Ian P Howard


Ph.D. D.Sc.


  • Ian P Howard · Yoshitaka Fujii · Robert S Allison
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    ABSTRACT: Information about the motion in depth of an object along the midline of a stationary observer is provided by changes in image size (looming), changes in vergence produced by changes in binocular disparity of the images of the object, and changes in relative disparity between the moving object and a stationary object. Each of these cues was independently varied in the dichoptiscope, which is described in Howard, Fukuda, and Allison (2013). The stimuli were a small central dot and a textured surface moving to and fro in depth along the midline. Observers tracked the motion with the unseen hand. Image looming was normal or absent. The change in vergence was absent, normal, more than normal, or reversed relative to normal. Changing relative disparity between the moving stimulus and a stationary surface was present or absent. Changing vergence alone produced no motion in depth for the textured surface but it produced some motion of the dot. Looming alone produced strong motion in depth for the texture but not for the dot. When the direction of motion indicated by looming was opposite that indicated by changing relative disparity observers could use either cue. The cues dissociated rather than combined.
    Journal of Vision 02/2014; 14(2). DOI:10.1167/14.2.14 · 2.39 Impact Factor
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    ABSTRACT: The brain receives disparate retinal input owing to the separation of the eyes, yet we usually perceive a single fused world. This is because of complex interactions between sensory and oculomotor processes that quickly act to reduce excessive retinal disparity. This implies a strong link between depth perception and fusion, but it is well established that stereoscopic depth percepts are also obtained from stimuli that produce double images. Surprisingly, the nature of depth percepts from such diplopic stimuli remains poorly understood. Specifically, despite long-standing debate it is unclear whether depth under diplopia is owing to the retinal disparity (directly), or whether the brain interprets signals from fusional vergence responses to large disparities (indirectly). Here, we addressed this question using stereoscopic afterimages, for which fusional vergence cannot provide retinal feedback about depth. We showed that observers could reliably recover depth sign and magnitude from diplopic afterimages. In addition, measuring vergence responses to large disparity stimuli revealed that that the sign and magnitude of vergence responses are not systematically related to the target disparity, thus ruling out an indirect explanation of our results. Taken together, our research provides the first conclusive evidence that stereopsis is a direct process, even for diplopic targets.
    Proceedings of the Royal Society B: Biological Sciences 01/2014; 281(1776):20132118. DOI:10.1098/rspb.2013.2118 · 5.05 Impact Factor
  • Ian P Howard · Kazuho Fukuda · Robert S Allison
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    ABSTRACT: A stereoscope displays 2-D images with binocular disparities (stereograms), which fuse to form a 3-D stereoscopic object. But a stereoscopic object creates a conflict between vergence and accommodation. Also, motion in depth of a stereoscopic object simulated solely from change in target vergence produces anomalous motion parallax and anomalous changes in perspective. We describe a new instrument, which overcomes these problems. We call it the dichoptiscope. It resembles a mirror stereoscope, but instead of stereograms, it displays identical 2-D or 3-D physical objects to each eye. When a pair of the physical, monocular objects is fused, they create a dichoptic object that is visually identical to a real object. There is no conflict between vergence and accommodation, and motion parallax is normal. When the monocular objects move in real depth, the dichoptic object also moves in depth. The instrument allows the experimenter to control independently each of several cues to motion in depth. These cues include changes in the size of the images, changes in the vergence of the eyes, changes in binocular disparity within the moving object, and changes in the relative disparity between the moving object and a stationary object.
    Journal of Vision 12/2013; 13(14). DOI:10.1167/13.14.1 · 2.39 Impact Factor
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    ABSTRACT: Shape constancy is the ability to perceive that a shape remains the same when seen in different orientations. It has usually been measured by asking subjects to match a shape in the frontal plane with an inclined shape. But this method is subject to ambiguity. In Experiment 1 we used a canonical-shape method, which is not subject to ambiguity. Observers selected from a set of inclined trapezoids the one that most resembled a rectangle (the canonical shape). This task requires subjects to register the linear perspective of the image, and the distance and inclination of the stimulus. For inclinations of 30˚ and 60˚ and distances up to 1 m subjects were able to distinguish between a rectangle and a trapezoid tapered 0.4˚. As the distance of the stimulus increased to 3 m, linear perspective became increasingly perceived as taper. In Experiment 2 subjects matched the perceived inclination of an inclined rectangle, in which the only cue to inclination was disparity, to the perceived inclination of a rectangle with all depth cues present. As the distance of the stimulus increased, subjects increasingly underestimated the inclination of the rectangle. We show that this pattern of inclination underestimation explains the distance-dependent bias in taper judgments found in Experiment 1.
    Vision research 11/2013; 94. DOI:10.1016/j.visres.2013.10.021 · 1.82 Impact Factor
  • Philip A Duke · Ian P Howard
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    ABSTRACT: A textured surface appears slanted about a vertical axis when the image in one eye is horizontally enlarged relative to the image in the other eye. The surface appears slanted in the opposite direction when the same image is vertically enlarged. Two superimposed textured surfaces with different horizontal size disparities appear as two surfaces that differ in slant. Superimposed textured surfaces with equal and opposite vertical size disparities appear as a single frontal surface. The vertical disparities are averaged. We investigated whether vertical size disparities are averaged across two superimposed textured surfaces in different depth planes or whether they induce distinct slants in the two depth planes. In Experiment 1, two superimposed textured surfaces with different vertical size disparities were presented in two depth planes defined by horizontal disparity. The surfaces induced distinct slants when the horizontal disparity was more than ±5 arcmin. Thus, vertical size disparities are not averaged over surfaces with different horizontal disparities. In Experiment 2 we confirmed that vertical size disparities are processed in surfaces away from the horopter, so the results of Experiment 1 cannot be explained by the processing of vertical size disparities in a fixated surface only. Together, these results show that vertical size disparities are processed separately in distinct depth planes. The results also suggest that vertical size disparities are not used to register slant globally by their effect on the registration of binocular direction of gaze.
    Journal of Vision 08/2012; 12(8):10. DOI:10.1167/12.8.10 · 2.39 Impact Factor
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    Ian P. Howard
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    ABSTRACT: Volume 3 deals with all depth-perception mechanisms other than stereoscopic vision. It first deals with the visual depth cues of accommodation, vergence eye movements, perspective, interposition, shading, and motion parallax. Ways in which depth cues interact are discussed. These interactions improve discrimination of depth intervals and motion in depth. They also allow us to perceive constancy of size, shape, and relative depth. Pathologies of visual depth perception are described, including visual neglect, and albinism. An account is given of how visual information is used to guide movements of the hand and of the body. Non-visual mechanisms of depth perception are then described. These include audition, echolocation by bats and marine mammals, electrolocation in electric fish, and thermal organs in snakes. The book ends with an account of mechanisms that animals use in navigation and migration.
    03/2012; Oxford University Press., ISBN: 978-0-19-976416-7
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    Ian P. Howard · Brian J. Rogers
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    ABSTRACT: Volume 2 deals with stereoscopic vision in cats, monkeys and humans. It starts with a review of physiological mechanisms of stereoscopic vision. Stereoscopic vision depends on inputs from the two eyes converging in the visual cortex. The mechanisms of binocular rivalry and of other ways in which binocular images interact are reviewed. The images of objects on the horopter are combined so that corresponding parts are brought into register. Once the images are in register, differences between the images are used to code depth. An account is provided of the nature of these differences, the precision with which they are detected (stereoacuity), and the use to which they are put. Two chapters describe how impressions of depth created by binocular disparity are modified by depth contrast, figure-ground interactions, motion, and attention. The book ends with a review of stereoscopic techniques used to create three-dimensional displays and the practical applications of stereoscopic devices.
    03/2012; Oxford University Press., ISBN: 978-0-19-976415-0
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    Ian P. Howard
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    ABSTRACT: Volume 1 deals with basic visual mechanisms underlying depth perception. It starts with a review of the history of the subject from the Greeks to the present. Four chapters review psychophysical procedures, visual coding, and the physiology of the visual system. Other chapters deal with accommodation, vergence eye movements, and the development of the visual system, including the effects of early visual deprivation. Volume 1 ends with a review of depth-detection mechanisms throughout the animal kingdom.
    03/2012; Oxford University Press., ISBN: 978-0-19-97641-3
  • Yuichi Sakano · Robert S Allison · Ian P Howard
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    ABSTRACT: We examined whether a negative motion aftereffect occurs in the depth direction following adaptation to motion in depth based on changing disparity and/or interocular velocity differences. To dissociate these cues, we used three types of adapters: random-element stereograms that were correlated (1) temporally and binocularly, (2) temporally but not binocularly, and (3) binocularly but not temporally. Only the temporally correlated adapters contained coherent interocular velocity differences while only the binocularly correlated adapters contained coherent changing disparity. A motion aftereffect in depth occurred after adaptation to the temporally correlated stereograms while little or no aftereffect occurred following adaptation to the temporally uncorrelated stereograms. Interestingly, a monocular test pattern also showed a comparable motion aftereffect in a diagonal direction in depth after adaptation to the temporally correlated stereograms. The lack of the aftereffect following adaptation to pure changing disparity was also confirmed using spatially separated random-dot patterns. These results are consistent with the existence of a mechanism sensitive to interocular velocity differences, which is adaptable (at least in part) at binocular stages of motion-in-depth processing. We did not find any evidence for the existence of an "adaptable" mechanism specialized to see motion in depth based on changing disparity.
    Journal of Vision 01/2012; 12(1). DOI:10.1167/12.1.11 · 2.39 Impact Factor
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    Ian P Howard · Robert S Allison
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    ABSTRACT: Before methods for drawing accurately in perspective were developed in the 15th century, many artists drew with divergent perspective. But we found that many university students draw with divergent perspective rather than with the correct convergent perspective. These experiments were designed to reveal why people tend to draw with divergent perspective. University students drew a cube and isolated edges and surfaces of a cube. Their drawings were very inaccurate. About half the students drew with divergent perspective like artists before the 15th century. Students selected a cube from a set of tapered boxes with great accuracy and were reasonably accurate in selecting the correct drawing of a cube from a set of tapered drawings. Each subject's drawing was much worse than the drawing selected as accurate. An analysis of errors in drawings of a cube and of isolated edges and surfaces of a cube revealed several factors that predispose people to draw in divergent perspective. The way these factors intrude depends on the order in which the edges of the cube are drawn.
    Perception 01/2011; 40(9):1017-33. DOI:10.1068/p6876 · 0.91 Impact Factor
  • I. P. Howard · V. A. Nguyen · B. Cheung
    Journal of Vision 09/2010; 5(8):192-192. DOI:10.1167/5.8.192 · 2.39 Impact Factor
  • K. Fukuda · I. P. Howard · R. S. Allison
    Journal of Vision 08/2010; 9(8):631-631. DOI:10.1167/9.8.631 · 2.39 Impact Factor
  • Journal of Vision 06/2010; 6(6):626-626. DOI:10.1167/6.6.626 · 2.39 Impact Factor
  • K. Fukuda · L. M. Wilcox · R. S. Allison · I. P. Howard
    Journal of Vision 05/2010; 8(6):1086-1086. DOI:10.1167/8.6.1086 · 2.39 Impact Factor
  • N. T. Daniels · I. P. Howard · R. S. Allison
    Journal of Vision 05/2010; 8(6):646-646. DOI:10.1167/8.6.646 · 2.39 Impact Factor
  • V. A. Nguyen · I. P. Howard · R. S. Allison
    Journal of Vision 01/2010; 4(8):461-461. DOI:10.1167/4.8.461 · 2.39 Impact Factor
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    ABSTRACT: The stereoscopic system tolerates some vertical misalignment of the images in the eyes. However, the reported tolerance for an isolated line stimulus (approximately 4 degrees) is greater than for a random-dot stereogram (RDS, approximately 45 arcmin). We hypothesized that the greater tolerance can be attributed to monoptic depth signals (E. Hering, 1861; M. Kaye, 1978; L. M. Wilcox, J. M. Harris, & S. P. McKee, 2007). We manipulated the vertical misalignment of a pair of isolated stereoscopic dots to assess the contribution of each depth signal separately. For the monoptic stimuli, where only one half-image was present, equivalent horizontal and vertical offsets were imposed instead of disparity. Judgments of apparent depth were well above chance, though there was no conventional disparity signal. For the stereoscopic stimuli, one element was positioned at the midline where monoptic depth perception falls to chance but conventional disparity remains. Subjects lost the depth percept at a vertical misalignment of between 44 and 88 arcmin, which is much smaller than the limit found when both signals were provided. This tolerance for isolated stimuli is comparable to the reported tolerance for RDS. We conclude that previous reports of the greater tolerance to vertical misalignment for isolated stimuli arose from the use of monoptic depth signals.
    Journal of Vision 02/2009; 9(2):1.1-8. DOI:10.1167/9.2.1 · 2.39 Impact Factor
  • Ian P Howard
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    ABSTRACT: It is known that the vergence state of the eyes can serve as a cue to distance. However, it has been claimed that changes in vergence induced by modulations of the binocular disparity of a random-dot display do not create a sensation of motion in depth. The present experiment tests the hypothesis that modulation of binocular disparity in a random-dot display does not create a sensation of motion in depth because, in such a display, looming of the image that normally accompanies motion in depth is absent. The image of a radial pattern remains self-similar when it moves in depth, so the absence of looming in such an image should not inhibit the effects of modulation of disparity. Subjects tracked with unseen hand the perceived motion in depth created by modulations of the disparity of a display of random spots, a single spot, and a radial pattern. As previously reported, the random-spot display produced almost no motion in depth. However, the single spot and the radial pattern produced motion in depth. It is concluded that modulations of vergence and/or of absolute disparity can create a sensation of motion in depth when the effects of looming are weakened or removed. However, strong sensations of motion in depth of isolated objects require the presence of looming.
    Spatial Vision 02/2008; 21(6):581-92. DOI:10.1163/156856808786451417 · 1.04 Impact Factor
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    ABSTRACT: An impression of a surface seen through holes is created when one fuses dichoptic pairs of discs, with one member of each pair black and the other member white. This is referred to as the 'sieve effect'. The stimulus contains no positional disparities. Howard (1995, Perception 24 67-74) noted qualitatively that the sieve effect occurs when the rivalrous regions are within the range of sizes, contrasts, and relative sizes where exclusive rivalry occurs, rather than binocular lustre, stimulus combination, or dominant rivalry. This suggests that perceived depth in the sieve effect should be at a maximum when exclusive rivalry is most prominent. We used a disparity depth probe to measure the magnitude of perceived depth in the sieve effect as a function of the sizes, contrasts, and relative sizes of the rivalrous regions. We also measured the rate of exclusive rivalry of the same stimuli under the same conditions. Perceived depth and the rate of exclusive rivalry were affected in the same way by each of the three variables. Furthermore, perceived depth and the rate of exclusive rivalry were affected in the same way by changes in vergence angle, although the configuration of the stimulus surface was held constant. These findings confirm the hypothesis that the sieve effect is correlated with the incidence of exclusive rivalry.
    Perception 02/2007; 36(7):990-1002. DOI:10.1068/p5749 · 0.91 Impact Factor
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    Stephen Palmisano · Robert S Allison · Ian P Howard
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    ABSTRACT: We report a novel illusory distortion of the visual scene, which became apparent during both: (i) observer rotation inside a furnished stationary room; and (ii) room rotation about the stationary observer. While this distortion had several manifestations, the most common experience was that scenery near fixation appeared to sometimes lead and other times lag more peripheral scenery. Across a series of experiments, we eliminated explanations based on eye-movements, distance misperception, peripheral aliasing, differential motion sensitivity and adaptation. We found that these illusory scene distortions occurred only when the observer perceived (real or illusory) changes in self-tilt and maintained a stable fixation.
    Vision Research 12/2006; 46(23):4048-58. DOI:10.1016/j.visres.2006.07.020 · 1.82 Impact Factor

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