Biomedical subjects

A M Rohaly

Publications and source records attributed to A M Rohaly.

7 recordsLinked to original sources

PubMed1999

The effects of contrast on perceived depth and depth discrimination.

The contrast dependence of perceived depth was quantified through a series of depth matching experiments. Perceived depth was found to be a power law function of contrast. In addition, subjects exhibited a large uncrossed depth bias indicating that low contrast test patterns appeared much farther away than high contrast patterns of equal disparity. For disparities in the range of +/- 4.0 arc min, matching disparities for low contrast patterns were shifted in the uncrossed direction by the same amount. In other words, while the magnitude of the uncrossed depth bias is a power law function of contrast, it is constant with respect to disparity. In a second series of experiments, the contrast dependence of stereo increment thresholds was measured. Like perceived depth and stereoacuity, stereo increment thresholds were found to be a power law function of contrast. These results suggest that contrast effects occur at or before the extraction of depth and have implications for the response properties of disparity-selective mechanisms.

Contrast Sensitivity↗

PubMed1997

Object detection in natural backgrounds predicted by discrimination performance and models.

Many models of visual performance predict image discriminability, the visibility of the difference between a pair of images. We compared the ability of three image discrimination models to predict the detectability of objects embedded in natural backgrounds. The three models were: a multiple channel Cortex transform model with within-channel masking; a single channel contrast sensitivity filter model; and a digital image difference metric. Each model used a Minkowski distance metric (generalized vector magnitude) to summate absolute differences between the background and object plus background images. For each model, this summation was implemented with three different exponents: 2, 4 and infinity. In addition, each combination of model and summation exponent was implemented with and without a simple contrast gain factor. The model outputs were compared to measures of object detectability obtained from 19 observers. Among the models without the contrast gain factor, the multiple channel model with a summation exponent of 4 performed best, predicting the pattern of observer d's with an RMS error of 2.3 dB. The contrast gain factor improved the predictions of all three models for all three exponents. With the factor, the best exponent was 4 for all three models, and their prediction errors were near 1 dB. These results demonstrate that image discrimination models can predict the relative detectability of objects in natural scenes.

Adolescent↗

PubMed1994

Disparity averaging across spatial scales.

To determine whether disparity averaging occurs across spatial scales, a series of depth matching experiments was performed using stimuli composed of the sum of two cosine gratings of different spatial frequency and retinal disparity. Although there were large individual differences in the data, averaging was observed when the relative frequency of the cosines were separated by 3.5 octaves or less and when the relative orientation was less than 30 deg. Beyond these limits, averaging never occurred and the stimulus components were perceived transparently in depth. Furthermore, as the relative contrast of the gratings was varied, perceived depth varied smoothly in the direction of the higher contrast component. This dependence on contrast can be accounted for by a multi-channel model in which separate disparity estimates are generated on each spatial scale and then combined to determine the overall perceived depth of the stimulus.

Contrast Sensitivity↗

PubMed1993

Modeling the contrast-sensitivity functions of older adults.

To determine whether a parabolic template is a good description of the contrast-sensitivity functions (CSF's) exhibited by older adults, the curve-fitting method of Pelli et al. [J. Opt. Soc. Am. A 3(13), P56 (1986)] was applied to contrast-sensitivity data from 100 older subjects (ages 53-85 years). Although the method resulted in reasonable fits for most subjects, closer inspection revealed that this technique may be problematic. A significant number of observers had functions that were nonparabolic, and for many subjects the error tended to be concentrated at the peak of the CSF. In addition, in contrast to the study of Pelli et al., the peak contrast sensitivities of the subjects were only weakly related to Pelli-Robson contrast sensitivity and letter acuity. The data were also fitted with an asymmetric function of variable shape. Whereas this function provided a better fit to the nonparabolic CSF's, it resulted in inferior fits to most of the remaining data. These results demonstrate that the spatial CSF's of older adults cannot be described by a single parametric curve such as a parabola or a function of an exponential and that Pelli-Robson contrast sensitivity and letter acuity are not adequate predictors of their peak contrast sensitivities.

Adolescent↗

PubMed1993

Nature of coarse-to-fine constraints on binocular fusion.

Several stereo-matching algorithms posit that processing on coarse (low spatial frequency) scales constrains processing on fine (high spatial frequency) scales by shifting the disparity range over which fine mechanisms operate. If this were the case, stereo increment thresholds for high frequencies in the presence of low frequencies would be constant, regardless of base disparity. In contrast, we find that in the presence of low spatial frequencies, increment thresholds for high spatial frequencies increase with base disparity, as do increment thresholds for high frequencies presented alone. As a further test of whether information on coarse scales enhances processing on fine scales, diplopia thresholds were measured. In the presence of low frequencies, fusion ranges for high frequencies were severely reduced. These constraints were found to exhibit a power-law dependence on low frequency contrast and to operate over relatively localized areas of the visual field.

Algorithms↗

PubMed1989

Global spatiochromatic mechanism accounting for luminance variations in contrast sensitivity functions.

Psychophysical luminance and chromatic spatial contrast sensitivity functions (CSF's) exhibit a number of variations that depend on the average luminance of the stimulus grating. The contrast sensitivity at each spatial frequency and the maximum resolvable spatial frequency decrease with decreasing average luminance. Most prominently, luminance CSF's have peaks that shift toward lower frequencies, broaden, and disappear as luminance decreases. Chromatic CSF's are flat at low frequencies and do not exhibit luminance-related variations in shape. In this paper, we account for the luminance-dependent variations of CSF's by incorporating them into an earlier model that accounts for the basic high-luminance shapes of psychophysical luminance and chromatic spatial CSF's. These variations are modeled by increasing the spatial extent of a global center mechanism while decreasing the inhibitory effect of a global surround on that center. The results support the hypothesis that the global center mechanism changes size as a means of maintaining the dynamic range and controlling the signal-to-noise ratio as luminance varies.

Biomechanical Phenomena↗

PubMed1988

Inference of global spatiochromatic mechanisms from contrast sensitivity functions.

A single spatiochromatic signal can describe both chromatic and luminance cosine gratings by varying the spatial phase of that signal. This signal representation permits chromatic and luminance spatial contrast sensitivity functions to be described as linear combinations of two global spatiochromatic mechanisms. If the spatiochromatic mechanisms are assumed to have Gaussian spatial profiles, we obtain one mechanism with broad spatial frequency tuning and one with narrow tuning. We find that the relative spatial shapes and extents of these mechanisms are in agreement with the center-surround organization of receptive fields of single retinal and geniculate cells. Furthermore, the chromatic responses of the derived mechanisms are in accord with the color-opponent nature of spatially antagonistic receptive fields.

Biomechanical Phenomena↗