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Biomedical subjects

R F Hess

Publications and source records attributed to R F Hess.

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

Contrast vision and optic neuritis: neural blurring.

In order to understand the vision of patients with long standing optic neuritis, contrast perception was analysed in some detail for three patients with uniocular anomalies. Contrast detection and contrast matching experiments each demonstrate anomalies in contrast processing for eyes with optic neuritis. In this regard, optic neuritis differs from all other visual anomalies so far investigated where the suprathreshold deficits (from matching experiments) are slight compared with those measured under threshold conditions (from detection experiments). Contrast discrimination is also impaired to a mild degree in optic neuritis but this cannot be predicted on the basis of the matching results. These results bear upon the currently proposed models for contrast coding in the normal visual system as well as the nature of the underlying pathology in optic neuritis.

Adult↗
PubMed1983

The effect of temporal frequency variation on threshold contrast sensitivity deficits in optic neuritis.

The effect of increasing temporal frequency on contrast sensitivity anomalies in unilateral optic neuritis has been investigated. For 4 c/deg gratings no change in the deficit was observed at any temporal frequency whereas there was a tendency for the deficit to decrease with increase in temporal frequency for 0-5 c/deg gratings. The latter effect was not observed in a case of optic neuritis with severe deficit and not in two cases of other optic nerve pathology. An explanation based on the assumption that "threshold scotomata" might be present in cases of demyelinating optic nerve disease is proposed.

Adult↗
PubMed1982

The discriminability of spatial phase relationships in amblyopia.

Previous work has shown that anomalies in the contrast sensitivity functions of amblyopes are often insufficient to explain the degree of visual deficit in more complex tasks. Our stimuli were compound gratings composed of a fundamental and its third harmonic, added in either square-wave or triangle-wave phase. At medium to high spatial frequencies we find that many amblyopes, unlike normal observers, are unable to distinguish between such gratings which have identical power spectra, but different phase spectra. In this frequency range they can, however, easily distinguish between a compound grating and its fundamental component alone. It seems that in amblyopia visual processing occurs over a more truncated frequency range than is implied by detection experiments. Various explanations of this observation are considered.

Adolescent↗
PubMed1982

Developmental sensory impairment: amblyopia or tarachopia?

Amblyopia is a relatively common visual disorder that affects about 6% of the population. It involves a substantial visual debility from a developmental etiology and its basis is very poorly understood. The vast majority of research to date has concentrated on the detection deficit for stimuli presented in isolation. Some of this work is reviewed and it is argued that these anomalies in detection, though interesting and useful, do not really explain amblyopia. A new approach is suggested in which discrimination tasks are employed for stimuli having the same amplitude spectra but different phase spectra. For this task amblyopes exhibit a dramatic and selective impairment.

Amblyopia↗
PubMed1981

Residual vision in humans who have been monocularly deprived of pattern stimulation in early life.

Spatio-temporal sensitivity at and above threshold was investigated in a group of patients who exhibited visual loss secondary to uniocular congenital cataract which was present within the first year of life and later removed. The results fall into two general categories depending upon the severity of the visual loss and in particular upon the nature of the temporal loss. In the group exhibiting less severe amblyopia, contrast sensitivity for high and medium spatial frequencies was attenuated to a similar extent for all temporal frequencies. In the group exhibiting more severe amblyopia no form vision a was present; only temporal perception remained. In these cases a greater loss of flicker threshold sensitivity occurred at higher temporal frequencies. Supra-threshold tests revealed that movement perception was effectively normal in both of these groups. These results demonstrate that stimulus deprivation amblyopia, while different in some respects from anisometropic amblyopia is more similar to that condition than to strabismic amblyopia Previous animal results derived from monocular lid suture in cat and monkey are compared with these findings.

Adolescent↗
PubMed1980

Contrast perception above threshold is only minimally impaired in human amblyopia.

Amblyopia is a condition occurring at birth, or in early childhood, which affects up to 5% of the population and is characterized by degraded vision in one eye not able to be corrected by spectacles and having no observable lesion in the eye itself. Its cause is therefore uncertain, yet indirect evidence suggests a neural one, very likely in the visual areas of the brain. Ambyopia is usually associated with either a strabismus (eye misalignment) or anisometropia (unequal refractive error). Investigation into its neural basis has involved two approaches: psychophysical research in humans with amblyopia has shown marked contrast deficits for threshold stimuli, suggesting deficient contrast coding at and above threshold in amblyopia. Neurophysiological research on animals deprived in early life with either a surgically induced squint or an optically induced anisomeropia has suggested that 'blur' may be the common etiological basis for these two amblyopias. We have investigated here whether amblyopes exhibit contrast deficiencies for other than threshold stimuli. Our measurements show that the threshold deficits measured in amblyopia bear no simple relationship to what happens above threshold. There is no contrast coding abnormality in amblyopia in the high-contrast range typical of everyday vision. We also give suprathreshold evidence for anisometropic and strabismic amblyopia having different neural bases.

Amblyopia↗
PubMed1979

Contrast sensitivity assessment of functional amblyopia in humans.

Contrast threshold measurements have extended our perspective of human amblyopia by focusing our attention on the perception of objects larger than the resolution limit. This approach has shown that there are two categories of spatial loss in amblyopia, that anisometropic and strabismic amblyopia have a different neural basis, and that suprathreshold anomalies can occur independent of threshold defects.

Amblyopia↗
PubMed1979

Vision through an abnormal cornea: a pilot study of the relationship between visual loss from corneal distortion, corneal edema, keratoconus, and some allied corneal pathology.

Visual function was assessed by classic acuity measures as well as contrast thresholds over a wide spatial frequency range for subjects with experimentally induced corneal distortion and induced corneal edema. These results were compared with similar results for patients with uniocular keratoconus, bilateral keratoconus, and allied corneal pathology. Distortion and edema were found to produce characteristically different types of contrast attenuation at threshold (amplitude) and have quite different suprathreshold abnormalities (phase) for objects within the resolution limit and therefore could form the basis of a useful functional classification of the visual degradation from corneal pathology.

Cornea↗
PubMed1978

On the nature of the neural abnormality in human amblyopia; neural aberrations and neural sensitivity loss.

In this investigation contrast threshold measurements are compared with supra-threshold perception for a group of human amblyopes. The results indicate that human amblyopia involves, in some cases, not only loss of sensitivity but spatial distortion. Thus a new group of amblyopes can now be identified in which only distortion occurs. These results have important physiological implications for both the normal and abnormal visual systems. Neurophysiologists investigating visual loss from deprivation should assess whether similar distortions occur in animals. This question may hold the answer to whether the present animal models are relevant to the human condition.

Amblyopia↗