Biomedical subjects
B Timney
Publications and source records attributed to B Timney.
The retinal ganglion cell layer and visual acuity of the camel.
We examined the retinal ganglion cell layer of the dromedary camel, Camelus dromedarius. We have estimated that there are 8 million neurons in the ganglion cell layer of this large retina (mean area of 2,300 mm(-2)). However, only approximately 1 million are considered to be ganglion cells. The ganglion cells are arranged as two areas of high cell density, one in the temporal and one in the nasal retina. Densities of ganglion cells between these two high density regions is much lower, often less than 100 per mm(-2). In between these two high density regions, on the nasal side of the optic nerve head, is a unique and dense vertical streak of mostly non-ganglion cells; the function of this specialization is unknown. On the basis of ganglion cell density we estimate that the peak acuity in the dromedary camel is about 10 and 9.5 cycles per degree in the temporal and nasal high density regions respectively and falls to 2-3 cycles per degree in the central retina. Behavioral acuity was estimated for one bactrian camel and was found to be approximately 10 cyc deg(-1). The camel has a retina with a mean thickness of 104 microm, less than the 143 microm thickness that has previously been thought to be necessary for a retinal vasculature. Nevertheless, there is an extensive vitreal vasculature that does not appear to spare any retinal region.
Behavioural assessment of visual acuity in bumblebees (Bombus impatiens).
The present study used previously developed techniques to measure resolution acuity in bumblebees (Bombus impatiens). Bees were required to discriminate between horizontal and vertical gratings in a Y-maze apparatus. The gratings had a mean luminance of 9 cd m(-)(2) and a Michelson contrast of 84 %. For different bees, either the horizontal or vertical grating was rewarded. Rewarded gratings were associated with a sucrose and water solution (30 % sucrose by volume) and unrewarded gratings with plain water. Acuity estimates were established at several different viewing distances over several sessions using a method of constant stimuli. Visual acuity functions were generated from the performance data, and acuity thresholds were interpolated at a performance level of 65 % correct. When corrected for viewing distance, best angular acuity obtained for horizontal and vertical gratings was 0.35 and 0.36 cycles degree(-)(1), respectively. These results are compared with those of the honeybee and discussed in the context of the bumblebee's foraging behaviour.
Local and global stereopsis in the horse.
Although horses have laterally-placed eyes, there is substantial binocular overlap, allowing for the possibility that these animals have stereopsis. In the first experiment of the present study we measured local stereopsis by obtaining monocular and binocular depth thresholds for renal depth stimuli. On all measures, the horses' binocular performance was superior to their monocular. When depth thresholds were obtained, binocular thresholds were several times superior to those obtained monocularly, suggesting that the animals could use stereoscopic information when it was available. The binocular thresholds averaged about 15 min arc. In the second experiment we obtained evidence for the presence of global stereopsis by testing the animals' ability to discriminate between random-dot stereograms with and without consistent disparity information. When presented with such stimuli they showed a strong preference for the cyclopean equivalent of the positive stimulus with the real depth. These results provide the first behavioral demonstration of a full range of stereoscopic skills in a lateral-eyed mammal.
Differential effects of alcohol on rod and cone temporal processing.
OBJECTIVE: One of the more reliable effects of alcohol on sensory processing is the reduction of the critical flicker fusion frequency (CFF). To gain a better understanding of the mechanisms through which alcohol influences the processing of temporal information, we explored the effects of alcohol on CFF at a high level of illumination when performance is mediated by the cones (photopic), and at a lower level when performance is dominated by the rods (low mesopic). METHOD: Measurements of CFF were made under placebo and alcohol conditions (0.06% blood alcohol concentration) in six subjects (three men, three women). Assessments were obtained in both the rising and falling phases of absorption for both light levels at several retinal locations. RESULTS: Alcohol was found to reduce CFF by approximately the same amount at all retinal locations for photopic viewing levels. Mesopic CFF was lower than photopic, as expected. However, it was only minimally affected by alcohol, showing only a slight decline for central viewing. CONCLUSIONS: These results imply a selective effect on alcohol on cone function. In considering the possible mechanisms of such a selective action, we suggest that this effect may be an indirect one. That is, the poorer performance under photopic conditions may be a consequence of an alcohol-induced reduction in the inhibitory interactions that are more prevalent at photopic light levels.
Effects of moderate blood alcohol concentrations on spatial and temporal contrast sensitivity.
OBJECTIVE: In order to gain a more complete picture of the influence of alcohol on visual performance, we measured contrast sensitivity for a range of spatial and temporal frequencies in individuals with moderate blood alcohol concentrations (BACs). METHOD: Subjects were tested at blood alcohol concentrations of 0.06% in both the rising and falling phases of absorption. In the first part of the study, the performance of two men and four women on a number of simple screening measures and static contrast sensitivity was measured. In the second part (four men and three women), the grating patterns were contrast reversed at four different rates, allowing us to assess the effects of alcohol on temporal processing. The second study examined the relationship between blood alcohol concentrations and contrast sensitivity. RESULTS: Although few significant changes in performance were found on the simple screening tests, we observed a significant reduction (43%) in stationary contrast sensitivity at the 0.06% BAC. This change in performance was greater at low and high spatial frequencies than at moderate ones. At high temporal frequencies, the reduction in sensitivity was 2.5 times that seen for lower temporal frequencies. With higher blood alcohol concentrations, the decrease in performance was found to be greatest for the high and low spatial frequencies. CONCLUSIONS: These data suggest that alcohol produces visual deficits that are not attributable to pursuit eye movements. It is suggested that these visual deficits, combined with changes in ocular-motor control and attentional deficits, may have a strong effect on performance under the influence of alcohol.
The effect of dark and equiluminant occlusion on the interocular transfer of visual aftereffects.
Lehmkuhle and Fox [(1976) Vision Research, 16, 428-430] reported that interocular transfer (IOT) of a translational motion aftereffect (MAE) was greater if the non-adapting eye viewed an equiluminant field than if it viewed a dark field. They recommended equiluminant occlusion of the non-adapted eye when measuring IOT of aftereffects. We tested this proposal in three experiments. First, we assessed IOT with equiluminant and dark occlusion for three different classes of aftereffects. Although transfer was greater with equiluminant occlusion for the translational MAE, there was no significant difference in the amount of transfer for the tilt aftereffect or the contrast threshold elevation effect. Second, we tested the hypothesis that spuriously large IOT could be the result of an aftereffect from tracking eye movements in the non-adapting eye. When potential tracking movements were reduced by using rotating spokes, a rotating spiral or contracting concentric circles, there was a corresponding reduction in the occlusion-dependent transfer. Third, we found that luminance shifts had no influence on the amount of transfer when all contours were eliminated from the non-adapting eye. We conclude that the type of occlusion used for measuring IOT of the translational MAE is important only when visible contours in the non-adapting eye contribute to the adapting process.
The aftereffect to relative motion does not show interocular transfer.
The motion aftereffect is strongest after viewing a moving field embedded in a patterned stationary surround, which suggests that relative motion is an important signal for its generation. The contribution of relative motion to binocular aspects of the motion aftereffect was assessed. Subjects viewed uniformly moving random dots surrounded by a stationary random-dot annulus. These displays could be presented in a variety of combinations to each eye separately or to both eyes, during adaptation and test. It was found that, although the presence of relative motion during adaptation significantly extended the duration of the monocular motion aftereffect, it did not augment interocular transfer. The presence of stationary surround contours in the nonadapting eye did not influence the aftereffect in the adapting eye. The enhancement provided by stationary surround contours is largely dependent on their presence during adaptation. The presence or absence of surround contours during the test phase did not influence the duration of the aftereffect. These findings are consistent with previous suggestions that the motion aftereffect is, in part, the result of adaptation to relative motion that occurs relatively early in the visual pathway-before binocular integration.
Horses are sensitive to pictorial depth cues.
In an attempt to demonstrate whether horses could make use of pictorial cues to depth, two were trained initially to make a relative-line-length discrimination between two lines placed one above the other. Psychophysical measurement of their discrimination thresholds showed that from a viewing distance of approximately 160 cm they could reliably distinguish a lower line of 10 cm from an upper one of 14 cm. In the second phase of the experiment, two lines of equal length were superimposed on a photograph of a set of railway tracks with many pictorial cues to depth, or a photograph of a pastoral scene with fewer obvious depth cues. To humans, the railway tracks created a Ponzo illusion, making the upper line-appear longer. When the horses were allowed to choose between the photographs, they overwhelmingly chose the display containing the converging railway tracks. Control experiments ruled out alternative explanations, leading to the conclusion that horses are susceptible to a Ponzo illusion created by depth cues in photographs.
Spatial knowledge in blind and sighted children.
Spatial knowledge was evaluated in sighted and congenitally blind children using a large-scale four-location navigation task adapted from the work of Landau, Spelke, and Gleitman (1984). From video records we coded the exact path taken and determined accuracy of initial turn, closest position, and final position, relative to target location. We then computed a score to index the efficiency of the path taken. For the sighted sample, after the navigation task, children constructed a tactile map of the test space without the aid of vision and, following removal of the blindfold, drew from memory the spatial layout of the test space. Performance on the navigation and mapping tasks consistently indicated increasing cognitive mapping skills with age in sighted children. Blind children performed comparably to the sighted on all measures except accuracy at final position, for which their performance was worse than that of the sighted. Analysis of the directness of novel paths and other measures taken suggest caution in ascribing well developed Euclidean coding skills to very young children. Results are discussed in light of Landau et al.'s (1984) conclusions.
On the contribution of a binocular 'AND' channel at contrast threshold.
Three experiments are reported in which an attempt was made to isolate the contribution of an AND channel by measuring aftereffects following alternating monocular adaptation. The first two were designed to test Wolf and Held's proposal that the binocular AND channel does not respond at contrast threshold. In the first experiment the relative sizes of monocular and binocular contrast threshold elevation were compared with the pattern of aftereffects obtained in a study of the suprathreshold tilt aftereffect. Identical patterns of results were obtained under the two adaptation conditions. In the second experiment, the monocular and binocular contrast-reduction aftereffect reported by Blakemore et al was measured over a wide range of reference contrasts. As in the previous experiment, the monocular effect was greater than the binocular effect. This occurred at all reference contrasts. These data support the conclusion that the AND channel contributes to visual performance in the same manner, irrespective of stimulus contrast. In the final experiment an alternative explanation for existing evidence against the existence of an AND channel was assessed.
Tactual object exploration and recognition in blind and sighted children.
Development of the haptic system was evaluated by examining object exploration and recognition in sighted children between the ages of 3 and 8 years. To determine the importance of visual experience for these abilities, the performance of seven congenitally blind children was compared with that of sighted peers matched for age and gender. Performance was evaluated in terms of the speed and correctness of object identification, thoroughness of exploration of object parts, representation of the global form versus local parts of objects, and the possible role of critical parts in object identification. Four types of common objects were presented: normal-sized, miniaturized small, miniaturized large, and oversized objects. All subjects were required to manipulate and identify these objects haptically, without the aid of vision. Results revealed the emergence of a developmental pattern in all performance measures for sighted children. Older sighted children were not only able to recognize more objects and to do so more quickly, but also were more thorough in their exploration patterns. With increasing age, children appear to change their representation of objects from one based predominantly on global shape to one that incorporates a balance of global shape and specific local parts. In agreement with this, critical parts also played a role in object identification, particularly in older children. Blind and sighted children did not differ in any performance measures, which suggests that previous visual experiences do not determine tactile exploration strategies and are not essential for haptic object recognition.
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.
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.
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.
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.
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.
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.