Biomedical subjects

B Timney

Publications and source records attributed to B Timney.

18 recordsLinked to original sources

PubMed1992

Visual acuity in the horse.

We assessed the ease with which horses could learn visual discriminations and measured their resolution acuity. We trained three horses to press their noses against one of two large wooden panels to receive a small food reward. Following training on a series of two-choice discrimination tasks, resolution acuity was measured. Although there was some variability between animals, the best acuity obtained was 23.3 c deg-1. Within the margin of error imposed by limited anatomical data, the obtained values are consistent with predictions based on retinal ganglion cell density estimates and posterior nodal distance/axial length ratios. They suggest that the resolution acuity of the horse is limited by ganglion cell density in the temporal portion of the narrow visual streak.

Animals↗

PubMed1990

Effects of brief monocular deprivation on binocular depth perception in the cat: a sensitive period for the loss of stereopsis.

The period of susceptibility for binocular depth vision was studied in kittens by subjecting them to periods of monocular deprivation beginning at different ages. In an initial study, we found that normally reared kittens can learn a depth-discrimination task much more rapidly when tested binocularly than monocularly, even when testing is begun as early at 30 d. In subsequent experiments, kittens were monocularly deprived by eyelid suture, following which their monocular and binocular depth thresholds were measured using the jumping-stand procedure. We obtained the following results: (1) When monocular deprivation is applied before the time of natural eye opening but is discontinued by no later than 30 d, there is very little effect on binocular depth thresholds. (2) When deprivation is begun at 90 d, binocular depth thresholds are unaffected. (3) When deprivation is begun between these two ages, the magnitude of the deficit varies with the period of deprivation and the age at which it begins. (4) By imposing brief (5 or 10 d) periods of deprivation, beginning at different ages, we were able to demonstrate that the peak of the sensitive period is between the ages of 35 and 45 d, with a fairly rapid decline in susceptibility outside those age limits. (5) Even with as little as 5 d of deprivation, substantial permanent deficits in binocular depth vision can be induced.

Animals↗

PubMed1990

Measurement of visual aftereffects and inferences about binocular mechanisms in human vision.

There is conflicting evidence concerning the characteristics of binocular channels in the human visual system with respect to the existence of a 'pure' binocular channel that responds only to simultaneous stimulation of both eyes. Four experiments were conducted to resolve these discrepancies and to evaluate the evidence for the existence of such an exclusive binocular channel. In the first three studies, tilt aftereffects were measured after monocular adaptation. The relative sizes of the direct, interocularly transferred, and binocular aftereffects were not influenced by the configuration of the adapting pattern (experiment 1), or by the eye used for adaptation (experiment 2). There were also consistent interobserver differences in the relative sizes of the aftereffect seen after monocular adaptation (experiment 3). Taken together, these data raise questions about the appropriateness of a monocular adaptation paradigm for evaluating the presence of a pure binocular channel in observers with normal binocular vision. In experiment 4, in which the paradigm of alternating monocular adaptation was used, data were obtained that are consistent with the presence of a pure binocular channel.

Adolescent↗

PubMed1989

Binocular depth perception, visual acuity and visual fields in cats following neonatal section of the optic chiasm.

We studied the role of the transcallosal pathway in stereopsis by measuring binocular and monocular depth perception in two cats that had undergone section of the optic chiasm at the age of 21 d. To ensure that the surgery did not impair vision to the extent that depth perception could not be evaluated, visual acuity and visual fields were also measured. In both of the chiasm-sectioned animals the visual fields were reduced and the visual acuity was substantially lower than in normal cats, with a maximum of about 2 cyc deg-1. Binocular depth thresholds of the chiasm-sectioned cats were worse than those of the normal cat but were better than their own monocular thresholds. These results suggest that the chiasm-sectioned animals were still able to use binocular cues to judge depth and indicate that the indirect pathway through the corpus callosum is sufficient to mediate binocular depth perception.

Depth Perception↗

PubMed1989

Spatial frequency discrimination at different orientations.

Hirsch and Hylton (1984) have reported a meridional anisotropy for spatial frequency discrimination with a periodicity containing both square and hexagonal components. We measured the difference limen for spatial frequency for sine-wave gratings whose orientations varied over a 180 deg range and whose nominal spatial frequency varied between 2.5 and 10.0 c.deg.-1. Although some periodicity was evident at the lowest spatial frequency, it was not consistent between observers and it did not contain a hexagonal component. At the higher spatial frequencies there was no evidence for any meridional anisotropy.

Adult↗

PubMed1989

Width discrimination for band-pass stimuli.

There is a disagreement in the literature about whether the discrimination function for width is a continuous or a segmented function of target size (Hirsch & Hylton, 1982; Mayer & Kim, 1986). We determined the Weber fraction for width for difference of Gaussian (DOG) luminance distributions. Data for seven different standard widths were collected simultaneously. Although the value of the Weber fraction varied with target size, there was no evidence for any discontinuities in the discrimination function, either in the fovea or at 2.0 deg eccentricity.

Humans↗

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↗

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↗

PubMed1978

The development of vision in cats after extended periods of dark-rearing.

The time course of development of visual acuity for square wave gratings was measured behaviourally in a number of cats that were reared in total darkness until they were either 4 (5 cats) or 6 (1 cat) months of age. Less extensive measurements were also made on animals reared in a similar manner until they were either 1 1/2 or 10 months old. Initially, all the animals appeared to be blind, but signs of vision became evident after periods of time in an illuminated environment that ranged from a few days, in the case of animals dark-reared for only 1 1/2 to 4 months, to 1 to 2 months for those animals that were deprived for 6 months or more. Thereafter, visual acuity as measured on a jumping stand progressively improved, reaching, in the case of animals deprived for 4 months, values that were comparable to those of normal animals (6.9 cycles/deg) after 4 months of exposure to light. The animal deprived for 6 months remained apparently blind for a month and eventually attained an acuity (5.7 cycles/deg) that was somewhat less than that of normal animals. The fact that such high acuities can be achieved after periods of binocular deprivation that extend throughout the classically defined "critical period" suggest that the effect of dark-rearing is somehow to impede the natural decline with age in the degree to which cortical neurones are susceptible to environmental modification.

Animals↗

PubMed1977

Measurement of visual preferences in cats.

A technique is described for measuring visual preferences in cats. The method involves a self-presentation paradigm, but provides strict control of the visual field and orientation of contours in the field. Two procedures are presented: a single-choice design and a multiple-choice design, both of which use duration of self-produced exposure as the dependent measure. Although both procedures may serve as indices of preference order, the multiple-choice method is preferable for measuring preference strength. The application of the technique as a behaviorual index of the effects of early environmental manipulation is suggested. Preliminary data from two normal cats indicate that, although homogeneous illumination is reinforcing, the presence of features, especially oriented contours, appears to increase the 'interest' value of the presentation.

Animals↗

PubMed1977

A behavioural technique for the rapid assessment of the visual capabilities of kittens.

A behavioural method is described for the measurement of various visual spatial acuities in kittens as young as thirty days of age. Expamples are given of applications of the technique to measurement of the visual acuity for gratings in normal kittens as well as to studies of the time course of behavioural recovery from the effects of early monocular visual deprivation.

Age Factors↗

PubMedDate not supplied

Binocular depth perception following early experience with interocular torsional disparity.

The relationship between the behavioral and physiological consequences of rearing with optically induced cyclotropia was assessed. Beginning at the age of 4 weeks, kittens wore goggles that rotated the visual field in opposite directions in each eye for several hours each day over a period of several weeks. The amounts of interocular rotation were 0 deg (control), 16 deg, and 32 deg. Subsequently, they were tested to determine their monocular and binocular depth thresholds and, in some cases, visual acuity. In several kittens recordings were also made from the visual cortex. Binocular performance of all kittens in the 0-deg condition and three out of six kittens in the 16-deg condition was comparable to, although slightly lower than, that of normally reared kittens. In contrast, none of the 32-deg kittens showed any evidence of the binocular superiority that would suggest the presence of stereopsis. Extracellular unit recordings from the visual cortex confirmed our earlier results with goggle-reared kittens. In 16-deg kittens, the distribution of the cells' preferred interocular disparities (IOD) in receptive-field orientation showed a compensating shift so that the mean matched the experienced rotational disparity. In the 32-deg kittens, binocularity was greatly disrupted and there was no compensatory shift in the IOD distribution. Two 32-deg kittens were afforded 3 years of subsequent normal visual experience. Both the behavioral and the physiological findings were unaffected by normal visual exposure in adulthood. Control measurements of acuity indicated that any deficits in depth perception were not due to reduced spatial-resolution abilities. The data indicate that the kitten visual system is able to maintain functional binocularity sufficient to subserve a moderate level of stereoacuity with interocular rotations of up to at least 16 deg.

Animals↗