Despite enormous informational complexity in the optical environment, the visual world is effortlessly seen as coherent. Indeed, an object may change in virtually all of its physical properties and in its spatial location and yet maintain a constant perceptual identity. Apparently pieces of information registered in different segments of space-time, but referring to the same object, are perceptually integrated. Kahneman, Treisman and Gibbs (in progress) explored the cognitive organization corresponding to this perceptual organization; the present thesis represents an extension of their work. To study the spatiotemporal integration of information regarding moving objects they developed the preview paradigm. The prototypical visual display of this paradigm consists of three phases: (a) Letters are presented, each within a line-figure object, and are then removed (field-1), (b) the empty objects move to new positions, (c) letters are again presented in the objects and a marker appears, cueing one of them (field-2). The task is to name the letter in the cued object. The critical reaction time (RT) comparison is between consistent conditions (the target letter is previewed in the target object) and inconsistent conditions (the target letter is previewed, but in another object). An RT advantage for consistent conditions is termed the object effect because it represents object-specific facilitation. Object effects were generated in many experiments, including one utilizing only apparent motion to create objects. Certain experiments suggested that the object effect does not occur at a lexical or semantic level, but involves information concerning physical properties. The present thesis further explores the physical nature of the information integration underlying the object effect. Preview experiments were conducted, typically not with a letter-naming task, but with tasks requiring stimulus identification on the basis of a particular physical property. In experiments utilizing four moving line figures, object effects were obtained with presence and size. These effects were not artifacts of attending to field-1 or of confusing field-1 with field-2. In experiments utilizing apparent motion, object effects were obtained with color and with letters. Duodimension experiments elaborated the paradigm by introducing variation on a response-irrelevant dimension. The presence object effect was reduced by response-irrelevant shape inconsistency; the size object effect was eliminated by response-irrelevant shape inconsistency; the color object effect was unaffected by response-irrelevant letter-shape inconsistency; the letter object effect was slightly reduced by response-irrelevant color inconsistency. The duodimension results suggest that the object-specific representation underlying the object effect consists of somewhat conjoined properties. This has implications for the role of attention in the object effect, and inspires the speculation that motion might be special with respect to attention. Accounts of the object effect rival to Kahneman et al.'s can be proposed: that it results from the integration of response tendencies rather than stimulus information, that it is based on a decrease in apparent distance between stimuli rather than on their unitization, and that its seeming retroactivity is an illusion produced by the relative quickness with which low spatial frequencies are processed. The present results support arguments against each of these accounts. The general conclusion of this thesis is that the spatiotemporal integration underlying the object effect does involve information about physical properties. / Arts, Faculty of / Psychology, Department of / Graduate

The aim of this study is to investigate the processes that underlie image motion detection in human vision. To do this I have investigated motion perception for a wide range of stimulus velocities across the visual field, and have made use of different stimuli. Two mechanisms were revealed at different positions across the visual field as a result of the examination of the temporal properties of the Lower Threshold of Motion (LTM), that is, the lowest velocity that is reliably detected. The results for central vision showed that the LTM is mediated by a code that utilizes the spatial displacement transversed by the stimulus. For peripheral vision, the LTM is mediated by a code that utilizes the velocity or temporal frequency of the stimulus. This raised the question, do these two processes underlie image motion processing at all eccentricities with different sensitivities at threshold? To investigate this question, a wide velocity range was used to assess the ability of the visual system to discriminate different speeds. The temporal and spatial properties of the stimulus were individually disrupted to reveal the critical cues for velocity discrimination. The results show the presence of two processes at all eccentricities. The two processes can be characterised as a displacement code, and a velocity code. Evidence is shown that the velocity code uses 'velocity' information and not solely temporal frequency information. For central vision, the displacement code is most sensitive for short stimulus durations. The duration at which it is most sensitive is inversly proportional to the velocity of the stimulus. The velocity code is most sensitive at long term regions of the visual field. However, the range of velocities to which each mechanism is sensitive changes at different rates across the visual field. This leaves a range of low velocities which are detected only by the velocity mechanism at large eccentricities. Further investigation into the displacement code has revealed that this code can be characterised by an optimal displacement. This is less than the 1/4 of a spatial cycle of the stimulus which is proposed value for quadrature phase. Also luminance contrast was found to be an important parameter of the motion process. The two codes described above could be mediated by two motion areas of the primate visual cortex: the striate and prestriate cortex. From recent single cell studies, the emerging properties of neurons in these two parts of the visual cortex suggest that the displacement code may be mediated by the striate cortex, and the velocity code by the middle temporal area of the pre striate cortex.

Thesis (M.Sc.)--York University, 2004. Graduate Programme in Computer Science. / Typescript. Includes bibliographical references (leaves 130-144). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL:http://gateway.proquest.com/openurl?url%5Fver=Z39.88-2004&res%5Fdat=xri:pqdiss&rft%5Fval%5Ffmt=info:ofi/fmt:kev:mtx:dissertation&rft%5Fdat=xri:pqdiss:MQ99409

The human visual system comprises two neural pathways, the magnocellular/M and parvocellular/P pathways that process aspects of motion and form perception, respectively. Amblyopia is a developmental condition which may affect an otherwise healthy eye if it experiences abnormal visual stimulation due to ocular misalignment (strabismus), unequal refractive errors (anisometropia), or both. Amblyopia has been associated with deficits in both form and motion perception. Random-dot kinematograms (RDKs) which are created by shifting a computer-generated dot display in one direction by a given displacement can be used to assess motion processing. Maximum motion displacement (Dmax) is the largest dot displacement at which the direction of motion for a RDK can be correctly discriminated. Strabismic and anisometropic amblyopia represent two distinct subtypes of amblyopia and have been proposed to have different neural substrates. They have also been reported to have different Dmax deficits (Ho et al., 2005). The intentions of this thesis were: 1) to characterize deficits in Dmax for direction discrimination in the fellow and amblyopic eyes of participants with anisometropic and strabismic amblyopia using psychophysical methods; and 2) to investigate the relationship between psychophysical Dmax deficits and dysfunction in motion-sensitive extrastriate cortex of the M pathway using functional MRI techniques. The psychophysical results showed that Dmax thresholds are smaller in both amblyopic and fellow eyes for both subtypes of amblyopia relative to controls, although the deficits were greatest for strabismic amblyopia. Functional MRI results revealed decreased extrastriate cortical activation in both the strabismic and anisometropic groups relative to the control group when either eye viewed the RDK stimulus, although the lack of cortical activation was greatest for strabismic amblyopia. Taken together, this evidence suggests that dysfunctional binocular motion processing mechanisms in extrastriate cortex are part of the neural deficit underlying anisometropic and strabismic amblyopia and implies that strabismic amblyopia may be affected to a greater degree. For both amblyopic groups, there was a robust correlation between depth perception (stereoacuity) and Dmax thresholds. Specifically, direction discrimination was better when stereoacuity was worse. Abnormal binocular integration may have a significant role in predicting motion deficits in both anisometropic and strabismic amblyopia.

Thesis (M.Sc.)--York University, 2006. Graduate Programme in Computer Science and Engineering. / Typescript. Includes bibliographical references (leaves 124-134). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR29617

Perceiving the path we are travelling on is important for successful navigation. Relative motion between the world and the observer generates optical flow on the retinae (retinal flow). Gibson (1950) pointed out that when travelling on a straight path with no eye, head, or body rotation, retinal flow is radial and the stationary point indicates the instantaneous direction of travelling, or heading, of the observer. The straight path can then be recovered as it coincides with heading. Nevertheless, it is rarely the case that people travel with no rotation. Instead, they normally look at different points of interest when they are navigating. The result of changing one's gaze or rotating one's head is the addition of a rotational component, which is a laminar flow, to the flow field. The rotational component shifts the stationary point from heading and makes heading perception difficult. Extensive research has been conducted on how the human visual system removes the rotational component of the retinal flow and how extra-retinal information, such as efferent copies of eye muscle commands, may contribute to this process. The paths on which people travel are not always straight, but often curved. When a path is curved, it no longer coincides with heading. In this case, heading is the tangent of the path. Researchers have proposed theories to explain how curved paths are perceived. Each of them requires different visual information and gaze conditions (e.g., fixating on a target or gazing along the heading direction). They can be categorized by whether or not path perception depends on heading perception. The goal of this thesis is to systematically examine different theories of path perception and determine how humans perceive curved paths. Study 1 examined different path perception theories by comparing human path perception performance in various gaze conditions and with the availability of various optic flow information. Study 2 investigated whether path perception depends on heading perception. Study 3 examined the contribution of reference objects to path perception. Study 4 investigated how extra-retinal informationcontribute to path perception. The experiments that I present here show that (a) when there is no extra-retinal information, path perception is accurate only when one's gaze is along heading such that the rotation in the flow field is equal to path rotation; (b) when one's gaze is not along heading such that the rotation in the flow field is not equal to path rotation, path perception is inaccurate. Adding more visual information, such as acceleration, dense flow field, and/ or reference objects does not improve the accuracy; (c) eye movement signals support accurate path perception only in the natural case of self-motion in which body orientation is aligned with heading such that eye movement signals help to stabilize heading in the body-centric coordinate system. / published_or_final_version / Psychology / Doctoral / Doctor of Philosophy

Human observers can perceive self-motion from optic flow for both circular paths and straight paths. However, when view does not rotate during travel on a circular path, observers often show significant bias in path judgments from optic flow. A proposed explanation is that human observers use the velocity field of optic flow for perception of self-motion and assume that path curvature is accompanied by body rotation. Use of a scene-based strategy, as opposed to optic flow, could potentially reduce the bias in judgments of circular path with no view rotation. It has been shown that human observers can judge heading direction from a sequence of static pictures of a scene, suggesting that observers can perceive self-motion from change in observer position relative to a scene. Another previous study found that showing the same configuration of landmarks repeatedly improved performance in a steering task, suggesting that familiarity of a scene may further improve perception of self-motion and steering control. In this thesis, I explored the effect of scene information on judgments of circular path and steering control. Experiments 1 and 2 tested path judgments for simulated travel along a circular path. Observers viewed 1 s presentations of optic flow and adjusted a response pole to lie on their future path, i.e. where they would intercept the pole if they keep travelling on the current path. Precision and accuracy of judgments were compared across conditions that varied in the presence and consistency of landmarks. I found no effect of landmarks on precision and accuracy, and curvature underestimation remained present in all conditions. Experiment 3 tested whether familiarity with a scene can improve steering control. I used the same steering task by Andersen and Enriquez (2006). During a trial, heading direction was perturbed horizontally in a smooth, pseudo-random manner, as if unexpectedly wind perturbed a car and caused it to slide to the left and right while driving straight ahead. Observers were instructed to counteract these perturbations and steer to maintain a straight path of travel. Contrary to previous findings, I found no effect of scene familiarity on steering control. I found no benefit from landmarks in any of the conditions tested in this thesis. These results suggest that scene information has a limited role for path judgments and steering control. / published_or_final_version / Psychology / Master / Master of Philosophy

When we stand still and do not move our eyes and head, the motion of an object in the world or the absence thereof is directly given by the motion or quiescence of the retinal image. Self-motion through the world however complicates this retinal image. During self-motion, the whole retinal image undergoes coherent global motion, called optic flow. Self-motion therefore causes the retinal motion of objects moving in the world to be confounded by a motion component due to self-motion. How then do we perceive the motion of an object in the world when we ourselves are also moving? Although non-visual information about self-motion, such as provided by efference copies of motor commands and vestibular stimulation, might play a role in this ability, it has recently been shown that the brain possesses a purely visual mechanism that underlies scene-relative object motion perception during self-motion. In the flow parsing hypothesis developed by Rushton and Warren (2005; Warren & Rushton, 2007; 2009b), the brain uses its sensitivity to optic flow to detect and globally remove retinal motion due to self-motion and recover the scene-relative motion of objects. Research into this perceptual ability has so far been of a qualitative nature. In this thesis, I therefore develop a retinal motion nulling paradigm to measure the gain with which the flow parsing mechanism uses the optic flow to remove the self-motion component from an object’s retinal motion. I use this paradigm to investigate how accurate scene-relative object motion perception during self-motion can be based on only visual information, whether this flow parsing process depends on a percept of the direction of self-motion and the tuning of flow parsing, i.e., how it is modulated by changes in various stimulus aspects. The results reveal that although adding monocular or binocular depth information to the display to precisely specify the moving object’s 3D position in the scene improved the accuracy of flow parsing, the flow parsing gain was never up to the extent required by the scene geometry. Furthermore, the flow parsing gain was lower at higher eccentricities from the focus of expansion in the flow field and was strongly modulated by changes in the motion angle between the self-motion and object motion components in the retinal motion of the moving object, the speeds of these components and the density of the flow field. Lastly, flow parsing was not affected by illusory changes in the perceived direction of self-motion. In conclusion, visual information alone is not sufficient for accurate perception of scene-relative object motion during self-motion. Furthermore, flow parsing takes the 3D position of the moving object in the scene into account and is not a uniform global subtraction process. 8e observed tuning characteristics are different from those of local perceived motion interactions, providing evidence that flow parsing is a separate process from these local motion interactions. Finally, flow parsing does not depend on a prior percept of self-motion direction and instead directly uses the input retinal motion to construct percepts of scene-relative object motion during self-motion. / published_or_final_version / Psychology / Doctoral / Doctor of Philosophy

Human use both optic flow and target egocentric direction cues to guide selfmotion at walking speed. The first study examined whether people change their reliance on the two cues when they are controlling high-speed steering, e.g. driving a car? In Experiment 1, I simulated two viewing conditions. In the opticflow only condition, the participant's virtual gaze direction was fixed on the target which was placed 10° away from their straight ahead. The target egocentric direction was fixed during steering and was thus unavailable for steering control. Participants steered to align their heading but not their path of forward travel with the target at all travel speeds tested. In the optic flow-plus-egocentric direction condition, the participant's virtual gaze direction was aligned with heading which was displaced 10° away from their straight ahead. The target egocentric direction changed during steering and was thus available for steering control. Participants’ steering was affected by the heading displacement at all travel speeds tested. The faster and larger reduction of heading error observed at higher travel speed for both viewing conditions was mirrored by an increase in the precision of heading judgment in the corresponding heading perception experiment (Experiment 2). The findings support that while people are increasingly more accurate and efficient in using heading specified by optic flow for steering when travel speed increases, high-speed travel does not affect the type of visual strategy used for the control of steering toward a goal. The second study examined how different attentional loads affected people’s steering toward a goal at both low and high travel speeds. In Experiment 3, I used the same display setting as the optic flow-plus-egocentric direction condition in Experiment 1 for the steering task. Participants were asked to steer toward a red post target with (1) no attention tracking task, (2) concurrently tracking one dot (low attentional load), or (3) three dots (high attentional load) among eight dots that randomly moved inside the red circle on top of the target post. I found that attentional load affected the early stage of steering control but did not affect the overall visual strategy. Experiment 4 replicated Experiment 3 except that participants were specifically instructed to center the target straight ahead or align the target with heading for the steering task. I found that attentional load only affected the early stage of steering for the centering-the-target instruction group, but affected the steering performance throughout the trial for the aligning the-target-with-heading instruction group. In both Experiments 3 and 4, while the tracking accuracy was high (> 90%) and not affected by the travel speed when the attentional load was low, it decreased more at higher travel speed when the attentional load was high. The findings suggest that people have more difficulty in dealing with high attention demanding task at high than low travel speed. Increasing attentional load affects the accuracy in using heading specified by optic flow but does not change the natural visual strategy for goal-directed selfmotion control. / published_or_final_version / Psychology / Master / Master of Philosophy

The human visual system comprises two neural pathways, the magnocellular/M and parvocellular/P pathways that process aspects of motion and form perception, respectively. Amblyopia is a developmental condition which may affect an otherwise healthy eye if it experiences abnormal visual stimulation due to ocular misalignment (strabismus), unequal refractive errors (anisometropia), or both. Amblyopia has been associated with deficits in both form and motion perception. Random-dot kinematograms (RDKs) which are created by shifting a computer-generated dot display in one direction by a given displacement can be used to assess motion processing. Maximum motion displacement (Dmax) is the largest dot displacement at which the direction of motion for a RDK can be correctly discriminated. Strabismic and anisometropic amblyopia represent two distinct subtypes of amblyopia and have been proposed to have different neural substrates. They have also been reported to have different Dmax deficits (Ho et al., 2005). The intentions of this thesis were: 1) to characterize deficits in Dmax for direction discrimination in the fellow and amblyopic eyes of participants with anisometropic and strabismic amblyopia using psychophysical methods; and 2) to investigate the relationship between psychophysical Dmax deficits and dysfunction in motion-sensitive extrastriate cortex of the M pathway using functional MRI techniques. The psychophysical results showed that Dmax thresholds are smaller in both amblyopic and fellow eyes for both subtypes of amblyopia relative to controls, although the deficits were greatest for strabismic amblyopia. Functional MRI results revealed decreased extrastriate cortical activation in both the strabismic and anisometropic groups relative to the control group when either eye viewed the RDK stimulus, although the lack of cortical activation was greatest for strabismic amblyopia. Taken together, this evidence suggests that dysfunctional binocular motion processing mechanisms in extrastriate cortex are part of the neural deficit underlying anisometropic and strabismic amblyopia and implies that strabismic amblyopia may be affected to a greater degree. For both amblyopic groups, there was a robust correlation between depth perception (stereoacuity) and Dmax thresholds. Specifically, direction discrimination was better when stereoacuity was worse. Abnormal binocular integration may have a significant role in predicting motion deficits in both anisometropic and strabismic amblyopia.