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Biomedical subjects

T E Cohn

Publications and source records attributed to T E Cohn.

At least 19 recordsLinked to original sources
PubMed1991

Postural stability and stereo-ambiguity in man-designed visual environments.

Our modern rectilinear visual environment contains visual stimuli for which evolution has not had time to optimally shape visual processing. One such stimulus, periodic stripes, is known to lead to visual depth ambiguity. In this paper we show that postural instability, as measured by the variance of fore and aft sway, is increased by viewing such stimuli. This instability may be the precursor of falls. Designers must evaluate the visual impressions conveyed by their systems in order to avoid postural instability due to visual ambiguity.

Adult↗
PubMed1991

Saccadic suppression and stimulus uncertainty.

Saccadic suppression is a decline in detectability of a weak flash presented during a saccadic eye movement. We examined the hypothesis of Matin [Psychol. Bull. 81, 899 (1974)] that saccadic suppression may be due to increased stimulus uncertainty during the saccade. Uncertainty could arise from variability and inhomogeneities in the visual frame of reference translation that must accompany a saccade. We measured an average 0.6-log-unit suppression for a brief foveal 1 degree flash in a light-adapted detection task. Receiver-operating-characteristic (ROC) slopes for flash detection during saccades, compared with those when fixating, were reduced, indicating the presence of increased uncertainty. The magnitude of this uncertainty change was estimated and found to be consistent with that required to account for the measured detectability decline. When a flashed pedestal was employed to reduce the effect of uncertainty, there was no saccadic suppression and no ROC slope change. Also, spatially separate flashed markers, intended to reduce uncertainty, led to a significant reduction in saccadic suppression for one of two subjects. Our results are consistent with the hypothesis that a saccade leaves the observer with increased uncertainty as to which subjective visual direction to attend for a stimulus of fixed retinal locus. The magnitude of this uncertainty change can account fully for the saccadic suppression measured.

Eye Movements↗
PubMed1990

Wallpaper illusion: cause of disorientation and falls on escalators.

The wallpaper illusion, first described over a century ago, can occur when a person with normal binocular vision views a pattern that is periodic in the horizontal meridian of the visual field. Escalator trends present such a pattern. Evidence is presented favoring the view that disorientation experienced by escalator riders is caused by this illusion. Possibly some of the estimated 60,000 escalator falls occurring in the United States each year are linked to it.

Accidental Falls↗
PubMed1989

Are rods and cones of dyslexics anomalous?

Grosser and Spafford (in this journal, 1989) have advanced an hypothesis and presented measurements which they believe support the idea of an excess of cones in the peripheral retinae of dyslexics. This note points out that their hypothesis is based on the erroneous assumption that normals have no peripheral cones. Further, their data can be explained by at least two alternative, though uninteresting, methodological hypotheses, that uncontrolled eye movements or experimenter suggestion (or both) could have produced their results. Finally, the requisite methods for assessing color vision, and the cones, were not met in the study.

Child↗
PubMed1986

Analysis of human color mechanisms using sinusoidal spectral power distributions.

We examined the effects of probing human color mechanisms using sinusoidal spectral power distributions (SPD's) varying in frequency (i.e., from 0.1 to 5.0 cycles/300 nm for a constant starting phase) and phase (i.e., from 0 to 360 deg for a fixed frequency of 1 cycle/300 nm) through computer simulation using several color models. Predicted modulation sensitivity functions (MSF's) in spectral frequency and phase differ among the models and indicate that measurements of the minimum amplitudes necessary to detect sinusoidal SPD's would be useful for distinguishing among theories of color vision. MSF's obtained from similar analyses of dichromats' color mechanisms reveal characteristic patterns of modulation sensitivities and suggest that such measures could serve to distinguish type and degree of color-vision defect. Some implications based on sinusoidal approximations to illuminant and reflectance spectra are discussed along with more general considerations regarding sine-wave SPD's as a probe for mechanisms of color vision.

Color Perception↗
PubMed1985

Site of the accelerating nonlinearity underlying luminance-change detection.

The visual psychometric function for luminance-increment detectability (d') is known to be an accelerating function of stimulus energy. Two different models have been suggested to explain this fact and also why a luminance-increment pedestal linearizes the psychometric function. In the present experiment it is shown that a dichoptically presented increment linearizes the psychometric function and facilitates detection of weak signals. Since the dichoptic pedestal combines with the signal centrally, the nonlinearity must then originate more centrally. This result is compatible with the uncertainty model of the nonlinear psychometric function but not with a model that requires a nonlinear transducer.

Humans↗
PubMed1985

Discrimination of luminance increments and decrements.

Discrimination of pulsed increments from decrements improves relative to detection when the adaptation level is decreased or stimulus duration is increased. Using Tanner's theory of recognition [J. Acoust. Soc. Am. 28, 882 (1956)], we show that the subject's internal decision variables corresponding to the increment and decrement stimuli also show increased statistical independence as adaptation is decreased or stimulus duration increases. We infer from this result that at high adaptation levels detection is based predominantly on information in one channel, whereas at low adaptation levels there is information from two channels. Similarly, for long durations, we infer two independent channels, which contribute little information about the sign of the stimulus at short durations.

Discrimination, Psychological↗
PubMed1985

Effect of large spatial uncertainty on foveal luminance increment detectability.

In principle, the ability to detect a luminance increment is lowered when there is uncertainty for its spatial location. Frequency-of-seeing curves were generated for small foveal targets. When fixed in space the target's detectability was more than 10 times higher than when it could occur at one of 140 locations.

Fovea Centralis↗
PubMed1985

Detection and recognition of visual targets.

To aid the development of detection-based interpretations of visual resolution, we evaluated theorems that (1) relate observers' performance in detection of a single target to that in 1-of-m signal detection and (2) predict recognition performance from measured performance at 1-of-m detection. These theorems require that the sensory effects of the stimuli be continuous and that the m signals be equally detectable and mutually orthogonal. To evaluate the theorems, we tested observers in simple detection, 1-of-m detection, and recognition of Landolt C targets and compared predicted and observed performance. Predictions of 1-of-m detection performance from that for simple detection and predictions of recognition from the 1-of-m receiver operating characteristic (ROC) were both accurate to within 0.03 in P(A). In addition, predictions of recognition based on the 1-of-m ROC predicted from simple detection were generally accurate. Thus, under restricted experimental conditions, recognition is determined completely by that for simple detection.

Astigmatism↗
PubMed1985

Visual depth illusion and falls in the elderly.

This article investigates the relation of vision and the effects of age on the maintenance of posture. This relationship in the elderly is explored within the context of visual depth illusions induced by repeating patterns that occur on escalator treads and elsewhere in the environment. Age does not appear to reduce the susceptibility of the elderly to visual depth illusions. However, if age is coupled with declines in motor control and strength, the elderly are probably more susceptible to falls.

Accidents↗
PubMed1978

Effect of chromatic uncertainty on detectability of a visual stimulus.

The detectability of a brief color shift towards red or towards green of a foveally viewed yellow target is less by 25% if the observer is uncertain as to the direction of the color shift. This result matches a prediction of the theory of signal detectability: When signal parameters become uncertain, detectability declines.

Color Perception↗
PubMed1976

Binocular vision: two possible central interactions between signals from two eyes.

Both foveae of light-adapted subjects were stimulated at the same time with monocularly presented lights of increasing or decreasing luminance. Combinations judged just detectable violated predictions of the energy summation and the probability summation hypotheses of binocular interaction. Rather, the results can be explained by independent central neural mechanisms that signal the sum or the difference of stimuli to two eyes.

Humans↗
PubMed1975

Receiver operating characteristic analysis. Application to the study of quantum fluctuation effects in optic nerve of Rana pipiens.

Receiver operating characteristic (ROC) analysis of nerve messages is described. The hypothesis that quantum fluctuations provide the only limit to the ability of frog ganglion cells to signal luminance change information is examined using ROC analysis. In the context of ROC analysis, the quantum fluctuation hypothesis predicts (a) the detectability of a luminance change signal should rise proportionally to the size of the change, (b) detectability should decrease as the square root of background, an implication of which is the deVries-Rose law, and (c) ROC curves should exhibit a shape particular to underlying Poisson distributions. Each of these predictions is confirmed for the responses of dimming ganglion cells to brief luminance decrements at scotopic levels, but none could have been tested using classical nerve message analysis procedures.

Animals↗