Search PubMed⌕ Search

Biomedical subjects

R F Hess

Publications and source records attributed to R F Hess.

At least 127 records · Page 7Linked to original sources
PubMed1987

Evidence for spatially local computations underlying discrimination of periodic patterns in fovea and periphery.

Human visual sensitivity for discriminating between, on the one hand stimuli composed of components (F) and (F + 3F) (compound detection), and on the other hand (F + 3F) and (F - 3F) (phase discrimination), was measured as a function both of stimulus contrast and eccentricity. Performance under these particular conditions was found to depend upon whether the (F) or (3F) component was dominant in the pattern. When the (F) component was high in contrast, visual performance was well modelled by an edge-blur discrimination, whereas when the (3F) component was high in contrast, visual performance was well modelled by a contrast discrimination involving local spatial features within each waveform. These conclusions were valid for both foveal and peripheral vision. The finding that these suprathreshold compound stimuli are discriminated on the basis of the local spatial features, and not on differences in their phase spectra as previously thought, allows a reinterpretation of the importance of phase coding in normal vision and of the selective loss of these discriminations that have been previously reported for peripheral vision and in amblyopia.

Adult↗
PubMed1987

Regional threshold contrast sensitivity within the central visual field in optic neuritis.

Contrast sensitivity was measured at nine locations within the central 10 degrees of the visual field in cases of recovered optic neuritis having varying degrees of residual deficit. A sample of 82 patches of visual field was obtained in 14 cases. Circular patches of vertically orientated sinusoidal gratings, 2.5 degrees in diameter, were used. The gratings were modulated in time at 8 Hz and the effect of spatial frequency on the threshold loss determined at each visual field location. As anticipated from what is known of visual field changes in the disorder there was considerable variation in the magnitude of the contrast threshold elevation at different locations in the visual field in any one case. The variability was more marked in cases with greater overall deficit. Three types of spatial loss were encountered. The most common was a loss which increased at higher spatial frequencies, found in 65 of the 82 patches of visual field examined. In 11 the loss was unaffected by spatial frequency and in the remaining 6 the loss was maximal at an intermediate spatial frequency. There was no instance of a loss maximal at low spatial frequencies. Overall the results indicate that sensitivity to higher spatial frequencies is more likely to be impaired following an attack of optic neuritis. In the combined results the effect of spatial frequency on the threshold elevation was statistically significant at all eccentricities (P less than 0.001). Analysis of the combined results revealed no difference in the mean contrast sensitivity loss at eccentricities of zero, 3.75 degrees or 7.5 degrees for intermediate and low spatial frequencies. There is no evidence from these results to suggest that the central foveal projection (papillomacular bundle) is more likely to be affected following an attack of optic neuritis than the projections of other eccentricities within the central 10 degrees as far as mechanisms subserving luminance vision are concerned at these spatial frequencies. Overall there was slightly greater reduction in acuity within the central 5 degrees than at 7.5 degrees eccentricity (P less than 0.05). This may be accounted for by the finding that higher spatial frequencies are more affected, rather than being related to eccentricity per se.

Demyelinating Diseases↗
PubMed1987

Regional variation of contrast sensitivity across the retina of the achromat: sensitivity of human rod vision.

1. Detection thresholds for two-dimensional Gabor functions of varying spatial and temporal frequency were used to investigate the post-receptoral sensitivity across the retina of the typical and complete achromat. 2. Under photopic conditions there is no evidence for post-receptoral cone function at any retinal eccentricity investigated. Sensitivity saturates in a way consistent with known psychophysical and electrophysiological measures of rod saturation. This occurs in a unitary fashion across the retina. 3. Under scotopic conditions the regional fall-off in spatio-temporal sensitivity is similar for the achromat and duplex retina. This suggests that the rods in the achromat make normal neural connections. 4. Taken together this supports the contention that the typical and complete achromat is a functional rod monochromat and hence can be used to explore the sensitivity of the isolated rod post-receptoral mechanism under mesopic conditions where its sensitivity is optimal. This is where its contribution is most difficult to isolate in the duplex retina. 5. For the human rod mechanism, mesopic post-receptoral sensitivity for all spatio-temporal stimuli is optimal in the central region of the retina and falls off as a function of eccentricity. 6. For localized stimuli, peripheral spatial sensitivity is reduced evenly at all spatial frequencies compared with that of the central retina. A similar displacement of the spatial sensitivity function of the rod mechanism occurs as illuminance is reduced. 7. For localized stimuli, temporal acuity of the rod mechanism is around 20-25 Hz irrespective of retinal position. As the illuminance is further lowered dynamics of the rod pathway are reduced irrespective of retinal position and the sensitivity function maintains a bandpass shape. 8. The regional distribution sensitivity of the rod mechanism changes as illuminance is reduced from mesopic to scotopic levels.

Color Vision Defects↗
PubMed1986

The pattern evoked electroretinogram in optic neuritis. A combined psychophysical and electrophysiological study.

The electroretinogram (ERG) and the occipital visual evoked potential (VEP) evoked by sinusoidal grating stimuli have been recorded in cases with a past history of optic neuritis. Although a significant reduction in pattern ERG amplitude was found in the patient group there was considerable overlap between the patient and the control populations. The pattern ERG was not found to be a sensitive test in providing evidence of a previous attack of optic neuritis. Reduction in VEP amplitude was considerably greater than was the reduction in ERG amplitude. The residual visual deficit in the patients was quantified by measuring contrast sensitivity. Overall, the greater the residual deficit the greater was the ERG abnormality, but in some individual patients it was found that the contrast sensitivity loss and the ERG amplitude reduction were not well correlated. No evidence was found for a latency change peripheral to the generators of the pattern ERG which might contribute to the VEP latency changes which occur following an attack of optic neuritis.

Electroretinography↗
PubMed1986

Spatial and temporal limits of vision in the achromat.

Threshold detection for sine-wave grating stimuli of varying spatial and temporal frequency was used to investigate the nature of spatial and temporal post-receptoral sensitivity in the typical, complete achromat. Threshold spatial and temporal sensitivities under low photopic conditions show no evidence of cone function. The abrupt fall-off in sensitivity is consistent with known psychophysical and electrophysiological measures of rod saturation. Threshold spatial and temporal sensitivities under scotopic conditions are identical in form and absolute sensitivity to that of the normal scotopic system. Spatial and temporal acuity measurements substantiate the above conclusions based on contrast sensitivity. We conclude that the typical and complete achromat possesses only a normal functioning rod post-receptoral mechanism.

Color Vision Defects↗
PubMed1986

Spatial and temporal properties of human rod vision in the achromat.

The spatial and temporal properties of rod vision were measured for stimuli at and above the detection threshold in an achromat whose spectral sensitivity, dark adaptation, spatial and temporal thresholds and Stiles-Crawford effect suggest the presence of only a normally functioning rod system. The properties of rod and cone vision were compared at illuminances where their respective sensitivities were optimum. The threshold spatial sensitivity of the rod mechanism under optimum illumination (180 scotopic trolands) exhibits bandpass properties with a peak sensitivity of around 80 at 0.5 cycles/deg and a spatial acuity of 6-7 cycles/deg. The threshold temporal sensitivity also exhibits bandpass properties under these conditions with a peak sensitivity of around 80 at 5 Hz and a temporal acuity of 30 Hz. For stimuli of low spatial frequency (less than 0.3 cycles/deg) and low temporal frequency, the threshold sensitivities of rod- and cone-mediated vision are identical. Rod- and cone-mediated vision display comparable spatial and temporal discrimination for targets of equal suprathreshold contrast over the low to mid spatial and temporal range that they share. Rod-mediated discriminations fall below those of cone vision above 1 cycle/deg for spatial judgements and above 15 Hz for temporal judgements. The number of discriminable steps in spatial frequency and temporal frequency at threshold is similar for rod and cone vision over the spatio-temporal frequency range that they share. Over this range rod- and cone-mediated vision can discriminate four steps in spatial frequency and one step in temporal frequency. These results suggest that rod vision shows comparable spatio-temporal discrimination performance to cone vision and that it is subserved by at least five spatial and two temporal labelled detectors. The response of the highest spatial frequency filter subserving rod vision extends from 0.5 to 6 cycles/deg.

Color Vision Defects↗
PubMed1986

Differences between electroretinograms of cat and primate.

We compared the electroretinogram (ERG) evoked by pattern and uniform field stimulation using steady-state analysis in cat, monkey, and human. Evidence is provided that the pattern-evoked ERG is different in cat and primate. In primate it exhibits a resonance at 8 Hz, a spatial band-pass characteristic, contrast linearity, and no scotopic component. None of these properties are seen in the response to 8-Hz modulation in cat. The ERG evoked by a sinusoidally modulated uniform field of light is composed of a fundamental and a second harmonic component. Although the properties of the fundamental response are similar in cat and primate, the second harmonic response exhibits important differences in its temporal response and luminance dependence. The correspondence between the properties of the pattern-evoked ERG and those of the second harmonic component of the uniform field stimulus in primates suggests a common generator that is different from that of the fundamental response to uniform field stimulation. These differences in the properties of the pattern ERG in cat and primate may suggest either a different generator in cat or one with substantially different properties. This should be taken into account in animal models for the generators of the human pattern ERG response.

Animals↗
PubMed1985

Differences in the neural basis of human amblyopia: the distribution of the anomaly across the visual field.

Spatio-temporal contrast sensitivities to horizontally-oriented Gaussian-weighted patches of sinusoidal grating stimuli were determined across the nasal and temporal visual fields of strabismic and non-strabismic, anisometropic amblyopes. The visual field distribution of the amblyopic anomaly differs in strabismic and non-strabismic, anisometropic eyes. In strabismus the peripheral region of one or both hemifields is spared; in non-strabismic, anisometropic cases the loss is evenly distributed across the binocular visual field but is not present in the monocular temporal field. These findings suggest that the non-strabismic forms of amblyopia in humans result from binocular competitive imbalance in early life. The strabismic results pose two problems for the present competitive model of amblyopia: in strabismus, amblyopia is mainly limited to central vision and shows an asymmetric distribution.

Adolescent↗
PubMed1985

Temporal frequency discrimination in human vision: evidence for an additional mechanism in the low spatial and high temporal frequency region.

Temporal frequency discrimination at and above the detection threshold has been studied using gratings of low (0.2 c/deg) and medium (2 c/deg) spatial frequencies. At 2 c/deg the results of previous investigators are confirmed: The results being consistent with the existence of two broadly tuned and directionally selective temporal mechanisms (up to 32 Hz). For the lower spatial frequency an additional temporal frequency discrimination at threshold can be made between 4 and 32 Hz and enhanced temporal frequency discrimination at suprathreshold levels occurs above 24 Hz. One interpretation of this result is the existence of one or more additional temporal mechanisms with restricted spatial acuity responding to higher temporal frequencies.

Discrimination, Psychological↗
PubMed1985

Temporal frequency discrimination in optic neuritis and multiple sclerosis.

The temporal contrast sensitivity function and temporal frequency discrimination have been studied in normals and in 7 cases of multiple sclerosis or optic neuritis. Sinusoidal gratings of two spatial frequencies (0.2 and 2.0 cycles/deg) were used. The abnormalities demonstrated in the patients varied between individuals but overall a picture has emerged of four principal anomalies. The temporal transfer function of the 2 cycles/deg medium spatial frequency grating has a band-pass appearance, whereas it is low-pass in normal subjects (5 out of 8 eyes). The peak of temporal frequency discrimination function (i.e. the temporal frequency at which discrimination is most acute) occurs at a lower temporal frequency in the patients than in normal subjects particularly at the higher spatial frequency (5 out of 8 eyes for a spatial frequency of 2.0 cycles/deg; 2 out of 8 eyes for a spatial frequency of 0.2 cycles/deg). It was found that normal subjects were able to discriminate between the maximum temporal frequency used as a standard in the discrimination experiments (16 Hz) and a higher temporal frequency. In the patients, however, a cut-off in the discrimination function occurred at a temporal frequency well below this (8 out of 8 eyes for a spatial frequency of 2.0 cycles/deg; 2 out of 8 eyes for a spatial frequency of 0.2 cycles/deg). Further to the above result, in a number of patients an attempt was made to ascertain how the gratings were perceived in this abnormally extensive region of ambiguity in the high temporal frequency range. It was found that high temporal frequencies, at a constant contrast level above threshold and constant apparent contrast, were perceived either as flickering at a slower rate than did an intermediate temporal frequency or even appeared stationary. This effect was rare at the lower spatial frequency (1 out of 8 eyes) but was demonstrated in 4 out of 8 eyes at the spatial frequency of 2.0 cycles/deg. It was not specifically sought in all individuals. These results are discussed in the context of an information channelling model of temporal frequency processing in the visual system.

Discrimination, Psychological↗
PubMed1985

Effects of flanking noise bands on the rate of growth of loudness of tones in normal and recruiting ears.

Five subjects with unilateral cochlear hearing impairments and three normally hearing subjects made loudness matches between tones presented alternately to two ears, as a function of the intensity of the tone in the impaired ear (or the left ear of the normal subjects). The impaired ears showed recruitment; the rate of growth of loudness with increasing intensity was more rapid in the impaired ear than the normal ear. Presenting the tone in the impaired ear with two noise bands on either side of the tone frequency, at a fixed signal-to-noise ratio, did not abolish the recruitment. This suggests that recruitment is not caused by an abnormally rapid spread of excitation in the peripheral auditory system. At low signal-to-noise ratios, a continuous background noise reduced the loudness of the tone more than a noise gated with the tone, suggesting that the continuous noise induces adaptation to the tone. The noise had a greater effect on the loudness of the tone in normal ears than in impaired ears. It is possible that the loudness reduction of the tone in noise is mediated by suppression; suppression is weak or absent in impaired ears, and so the loudness reduction is smaller.

Aged↗
PubMed1985

Contrast attenuation characteristics of iris clipped intraocular lens implants in situ.

In this study contrast thresholds were monitored over a wide range of spatial frequencies for patients with iris clipped implants or Fyodorov and Binkhorst types. These results were compared with those of an age matched normal group and attenuation characteristics derived for the implants and media environment alone. These results show that lenticular implants of this type can produce large reductions in contrast with a wide variety of transfer functions, for example, low pass, band reject, and band pass. These findings highlight the potential inadequacies of the assessment of the visual capabilities of these devices by acuity measures alone. We suggest that a more realistic and abbreviated approach should be based upon supplementing acuity assessment with monitoring of threshold sensitivity for a low, medium, and high spatial frequency sinewave grating.

Humans↗
PubMed1985

The pattern evoked electroretinogram: its variability in normals and its relationship to amblyopia.

Electroretinograms evoked by pattern stimuli (contrast reversing gratings) were measured under steady state conditions in the normal and amblyopic eyes of 14 amblyopic individuals having Snellen acuities in the range 20/100 to 20/600. These ERGs were measured as a function of spatial frequency, and compared with the psychophysical threshold losses to the same stimuli. In all cases the authors compared the normal and fellow amblyopic eye's response while taking into account the variability of right-left eye comparisons of normal individuals for these psychophysical and electrophysiological tests. When factors such as optical focus, fixation alignment, and fixation stability have been individually optimized, no pattern ERG deficit was observed in a spatial frequency range where there were obvious psychophysical deficits to the same stimuli. Our results do not substantiate previous claims of a pattern ERG anomaly in many severely amblyopic eyes.

Adolescent↗
PubMed1984

Assessment of retinal function in severely amblyopic individuals.

A quantitative psychophysical and electroretinographic assessment of severely amblyopic human eyes revealed no electrophysiological retinal analogue of the psychophysical deficit, when objectively refracted. Electroretinograms to pattern and uniform field flicker were normal in all but one amblyope. In this amblyope, although the electroretinographic deficit was uncorrelated with the psychophysical deficit, the reduced amplitude of the pattern-evoked response was also seen in the second harmonic component to uniform field stimulation. The deficit for each stimulus was only present at high signal levels. The similarity of the pattern evoked response and the second harmonic response to uniform field stimulation in normal and amblyopic eyes suggests a similar underlying process.

Adult↗
PubMed1984

Human pattern-evoked electroretinogram.

We have recorded electroretinographic (ERG) responses to grating patterns whose spatial, temporal, and contrast parameters were varied. The resultant evoked potential is dependent on spatial frequency and it exhibits a spatial band-pass characteristic. The peak frequency is dependent on retinal eccentricity. These findings are independent of temporal frequency, contrast, or mean luminance in the photopic range. These results suggest that the pattern ERG originates from a postreceptoral site.

Cerebral Cortex↗
PubMed1984

Linear and nonlinear components of human electroretinogram.

We compared electroretinographic (ERG) responses to uniform-field and a variety of pattern stimuli using both transient and steady-state analyses. Evidence is provided that for all of these stimuli, the peak at high temporal frequencies in the steady-state response corresponds to the fast wave of the transient response and that the peak at low temporal frequencies corresponds to the slow wave of the step response. A variety of contrast-modulated grating stimuli were used to demonstrate that the fast, high-frequency response can be regarded as the sum of two components, an "odd-symmetric" component, which behaves linearly and is independent of spatial frequency, and an "even-symmetric" component, which behaves nonlinearly and has a band-pass spatial-frequency dependence. The prevailing distinction that is made between pattern and uniform-field ERGs is a consequence of the fact that the uniform-field ERG is dominated by the odd-symmetric (linear) component, whereas the so-called pattern (contrast reversal) ERG reveals the even-symmetric (nonlinear) component in isolation. Since a uniform field can also drive the nonlinear component, the present dichotomy ("luminance" versus "pattern") can be better understood in terms of the linear and nonlinear components of the response rather than in terms of the stimuli that produce them.

Electroretinography↗