Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Motion of glossy objects does not promote separation of lighting and surface colour

The surface properties of an object, such as texture, glossiness or colour, provide important cues to its identity. However, the actual visual stimulus received by the eye is determined by both the properties of the object and the illumination. We tested whether operational colour constancy for glossy objects (the ability to distinguish changes in spectral reflectance of the object, from changes in the spectrum of the illumination) was affected by rotational motion of either the object or the light source. The different chromatic and geometric properties of the specular and diffuse reflections provide the basis for this discrimination, and we systematically varied specularity to control the available information. Observers viewed animations of isolated objects undergoing either lighting or surface-based spectral transformations accompanied by motion. By varying the axis of rotation, and surface patterning or geometry, we manipulated: (i) motion-related information about the scene, (ii) relative motion between the surface patterning and the specular reflection of the lighting, and (iii) image disruption caused by this motion. Despite large individual differences in performance with static stimuli, motion manipulations neither improved nor degraded performance. As motion significantly disrupts frameby-frame low-level image statistics, we infer that operational constancy depends on a high-level scene interpretation, which is maintained in all condition

3D motion : encoding and perception

The visual system supports perception and inferences about events in a dynamic, three-dimensional (3D) world. While remarkable progress has been made in the study of visual information processing, the existing paradigms for examining visual perception and its relation to neural activity often fail to generalize to perception in the real world which has complex dynamics and 3D spatial structure. This thesis focuses on the case of 3D motion, developing dynamic tasks for studying visual perception and constructing a neural coding framework to relate neural activity to perception in a 3D environment. First, I introduce target-tracking as a psychophysical method and develop an analysis framework based on state space models and the Kalman filter. I demonstrate that target-tracking in conjunction with a Kalman filter analysis framework produce estimates of visual sensitivity that are comparable to those obtained with a traditional forced-choice task and a signal detection theory analysis. Next, I use the target-tracking paradigm in a series of experiments examining 3D motion perception, specifically comparing the perception of frontoparallel motion with the perception of motion-through-depth. I find that continuous tracking of motion-through-depth is selectively impaired due to the relatively small retinal projections resulting from motion-through-depth and the slower processing of binocular disparities. The thesis then turns the neural representation of 3D motion and how that underlies perception. First I introduce a theoretical framework that extends the standard neural coding approach, incorporating the environment-to-retina transformation. Neural coding typically treats the visuals stimulus as a direct proxy for the pattern of stimulation that falls on the retina. Incorporating the environment-to-retina transformation results in a neural representation fundamentally shaped by the projective geometry of the world onto the retina. This model explains substantial anomalies in existing neurophysiological recordings in primate visual cortical neurons during presentations of 3D motion and in psychophysical studies of human perception. In a series of psychophysical experiments, I systematically examine the predictions of the model for human perception by observing how perceptual performance changes as a function of viewing distance and eccentricity. Performance in these experiments suggests a reliance on a neural representation similar to the one described by the model. Taken together, the experimental and theoretical findings reported here advance the understanding of the neural representation and perception of the dynamic 3D world, and adds to the behavioral tools available to vision scientists.Neuroscienc

Relation of Parkinson\u27s Disease Subtypes to Visual Activities of Daily Living

Visual perceptual problems are common in Parkinson\u27s disease (PD) and often affect activities of daily living (ADLs). PD patients with non-tremor symptoms at disease onset (i.e., rigidity, bradykinesia, gait disturbance or postural instability) have more diffuse neurobiological abnormalities and report worse non-motor symptoms and functional changes than patients whose initial symptom is tremor, but the relation of motor symptom subtype to perceptual deficits remains unstudied. We assessed visual ADLs with the Visual Activities Questionnaire in 25 non-demented patients with PD, 13 with tremor as the initial symptom and 12 with an initial symptom other than tremor, as well as in 23 healthy control participants (NC). As expected, the non-tremor patients, but not the tremor patients, reported more impairment in visual ADLs than the NC group, including in light/dark adaptation, acuity/spatial vision, depth perception, peripheral vision and visual processing speed. Non-tremor patients were significantly worse than tremor patients overall and on light/dark adaptation and depth perception. Environmental enhancements especially targeted to patients with the non-tremor PD subtype may help to ameliorate their functional disability

Cue integration outside central fixation: A study of grasping in depth.

We assessed the usefulness of stereopsis across the visual field by quantifying how retinal eccentricity and distance from the horopter affect humans' relative dependence on monocular and binocular cues about 3D orientation. The reliabilities of monocular and binocular cues both decline with eccentricity, but the reliability of binocular information decreases more rapidly. Binocular cue reliability also declines with increasing distance from the horopter, whereas the reliability of monocular cues is virtually unaffected. We measured how subjects integrated these cues to orient their hands when grasping oriented discs at different eccentricities and distances from the horopter. Subjects relied increasingly less on binocular disparity as targets' retinal eccentricity and distance from the horopter increased. The measured cue influences were consistent with what would be predicted from the relative cue reliabilities at the various target locations. Our results showed that relative reliability affects how cues influence motor control and that stereopsis is of limited use in the periphery and away from the horopter because monocular cues are more reliable in these regions. Keywords: binocular vision, spatial vision, 3D surface and shape perception, grasping, cue integration Citation: Introduction Most conclusions about visual perception have been based on foveal vision since this is where visual acuity and thus performance on most tasks is best, and it is well established that stereopsis contributes to perception and motor control when stimuli are in the central portion of the visual field. However, peripheral regions of the visual field also significantly impact how we navigate through and interact with the world. Information from the periphery is particularly important for planning and executing reaching movements. It helps us plan both the saccades that will move the eyes so that the desired objects project onto the foveae and the reaching movements themselves Surprisingly few studies have focused on stereoacuity, the ability to use binocular disparity as a depth cue, away from the fovea, although it is agreed that thresholds for stereopsis increase with retinal eccentricity. This decrease in sensitivity appears to reflect decreases in the amount of cortical representation in the periphery rather than the visual angle per se Journal of Vision We tested these predictions in three experiments that required human subjects to use monocular and binocular information to estimate the 3D orientations of stimuli at different retinal eccentricities and distances from the horopter. Our first experiment separately measured monocular and binocular thresholds for 3D orientation discrimination at different retinal eccentricities along the theoretical horopter. Then, we used a grasping task to quantify how subjects integrated monocular and binocular information about 3D orientation at these same positions and compared the cue integration strategies we observed with those predicted by sensitivity to the individual cues at each retinal location. In Experiment 3, we investigated Journal of Vision (2009) 9(2):11, 1-16 Greenwald & Knill 2 how increasing the targets' distance from the theoretical horopter affected the contribution of stereopsis to subjects' 3D orientation estimates. Experiment 1: Eccentric monocular and binocular slant thresholds We separately measured 3D orientation thresholds from aspect ratio, a monocular cue, and disparity, a binocular cue, at the fixation point and at two points in the periphery. This enabled us to predict how the relative influences of the cues should change as a function of eccentricity. Method Subjects The ten subjects in this experiment were laboratory staff, graduate students, or postdoctoral fellows in the Department of Brain & Cognitive Sciences and/or the Center for Visual Science at the University of Rochester. All subjects had normal or corrected-to-normal vision and binocular acuity of at least 40 arc seconds, provided written informed consent, and were paid /10 per hour. We used experienced psychophysical observers to obtain the best possible threshold estimates; although they were aware that the purpose of the experiment was to estimate psychophysical thresholds, they were not informed of the details of the staircases we used or of our hypotheses. All experiments reported here followed protocols specified by the University of Rochester Research Subjects Review Board. Apparatus Participants viewed a 20 in. display (1152 Â 864 resolution, 118 Hz refresh rate) through a half-silvered mirror as shown in Journal of Vision (2009) 9(2):11, 1-16 Greenwald & Knill 3 and we occluded their left eye with a patch during monocular trials. Each subject viewed the monocular stimuli with their right eye because stimuli appeared to the right of fixation and Calibration procedures We first identified the locations of each subject's eyes relative to the monitor. At the beginning of each session, the backing of the half-silvered mirror was removed, which allowed subjects to see the monitor and their hand simultaneously. Subjects positioned an infrared marker at a series of visually cued locations so that the marker and a symbol presented monocularly on the monitor appeared to be aligned. Thirteen positions were matched for each eye at two different depth planes, and we calculated the 3D position of each eye relative to the center of the display by minimizing the squared error between the measured position of the marker and the position we predicted from the estimated eye locations. Subjects then moved the marker around the workspace and confirmed that a symbol presented binocularly in depth appeared at the same location as the infrared marker. To calibrate the eyetracker, we recorded the positions of both eyes for binocular conditions or the right eye for monocular conditions as subjects fixated points in a 3 Â 3 grid displayed on the screen. The eyetracker was calibrated at the start of each experimental block and after subjects removed their head from the chinrest, and drift corrections were performed after every five fixation losses or as needed. Fixation losses occurred when subjects looked away from the fixation target or when their measured eye positions drifted significantly from the calibrated positions. Stimuli The stimulus in monocular trials (see In both conditions, a red wireframe sphere (RGB = (0.8,0,0)) with a diameter of 1 cm (approximately 1-of visual angle) served as a fixation target. It appeared 4 cm to the left and 8 cm below the center of the display and 4 cm behind the accommodative plane of the display so that stimuli were near this plane at all retinal eccentricitie

Naturalistic depth perception

textMaking inferences about the 3-dimensional spatial structure of natural scenes is a critical visual function. While spatial discrimination both in depth and on the image plane has been well characterized for simple stimuli, little is known about our ability to discriminate depth in natural scenes, particularly at far distances. To begin filling in this gap we: (i) developed a database of 80 stereoscopic images paired with the corresponding measured distance information, (ii) used these scenes as psychophysical stimuli and measured near-far discrimination acuity in 4 observers as a function of distance and the visual angle separating the targets, (iii) made additional measurements under patched-eye (monocular) viewing conditions to evaluate the importance of binocular vision in depth discrimination as a function of viewing geometries. We find that binocular thresholds are roughly a constant Weber fraction of the distance for absolute distances ranging from 4 to 28 meters. Further, measured thresholds were around 1% for small separations, and increased to 4% for stimuli separated by 10 deg. Thus, the ability to discriminate depth in natural scenes is very good out to considerable distances. To investigate the basis of this discrimination ability, monocular thresholds were measured. We found that monocular thresholds were elevated for distances less than 15 meters, but were comparable to binocular thresholds for greater distances. Accurate depth perception depends on combining (fusing) multiple sources of sensory information. Thus binocular thresholds probably involve fusing separate monocular and disparity-derived estimates. Under the assumption of Gaussian distributed independent estimates, Bayes rule provides a simple reliability-weighted summation model of cue combination. Using disparity threshold measurements by Blakemore (1970), and the current monocular thresholds, parameter-free predictions were generated for the current binocular thresholds. These predictions were in broad agreement with the data, suggesting that the disparity and monocular cues are separable and combined optimally in natural scenes.Psycholog

Human amblyopia and its perceptual consequences

The presence of ocular defects of an optical or muscular nature during early childhood can cause amblyopia: a reduction in visual acuity of the defective eye. The research reported in this thesis investigated the impact of amblyopia on some aspects of visual perception by evaluating three main perceptual functions: precision of judgement of spatial relationships (in three-dimensional space), ability to detect depth in tests of stereopsis, and contrast sensitivity. In some experiments amblyopic subjects were paired with non-amblyopic subjects who had monocular acuity deficits owing to uncorrected refractive errors, in order to assess the importance of the acuity deficit as a determinant of other perceptual losses suffered by amblyopes. In an alignment task non-amblyopes with monocularly reduced acuity performed significantly better than amblyopes, suggesting that the acuity deficit was not solely responsible for amblyopes' perceptual deficit in this task. However, in another experiment in which a greater variety of spatial cues was provided amblyopes performed as well as non- amblyopes. Thus their perceptual skills would seem to be adequate for efficient functioning in most normal environments where spatial cues are abundant. Previous reports that amblyopes generally lack stereopsis were confirmed in two experiments with a few interesting exceptions, whose cases are discussed. The data obtained in the four experiments on space perception and stereopsis in amblyopia provided support for most current theories in these areas. Experiments on contrast sensitivity showed that the losses suffered by amblyopes, as measured by interocular comparison, varied between individuals, both in depth and in bandwidth (the range of spatial frequencies affected). This variation was not directly related to the extent of acuity deficit, or to the condition which originally gave rise to amblyopia, but did seem connected with the age at which the subject first received treatment for the primary causative ocular defect. A similarity between the contrast sensitivity functions of amblyopic eyes and those of infant eyes is considered as a basis for explaining the nature of contrast sensitivity loss in amblyopia. Some preliminary attempts to measure contrast sensitivity in infancy by methods suitable for screening purposes are described in the final chapter. The thesis includes a historical review of theories of amblyopia derived from clinical and experimental work on human subjects, and a critical evaluation of experimental work in which animals were visually deprived with a view to measuring the contributions of experience to visual development. The claims of some authors that such work may have clinical relevance for preventing or treating amblyopia are refuted, since clinical experience has already furnished sufficient evidence to achieve these ends. The perceptual consequences of human amblyopia, as characterised in the present research have important practical implications for the amblyope, and important theoretical implications for models and mechanisms of visual perception and its development

Psychological and Physiological Processes Underlying Perception and Attention: A Study of Binocular Rivalry.

This thesis is concerned with an investigation of certain psychological and physiological processes underlying perception and attention. In this context binocular rivalry is selected for close investigation since it has at different times been related to both perception and attention. This relationship is demonstrated by a series of investigations which show that the stimulus that is currently non-dominant in rivalry is nevertheless fully analysed. The nature of rivalry indicates that two complementary visual systems contribute to perception and attention. Whilst one system (superior colliculus - posterior association cortex) is responsible for monitoring unperceived/unattended information and initiating a shift in attention, the other system (geniculo-striate cortex) is concerned with currently perceived/attended information. In the terminology of control theory, these two visual systems contribute to feedforward and feedback control respectively. The interaction between the two is considered to be the correlate of conscious perception and attention, reflecting the sampling of sensory information by a process that matches this information against the expectations based on a model of the world. Confirmation of a number of predictions refines and further anchors the theory to physiological mechanisms

Neural mechanisms for reducing uncertainty in 3D depth perception

In order to navigate and interact within their environment, animals must process and interpret sensory information to generate a representation or ‘percept’ of that environment. However, sensory information is invariably noisy, ambiguous, or incomplete due to the constraints of sensory apparatus, and this leads to uncertainty in perceptual interpretation. To overcome these problems, sensory systems have evolved multiple strategies for reducing perceptual uncertainty in the face of uncertain visual input, thus optimizing goal-oriented behaviours. Two available strategies have been observed even in the simplest of neural systems, and are represented in Bayesian formulations of perceptual inference: sensory integration and prior experience. In this thesis, I present a series of studies that examine these processes and the neural mechanisms underlying them in the primate visual system, by studying depth perception in human observers. Chapters 2 & 3 used functional brain imaging to localize cortical areas involved in integrating multiple visual depth cues, which enhance observers’ ability to judge depth. Specifically, we tested which of two possible computational methods the brain uses to combine depth cues. Based on the results we applied disruption techniques to examine whether these select brain regions are critical for depth cue integration. Chapters 4 & 5 addressed the question of how memory systems operating over different time scales interact to resolve perceptual ambiguity when the retinal signal is compatible with more than one 3D interpretation of the world. Finally, we examined the role of higher cortical regions (parietal cortex) in depth perception and the resolution of ambiguous visual input by testing patients with brain lesions

Naturalistic depth perception and binocular vision

Humans continuously move both their eyes to redirect their foveae to objects at new depths. To correctly execute these complex combinations of saccades, vergence eye movements and accommodation changes, the visual system makes use of multiple sources of depth information, including binocular disparity and defocus. Furthermore, during development, both fine-tuning of oculomotor control as well as correct eye growth are likely driven by complex interactions between eye movements, accommodation, and the distributions of defocus and depth information across the retina. I have employed photographs of natural scenes taken with a commercial plenoptic camera to examine depth perception while varying perspective, blur and binocular disparity. Using a gaze contingent display with these natural images, I have shown that disparity and peripheral blur interact to modify eye movements and facilitate binocular fusion. By decoupling visual feedback for each eye, I have found it possible to induces both conjugate and disconjugate changes in saccadic adaptation, which helps us understand to what degree the eyes can be individually controlled. To understand the aetiology of myopia, I have developed geometric models of emmetropic and myopic eye shape, from which I have derived psychophysically testable predictions about visual function. I have then tested the myopic against the emmetropic visual system and have found that some aspects of visual function decrease in the periphery at a faster rate in best-corrected myopic observers than in emmetropes. To study the effects of different depth cues on visual development, I have investigated accommodation response and sensitivity to blur in normal and myopic subjects. This body of work furthers our understanding of oculomotor control and 3D perception, has applied implications regarding discomfort in the use of virtual reality, and provides clinically relevant insights regarding the development of refractive error and potential approaches to prevent incorrect emmetropization