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R Blake

Publications and source records attributed to R Blake.

At least 109 records · Page 6Linked to original sources
PubMed1988

Limits of binocular fusion in the short wave sensitive ("blue") cones.

Stereoscopic depth perception is possible when the short wave sensitive (SWS or "Blue") cones are isolated using a yellow adapting field. We have measured the maximum disparity that can be fused (the diplopia threshold) as a function of the separation between pairs of dots or lines. Under all conditions, these diplopia thresholds are the same for the isolated SWS cones as for the entire visual system. In addition, SWS diplopia thresholds vary as a linear function of dot or line separation, so that they exhibit disparity scaling. Further experiments show that disparity scaling is dependent upon the presence of low spatial frequencies in the stimulus and not upon the retinal eccentricity of stimulation. These data indicate that the SWS cones provide information to the disparity processing system through more than one low spatial frequency channel but not through high frequency ones.

Adaptation, Ocular↗
PubMed1988

Mislocalization of diplopic images.

When observers dichoptically view bar stimuli at disparities beyond the fusion limit, the bars are perceived as being closer together than they really are. When 6.0-cycle/deg vertical derivative-of-Gaussian bars are adjusted into apparent alignment with binocularly presented spots, the mislocalization of each bar's half-image is typically 2-5 arcmin for disparities below 60 arcmin. The effect does not generalize to an additional monocular probe bar at more-eccentric locations, thus excluding reflex vergence eye movements as an explanation. Instead, the results indicate a process of interocular matching that seeks to attribute similar directions as well as depths to matching half-images.

Depth Perception↗
PubMed1988

Disparity range for binocular summation.

Binocular summation of contrast and stereopsis have been linked because they both disappear under certain pathological conditions. The dependence of stereopsis on spatial frequency prompted us to examine how binocular summation varies with both spatial frequency and binocular disparity. We therefore measured binocular summation at different disparities using spatially localized stimuli which were also restricted in their Fourier composition. Contrast thresholds were measured using three interleaved forced-choice staircases for left and right eye monocular stimuli and a binocular stimulus composed of the two monocular stimuli presented simultaneously. At zero disparity binocular thresholds were 1.4 to 1.6 times lower than monocular. As disparity was increased the ratio between the thresholds became smaller, such that at large disparities it was near 1.2, the value expected from probability summation. The range of disparities over which probability summation was exceeded varied with the spatial frequency of the stimulus. At 6.0 cpd the range was 2-3 deg, but at 2.0 cpd or 0.75 cpd the range increased to 4-6 deg. These values closely parallel the range of disparities over which stereoscopic depth sensations occur, but they exceed the limits within which disparate images of an object can be fused into a single percept. The results support the contentions that "neural" summation occurs in the mechanism for stereopsis, that this mechanism uses spatial frequency selective channels, and that this mechanism is separate from the mechanism which mediates fusion.

Adult↗
PubMed1988

Clinical suppression and amblyopia.

In individuals with abnormal binocular vision, such as strabismics and anisometropes, it is common for all or part of one eye's view to be suppressed so binocular confusion and diplopia are eliminated. We examined the relation between the depth of suppression (the amount by which the monocular contrast increment threshold for an eye was elevated by stimulation in the contralateral eye) and the degree of amblyopia (difference in monocular contrast thresholds for the two eyes). There was a significant negative correlation between suppression and amblyopia, so that clinical suppressors with no amblyopia exhibited deep suppression (ie, large threshold elevation) while observers with amblyopia exhibited weaker or no suppression. This negative correlation was found when the two eyes viewed orthogonally oriented contours as well as identically oriented contours. These results suggest that when an eye is amblyopic there is no longer a need for strong suppression of that eye by the contralateral eye.

Amblyopia↗
PubMed1987

What causes stereoscopic tilt from spatial frequency disparity.

A controversy still exists concerning whether the tilt created with interocular spatial frequency disparity arises from a computation of spatial frequency differences or from cumulative positional disparity. In a first experiment, we examined the influence of positional disparity on tilt created with frequency disparity, reasoning that if tilt were computed from spatial frequency differences, the perceived angle should remain unaltered since adding a positional disparity does not change the harmonic content of the stimulus. The results indicated that positional disparity weakened perceived tilt. In a second experiment, we tested the idea that tilt results from the calculation of increasing positional disparity across the display, arguing if local matches of features in the two eyes are made in computing tilt, then the solution to binocular correspondence may be less ambiguous if the same number of cycles was displayed for both spatial frequencies. Perceived tilt increased when the number of cycles was equal, although the angle of tilt still decreased with positional disparity. In Experiment 3, we further reduced potential sources of ambiguity for the binocular matching process by employing D10s (the tenth derivative of a Gaussian) instead of grating patterns. Positional disparity exerted essentially no influence on the perceived angle of tilt of the D10s. Taken together, the results of these experiments suggest that tilt from frequency disparity can be explained solely on the basis of positional disparity.

Depth Perception↗
PubMed1987

Boarding home residents: new underclass in the mental health system.

Boarding homes have proliferated in recent times, spawned by the deinstitutionalization movement. This study explored the realities of living in these homes by using a quantified assessment procedure to examine their programs and policies. Findings suggest an absence of support and stimulation for the residents of homes and a need for outreach and advocacy on the part of social workers in community mental health practice.

Activities of Daily Living↗
PubMed1987

Are stereoacuity and binocular rivalry related?

Several lines of evidence suggest that the processes of excitation and inhibition associated with good stereoacuity may also underlie binocular rivalry, implying that performance on these two tasks could be related. To test this possibility, we measured stereoacuity and rivalry under similar stimulus conditions in 40 observers. To estimate stereoacuity, a two-alternative, forced-choice procedure was used, wherein observers determined which of two sinusoidal grating patterns appeared displaced in depth. To measure rivalry, observers reported the occurrences of exclusive right- and left-eye dominance; dominance durations and alternation rates were recorded. The results showed that stereoacuity was significantly correlated with binocular rivalry, suggesting that stereoacuity and rivalry may share, at least in part, common neural mechanisms.

Depth Perception↗
PubMed1986

Selective losses in binocular vision in anisometropic amblyopes.

Human anisometropic amblyopes typically exhibit reduced contrast sensitivity in the amblyopic eye, especially at higher spatial frequencies. We determined whether this spatial frequency selective loss in contrast sensitivity is accompanied by selective losses in binocular function. Binocular summation (the improvement in one eye's detection performance produced by a subthreshold pattern presented to the fellow eye) was measured at several spatial frequencies. Normal observers exhibited equivalent binouclar summation at all spatial frequencies, whereas all anisometropic amblyopes exhibited normal summation at low spatial frequencies but none at high spatial frequencies. Stereoacuity (minimum resolvable disparity) was also measured as a function of spatial frequency. For normal observers, stereoacuity was best at the highest spatial frequency; for anisometropes stereoacuity was normal at low spatial frequencies, subnormal at intermediate spatial frequencies, and unmeasurable at higher spatial frequencies. Anisometropia may represent a form of selective binocular deprivation that affects neural mechanisms underlying binocular summation and stereopsis.

Adult↗
PubMed1986

Spatial-frequency discrimination in cats.

Spatial-frequency discrimination thresholds were measured in two cats by using a two-alternative forced-choice procedure. A range of standard spatial frequencies centered around the peak of the cat's contrast-sensitivity function was sampled. For comparison, discrimination thresholds were also measured in two human observers at spatial frequencies centered around the peak of the human contrast-sensitivity function. For the humans, spatial-frequency discrimination thresholds averaged 4%, while for cats discrimination thresholds averaged 50%. The relatively poorer performance of the cats cannot be entirely explained on the basis of the larger sample spacing of the feline retinal mosaic. More accurate predictions can be obtained from a model that assumes that the cat bases its discrimination performance on the level of activity within that mechanism maximally activated by the standard spatial frequency.

Animals↗
PubMed1985

Visual motion, binocular correspondence and binocular rivalry.

Human observers dichoptically viewed displays consisting of isotropic random dots, with the dots in each eye's view all moving in a given direction or appearing stationary. When the interocular difference in direction of motion was less than 30 deg, a stable, fused percept resulted. Once this interocular difference was exceeded, binocular rivalry ensued. Rivalry was also obtained when dots seen by the two eyes moved in identical directions but at different velocities. Under this condition, the proportion of time that rivalry was experienced increased with the interocular difference in velocity. Moving dots predominated over stationary ones, and when both sets of dots moved there was no clear advantage of one speed of motion over the other. Contrary to some earlier reports, these results show that motion is not immune to binocular rivalry. Evidently the process responsible for establishing binocular correspondence between images received by the two eyes is sensitive to disparities in direction and velocity of motion.

Functional Laterality↗