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Andrew Glennerster

Publications and source records attributed to Andrew Glennerster.

11 recordsLinked to original sources
PubMed2006

Stereo and motion parallax cues in human 3D vision: can they vanish without a trace?

In an immersive virtual reality environment, subjects fail to notice when a scene expands or contracts around them, despite correct and consistent information from binocular stereopsis and motion parallax, resulting in gross failures of size constancy (A. Glennerster, L. Tcheang, S. J. Gilson, A. W. Fitzgibbon, & A. J. Parker, 2006). We determined whether the integration of stereopsis/motion parallax cues with texture-based cues could be modified through feedback. Subjects compared the size of two objects, each visible when the room was of a different size. As the subject walked, the room expanded or contracted, although subjects failed to notice any change. Subjects were given feedback about the accuracy of their size judgments, where the "correct" size setting was defined either by texture-based cues or (in a separate experiment) by stereo/motion parallax cues. Because of feedback, observers were able to adjust responses such that fewer errors were made. For texture-based feedback, the pattern of responses was consistent with observers weighting texture cues more heavily. However, for stereo/motion parallax feedback, performance in many conditions became worse such that, paradoxically, biases moved away from the point reinforced by the feedback. This can be explained by assuming that subjects remap the relationship between stereo/motion parallax cues and perceived size or that they develop strategies to change their criterion for a size match on different trials. In either case, subjects appear not to have direct access to stereo/motion parallax cues.

Adult↗
PubMed2006

Disparity with respect to a local reference plane as a dominant cue for stereoscopic depth relief.

Earlier studies showed that the disparity with respect to other visible points could not explain stereoacuity performance, nor could various spatial derivatives of disparity [Glennerster, A., McKee, S. P., & Birch, M. D. (2002). Evidence of surface-based processing of binocular disparity. Current Biology, 12:825-828; Petrov, Y., & Glennerster, A. (2004). The role of the local reference in stereoscopic detection of depth relief. Vision Research, 44:367-376.] Two possible cues remain: (i) local changes in disparity gradient or (ii) disparity with respect to an interpolated line drawn through the reference points. Here, we aimed to distinguish between these two cues. Subjects judged, in a two AFC paradigm, whether a target dot was in front of a plane defined by three reference dots or, in other experiments, in front of a line defined by two reference dots. We tested different slants of the reference line or plane and different locations of the target relative to the reference points. For slanted reference lines or plane, stereoacuity changed little as the target position was varied. For judgments relative to a frontoparallel reference line, stereoacuity did vary with target position, but less than would be predicted by disparity gradient change. This provides evidence that disparity with respect to the reference plane is an important cue. We discuss the potential advantages of this measure in generating a representation of surface relief that is invariant to viewpoint transformations.

Cues↗
PubMed2006

Humans ignore motion and stereo cues in favor of a fictional stable world.

As we move through the world, our eyes acquire a sequence of images. The information from this sequence is sufficient to determine the structure of a three-dimensional scene, up to a scale factor determined by the distance that the eyes have moved. Previous evidence shows that the human visual system accounts for the distance the observer has walked and the separation of the eyes when judging the scale, shape, and distance of objects. However, in an immersive virtual-reality environment, observers failed to notice when a scene expanded or contracted, despite having consistent information about scale from both distance walked and binocular vision. This failure led to large errors in judging the size of objects. The pattern of errors cannot be explained by assuming a visual reconstruction of the scene with an incorrect estimate of interocular separation or distance walked. Instead, it is consistent with a Bayesian model of cue integration in which the efficacy of motion and disparity cues is greater at near viewing distances. Our results imply that observers are more willing to adjust their estimate of interocular separation or distance walked than to accept that the scene has changed in size.

Computer Simulation↗
PubMed2006

Quantitative analysis of accuracy of an inertial/acoustic 6DOF tracking system in motion.

An increasing number of neuroscience experiments are using virtual reality to provide a more immersive and less artificial experimental environment. This is particularly useful to navigation and three-dimensional scene perception experiments. Such experiments require accurate real-time tracking of the observer's head in order to render the virtual scene. Here, we present data on the accuracy of a commonly used six degrees of freedom tracker (Intersense IS900) when it is moved in ways typical of virtual reality applications. We compared the reported location of the tracker with its location computed by an optical tracking method. When the tracker was stationary, the root mean square error in spatial accuracy was 0.64 mm. However, we found that errors increased over ten-fold (up to 17 mm) when the tracker moved at speeds common in virtual reality applications. We demonstrate that the errors we report here are predominantly due to inaccuracies of the IS900 system rather than the optical tracking against which it was compared.

Acceleration↗
PubMed2006

Stereoscopic acuity and observation distance.

The amount of depth perceived from a fixed pattern of horizontal disparities varies with viewing distance. We investigated whether thresholds for discriminating stereoscopic corrugations at a range of spatial frequencies were also affected by viewing distance or whether they were determined solely by the angular disparity in the stimulus prior to scaling. Although thresholds were found to be determined primarily by disparity over a broad range of viewing distances, they were on average a factor of two higher at the shortest viewing distance (28.5 cm) than at larger viewing distances (57 to 450 cm). We found the same pattern of results when subjects' accommodation was arranged to be the same at all viewing distances. The change in thresholds at close distances is in the direction expected if subjects' performance is limited by a minimum perceived depth.

Accommodation, Ocular↗
PubMed2005

Systematic distortions of perceptual stability investigated using immersive virtual reality.

Using an immersive virtual reality system, we measured the ability of observers to detect the rotation of an object when its movement was yoked to the observer's own translation. Most subjects had a large bias such that a static object appeared to rotate away from them as they moved. Thresholds for detecting target rotation were similar to those for an equivalent speed discrimination task carried out by static observers, suggesting that visual discrimination is the predominant limiting factor in detecting target rotation. Adding a stable visual reference frame almost eliminated the bias. Varying the viewing distance of the target had little effect, consistent with observers underestimating distance walked. However, accuracy of walking to a briefly presented visual target was high and not consistent with an underestimation of distance walked. We discuss implications for theories of a task-independent representation of visual space.

Computer Simulation↗
PubMed2004

Sensitivity to depth relief on slanted surfaces.

The finest stereoacuity is known to depend on the disparity of a target relative to other visible points. Here we show that a more important factor in determining sensitivity to displacement can be the disparity of a target relative to an invisible interpolation plane through other neighboring points. We tested the sensitivity of observers to displacements of the central column of a regular grid of dots that was either fronto-parallel or slanted about a vertical axis. We found that subjects' sensitivity to displacement was better predicted by a model based on the disparity of a target with respect to the grid plane than it was by a model based on disparity with respect to other reference points. In control conditions carried out on one subject, we found that this result did not depend on adaptation to the grid slant because it also occurred when the direction of grid slant varied from trial to trial. Nor did it depend on the perception of slant, because the data were similar for trials on which the grid was perceived as approximately fronto-parallel or markedly slanted. Our results indicate that sensitivity to the depth component of the target displacement is based on disparity relative to a local reference plane.

Cues↗
PubMed2004

The role of a local reference in stereoscopic detection of depth relief.

Stereoacuity thresholds have been shown to depend on the disparity of a point with respect to a slanted reference plane through neighbouring points [Curr. Biol. 12 (2002) 825]. Here we explored a wider range of conditions, including slanting the reference points about a horizontal axis and varying the spacing of the reference dots, allowing alternative hypotheses for the effect to be distinguished. The stimulus consisted of three dots; the outer two defined a line that was slanted in depth. Observers judged in which of two intervals the third, central dot was displaced from the location midway between the outer reference dots. The displacement consisted of both a disparity and a shift in the fronto-parallel plane. We compared performance for pairs of conditions in which the disparity was the same but the fronto-parallel shifts were in opposite directions. Models based purely on relative disparity predict that performance should be the same for these conditions. We found consistent differences: performance was always better when the target had a greater disparity with respect to the line joining the reference dots. The other stimulus parameters varied were: target disparity (concave/convex), stimulus size (large/small), slant sign (sky/ground) and axis (vertical/horizontal). The results suggest that either (a) disparity with respect to the line drawn through the outer reference dots or (b) difference in disparity gradients on either side of the target determines the depth discrimination threshold for these stimuli.

Cues↗
PubMed2003

Pooling speed information in complex tasks: estimation of average speed and detection of nonplanarity.

To gain insight into how speeds are combined in structure-from-motion, we compared performance for estimating the mean speed and performance for detecting deviations from planarity. The stimuli showed a center dot surrounded by an annulus of dots. In one (plane) condition, the stimuli simulated a rotating plane. In a two alternative forced choice (2AFC) task, the subject had to choose in which of two stimuli the center dot moved in the plane. In another (cloud) condition, the same dot locations and speeds were used but now assigned to different dots. Such a stimulus resembles a translating and rotating cloud of dots. In this case, the subject had to choose the stimulus in which the center dot moved with the mean speed of the surrounding dots. Performance was measured as a function of deformation/slant. Although location and speeds were the same in both conditions, performance was much poorer in the cloud condition. Subsequent experiments and an ideal observer model point to a plausible explanation: in detecting deviations from planarity, the visual system can focus on the most reliable pieces of information (the slower dots, closest to the test dot). Although performance could benefit by taking more dots into account, performance barely improved with an increase in the number of dots. This may reflect a limited processing capacity of the visual system.

Humans↗
PubMed2002

Evidence for surface-based processing of binocular disparity.

It is convenient to think of an object's location as a point within a Cartesian framework; the x axis corresponds to right and left, the y axis to up and down, and the z axis to forward or backward. When an observer is looking straight ahead, binocular disparities provide information about distance along the z axis from the fixation plane. In this coordinate system, changes in disparity are treated as independent of changes in location along the orthogonal x and y axes. Does the human visual system use this three-dimensional coordinate system, or does it specify feature location in a coordinate frame determined by other nearby visible features? Here we show that the sensitivity of the human stereo system is determined by the distance of points with respect to a local reference plane, rather than by the distance along the z axis with respect to the fixation plane. There is a distinct advantage to using a local frame of reference for specifying location. It obviates the need to construct a complex three-dimensional space in either eye-centered or head-centered coordinates that must be updated with every shift of the eyes and head.

Biophysical Phenomena↗