Motion-transparent inducers have different effects on induced motion and motion capture.
(25/1300)
To assess the relationship among the underlying mechanisms of induced motion, motion capture, and motion transparency, directions of the former two illusions in the presence of motion-transparent inducers were examined. Two random-dot patterns (inducers) were superimposed upon a stationary disk (target), and moved in orthogonal directions. Either a high-contrast target (for induced motion) or a low-contrast target (for motion capture) was used. The task was to report the perceived direction of the target. The depth order of inducers was controlled either by adding binocular disparity or by asking the subject to report subjective depth order. For induced motion, the target appeared to move in the direction opposite to the inducer that had a disparity closer to the target; when there was no difference in disparity, induced motion occurred oppositely to the 'vector sum' of the inducers' directions. For motion capture, the target was captured by the inducer that subjectively appeared behind. These results suggest that the underlying mechanism of motion capture utilizes the output from the process for motion transparency, whereas induced motion has no clear relationship to the output of the process for motion transparency. (+info)
The role of perspective information in the recovery of 3D structure-from-motion.
(26/1300)
When investigating the recovery of three-dimensional structure-from-motion (SFM), vision scientists often assume that scaled-orthographic projection, which removes effects due to depth variations across the object, is an adequate approximation to full perspective projection. This is so even though SFM judgements can, in principle, be improved by exploiting perspective projection of scenes on to the retina. In an experiment, pairs of rotating hinged planes (open books) were simulated on a computer monitor, under either perspective or orthographic projection, and human observers were asked to indicate which they perceived had the larger dihedral angle. For small displays (4.6 x 6.0 degrees) discrimination thresholds were found to be similar under the two conditions, but diverged for all larger stimuli. In particular, as stimulus size was increased, performance under orthographic projection declined and by a stimulus size of 32 x 41 degrees performance was at chance for all subjects. In contrast, thresholds decreased under perspective projection as stimulus size was increased. These results show that human observers can use the information gained from perspective projection to recover SFM and that scaled-orthographic projection becomes an unacceptable approximation even at quite modest stimulus sizes. A model of SFM that incorporates measurement errors on the retinal motions accounts for performance under both projection systems, suggesting that this early noise forms the primary limitation on 3D discrimination performance. (+info)
Second-order processing of four-stroke apparent motion.
(27/1300)
In four-stroke apparent motion displays, pattern elements oscillate between two adjacent positions and synchronously reverse in contrast, but appear to move unidirectionally. For example, if rightward shifts preserve contrast but leftward shifts reverse contrast, consistent rightward motion is seen. In conventional first-order displays, elements reverse in luminance contrast (e.g. light elements become dark, and vice-versa). The resulting perception can be explained by responses in elementary motion detectors turned to spatio-temporal orientation. Second-order motion displays contain texture-defined elements, and there is some evidence that they excite second-order motion detectors that extract spatio-temporal orientation following the application of a non-linear 'texture-grabbing' transform by the visual system. We generated a variety of second-order four-stroke displays, containing texture-contrast reversals instead of luminance contrast reversals, and used their effectiveness as a diagnostic test for the presence of various forms of non-linear transform in the second-order motion system. Displays containing only forward or only reversed phi motion sequences were also tested. Displays defined by variation in luminance, contrast, orientation, and size were effective. Displays defined by variation in motion, dynamism, and stereo were partially or wholly ineffective. Results obtained with contrast-reversing and four-stroke displays indicate that only relatively simple non-linear transforms (involving spatial filtering and rectification) are available during second-order energy-based motion analysis. (+info)
Measuring perceived 3D shape at multiple spatial scales.
(28/1300)
We present and test a novel multiscale representation of perceived 3D surface orientation: the orientation path. Using a multiscale probe, we measure perceived surface orientation at multiple spatial scales; linking the measurements for a given surface location yields that location's orientation path. The multiscale data obtained show that observers consistently see different surface orientations at different spatial scales. We demonstrate that such multiscale data can reveal multiscale differences between observers' percepts of a stimulus and the stimulus geometry. We also demonstrate the use of the orientation path in evaluating the multiscale effects of adding a depth cue to a 3D display. (+info)
Local and global stereopsis in the horse.
(29/1300)
Although horses have laterally-placed eyes, there is substantial binocular overlap, allowing for the possibility that these animals have stereopsis. In the first experiment of the present study we measured local stereopsis by obtaining monocular and binocular depth thresholds for renal depth stimuli. On all measures, the horses' binocular performance was superior to their monocular. When depth thresholds were obtained, binocular thresholds were several times superior to those obtained monocularly, suggesting that the animals could use stereoscopic information when it was available. The binocular thresholds averaged about 15 min arc. In the second experiment we obtained evidence for the presence of global stereopsis by testing the animals' ability to discriminate between random-dot stereograms with and without consistent disparity information. When presented with such stimuli they showed a strong preference for the cyclopean equivalent of the positive stimulus with the real depth. These results provide the first behavioral demonstration of a full range of stereoscopic skills in a lateral-eyed mammal. (+info)
Computational analysis of disparity modulation sensitivity: an explanation in terms of a Bayesian surface reconstruction.
(30/1300)
Sensitivity to binocular disparity modulation has been shown to have a bandpass nature. This paper presents a computational account for the disparity modulation function (DMF) in terms of a Bayesian surface reconstruction. The Bayesian approach suggests that prior assumptions about surface structure will affect the perception of disparity modulation. Taking into account a prior constraint of surface smoothness being imposed on the perceived surfaces, we propose that computing the first derivatives of the surfaces determines the bandpass shape of the DMF. Based on this idea, we derive an analytical prediction of the DMF. It is then shown that the prediction gives a good fit to the empirical data. Implications for possible mechanisms underlying the DMF are discussed. (+info)
Stereopsis from contrast envelopes.
(31/1300)
We report two experiments concerning the site of the principal nonlinearity in second-order stereopsis. The first exploits the asymmetry in perceiving transparency with second-order stimuli found by Langley et al. (1998) (Proceedings of the Royal Society of London B, 265, 1837-1845) i.e. the product of a positive-valued contrast envelope and a mean-zero carrier grating can be seen transparently only when the disparities are consistent with the envelope appearing in front of the carrier. We measured the energy at the envelope frequencies that must be added in order to negate this asymmetry. We report that this amplitude can be predicted from the envelope sidebands and not from the magnitude of compressive pre-cortical nonlinearities measured by other researchers. In the second experiment, contrast threshold elevations were measured for the discrimination of envelope disparities following adaptation to sinusoidal gratings. It is reported that perception of the envelope's depth was affected most when the adapting grating was similar (in orientation and frequency) to the carrier, rather than to the contrast envelope. These results suggest that the principal nonlinearity in second-order stereopsis is cortical, occurring after orientation- and frequency-selective linear filtering. (+info)
First and second-order contributions to surface interpolation.
(32/1300)
Comparisons of 1st- and 2nd-order stereopsis have typically employed isolated, or local, narrow-band targets. While these experiments have revealed a great deal about the distinction between these two types of processing, such stimuli are rare in the natural environment. Instead, local disparity signals are more likely to be part of extended surfaces that very smoothly in depth. The aim of the experiments presented here is to determine the relative contribution of 1st- and 2nd-order stereopsis to the perception of depth-modulated surfaces. Stereothresholds were measured under a range of conditions designed to isolate either 1st- or 2nd-order processing. The results demonstrate that while 2nd-order stereopsis provides local depth estimates for individual texture elements, 1st-order processing is essential to the global interpolation of those estimates across surfaces. (+info)