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Biomedical subjects

B Timney

Publications and source records attributed to B Timney.

At least 37 records · Page 2Linked to original sources
PubMed1989

On the evidence for a 'pure' binocular process in human vision.

Wolfe (1986, Psychol. Rev. 93, 269-282) proposed a model of human binocular vision based on the assumption of two functionally distinct classes of binocular neuron. These neurons may be regarded as logical AND and OR gates. In the present paper we assess the evidence relevant to this assumption. We find that while both types of binocular neuron have been described in the cortex of cat and monkey, there is no indication that they form functionally separate populations. Critical analysis of the psychophysical evidence for AND and OR channels in human vision suggests that much of the data presented in favor of an AND channel is subject to alternative interpretations. We conclude that the available data are not consistent with the existence of separate channels as proposed by Wolfe.

Adaptation, Ocular↗
PubMed1988

Meridional anisotropies of orientation discrimination for sine wave gratings.

The difference limen for perceived stimulus orientation was measured for thin lines, and for sine wave gratings between 2.5 and 20.0 c/deg. All observers exhibited a marked meridional anisotropy on this task with both the thin line and grating test targets. For the sine wave gratings orientation discrimination was not found to depend on their spatial frequency. Contrast threshold measurements with the same set of stimuli confirmed earlier reports that the meridional anisotropy for contrast detection increases with test spatial frequency. The data are consistent with published hypotheses (Regan and Beverley, 1985) that detection and discrimination of spatial patterns may be processed differently by orientation selective elements of the visual system and that there are fewer of these mechanisms subserving oblique orientations than either vertical or horizontal orientations.

Discrimination, Psychological↗
PubMed1987

Visual acuity in esotropic cats following occlusion of the non-deviating eye.

The present study examined the possibility that strabismic amblyopia may be explained in terms of binocular competition. This was done by measuring visual acuity in strabismic kittens whose deviating eye was placed at an advantage over the other. Four groups of kittens were studied: Normal/Normal (N/N); Lid-sutured/Normal (LS/N); Strabismic/Normal (S/N); Strabismic/Lid-sutured (S/LS). In conditions 2-4 kittens underwent lateral rectus section and/or unilateral lid suture around the time of natural eye opening. Visual acuity measurements were made using the jumping stand technique and the following results were obtained: all S/N kittens showed an acuity deficit in the strabismic eye; each S/LS kitten had a better acuity in the deviating eye than any S/N kitten; the acuity of the LS/S kittens was slightly below that of the N/N kittens or the normal eye of the LS/N kittens. These data suggest that binocular competition can account for essentially all of the strabismic amblyopia observed in kittens.

Animals↗
PubMed1987

Changes in perceived contrast of suprathreshold gratings as a function of orientation and spatial frequency.

Orientation anisotropy for suprathreshold gratings of different spatial frequencies was measured using a contrast matching procedure. Observers matched the contrast of sine-wave gratings of various orientations to a vertical reference grating set at different reference contrasts. At threshold, the size of the anisotropy increased with spatial frequency, confirming previous results. When the reference grating contrast was set above threshold, the anisotropy declined, and eventually disappeared for gratings of medium spatial frequencies. At higher spatial frequencies, although the relative anisotropy became smaller, it did not disappear within the range of contrasts used in this study. For medium, but not for high spatial frequencies, the data are consistent with Kulikowski's (1976) model of effective contrast constancy.

Contrast Sensitivity↗
PubMed1987

The reliability of estimates of interhemispheric transmission times derived from unimanual and verbal response latencies.

In this study, the reliability of estimates of interhemispheric transmission times derived from verbal and unimanual responses was assessed. Responses were made to presentations of lateralized light flashes in a series of 20 experimental sessions, with responses being made to stimuli presented at three different retinal eccentricities. The reliability of estimates of interhemispheric transmission across 20 experimental sessions was generally poorer for verbal compared to unimanual responses. This was particularly true for stimuli presented close to the centre of the visual field. In addition, for unimanual response conditions, estimates of interhemispheric transmission times for the female subjects were longer in duration and exhibited poorer reliability than estimates found for the male subjects. In the verbal response conditions, there were no differences between the mean durations of the estimates found for the male and female subjects. The results suggest that for centrally located stimulus presentations, verbal responses do not produce reliable estimates of interhemispheric transmission time. This relatively poor reliability across experimental sessions may account for the mixed results seen in previous studies which have used verbal reaction times to estimate interhemispheric transmission time. The collection of unimanual responses to lateralized visual stimulus presentations is likely the best method of estimating interhemispheric transmission time in normal man.

Adult↗
PubMed1986

Interhemispheric transmission delays in human strabismics.

Experimentally induced strabismus in cats and monkeys leads to a variety of anatomical, physiological, and perceptual deficits. Given the similarities between the perceptual deficits of human strabismics and those of animal models, it is reasonable to suggest similarities in the underlying anatomical and physiological deficits. In cat, one anatomic anomaly associated with strabismus is a disruption of the normal development of the posterior corpus callosum. The present study provides evidence that interhemispheric transmission in human strabismics may be disrupted also. Simple unimanual reaction times were used to examine the transmission of information through callosal fibres. Estimates of cross-callosal transmission time for strabismics were consistently longer than for individuals with no history of strabismus. This difference was found only when targets were presented within 5 degrees of the fixation point. The results suggest that human strabismics may have abnormalities in those callosal fibres which are involved in the interhemispheric transmission of visual information.

Adolescent↗
PubMed1985

Binocular depth perception in the cat following early corpus callosum section.

The role of the corpus callosum in the mediation of binocular depth perception was examined by measuring monocular and binocular depth discrimination thresholds in cats which had undergone section of the corpus callosum shortly after birth. Three kittens had the posterior callosum sectioned at the age of eleven days. A fourth kitten underwent a sham operation and one additional animal served initially as an unoperated control. Monocular and binocular depth thresholds were measured for all kittens when they were between three and five months old. Although there was some individual variability, none of the callosum-sectioned kittens showed any deficits of binocular depth perception relative to normal animals. The initially unoperated kitten had its callosum sectioned at five months and was retested following surgery. Its performance did not change from preoperated levels. Finally, the three neonatal callosum-sectioned kittens underwent section of the optic chiasm when they were six months old, causing a complete breakdown in binocular depth discrimination. The results are interpreted to indicate that although the corpus callosum may be a sufficient pathway for the maintenance of stereopsis in cat, it is not necessary.

Animals↗
PubMed1984

Distance estimation in the Mongolian gerbil: the role of dynamic depth cues.

The role of dynamic depth cues in distance estimation was investigated in the Mongolian gerbil. Animals were trained to jump randomly varied distances on a jumping stand under both binocular and monocular conditions. Videotape analysis revealed that prior to jumping, the gerbils executed a series of vertical head movements, the amplitude and velocity of which were related to the gap distance and to each animal's accuracy. This suggested that the gerbils were employing motion parallax cues to judge distance. An inverse relation between the magnitude of forward movement and the frequency of vertical head movements suggested that loom cues were also being used to judge distance. This hypothesis received support from a second experiment in which forward movements were constrained by a short take-off platform. In this condition, frequency of vertical head movements increased, suggesting that a compensation had occurred for the loss of information from loom.

Animals↗
PubMed1984

Eye movements of human albinos.

Albino mammals are known to suffer from misrouted optic projections and there is a growing body of evidence suggesting that human albinos have similar aberrant anatomical pathways. The present study examined the possible consequences of such aberrant pathways on the oculomotor performance of five adult human albinos. Optokinetic nystagmus to drifting grating patterns and pursuit eye movements were measured. The subjects' congenital nystagmus was also measured under different conditions of gaze position and ambient room illumination. Two of the subjects showed clear instances of an inversion in the optokinetic response and there were probable inversions observed for a third subject. The magnitude of the optokinetic nystagmus was appropriate for the rate of pattern drift, but inverted in direction. In all cases smooth pursuit was severely impaired, but reversals of the appropriate direction of pursuit eye movements were not observed. Changes in the congenital nystagmus under conditions of light and darkness were found for four of the five subjects and varied greatly between subjects. The results suggest that human albinos share many of the oculomotor deficits found in other albino species.

Albinism↗
PubMed1983

Binocular depth perception in the meerkat (Suricata suricatta).

Although it has been widely assumed that mammals with frontally placed eyes have stereoscopic vision, there is actually a paucity of supporting evidence. We have measured monocular and binocular depth perception in the meerkat (Suricata suricatta), obtaining strong evidence for the presence of stereopsis. The data provide additional evidence for the generality of stereoscopic vision across many different species.

Animals↗
PubMed1983

The effects of early and late monocular deprivation on binocular depth perception in cats.

Binocular and monocular depth discrimination thresholds were obtained from cats which had been monocularly deprived either from the time of natural eye opening or else at the age of 4 months. Among normal cats, binocular depth thresholds typically are very much better than monocular thresholds, allowing the inference that normal cats have good stereopsis. For the early-deprived animals in the present study, only those whose deprived eyes were opened by 30 days of age showed any binocular advantage. Deprivation periods lasting to 35 days or older completely eliminated the binocular superiority, with no sign of any recovery. These results provide behavioral evidence that binocular visual mechanisms are extremely susceptible to disruption and, unlike those underlying visual acuity, do not have the potential for recovery. The effect of deprivation imposed later in life was quite different. Three cats, deprived for 1, 2 or 3 months respectively, beginning at the age of 4 months, showed no deficits in binocular depth perception. This latter finding implies the existence of a sensitive period for stereopsis which is over completely by the age of 4 months.

Aging↗
PubMed1982

Adaptation to optically reduced size.

Previous investigators have reported some difficulty in obtaining evidence for adaptation to optically reduced size. A study is reported in which a ballistic target-pointing task was used to examine such adaptation. In the first experiment size was reduced either by means of negative spherical lenses, or cylindrical lenses which reduced image size in only one dimension. Both lens types produced a negative aftereffect of adaptation, which increased as a function of lens strength. There was also a tendency for cylindrical lenses to give larger aftereffects. In a second experiment, in which a modified adaptation procedure was used, adaptation to cylindrical distortion was found to be significantly greater than adaptation to equivalent spherical distortion. These data are interpreted with respect to the relative amounts of distance distortion induced by viewing through the different lens types. The third experiment employed a cylindrical lens and a mental rotation procedure to determine whether adaptation in the first two experiments may have been visual or visuomotor in nature. Results indicated that adaptation was not purely visual. It is concluded that it is possible to demonstrate adaptation to size distortion by using an appropriate procedure.

Adaptation, Psychological↗
PubMed1981

Visual acuity in cats following surgically induced cyclotropia.

Visual acuity was measured, using a jumping stand technique, in cats which had undergone surgical cyclotorsion of one or both eyes. In cats with unilateral rotations acuity was depressed in the rotated eye and the extent of loss increased with larger amounts of rotation. Cats which had undergone bilateral rotations tended to have lower acuities than normal cats and there was a slight tendency for the eye with the greater rotation to show the larger deficit. Evidence is presented that the lowered acuities were not the result of peripheral damage and it is suggested that the defect may be considered as a form of strabismic amblyopia.

Amblyopia↗
PubMed1981

Development of binocular depth perception in kittens.

By means of the jumping stand technique, binocular and monocular depth thresholds were measured in kittens 4 weeks to 4 months old. Binocularly, performance improved very rapidly during the fifth and sixth weeks, coinciding with the maturation of cortical disparity-tuned neurons. Discriminations made monocularly took longer to learn and thresholds were consistently poor. The results demonstrate that kittens can use binocular cues for depth at a very early age.

Age Factors↗
PubMed1981

Sensitivity deficits consistent with aberrant crossed visual pathways in human albinos.

There is both anatomical and electrophysiological evidence that human albinos have abnormally crossed visual pathways. The present study examined some perceptual consequences of such pathways on the visual abilities of 12 oculocutaneous albinos and one ocular albino. Measurements were made of visual fields in all subjects. For seven subjects, contrast sensitivity functions were obtained for the central, nasal, and temporal retinas. There was a wide variation in the dimensions of the visual fields, but there was no evidence for a loss corresponding to the origin of the aberrant projections. On the basis of their contrast sensitivity functions, however, it was possible to classify the albinos into two groups. One group, with higher overall sensitivity, showed no differences between central, nasal, and temporal retina. The other group showed a marked depression in sensitivity for the temporal retina, indicating that in some albinos, information from the retinal region corresponding to the aberrant pathway is degraded.

Adult↗
PubMed1979

Assessment of depth perception in cats.

A behavioural method is described for the assessment of depth perception of kittens. Measurement is made of the smallest separation in depth that can be discriminated between two adjacent stimuli under both monocular and binocular viewing conditions. Normal animals can discriminate much smaller separations in depth when using two eyes than with monocular viewing, implying the presence of a cue to depth that is uniquely available with binocular viewing. The test provides a quick and reliable way of screening animals for stereopsis.

Animals↗