Biomedical subjects

Philip A Duke

Publications and source records attributed to Philip A Duke.

7 recordsLinked to original sources

PubMed2006

Flash lag in depth.

The perceived position of a moving target at a particular point in time, indicated by a flash, is often judged to be different from its actual location. Here, we show that the position of a target moving in depth is also systematically mislocalized. We used three types of targets moving in depth at a range of speeds from 2 to 16 cm/s. (i) A target realistically rendered that included concordant looming, disparity, and perspective cues. (ii) A random dot surface whose depth was defined by disparity, without concordant perspective or looming cues. (iii) A surface of dynamic random dots whose depth was defined by disparity with no consistent motion visible monocularly. Subjects viewed the targets moving either towards or away from them and indicated whether the targets appeared to be nearer or farther than a continuously present reference depth at the moment that a flash was presented. A staircase procedure was used to null, and thus measure, any perceptual displacement from the reference depth. A flash lag in depth was found in which the target appeared ahead of its true position, displaced by a constant amount of time depending on the stimulus type and the direction of motion (towards or away). The time displacement varied from 76 ms (for the realistic target moving away from the observer) to 263 ms (for static random dots moving towards). These effects may depend on the confidence with which subjects were able to judge the location of our various targets: greater confidence leading to a smaller temporal displacement.

Cues↗

PubMed2005

Depth aftereffects mediated by vertical disparities: evidence for vertical disparity driven calibration of extraretinal signals during stereopsis.

Perceptual adaptation often results in a repulsive aftereffect: stimuli are seen as biased away from the adaptation stimulus (). Here we report the absence of a repulsive aftereffect for a vertical gradient of vertical disparity (or vertical size ratio, VSR). We exposed observers to a binocular stimulus consisting of horizontal lines. This stimulus contains vertical, but not horizontal disparities. The visual system was able to measure the VSR of this stimulus: although the lines themselves always appeared unslanted, the VSR carried by the lines had a dramatic effect on the apparent slant of a horizontal row of dots, as predicted by recent accounts of Ogle's (1938) induced effect (e.g., Backus, Banks, van Ee, & Crowell, 1999). Yet we observed no repulsive aftereffect for the VSR signal: after adaptation to horizontal lines that were vertically larger in one eye, we found an attractive aftereffect, the magnitude of which was largest in stimuli that did not contain a VSR signal. We interpret these results as a case of recalibration: disagreement between extra-retinal eye position signals (EP) and VSR causes a recalibration in the use of EP as used in the stereoscopic perception of slant.

Adaptation, Physiological↗

PubMed2005

Vertical-disparity gradients are processed independently in different depth planes.

We examined the effects of vertical-disparity gradients on apparent depth curvature of textured surfaces. In Experiment 1, vertical disparities induced expected curvatures when the surface had a horizontal disparity of < +/-40.34'. A central row of elements, lacking vertical disparities, ceased to have the same apparent curvature as the surface when the horizontal disparity between row and surface exceeded +/-5'. In Experiment 2, vertical disparities were not pooled between superimposed surfaces separated by horizontal disparities > +/-10'. Thus, vertical-disparity gradients are not pooled over depth for curvature perception. Our results suggest that vertical disparities are used to determine distances to surfaces directly, rather than to estimate vergence.

Adult↗

PubMed2005

Spatial and temporal properties of stereoscopic surface interpolation.

It is well established that under a wide range of conditions when a sparse collection of texture elements varies smoothly in depth, the spaces between the elements are assigned depth values. This disparity interpolation process has been studied in an effort to define some of its fundamental spatial and temporal constraints. To assess disparity interpolation we employed two tasks: a novel task that relies on the bisection of illusory boundaries created when subjective stereoscopic surfaces intersect, and one that relies on a 3-D shape discrimination. The results of both experiments show that there is no improvement in performance when texture density is increased from near 0.20 to 0.85 or when exposure duration is increased from 50-100 to 1000 ms. This lack of dependence on the addition of features that define the interpolated surface, along with the abrupt decline in performance below a critical value, is consistent with the view that surface interpolation is an important function of human stereoscopic vision.

Adult↗

PubMed2003

Adaptation to vertical disparity induced-depth: implications for disparity processing.

Depth aftereffects produced by prolonged inspection of an object in depth can be mediated by monocular and binocular depth cues. The adapting mechanisms responsible for such effects have not yet been fully determined. Theories of binocular depth aftereffects typically posit a role of an adaptive horizontal disparity sensitive mechanism, implying multiple cue-specific mechanisms for depth aftereffects. Here we examined whether binocular depth aftereffects can be attributed to such a cue-specific mechanism. In Experiment 1 we did so using a technique allowing us to maintain horizontal disparities and vergence constant for our adaptation stimuli, whilst manipulating simulated depth by virtue of a vertical disparity induced-depth effect. We found that depth aftereffects were almost identical to those produced by adaptation to stimuli of equivalent depth produced by conventional horizontal disparity modulations. In Experiment 2, we examined depth aftereffects following adaptation to apparently frontal surfaces produced by different combinations of horizontal and vertical disparity modulations. Aftereffects were close to zero. These results suggest that binocular depth aftereffects are not due to adaptation of a horizontal disparity sensitive mechanism, and we argue that adaptation occurs at the level of a 3D shape sensitive mechanism derived from multiple cues. Experiment 3 was a control to examine whether the two types of adaptation stimuli in Experiment 1 were indeed perceptually the same, since in theory they may differ if vertical disparities influenced metric depth scaling. We found no evidence of this, and concluded that the two classes of stimuli used in Experiment 1, though consisting of very different patterns of disparity, were perceptually equivalent.

Adaptation, Physiological↗

PubMed2003

Monocular transparency generates quantitative depth.

Monocular zones adjacent to depth steps can create an impression of depth in the absence of binocular disparity. However, the magnitude of depth is not specified. We designed a stereogram that provides information about depth magnitude but which has no disparity. The effect depends on transparency rather than occlusion. For most subjects, depth magnitude produced by monocular transparency was similar to that created by a disparity-defined depth probe. Addition of disparity to monocular transparency did not improve the accuracy of depth settings. The magnitude of depth created by monocular occlusion fell short of that created by monocular transparency.

Adult↗

PubMed2003

Stereoscopic surface interpolation supports lightness constancy.

The human visual system has a remarkable ability to construct surface representations from sparse stereoscopic, as well as texture and motion, information. In impoverished displays where few points are used to define regions in depth, the brain often interpolates depth estimates across intervening blank regions to create a compelling sense of a solid surface. The set of experiments described here examined stereoscopic interpolation using a novel technique based on lightness constancy. The effectiveness of this method is notable because it stands as the only technique to date that unequivocally examines the perception of interpolated surfaces, and not surfaces inferred subjectively from depth information in the stimulus. Further, these data support the growing evidence that a primary function of the stereoscopic system is to define three-dimensional surface structure.

Adult↗