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S H Mahmud

Publications and source records attributed to S H Mahmud.

4 recordsLinked to original sources
PubMed2000

Parallel detection of orientation differences in the presence of orientation gradients.

We measured thresholds and reaction times for detecting a target, defined by orientation contrast, as a function of the number of elements displayed simultaneously and of the linear orientation gradient present in the display. This test served to evaluate how well the human visual system is able to ignore smooth gradients in orientation - similar to what it does with gradients of luminance or wavelength. Smooth orientation gradients are common in natural environments as opposed to the usual laboratory (search) experiments. It turns out that targets defined by a discontinuity in the transition between line orientations can be processed in parallel, i. e. that 'search' times increase by between 0.5 and 6 ms, on average, per additional element displayed, irrespective of the number of elements. But thresholds of orientation difference for the detection of the target increase linearly with the orientation gradient present in the display, and tend to increase more strongly for small gradients, indicating a special bonus for (near) collinearity. The averaged data follow a Weber-law type while this is not true for the individual observers' data. These results show that the visual system is indeed able to detect targets based on orientation contrast, rather than on absolute orientation [cf. Nothdurft (1985). Vision Research, 25, 551-560], but that the orientation gradient cannot be ignored.

Computer Graphics↗
PubMed1999

Relative mislocalization of briefly presented stimuli in the retinal periphery.

We studied the ability to localize flashed stimuli, using a relative judgment task. When observers are asked to localize the peripheral position of a probe with respect to the midposition of a spatially extended comparison stimulus, they tend to judge the probe as being more toward the periphery than is the midposition of the comparison stimulus. We report seven experiments in which this novel phenomenon was explored. They reveal that the mislocalization occurs only when the probe and the comparison stimulus are presented in succession, independent of whether the probe or the comparison stimulus comes first (Experiment 1). The size of the mislocalization is dependent on the stimulus onset asynchrony (Experiment 2) and on the eccentricity of presentation (Experiment 3). In addition, the illusion also occurs in an absolute judgment task, which links mislocalization with the general tendency to judge peripherally presented stimuli as being more foveal than they actually are (Experiment 4). The last three experiments reveal that relative mislocalization is affected by the amount of spatial extension of the comparison stimulus (Experiment 5) and by its structure (Experiments 6 and 7). This pattern of results allows us to evaluate possible explanations of the illusion and to relate it to comparable tendencies observed in eye movement behavior. It is concluded that the system in charge of the guidance of saccadic eye movements is also the system that provides the metric in perceived visual space.

Adult↗