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

Components of bottom-up gaze allocation in natural images

Recent research [Parkhurst, D., Law, K., & Niebur, E., 2002. Modeling the role of salience in the allocation of overt visual attention. Vision Research 42 (1) (2002) 107–123] showed that a model of bottom-up visual attention can account in part for the spatial locations fixated by humans while free-viewing complex natural and artificial scenes. That study used a definition of salience based on local detectors with coarse global surround inhibition. Here, we use a similar framework to investigate the roles of several types of non-linear interactions known to exist in visual cortex, and of eccentricity-dependent processing. For each of these, we added a component to the salience model, including richer interactions among orientation-tuned units, both at spatial short range (for clutter reduction) and long range (for contour facilitation), and a detailed model of eccentricity-dependent changes in visual processing. Subjects free-viewed naturalistic and artificial images while their eye movements were recorded, and the resulting fixation locations were compared with the models’ predicted salience maps. We found that the proposed interactions indeed play a significant role in the spatiotemporal deployment of attention in natural scenes; about half of the observed inter-subject variance can be explained by these different models. This suggests that attentional guidance does not depend solely on local visual features, but must also include the effects of interactions among features. As models of these interactions become more accurate in predicting behaviorally-relevant salient locations, they become useful to a range of applications in computer vision and human-machine interface design

Investigating the effect of texture edges in figure-ground segregation using psychophysical and eye tracking experiments

Although it happens infrequently in the natural world, the human visual system is able to perceive objects defined solely by a difference in texture i.e. with no accompanying change in colour or luminance. However, studies on texture perception have frequently used figure-ground patterns with abrupt texture variations, with few studies using patterns with smooth texture variations. The work presented in this thesis considers the contribution of the edge and centre regions of a texture figure to orientation-based texture segregation. To this end, we used psychophysical (Chapters 4 – 6) and eye tracking techniques (Chapters 8 – 10) to investigate texture detection and segmentation. For the psychophysics experiments, the primary goal was to investigate if either an edge-based or region-based mechanism can account for both smooth and abrupt texture variations. We first examined this in Chapter 4, where we studied the effects of texture edges on figure-ground segregation. Lower thresholds were found when the texture figure had orientation contrast information at the edge and centre of the figure. Data modeling supports the notion that texture segregation involves a large-scale second-order texture filter i.e. akin to a region-based mechanism, but where information is extracted over a large albeit fixed-size region. Various studies were also conducted to determine what aspects of a texture figure would change the size of the second-stage filter. The size and aspect ratio of the figure were manipulated (Chapter 5), and also the spatial frequency of the texture pattern, age of the participants, and the viewing distance (Chapter 6). We found that higher spatial frequencies resulted in larger integration regions i.e. feeds into large second-stage filters, but age, viewing distance, figure size and aspect ratio did not influence the size of the integration region. For the eye tracking studies, the general aim was to investigate what information of a texture target is extracted in order to produce signals for eye movement control. We measured eye movements made by participants while they searched for a texture figure embedded in a background. We found that irrespective of the types of orientation profiles, area-normalized data were that the centre region of a figure was looked at most often, and for longer durations. However, figures with information of orientation contrast at both the edge and centre of figure were easier to localise (Chapter 7), and produced the highest level of saliency in attracting eye movements (Chapter 10). In Chapter 9, we demonstrate that the visual system is also able to efficiently segregate a texture figure from the ground to accurately plan a saccade to the target figure, and these saccades are planned based on the representation of the whole figure shape as opposed to local salient regions. More specifically, saccades were directed to the centre of gravity of the target, with some degree of undershoot. Finally, the similar size of the integration region for the eye tracking (Chapter 10) and psychophysics experiments implies that the saccadic system receives input from the mechanism that segregates figure-ground texture stimuli

Goal Directed Visual Search Based on Color Cues: Co-operative Effectes of Top-Down & Bottom-Up Visual Attention

Focus of Attention plays an important role in perception of the visual environment. Certain objects stand out in the scene irrespective of observers\u27 goals. This form of attention capture, in which stimulus feature saliency captures our attention, is of a bottom-up nature. Often prior knowledge about objects and scenes can influence our attention. This form of attention capture, which is influenced by higher level knowledge about the objects, is called top-down attention. Top-down attention acts as a feedback mechanism for the feed-forward bottom-up attention. Visual search is a result of a combined effort of the top-down (cognitive cue) system and bottom-up (low level feature saliency) system. In my thesis I investigate the process of goal directed visual search based on color cue, which is a process of searching for objects of a certain color. The computational model generates saliency maps that predict the locations of interest during a visual search. Comparison between the model-generated saliency maps and the results of psychophysical human eye -tracking experiments was conducted. The analysis provides a measure of how well the human eye movements correspond with the predicted locations of the saliency maps. Eye tracking equipment in the Visual Perceptual Laboratory in the Center for Imaging Science was used to conduct the experiments

Eye movements used for the objective assessment of contrast sensitivity

Assessment of contrast sensitivity (CS) from eye movements has already been proposed as a possible method to gain objective information about this visual function. The currently submitted dissertation holding the title Eye movements used for the objective assessment of contrast sensitivity improves the currently proposed approaches of the eye-movement-based CS tests and proposes novel ones. The first project of this dissertation extends the applicability of microsaccadic rate signatures, already proposed as the tool for objective CS testing, into the clinical practice by stimulation of microsaccades under monocular conditions, while analyzing the eye movements as binocular events. In this approach an infra-red filter for covering the left eye of the tested subjects and an infrared eye-tracker were used, resulting in the stimulation of just one eye of the participant, however allowing the eye movement detection in both eyes. Because the microsaccadic events were found to be rare in some participants, the second project utilized optokinetic nystagmus in CS testing. In this project, the optokinetic nystagmus was detected in real-time when running the experiment, using a newly-developed life detection method, and along with a one-dimensional adaptive psychometric procedure (QUEST+) for the contrast level management enabled a fully automated CS examination. Furthermore, the examination was conducted for a range of spatial frequencies of a grating as well as a range of defocus conditions, replicating the well-known effects on CS from the clinical trials. The third project aimed to extend the eye-movement-based testing also to the peripheral visual field, and thus replicate the CS perimetry testing procedure in an objective way. In this project CS was tested using reflexive (reactive) saccades, occurring towards a newly presented target, which was located in one of the four cardinal directions in the patient’s visual field. Here the four independent QUEST+ psychometric procedures sought for the contrast threshold in the four directions individually, for each of the three tested eccentricity levels. Moreover, the measurements were conducted for the eye-movement-based and key-board-based measurements to run a rounded validation of this novel procedure, showing its feasibility for future clinical application. In conclusion, this thesis extends an improves the portfolio of eye movements used in CS testing, shows novel approaches to improve the time efficiency, and replicates the well-known effects from the clinical trials

A review of experimental task design in psychophysical eye tracking research

While eye tracking is a technique commonly used in the experimental study of higher-level perceptual processes such as visual search, working memory, reading, and scene exploration, its use for the quantification of basic visual functions (visual acuity, contrast sensitivity, color vision, motion detection) is less explored. The use of eye movement features as dependent variables in a psychophysical investigation can serve multiple roles. They can be central in studies with neurological patients or infants that cannot comply with verbal instructions, understand task demands, and/or emit manual responses. The technique may also serve a complementary role, determining the conditions under which a manual or verbal response is given, such as stimulus position in the visual field, or it can afford the analysis of new dependent variables, such as the time interval between oculomotor and manual responses. Our objective is to review the literature that applied the eye tracking technique to psychophysical problems. The two questions our review raises are: can eye movements (reflex or voluntary) be an objective index of stimulus detection in psychophysical tasks? If so, under what conditions, and how does it compare with traditional paradigms requiring manual responses? Our (non-systematic) methodological review selected studies that used video-oculography as the technique of choice and had a basic visual function as their primary object of investigation. Studies satisfying those criteria were then categorized into four broad classes reflecting their main research interest: (1) stimulus detection and threshold estimation, (2) the effects of stimulus properties on fixational eye movements, (3) the effects of eye movements on perception, and (4) visual field assessment. The reviewed studies support the idea that eye tracking is a valuable technique for the study of basic perceptual processes. We discuss methodological characteristics within each of the proposed classification area, with the objective of informing future task design

Object and feature based modelling of attention in meeting and surveillance videos

MPhilThe aim of the thesis is to create and validate models of visual attention. To this extent, a novel unsupervised object detection and tracking framework has been developed by the author. It is demonstrated on people, faces and moving objects and the output is integrated in modelling of visual attention. The proposed approach integrates several types of modules in initialisation, target estimation and validation. Tracking is rst used to introduce high-level features, by extending a popular model based on low-level features[1]. Two automatic models of visual attention are further implemented. One based on winner take it all and inhibition of return as the mech- anisms of selection on a saliency model with high- and low-level features combined. Another which is based only on high-level object tracking results and statistic proper- ties from the collected eye-traces, with the possibility of activating inhibition of return as an additional mechanism. The parameters of the tracking framework thoroughly investigated and its success demonstrated. Eye-tracking experiments show that high- level features are much better at explaining the allocation of attention by the subjects in the study. Low-level features alone do correlate signi cantly with real allocation of attention. However, in fact it lowers the correlation score when combined with high-level features in comparison to using high-level features alone. Further, ndings in collected eye-traces are studied with qualitative method, mainly to discover direc- tions in future research in the area. Similarities and dissimilarities between automatic models of attention and collected eye-traces are discusse