 2 eyes. However, while individuals lacking stereopsis showed an absence of binocular interaction for uniform-field flicker at threshold, they showed suprathreshold dichoptic temporal frequency masking which was similar to that found in normal persons.

Adult↗
PubMed1982

Psychophysical investigations of the temporal modulation sensitivity function in amblyopia: uniform field flicker.

The temporal modulation sensitivity function was studied in eight strabismic and/or anisometropic amblyopes and in two subjects with amblyopia resulting from monocular stimulus deprivation (cataract and ptosis). Half of the strabismic/anisometropic amblyopes showed a reduction in the sensitivity of the amblyopic eye. These differences were more marked at low and middle temporal frequencies. Two strabismic/anisometropic observers showed little difference in sensitivity between the two eyes, but in the remaining two observers the amblyopic eye was slightly more sensitive to low-frequency modulation than was the nonamblyopic eye. Four of the strabismic/anisometropic amblyopes also showed a small (5% to 10%) but statistically significant difference in the critical fusion frequency, with the sensitivity of the amblyopic eye being reduced. These differences in sensitivity were not related to the visual acuity loss. However, all differences in sensitivity between the two eyes were reduced or eliminated when the mean luminance of the test field was decreased by 2 log units form 32 to 0.32 cd/m2, or with suprathreshold stimulation. One of the two stimulus-deprivation amblyopes showed a decrease in sensitivity to low and middle temporal modulation frequencies similar to that found in four of in sensitivity to low and middle temporal modulation frequencies similar to similar to that found in four of the strabismic/anisometropic amblyopes, and the other observer showed a marked decrease in sensitivity at all temporal frequencies. The losses in temporal resolution of the amblyopic eye in both strabismic/anisometropic and stimulus-deprivation amblyopia are small in comparison with those reported in the spatial domain.

Adolescent↗
PubMed1982

Psychophysical investigations of the temporal modulation sensitivity function in amblyopia: spatiotemporal interactions.

Temporal modulation sensitivity functions were measured in response to counterphase sine-wave gratings of various spatial frequencies in seven strabismic and/or anisometropic amblyopes and in two stimulus-deprivation amblyopes. Both movement/flicker and pattern detection thresholds were determined. Strabismic and/or anisometropic amblyopes showed reduced sensitivity for movement/flicker and pattern detection in the amblyopic eyes, which were dependent on both the spatial and temporal frequencies of the stimulus and the detection criterion. At medium and high spatial frequencies, substantial reductions in movement/flicker detection were present, which were generally more marked for low temporal modulation frequencies. Reductions in pattern sensitivity were greater at the higher spatial frequencies, although some amblyopes showed losses in sensitivity at low spatial frequencies as well. The losses observed in the spatiotemporal transfer function of the amblyopic eyes were greater than those found with unstructured stimuli (uniform field flicker) and were not eliminated under reduced luminance. In subjects with amblyopia caused by stimulus deprivation, losses in spatiotemporal contrast sensitivity were either invariant across temporal frequencies or increased at higher rates of temporal modulations.

Amblyopia↗
PubMed1982

Differences in vernier discrimination for grating between strabismic and anisometropic amblyopes.

Two gratings composed of lines were displayed one above the other, and the vernier threshold was measured as a function of spatial frequency and vertical separation between gratings for 12 strabismic and/or anisometropic amblyopes. The vernier acuity of the anisometropic amblyopes was similar to that of nonamblyopic eyes when scaled proportionally to their reduced grating acuity. Strabismic amblyopes, on the other hand, have poorer vernier acuity than might be predicted from their grating resolution, even at low spatial frequencies. In addition, the strabismic amblyopes showed "crowding effects" for vernier gratings well within their resolution limit, while anisometropic amblyopes, like normals, showed no such effects.

Adult↗
PubMed1982

Spatial processing of complex stimuli in the amblyopic visual system.

Thresholds for detection and discrimination of the polarity (phase) of repetitive gratings with a ramp luminance profile were compared to sine-wave thresholds in normal and amblyopic observers. In the high-frequency range (4 cy/deg), normal observers detected ramp-wave gratings when the contrast of the fundamental spatial frequency was close to its independent threshold and discriminated the polarity (phase) of the ramp when the second harmonic reached its independent threshold. For the amblyopic eyes, detection of the ramp also occurred when the contrast of the fundamental frequency was near its independent threshold. In contrast, discrimination of the polarity (phase) of the ramp required contrast levels 2 to 10 times greater than needed to detect the second harmonic. The reduced ability of the amblyopic eye to discriminate the polarity of the ramp represents an abnormality in phase processing and appears to be roughly proportional to athe reduced optotype acuity of the amblyopic eyes.

Amblyopia↗
PubMed1981

Edge sensitive mechanisms in humans with abnormal visual experience.

Detection of broadband, aperiodic stimuli (edges) was investigated in normal observers, and in observers with abnormal visual experience which resulted in amblyopia. The spatial properties of the mechanisms used to detect an edge were investigated by a method of subthreshold addition. The method involved the determination of the threshold contrast for detecting an edge in the presence of a subthreshold line at various distances from the edge. In normal eyes, the one dimensional sensitivity profile of the edge detecting mechanism was: (1) approximately antisymmetric, (2) very localized, with sensitivity changes restricted to +/- 6'--8' on either side of the edge, and (3) phase dependent, showing an abrupt change in sign between +/- 1.5'. The sensitivity profiles of the amblyopic eyes were also approximately antisymmetric and showed the same steep rate of change from plus to minus as the fellow (nonamblyopic) eyes. However, in every case, the spatial extent of the profile was much broader than that of the nonamblyopic eyes. In normal eyes, the narrowest edge sensitivity profile was associated with the fovea; however, in two amblyopes with eccentric fixation, the narrowest edge sensitivity profile coincided with the locus of eccentric fixation. Moreover, the grating sensitivity function of the edge detecting mechanism of the amblyopic eye was similar to that of the non-amblyopic eye, but was shifted toward lower spatial frequencies. Control experiments show that these results are not accounted for on the basis of optics, eccentric fixation, or abnormal eye movements. The findings are discussed in terms of current models for the detection of aperiodic stimuli, and in the context of animal models of amblyopia.

Adult↗