Discrimination Of Spatial Phase Research Articles

Discrimination of spatial phase: The roles of luminance distribution and attention

We can easily discriminate certain phase relations in spatial patterns but not others. Phase perception has been found different in the fovea vs. periphery, and for single patterns vs. textures. Different numbers of mechanisms have been proposed to account for the regularities of phase perception.In this study, I attempt to better understand the mechanisms behind discrimination of spatial phase. In order to reveal the role of luminance cues, I use histogram matching of patterns with different phases. Possible effects of attention were studied using visual search experiments with varied stimulus set size. Simple and compound Gabor patches, broadband lines and edges, and textures composed of those patterns were used as stimuli.The experiments indicate that phase discrimination is mediated by two mechanisms. The first uses luminance differences and operates pre-attentively, in parallel across the visual field. The second compares relative positions of dark and bright segments within an image, and is strictly limited by capacity of attention.

Spatial phase discrimination in monkeys with experimental strabismus

Human strabismic amblyopes show deficits in spatial vision that are revealed in a variety of visual tasks. In particular, they show severe deficits in their ability to encode the relative spatial phase of the sinusoidal components in a compound grating. To investigate the neural basis of strabismic amblyopia we tested the ability of monkeys with experimentally induced strabismus to encode spatial phase relationships. First, we trained them to discriminate between compound gratings (made of a fundamental sinusoid and its third harmonic) that differed only in the relative phase of their components. These monkeys exhibited a pattern of severe deficits that resemble those described in the human population of strabismic amblyopes. We conclude that these animals represent a valid model of strabismic amblyopia. Second, we show that a model that had been used to account for the performance of normal human subjects and of humans with anisometropic amblyopia fails to predict the performance of monkeys with strabismic amblyopia.

Discrimination of spatial phase in central and peripheral vision

Sensitivity to relative phase was measured for central and peripheral vision using stimuli comprising 256 harmonics, smoothly filtered in amplitude. With these stimuli, peripheral phase sensitivity was much higher than that previously reported with two-harmonic stimuli. Sensitivity did not depend on the average phase of the stimuli, nor on their second-order statistics, irrespective of the spatial frequency of the stimulus or the position in the visual field. After scaling for size, peripheral sensitivity was as high as central sensitivity. The scaling factor required to equate phase sensitivity was the same as that required to equate contrast sensitivity and grating acuity. These results suggest that phase sensitivity decreases with eccentricity at a similar rate as contrast sensitivity and grating acuity, much more slowly than the positional acuities. This is consistent with the suggestion that phase discrimination is mediated by discriminating the amplitude of the response of quasi-linear filters, and does not require mechanisms that evaluate position. It is suggested that previous measurements on peripheral phase sensitivity may reflect positional uncertainty in the periphery, rather than a deficit in phase sensitivity per se.

Capabilities of monkey cortical cells in spatial-resolution tasks.

The performance of individual neurons in monkey striate cortex has been examined in three spatial-resolution tasks by making microelectrode recordings from single cells in anaesthetized, paralyzed animals. The statistical reliability of responses from cells was used to estimate threshold levels of performance. For each task (resolution acuity for high-contrast gratings, discrimination of gratings varying in spatial frequency, and localization ability, i.e., discrimination of spatial phase), performance approaching psychophysical thresholds was obtained from single cortical cells. The receptive-field organization underlying localization performance was examined in detail by the use of a linear model that relates localization ability to the sensitivity of the receptive field to luminance contrast. Calculations from this model agree well with direct measurements of localization performance and are comparable with psychophysical measurements of hyperacuity. Though it has been suggested that cells with nonoriented receptive fields in cortical layer IVc beta may be responsible for recovering fine-grain spatial information, our calculations indicate that these cells are poorer at localization than many other cells in the cortex.

Right-hemisphere superiority in the discrimination of spatial phase.

Visual field differences have been investigated in various detection and discrimination tasks for simple sinusoidal gratings or for complex gratings composed of two sinusoids of spatial frequencies f and 3f. Sinusoidal gratings were employed to evaluate contrast sensitivity, subthreshold summation effects, aftereffects of adaptation to a high-contrast grating, and spatial-frequency discrimination. The tasks with complex gratings were detection of the 3f component in the presence of a high-contrast f component and spatial-phase discrimination. The stimuli were presented either in the left or in the right visual hemifield. The results indicate a lack of lateralization for detection and spatial-frequency discrimination of sinusoidal gratings, and for the bandwidth of subthreshold summation effects and adaptation aftereffects, whereas the detection of the 3f component in the presence of a high-contrast f component, as well as spatial-phase discrimination of f +3f gratings, show a left-field advantage. This suggests a right-hemisphere superiority in the processing of spatial phase.

The effect of phase structures on spatial phase discrimination.

This study was concerned with the discrimination of multifrequency gratings which differed only in the relative phase of one sinewave component (the test frequency) relative to the other components (the background frequencies). The dependent variable was the contrast of the test frequency required to discriminate between the two gratings. This study found that increasing the test frequency's relative phase difference from 10 to 90 deg significantly increased an observer's contrast sensitivity. However, no overall change in contrast thresholds was measured for test gratings having a relative phase difference between 90 to 180 deg. Moreover, when the mean relative phase was changed from 0 to 45 deg, contrast thresholds did not change when discriminating between gratings having a 90 deg relative phase difference. The type of processing most likely to account for these results in discussed.

Interhemispheric difference in a visual task involving spatial phase discrimination

Interhemispheric difference in a visual task involving spatial phase discrimination