Biomedical subjects

R Blake

Publications and source records attributed to R Blake.

At least 37 records · Page 2Linked to original sources

PubMed1994

Interocular transfer of expansion, rotation, and translation motion aftereffects.

The motion aftereffect demonstrates the existence of direction-selective mechanisms in the visual system. However, direction-selective cells exist within many visual areas, including V1 and MT/V5. Can motion aftereffects be generated within each of these areas? In visual cortical areas beyond V1 almost all cells are binocular, whereas a smaller percentage are binocular in V1. The degree of binocularity can be revealed psychophysically by assessing interocular transfer. Interocular transfer of motion aftereffects generated from expanding rotating, and translating dynamic random-dot patterns were therefore compared, since these stimuli should activate cells in higher visual areas selectively. Partial interocular transfer was found that was greater for expansion and rotation than for translation. The results support the involvement of higher visual areas in motion aftereffects to complex animation sequences.

Female↗

PubMed1993

Visually guided attention is neutralized when informative cues are visible but unperceived.

The ability to voluntarily shift the focus of visual attention away from the focus of gaze was investigated in a novel paradigm designed to elaborate the stages of processing underlying this ability. A basic experimental method used to investigate guided visual attention involves measuring response times to targets presented at positions of which the observer has been informed by an orienting cue. Binocular rivalry was utilized to dissociate presentation of the orienting cue from visual awareness of that cue. The findings indicated that when an informative cue was presented to an eye during the dominance phase, thus reaching visual awareness, manual response times were significantly affected by cue validity. In contrast, when the same cue was presented to an eye during suppression, and thus was not seen by observers, response times were not influenced by cue validity. We conclude that to guide attention, neural signals registering informative visual cues must be processed at stages lying beyond the site of rivalry suppression. Implications for investigating the neural basis of visual attention are discussed.

Attention↗

PubMed1993

On the perceptual identity of dynamic stereopsis and kinetic depth.

This paper presents a set of experiments demonstrating novel interactions between kinetic depth (depth-from-motion) and dynamic stereopsis (depth-from-disparity). Previous research has shown that adaptation to a moving stereoscopic figure influences the subjective percept of a subsequently viewed kinetic depth figure. In this paper the interactions between kinetic depth and dynamic stereopsis are shown to be very robust and to occur in situations involving perceptual priming. It is also found that kinetic depth and dynamic stereo stimuli are indistinguishable when the stereoscopic stimulus has small, but perceptually salient, disparity. These results are consistent with the hypothesis that stimuli for kinetic depth and for dynamic stereopsis engage a common neural network.

Depth Perception↗

PubMed1993

Another means for measuring the motion aftereffect.

A new procedure for measuring the motion aftereffect (MAE) is described. The procedure involves adaptation to an animation sequence depicting dots moving in a given direction followed by presentation of a test sequence depicting dots moving in all possible directions. Under adaptation, the test sequence appears to have a directional bias opposite the direction experienced during adaptation. This MAE can be nullified by viewing an animation sequence in which a percentage of dots is constrained to move in a direction opposite the aftereffect. Using a method of constant stimuli, this percentage can be varied to find the value yielding incoherent motion. This dynamic MAE exhibits the same characteristics as the conventional MAE.

Adaptation, Ocular↗

PubMed1993

Visual alchemy: stereoscopic adaptation produces kinetic depth from random noise.

Observers perceive incoherent motion and no hint of depth when viewing stochastic motion, in which stimulus elements move in all possible directions. As earlier work has shown, depth can be specified by introducing a brief interocular delay between the presentation of corresponding animation frames of this 'noise' to the left and right eyes. A study is reported in which observers were adapted to a stereoscopic display consisting of coherent planes of motion at different depths. This stereoscopic adaptation caused incoherent depthless motion to take on the qualities of structure and depth, and it could nullify the depth induced by interocular delay. The findings are interpreted within the context of a neural model consisting of units selectively responsive to different directions of motion at different planes of depth.

Depth Perception↗

PubMed1992

Another perspective on the visual motion aftereffect.

Prolonged adaptation to motion in a given direction produces distinctly different visual motion aftereffects (MAEs) when viewing static vs. dynamic test displays. The dynamic MAE can be exactly simulated by real motion, whereas the static MAE cannot. In addition, the magnitude of the dynamic MAE depends on the bandwidth of motion directions experienced during adaptation, whereas the static MAE does not. Evidently a stationary pattern does not directly activate the neural mechanisms affected during motion adaptation, whereas a dynamic visual display does. These results imply that the traditional explanation of the MAE needs modification.

Adaptation, Ocular↗

PubMed1992

Mitochondrial glutathione in hypermetabolic rats following burn injury and thyroid hormone administration: evidence of a selective effect on brain glutathione by burn injury.

Cerebral cortex, heart, skeletal muscle, and liver mitochondrial glutathione (GSH) levels in severely burned rats are decreased to between approximately 50% to 70% of sham-operated, normally fed controls. In semistarved rats, weight-matched with burned rats, mitochondrial GSH levels in these tissues are decreased to between approximately 70% to 91% of those in sham-operated rats. Total GSH levels in peripheral tissues and brain are decreased to approximately 60% to 65% of control levels in rats with burn injury and food restriction, suggesting a higher mitochondrial GSH turnover in burned rats than in semistarved rats, probably because of higher "stress hormone" levels in burned rats than in semistarved rats. Cerebral cortex mitochondrial GSH levels are unaffected by variations in thyroid hormone status, but liver mitochondrial GSH levels are decreased by triiodothyronine and increased by propylthiouracil. The present results suggest that mitochondrial GSH is not only regulated by the rate of GSH synthesis in the cytosol, but seems to be under hormonal influence as well; stress hormones and triiodothyronine may decrease mitochondrial GSH by increasing mitochondrial oxygen consumption with increased reactive oxygen species formation or by increasing GSH exchange between mitochondria and the cytosol. These findings may be of importance therapeutically in increasing antioxidative defenses to limit oxidative stress injury in hypermetabolic patients.

Animals↗

PubMed1992

Misdirected visual motion in the peripheral visual field.

An object moving against a textured background is accurately perceived when viewed foveally, but when viewed peripherally the object's perceived direction of motion may deviate from veridical by as much as 90 deg. The illusory direction is oblique to the orientation of the background contours, which may themselves be moving or stationary. In several experiments, we examined the boundary conditions for occurrence of the illusion and tested hypotheses concerning its basis. This illusion of perceived direction dramatizes differences in motion processing between the fovea and the periphery.

Fixation, Ocular↗

PubMed1992

The contributions of figure and ground textures to segmentation.

Several models of texture segmentation use spatial gradients in the activity of early filters to locate texture boundaries. The models assume that these filters are identical to those involved in the detection and discrimination of near threshold patterns. The models differ in how activity gradients from different types of filters are combined. We examined this question by measuring the respective contributions of a figure and a ground texture to segmentation. Vertical and horizontal line segments were used to construct two perfectly discriminable textures and these textures were used to construct four types of displays. Each display contained an obliquely oriented figure, but the displays differed in the way this figure was defined. Displays consisted of either (1) a horizontally textured figure on a blank background, (2) a blank figure on a vertically textured background, (3) a horizontally textured figure on a vertically textured background or (4) a figure with a mixed texture (50% vertical lines, 50% horizontal lines) on a blank background. In a two-alternative forced-choice experiment, observers were asked to judge the figure's orientation (right or left oblique), and the contrast of the textures was varied across trials. The resulting psychometric functions for segmentation were very similar for the four types of displays, suggesting ways in which a simple model of segmentation should be modified.

Contrast Sensitivity↗

PubMed1992

Spatial zones of binocular rivalry in central and peripheral vision.

This paper presents results from psychophysical experiments on human binocular rivalry in central and peripheral vision. Results show that the incidence of periods of exclusive visibility of a given eye's rival target increased with decreasing target size, and for a given sized target exclusive visibility increased with retinal eccentricity. Control measures confirmed that these results were not attributable solely to reduced peripheral acuity, to Troxler's effect, or to spatial frequency. We computed the minimum-sized stimulus that would lead to a criterion level of exclusive visibility of one or the other eye; this we term the spatial zone of binocular rivalry. The change in estimated size of spatial zones of rivalry with eccentricity compares favorably with estimates of human cortical magnification. We propose a model that assumes concentrically organized zones of rivalry. These zones do not function independently, but instead exhibit a high degree of mutual excitatory cooperativity. The model has multiple solutions for the foveal zone size, but the best fits predict a diameter of 5.3 or 7.3 min of visual angle; these values dovetail nicely with our empirical estimates of the foveal zone size.

Humans↗

PubMed1992

Binocular rivalry suppression disrupts recovery from motion adaptation.

The motion aftereffect (MAE) lasts longer when the test period does not immediately follow adaptation, a phenomenon called storage. Does storage of the MAE occur if the test target is present but rendered phenomenally invisible owing to the presence of a rival target presented to the other eye during the storage period? Our experiment addressed this question. Following adaptation to a drifting grating, an intervening period preceded testing with a stationary grating. During this period, the adapted eye either viewed the test target immediately or was occluded, and the unadapted eye either viewed a high-contrast rival target or was occluded. Thus four conditions were employed. The duration of the residual MAE was found to be longer for the rivalry condition (grating and rival target viewed) than for the normal MAE condition (grating viewed), and comparable to that in the stored MAE condition (both eyes occluded). Thus, the MAE is stored when the test target is rendered invisible due to binocular rivalry, indicating that a suppressed target is ineffective at promoting decay of the MAE. So while suppression does not prevent information about the adapting grating from reaching the site of generation of the MAE (Lehmkuhle & Fox, 1975), it can prevent information about the test target from reaching the site of the stored MAE. Current models attribute the MAE to reduced responsiveness of direction-selective cortical neurons (Sutherland, 1961; Barlow & Hill, 1963). Thus, storage should reflect a differential return of these adapted cells to preadapted response levels, dependent on postadaptation stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗

PubMed1992

Spatial interactions in binocular rivalry.

Observers tracked binocular rivalry between a pair of small, foveally viewed gratings whose orientation differed between the 2 eyes. In Experiment 1, a textured annulus surrounding 1 eye's grating increased the total duration of exclusive visibility of the grating only when the grating-annulus separation was less than 0.5 degree. In Experiment 2, observers tracked the visibility of a monocular annulus that surrounded a foveally viewed grating that was either engaged in rivalry or fused with a grating alone viewed by the other eye. The visibility of the annulus was greater when the grating it surrounded was not undergoing rivalry fluctuations. In Experiment 3, the predominance of a rival grating was greater when the contours in the surrounding annulus were orthogonal to those of the rival grating. In Experiment 4, total exclusive visibility of a given grating-annulus target was greater when the grating and the annulus contained the same orientation.

Adult↗

PubMed1992

Do recognizable figures enjoy an advantage in binocular rivalry?

Five experiments examined whether recognizable stimuli predominate in binocular rivalry. It was found that a face predominated more than did a pattern equated for spatial frequency, luminance, and contrast; an objective reaction time procedure confirmed predominance of the face. The face was still liable to fragmentation as stimulus size increased. Observers tracked exclusive dominance of a picture of a camouflaged figure (a Dalmatian dog) prior to and then following discovery of the figure's presence; control observers received the same protocol with a scrambled version of the dog stimulus. Compared with control results, predominance of the dog picture was higher even before observers knew of the camouflaged figure. Inversion of the dog figure reduced its predominance. Binocular rivalry is sensitive to object-related, configural properties of a stimulus.

Attention↗

PubMed1992

On the variety of percepts associated with dichoptic viewing of dissimilar monocular stimuli.

Upon dichoptic viewing of dissimilar patterns, several distinct perceptual states may be experienced over time. One state is exclusive monocular dominance, wherein the view of only one eye is seen in its entirety for some period of time. Another state is characterized by a mosaic-like college consisting of portions of the view of each eye. Two other states involve simultaneous perception of both monocular images in their entirety. In one of these states, the two monocular stimuli appear to be superimposed without depth (a phenomenon we shall term 'superimposition'). In the other state, the two monocular stimuli appear to be located at different depth planes (which we shall term 'transparency'). This paper documents the stimulus conditions favoring these various perceptual states. Exclusive monocular dominance occurs most often when the two eyes view dissimilar patterns with the same spatial-frequency content, particularly when both patterns consist of low spatial frequencies. Superimposition also occurs most often when the two eyes view the same spatial frequencies, but predominantly when those spatial frequencies are high. Transparency is favored when the spatial-frequency difference between the eyes is great, particularly when the view of one eye consists of high spatial-frequency information.

Depth Perception↗

PubMed1992

A human visual disorder resembling area V4 dysfunction in the monkey.

We surveyed a broad range of visual functions in a man who complained of abnormal color experience and inability to recognize faces following bilateral damage in the visual cortex. A lesion in his right visual cortex caused complete left visual field loss. A lesion in his left visual cortex, located entirely below the calcarine fissure, affected the vision in his remaining hemifield, the right one. Psychophysical testing showed severely defective color vision and pattern processing, but relatively normal luminance contrast detection thresholds. The finding of normal spatial contrast sensitivity and static stereopsis did not resemble a parvocellular defect of the type described in the monkey. The abilities to detect global coherent motion among noise, structure from motion and dynamic stereopsis, and to pursue moving targets showed normal motion processing at several levels. Together with normal flicker perception, these results excluded magnocellular or MT-like defects. Altogether, the findings mimic area V4 dysfunction.

Adult↗

PubMed1992

Urea kinetics has limited relevance in assessing adequacy of dialysis in CAPD.

The application of urea kinetics to CAPD is controversial. Additional data is presented from our recent study on this topic. Different methods of calculating KT/V and normalized protein catabolic rate (PCRN) are compared and KT/V is shown to be on average 6.5% higher when V is calculated by Watson's formulae instead of by body weight alone. This discrepancy increases with time. It is also shown that standard methods may overestimate KT and underestimate PCRN. KT/V and PCRN by these different methods do not correlate with clinical outcomes. However, if V is calculated by Watson's formulae, there is a significant excess of deaths when KT/V is under 0.5 (weekly KT/V under 1.5). Survival curves show that neither initial KT/V nor PCRN predict failure on CAPD.

Female↗

PubMed1992

Chloroquine therapy in psoriatic arthritis.

Our aim was to examine the efficacy of chloroquine in psoriatic arthritis (PsA) and to assess whether chloroquine therapy exacerbated psoriasis. Thirty-two patients had been given chloroquine therapy while attending the University of Toronto Psoriatic Arthritis Clinic. Twenty-four patients continued therapy for at least 6 months, and 18, or 75%, demonstrated > 30% reduction in the actively inflamed joint count. Moreover, there was a significant decrease in the number of actively inflamed joints. A control group, consisting of 24 patients taking no remittive agents, seen during the same period of time, was identified. Only 14, or 58%, of these patients had a > 30% reduction in inflamed joint count over a 6-month period. This pattern was not significantly different (p = 0.19) from the chloroquine treated group. Of the 32 patients who had been given chloroquine, a total of 6 had an exacerbation of psoriasis, only 1 discontinued therapy. There was no case of exfoliative dermatitis. Six of the 24 control patients had an exacerbation of psoriasis. Our experience suggests that chloroquine may be an effective treatment in PsA, and that it does not exacerbate psoriasis. A prospective, randomized, double blind, controlled trial of antimalarial therapy in PsA is warranted.

Administration, Oral↗

PubMed1992

Relation between fibrositic and control site tenderness; effects of dolorimeter scale length and footplate size.

Recent data have suggested a correlation between the tenderness measured at tender and control sites, differing from earlier studies indicating site specific tenderness. In our study, the "constant control" model is tested against the "correlated control" model, in which control site tenderness varies with fibrositic site tenderness. Our study also addresses relevant technical issues in dolorimetry. We measured threshold of tenderness at 4 sites (2 fibrositic and 2 control) on 21 subjects, using dolorimeters with a 17 kg scale limit, and 3 different footplates, 0.7, 1.4 and 2.0 cm in diameter. To measure observer variation, we used multiple replications by multiple observers, obtaining in all 1,416 observations. There was a strong relationship between control and fibrositic site tenderness with control thresholds twice as high (half as tender). Scale length and dolorimeter footplate size each had important effects. The site specific approach is valuable diagnostically, but more broadly operative mechanisms should be the focus of research and therapy.

Adult↗