Biomedical subjects

U Tulunay-Keesey

Publications and source records attributed to U Tulunay-Keesey.

At least 19 recordsLinked to original sources

PubMed1996

Brightness of uniform stabilized fields.

Brightness of uniform fields during normal and stabilized viewing was determined as a function of adapting luminance, field size, and luminance gradient of the edges of the adapting field. In one set of experiments, it was found that, over a range of adapting luminances from 6 to 9600 td, a uniformly-illuminated 7.5 deg hemifield appeared about 1 log unit brighter in normal viewing than when it was retinally-stabilized. In the second set of experiments, it was found that the loss of brightness due to stabilized viewing was significantly greater for large fields with raised cosine edges than for small fields with step edges. Both sets of results can be accounted for by a two-stage model of light adaptation previously proposed to account for the fading time of stabilized images.

Adaptation, Ocular↗

PubMed1994

Adaptation with a stabilized retinal image: effect of luminance and contrast.

The addition of a uniform increment of luminance (L) to a faded retinally-stabilized target results in the subjective reappearance of the image with contrast opposite to that of the target. This phenomenon, called apparent phase reversal (APR), reveals a nonlinear gain mechanism in the adaptation process. The magnitude of the threshold increment to elicit APR (Lapr) is a measure of the state of stabilized adaptation. In the experiments reported here, Lapr was studied as a function of background luminance (Lo) and contrast (m) of the adapting stimulus. It was found that Lapr increases with increasing Lo, but does not depend on m. The data are analyzed within the context of a previously proposed model of stabilized image fading consisting of a multiplicative inverse gain followed by a subtractive process. It was found that the addition of a contrast processing stage was required to account for the relationship between Lapr and m.

Adaptation, Ocular↗

PubMed1994

Effect of orientation on spatiotemporal contrast sensitivity in multiple sclerosis.

Spatiotemporal contrast sensitivity at three orientations, vertical, horizontal and oblique, was studied in 18 patients with clinically definite and laboratory-confirmed definite multiple sclerosis (MS). Nineteen age-matched control subjects were also studied under identical experimental conditions. Contrast thresholds for detecting steady and counterphase modulated (5 Hz) gratings ranging in spatial frequency from 0.5 to 12 c/deg were measured by a modified psychophysical method of limits. With the exception of two patients (three eyes) whose Snellen acuity scores were 20/70, all observers had acuity scores of 20/30 or better. All subjects were corrected for astigmatism. Orientation, spatial frequency and temporal frequency interacted differently in determining contrast sensitivity in the two groups of observers. For the controls, an oblique effect was present for both the steady and counterphase modulated gratings of high spatial frequencies, and there was no orientation-dependent loss of sensitivity for low spatial frequencies. For the observers with MS, there was no oblique effect, but sensitivity was dependent on orientation for the low spatial frequencies. Most patients with MS had reduced contrast sensitivity, compared to the controls, at one or more orientations. Counterphase modulation increased sensitivity to the low spatial frequencies and decreased sensitivity to the high spatial frequencies for both normal controls and patients with MS. In patients with MS this effect of temporal modulation on contrast sensitivity was markedly enhanced.

Adult↗

PubMed1993

Fading time of retinally-stabilized images as a function of background luminance and target width.

Fading time of a retinally-stabilized difference-of-Gaussian (DOG) stimulus depends on the background luminance, contrast and spatial frequency content of the stimulus. A model of the visual system including a nonlinear multiplicative, non-local and fast process followed by a linear subtractive, local and slower process accounts for these effects. Analysis of the fading time data allows us to estimate the spatiotemporal characteristics of the proposed adaptation processes. The model is consistent with recent models of normal light adaptation from the probe-flash paradigm.

Adaptation, Ocular↗

PubMed1991

Dynamics of adaptation for vision with a stabilized image.

The addition of a uniform increment of light to a high-contrast image that has been stabilized on the retina reveals marked perceptual nonlinearities. When the increment is small, the pattern appears in its original phase (OP), large increments produce an apparent phase reversal (APR), and intermediate increments may yield an apparently blank field or an oscillation of the apparent phase. In the present series of studies the threshold values used to produce a stable OP and APR were determined as a function of adaptation time before the application of the increment. The stabilized target had a luminance profile consisting of the difference of two Gaussians. A model of detection incorporating a multiplicative gain controlled by a filtered version of the stimulus was used to account for the occurrence of the OP and the APR and the transitory phenomena following the uniform increment. It is argued that the midpoint of the transition zone between the OP and APR, corresponding to blanking, enables us to estimate the shape of the step response function of the gain filter independently of the subsequent detection processes.

Adaptation, Ocular↗

PubMed1988

Threshold and suprathreshold spatiotemporal response throughout adulthood.

The effect of age on spatiotemporal contrast sensitivity at both threshold and suprathreshold levels was studied in 63 adults ranging in age from 13 to 67 years. Sensitivity for low spatial frequencies modulated at 0 to 15 Hz was not affected by age, but a progressive age-related elevation of threshold was found for combinations of high spatial and temporal frequencies. The magnitude of the effect ascribed to age on spatial frequency was larger than the effect of age on temporal frequency. For most combinations of spatial and temporal frequencies, elevation of thresholds started at approximately 45 years of age. No age-related effects were observed in a contrast-matching task performed at suprathreshold levels.

Adolescent↗

PubMed1987

The role of eye movements in motion detection.

The roles of small eye movements of fixation, and of different kinds of background in motion detection were studied. Minimum detectable displacement for a luminous line oscillating either in a blank field or in the presence of three types of background was measured under two viewing conditions: normal, when eye movements generate normal movements of the image on the retina, and stabilized when these retinal image movements were nearly eliminated. It was demonstrated that eye movements enhance motion detection for a sinusoidally moving target when the target is superimposed on a patterned background; they are detrimental when there is no background. In addition, it was found that the function relating threshold amplitude to frequency of movement is band-pass when the image is stabilized or when the bar moves on a blank field, and is more low-pass when both the background and the test target are subject to the effects of eye movements.

Eye Movements↗

PubMed1987

Apparent phase reversal during stabilized image fading.

A pattern with a horizontal luminance profile described by the difference of Gaussians (DOG) was viewed under conditions of retinal image stabilization. When a uniform increment was applied after the image had disappeared, the pattern reappeared, but with the reversed phase. This phenomenon of apparent phase reversal (APR) is indicative of changes in local sensitivity. It was studied as a function of the time at which the increment was applied and the space constant of the DOG pattern. It was found that the threshold increment necessary to evoke an APR was an exponential function of time. A slight dependence on the spatial dimensions of the stimulus was also demonstrated. The data were examined according to a model of spatial vision that includes a gain that is inversely proportional to a spatially and temporally filtered version of the stimulus. The data provided estimates of the time constant and the spatial extent of the gain mechanism.

Humans↗

PubMed1986

Suprathreshold responses of the visual system in normals and in demyelinating diseases.

It is well-established that diseases affecting the visual pathway can result in the elevation of contrast thresholds. Little is known, however, about how people with decreased sensitivity to contrast perceive targets at suprathreshold levels of contrast. It is known that the normal visual response at suprathreshold levels cannot be linearly derived from the threshold contrast function. It may be expected that threshold abnormalities may not predict the quality of vision for high contrast images which prevail in normal operating conditions. In this investigation, the response to suprathreshold visual stimuli (vertical sine-wave gratings) in multiple sclerosis (MS) and optic neuritis patients with contrast sensitivity deficits was studied. Forty-eight normal eyes served as controls. Suprathreshold response was assessed with a matching procedure. Performance above threshold, for each individual, was estimated by a score which was the ratio of the contrast match to the contrast threshold for a given spatial frequency. The majority of the eyes studied showed that patients performed normally at high contrast levels. Indeed, in a number of cases, the scores implied that patients can perform better than normals. In some eyes, this enhancement of suprathreshold response was correlated with the threshold loss. These results suggest that a "compensation" mechanism helps make the perception of objects independent of the quality of detection at threshold for these patients. In other eyes, an abnormal decrease in the suprathreshold response measure was found, which was not related to the threshold deficit. Thus, suprathreshold responses of MS or optic neuritis patients cannot be predicted from their threshold contrast sensitivity.

Adolescent↗

PubMed1985

Spatiotemporal responses of the visual system in demyelinating diseases.

Previous studies have shown that, in patients with lesions of the visual pathway, contrast sensitivity (CS) measured with stationary sine-wave gratings can demonstrate deficits, that is, anomalies in pattern detection. The purpose of this study was to investigate whether temporal processing can be affected in demyelinating lesions of the visual pathway. CS was measured for eight spatial frequencies (SF), using stationary and temporally modulated stimuli in a group of 10 patients with multiple sclerosis or optic neuritis. A control group was composed of 48 normal eyes. With stationary stimuli, CS losses were found in 17 eyes of patients; 11 of these eyes were 'unaffected'. The importance of the CS measurements in detecting early subclinical visual damage is emphasized. The effect of temporal modulation in patients with CS deficits was different from that observed in normals: CS deficits were modified by temporal variation in three distinct manners. In 7 eyes (type 1 effect), temporal modulation reduced the deficit at all SFs. In type 2 (5 eyes), the effect was reversed at low and high SFs: the deficit was reduced at low SFs (below 2-3 cycles/deg) and aggravated at high SFs. In type 3 (4 eyes), CS deficit was aggravated over most SFs. Thus these patients exhibited anomalies of the spatiotemporal responses which could not be predicted by the CS to stationary stimuli. Some theoretical models of spatiotemporal processing proposed in normal vision might account for our results. Possible explanations of these findings are proposed and discussed.

Adolescent↗

PubMed1983

Response to the length of moving visual stimuli of the brisk classes of ganglion cells in the cat retina.

Response histograms were collected for brisk-sustained and brisk-transient ganglion cells in the cat retina as narrow bars were moved backwards and forwards across their receptive fields. When a bar of fixed length was moved across the centre of the receptive field with contrast proportional to velocity, a constant response was obtained as long as the centre of the receptive field was crossed within the summation time. However, if the length of the bar was such that it extended beyond the centre, then there was a small but steady increase in surround antagonism for an increase in velocity. The same response was produced by a brief whole-field flash as by an extended bar moving across the receptive field at high velocity if both stimulus conditions delivered the same energy uniformly across the receptive field. With brisk-sustained cells it was observed, for small bar lengths, that bar length and contrast could be exchanged to give a constant response, even when there was considerable non-linearity in the over-all stimulus-response relationship. Thus conditions that resulted in constant stimulus flux produced a constant response. This property was seen at both high and low velocities for the majority of brisk-sustained units. The stimulus-response relationship had a greater range of linearity at high velocities than at low velocities. From similar experiments with brisk-transient cells it was observed that bar length and contrast could only be exchanged to give a constant response at high velocities. At low velocities there was considerable non-linearity: there appeared to be saturation of the response from local regions and it was necessary to extend the bar outside such a region to obtain an increase in response. At lower velocities, despite the changes seen in length-response curves under different conditions of contrast and velocity, the degree of surround antagonism remained constant for a given cell. Further, both brisk-sustained and brisk-transient cells showed the same degree of surround antagonism.

Action Potentials↗

PubMed1983

Sensitivity to countermodulating gratings following spatiotemporal adaptation.

Contrast sensitivities to countermodulating gratings were measured with a two-alternative temporal forced-choice procedure following adaptation to a static grating of the same spatial frequency, a homogeneous flickering field of the same temporal frequency, or a countermodulating grating of identical spatial and temporal frequencies. At high spatial frequencies, the temporal-frequency content of the adaptation was not critical, that is, a countermodulating adaptation grating was only slightly more effective at raising threshold than was a static adaptation grating. At low spatial frequencies, the sensitivity to countermodulating test gratings could not be reduced by either a high-contrast stimulus matching the test in the spatial domain only or by one matching the test in the temporal domain only. Adapting to a high-contrast stimulus matching the countermodulating test grating in both spatial- and temporal-frequency domains was effective at reducing test sensitivity for one observer but not for another.

Adaptation, Ocular↗

PubMed1982

Fading of stabilized retinal images.

It is well known that targets whose images are stabilized on the retina by optical means, as well as afterimages that are naturally stabilized on the retina, fade and eventually disappear. Comparative data are presented on the rate of disappearance of stabilized images and afterimages as a function of contrast and spatial frequency. The main finding is that they disappear in a similar fashion only when target contrast is low.

Afterimage↗

PubMed1981

Residual vision in humans who have been monocularly deprived of pattern stimulation in early life.

Spatio-temporal sensitivity at and above threshold was investigated in a group of patients who exhibited visual loss secondary to uniocular congenital cataract which was present within the first year of life and later removed. The results fall into two general categories depending upon the severity of the visual loss and in particular upon the nature of the temporal loss. In the group exhibiting less severe amblyopia, contrast sensitivity for high and medium spatial frequencies was attenuated to a similar extent for all temporal frequencies. In the group exhibiting more severe amblyopia no form vision a was present; only temporal perception remained. In these cases a greater loss of flicker threshold sensitivity occurred at higher temporal frequencies. Supra-threshold tests revealed that movement perception was effectively normal in both of these groups. These results demonstrate that stimulus deprivation amblyopia, while different in some respects from anisometropic amblyopia is more similar to that condition than to strabismic amblyopia Previous animal results derived from monocular lid suture in cat and monkey are compared with these findings.

Adolescent↗

PubMed1981

Spatiotemporal characteristics of thresholds adjacent to a luminance edge.

Spatiotemporal incremental and decremental thresholds were measured for a thin vertical line (target) positioned adjacent to a briefly presented vertical edge. A significant difference between the stimuli used here and those used in previous studies is the background level against which the edge was presented. Here the edge was formed by briefly decreasing the luminance of the left side of a light background. This novel condition was compared with the more usual condition in which the edge is formed by briefly increasing the luminance of the right half of a dark background. In a further test the buildup of threshold after an edge was switched on was also measured. When the target was presented on the side of the edge that remained fixed in luminance, a small but reliable threshold change adjacent to the edge was measured. The effect was much larger for measurements made on the side of the edge that changed in luminance; however, the spread was comparable for the two conditions. For the target presented on the light side of the edge, decremental thresholds were much larger than incremental thresholds. This is attributed, at least in part, to the different types of tasks required of the observer. Maximum temporal threshold elevation occurs at or just before (i.e., less than 16 msec) 0 asynchrony between edge and target. The results are interpreted at a qualitative level as supporting a receptive-field type of model in which the edge, at various spatiotemporal locations relative to the target, inhibits or excites activity at the receptive field centered on the target.

Adult↗

PubMed1980

Phase selectivity of spatial frequency channels.

The phase selectivity of spatial frequency channels was measured, using an adaptation technique. Subjects first adapted to a grating of a given spatial frequency; subsequent threshold measurements were made at various spatial frequencies and phase shifts. Changes in the phase relationship between the test and adaptation gratings due to eye movements were circumvented by viewing the grating through an image stabilization apparatus. Local retinal adaptation was minimized by using an adaptation grating whose contrast flickered sinusoidally as a function of time. We were able to demonstrate channel-like frequency tuning for all conditions studied, but the threshold elevations following adaptation were always independent of the phase shift between the test and adaptation gratings. Our results imply that the channels which are selectively tuned to spatial frequency are not selectively tuned to spatial phase.

Adaptation, Ocular↗

PubMed1980

Thresholds at luminance edges under stabilized viewing conditions.

Increment thresholds were measured for a small, briefly presented test line as a function of distance from a high-contrast abrupt luminance edge. The experiments were carried out under both stabilized and unstabilized viewing conditions to determine the role of eye movements in the "edge threshold effect." It was found that the edge threshold effect (i.e., the rise in threshold at the luminance edge) was less pronounced under stabilized conditions. We conclude from this that a significant portion of this effect is mediated by the temporal transients that are brought about by eye movements. Little difference is found between stabilized and unstabilized conditions when the background is briefly presented. Narrow bright bands appear on the bright side of a sharp edge for unstabilized viewing, but disappear under stabilization.

Eye Movements↗

PubMed1980

Contrast sensitivity measures and accuracy of image stabilization systems.

Recently, it has been argued that the precision of image stabilization is reflected in the magnitude of the differences in contrast sensitivity measures obtained with and without image stabilization. Here we present two sets of data, one showing large and the other small differences in contrast sensitivity to sinusoidal gratings viewed under stabilized and unstabilized, normal conditions. Both sets of data were obtained by the use of the same apparatus optimized for image stabilization. Large differences occur between unstabilized and stabilized measures of sensitivity only when the observer is allowed to scan the unstabilized test grating, or to prolong inspection of the stabilized target thus allowing for disappearance of the stabilized image. On the other hand, when the target is presented for a few seconds and the observer fixates on it, normal image motion, which results from eye movements of fixation, is found to enhance contrast sensitivity by only a small amount. It would appear, therefore, that the extent of reduction of sensitivity for a stabilized grating cannot be used as an index of the precision of image stabilization.

Adult↗