Browsing by Subject "Visual perception"

Now showing 1 - 17 of 17

Open Access
Behavioral and computational investigation of the effect of prior knowledge on visual perception
(2021-01) Ürgen, Buse Merve
Visual perception results from the dynamic interaction of bottom-up and topdown processes. Top-down prior knowledge and expectations can guide us to predict upcoming events and even determine what we see in an ambiguous or noisy sensory stimulus. Despite the well-established facilitating effects of expectations on recognition or decision-making, whether and how early sensory processes are affected by expectations remain unclear. This dissertation attempts to investigate the effect of expectations on early visual processes. To this end, we used behavioral experiments to examine the effects of expectation on visual perception at the threshold level and implemented a recursive Bayesian model and a recurrent cortical model to unravel the computational mechanisms underlying those effects. In the behavioral experiments, we systematically manipulated expectation’s validity in separate sessions and measured duration thresholds, which is the shortest presentation time sufficient to achieve a certain success level. Our behavioral findings showed that valid expectations do not reduce the thresholds, rather unmet expectations lead them to increase. Next, using a recursive Bayesian updating scheme, we modeled the empirical data obtained in the behavioral experiments. Model fitting showed that higher thresholds observed in the unmet expectations are not due to a change in the internal parameters of the system. Instead, additional computations are required by the system to complete the sensory process. Finally, within the predictive processing framework, we implemented a recurrent cortical model to explain the behavioral findings and discuss possible neural mechanisms underlying the observed effects. The cortical model findings were in agreement with the Bayesian model results, revealing that longer processing is needed when expectations are not met. Overall, the computational models that are proposed in this study provide a parsimonious explanation for the observed behavioral effects. The proposed experimental paradigm and the computational models offer a novel framework that can be extended and used in other stimuli, tasks, and sensory modalities.
Open Access
Biased competition in semantic representations across the human brain during category-based visual search
(2017-01) Shahdloo, Mohammad
Humans can perceive thousands of distinct object and action categories in the visual scene and successfully divide their attention among multiple target categories. It has been shown that object and action categories are represented in a continuous semantic map across the cortical surface and attending to a specific category causes broad shifts in voxel-wise semantic tuning profiles to enhance the representation of the target category. However, the effects of divided attention to multiple categories on semantic representation remain unclear. In line with predictions of the biased-competition model, recent evidence suggests that brain response to two objects presented simultaneously can be described as a weighted average of the responses to individual objects presented in isolation, and that attention biases these weights in favor of the target object. We question whether this biased-competition hypothesis can also account for attentional modulation of semantic representations. To address this question, we recorded participants’ BOLD responses while they performed category-based search in natural movies that contained 831 distinct objects and actions. Three different tasks were used: search for “humans”, search for “vehicles”, and search for “both humans and vehicles” (i.e. divided attention). Voxel-wise category models were fit separately under each task, and voxel-wise semantic tuning profiles were then obtained using a principal components analysis on the model weights. Semantic tuning profiles were compared across the single-target tasks and the divided-attention task. We find that in higher visual cortex a substantial portion of semantic tuning during divided attention can be expressed as a weighted average of the tuning profiles during attention to single targets. We also find that semantic tuning in categoryselective regions is biased towards the preferred object category. Overall, these results suggest that the biased-competition theory accounts for attentional modulation of semantic representations during natural visual search.
Open Access
Biological motion perception in the theoretical framework of perceptual decision-making: an event-related potential study
(Elsevier Ltd, 2024-05) Oğuz, Osman Çağrı; Aydın, Berfin; Ürgen, Burcu Ayşen
Biological motion perception plays a critical role in various decisions in daily life. Failure to decide accordingly in such a perceptual task could have life-threatening consequences. Neurophysiology and computational modeling studies suggest two processes mediating perceptual decision-making. One of these signals is associated with the accumulation of sensory evidence and the other with response selection. Recent EEG studies with humans have introduced an event-related potential called Centroparietal Positive Potential (CPP) as a neural marker aligned with the sensory evidence accumulation while effectively distinguishing it from motor-related lateralized readiness potential (LRP). The present study aims to investigate the neural mechanisms of biological motion perception in the framework of perceptual decision-making, which has been overlooked before. More specifically, we examine whether CPP would track the coherence of the biological motion stimuli and could be distinguished from the LRP signal. We recorded EEG from human participants while they performed a direction discrimination task of a point-light walker stimulus embedded in various levels of noise. Our behavioral findings revealed shorter reaction times and reduced miss rates as the coherence of the stimuli increased. In addition, CPP tracked the coherence of the biological motion stimuli with a tendency to reach a common level during the response, albeit with a later onset than the previously reported results in random-dot motion paradigms. Furthermore, CPP was distinguished from the LRP signal based on its temporal profile. Overall, our results suggest that the mechanisms underlying perceptual decision-making generalize to more complex and socially significant stimuli like biological motion.
Open Access
A clustering-based method to estimate saliency in 3D animated meshes
(Elsevier Ltd, 2014) Bulbul, A.; Arpa, S.; Capin, T.
We present a model to determine the perceptually significant elements in animated 3D scenes using a motion-saliency method. Our model clusters vertices with similar motion-related behaviors. To find these similarities, for each frame of an animated mesh sequence, vertices' motion properties are analyzed and clustered using a Gestalt approach. Each cluster is analyzed as a single unit and representative vertices of each cluster are used to extract the motion-saliency values of each group. We evaluate our method by performing an eye-tracker-based user study in which we analyze observers' reactions to vertices with high and low saliencies. The experiment results verify that our proposed model correctly detects the regions of interest in each frame of an animated mesh. © 2014 Elsevier Ltd.
Open Access
The effect of graphic design materials on the retention level of viewers in prime time television newscasts
(1996) Ertep, Rifat Hakan
This study investigates the role of graphic design materials in improving the recall and retention level of television news viewers, and examines the capacity and power of these materials to shape or distort people's perception of reality. To this end, two experiments have been conducted with the aim of providing an empirical framework to the role of graphics in the construction of reality. It was found that graphics increase the recall and retention level of the viewers, and that they can alter viewers' recall of the content of the news stories.
Open Access
The effect of orientation-related prior probability information on contrast perception
(2019-07) Nazlı, İlayda
It is known that visual perception is the product of sensory input and prior probability information. Previous studies support well that expectation influence recognition and decision-making; however, we have limited knowledge about how expectation influences low level visual processing. In the current study, we examine the effect of expectation on early visual processing. That is, we provide task-irrelevant expectation cue related to the orientation of target Gabor grating, and we ask participants to indicate the spatial location of target grating while systematically manipulating its contrast level. In addition, we examine how different expectation validities (i.e. 75%, 50%, 100%) and neutral expectation cue affect visual perception. In Experiment 1, where the orientation of expectation cue and target gratings are vertical or horizontal, contrast threshold is lower in congruent trials at 75% validity condition. In Experiment 2, where the orientation of cue grating implies specific range (i.e. ±15 around 0 or 90), contrast threshold is lower in congruent trials at 75% and 50% validity condition. In Experiment 3B, where the orientation of expectation cue and target Gabor grating are within wider range (i.e. ±30 around 0 or 90), contrast threshold is lower in congruent trials at 75% validity condition. In Experiment 3A, where the orientation of expectation cue and target Gabor grating are within a specific range (i.e. ±15 around 0 or 90) contrast threshold is lower in congruent trials at 75% and 50% validity condition. In Experiment 3B, where the orientation of expectation cue and target Gabor grating are within wider range (i.e. ±30 around 0 or 90), contrast threshold is lower in congruent trials at 75% validity condition.
Open Access
Effects of context and expectations on dynamics of visual processing
(2023-09) Malik, Amna
Living organisms rely on the information they receive through their senses to survive and function in an environment that is constantly changing. However, incoming information from the senses is often ambiguous, noisy, or incomplete. Context, either spatial, temporal, or cognitive, is fundamental, especially in visual perception, to disambiguate and complete this noisy input and optimize behavior. The mechanisms by which contextual information influences visual perception, however, are not fully understood. Studies in the current thesis aim to add to the understanding of those mechanisms. It comprises two lines of work. The first line of work focuses on the spatial context, where using fMRI, we investigate the neural correlates of spatiotemporal properties of context-dependent lightness perception. Results show that activity in the primary visual cortex (V1) correlates with context-dependent lightness perception, providing evidence for low-level mechanisms underlying the contextual effects. The second line of work focuses on the cognitive context, where we systematically study the effect of expectations about dynamic material properties on perceptual decisions. To do so, we used behavioral methods, where we manipulated participants’ long-term and short-term expectations about material properties. Results show that expectations about material properties can impact relatively low-level perceptual decision-making processes. Furthermore, we found an interplay between long-term and newly learned expectations. In conclusion, the current thesis broadens our understanding of how context influences visual processes, particularly by pro-viding evidence that low-level processes are affected by the visual context. This knowledge has the potential to help develop more accurate models of visual perception, which in turn can have implications in clinical neuroscience, artificial intelligence, computer vision, and marketing.
Open Access
Effects of different lighting arrangements on space perception
(1997) Yücetaş, Banu
The importance given to the quality of lighting design has increased in recent years. This makes lighting design more an art than an engineering problem and it is closely related to the way a space is perceived. It is difficult to discuss perception as it is the subjective interpretation of a certain situation. This study concentrates on the quality of lighting and the relation between the lighting arrangements and perception. A room that has four different lighting arrangements was prepared and the differences between the perception of the participants under each lighting arrangement was analyzed in terms of the six impressions, clarity, spaciousness, relaxation, privacy, pleasantness and order. The variation in these terms due to the alterations in the lighting systems was studied. The age, gender and the eye deficiency of participants were collected as personal data and their relations to the process of perception were taken into account.
Open Access
Examining the effects of audiovisual associations on motion perception through task-based fMRI
(Yerkure Tanitim ve Yayincilik Hizmetleri A.S., 2018) Kafalıgönül, Hulusi
Examining the effects of audiovisual associations on motion perception through task-based fMRI Objective: Previous studies showed that associative learning can lead to drastic changes in perceptual experience and unexpected levels of sensory plasticity in the adult brain. However, how associative learning is involved in shaping perception and the underlying neural mechanisms are quite poorly understood. In the current study, by taking advantage of well-studied visual motion-processing hierarchy, the roles of different brain areas in audiovisual association-induced changes in motion perception are investigated. Method: Using a previously developed audiovisual associative paradigm, behavioral and Blood Oxygen Level Dependent (BOLD) data were collected from adult human participants (n=13) before and after the association phase. Behavioral data were collected through reports on visual motion direction. Functional magnetic resonance imaging (fMRI) was based on block design and the functional data were analyzed according to a general linear model. Results: Audiovisual associations, acquired within a short time and without any feedback, significantly affected the perception of motion direction. This effect was much more salient when the physical direction of visual motion was ambiguous. Moreover, fMRI findings pointed out that the BOLD activities across different cortical regions changed after the associative phase. Conclusion: Taken together, these findings indicate that low-level sensory, multisensory and high-level cognitive areas play a role in the effects of audiovisual associations on motion perception. In general, this suggests that our prior experiences acquired through associations may affect perceptual processing at different hierarchical levels and over different cortical areas.
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(1994) Cemal, Ahmet
Open Access
Human visual cortical responses to specular and matte motion flows
(Frontiers Media S. A, 2015) Kam, T.-E.; Mannion, D.J.; Lee, S.-W.; Doerschner, K.; Kersten, D.J.
Determining the compositional properties of surfaces in the environment is an important visual capacity. One such property is specular reflectance, which encompasses the range from matte to shiny surfaces. Visual estimation of specular reflectance can be informed by characteristic motion profiles; a surface with a specular reflectance that is difficult to determine while static can be confidently disambiguated when set in motion. Here, we used fMRI to trace the sensitivity of human visual cortex to such motion cues, both with and without photometric cues to specular reflectance. Participants viewed rotating blob-like objects that were rendered as images (photometric) or dots (kinematic) with either matte-consistent or shiny-consistent specular reflectance profiles. We were unable to identify any areas in low and mid-level human visual cortex that responded preferentially to surface specular reflectance from motion. However, univariate and multivariate analyses identified several visual areas; V1, V2, V3, V3A/B, and hMT+, capable of differentiating shiny from matte surface flows. These results indicate that the machinery for extracting kinematic cues is present in human visual cortex, but the areas involved in integrating such information with the photometric cues necessary for surface specular reflectance remain unclear. © 2015 Kam, Mannion, Lee, Doerschner and Kersten.
Open Access
Influence of complexity and Gestalt principles on aesthetic preferences for building façades: An eye tracking study
(Bern Open Publishing, 2024-08-09) Beder, Dilara; Pelowski, Matthew; İmamoğlu, Çağrı
Buildings are an integral part of our physical environment and have aesthetic significance with respect to the organizational integrity of architectural elements. While Gestalt principles are essential in design education, their relationship with architectural features remains understudied. The present study explored how Gestalt principles and complexity levels influence evaluations of building façades through the use of questionnaires and eye tracking. Twenty-four two-dimensional black and white façade drawings, manipulated using selected Gestalt principles (similarity and proximity) to achieve different levels of complexity (low, medium & high), were presented to 79 participants. The results suggested a negative linear relationship between aesthetic ratings and complexity levels across selected Gestalt principles. In addition, as expected, participants had the highest number of fixations, shortest fixation durations, and lowest aesthetic ratings for higher levels of complexity. Results involving Gestalt principles revealed that proximity-based designs received higher aesthetic ratings, demanded less time, elicited lower number of fixations, and resulted in shorter fixation durations. Conversely, similarity-based designs received lower aesthetic ratings, demanded more time, elicited higher number of fixations, and resulted in longer fixation durations. These findings offer insights into architectural aesthetic experiences and inform future research directions.
Open Access
An overview of research in 3DTV
(IEEE, 2007) Onural, Levent
3DTV is regarded by the experts and the general public as the next major step in video technologies. The ghost-like images of remote persons or objects are already depicted in many futuristic movies; both entertainment applications, as well as 3D video telephony, are among the commonly imagined utilizations of such a technology. As in every product, there are various different technological approaches also in 3DTV. By the way, 3D technologies are not new; the earliest 3DTV application is demonstrated within a few years after the invention of 2D TV. However, earlier 3D video relied on stereoscopy. Current work mostly focuses on advanced variants of stereoscopic principles like goggle-free autostereoscopic multi-view devices. However, holographic 3DTV and its variants are the ultimate goal and will yield the envisioned high-quality ghostlike replicas of original scenes once technological problems are solved. Stereoscopy is based on exploiting the human perception. Simply, two views, taken at two slightly different angles are then guided to left and right eyes. The two eyes, receiving the two different views of the same scene from two different angles, provide the visual signals to the brain; and then, the brain interprets the scene as 3D. However, there are many different 3D depth cues in perception, and usually, there are contradictory signals received by the brain. Viewers experience a motion-sickness-like feeling as a consequence of such mismatches. This is the major reason which kept 3D from becoming a popular mode of visual communications. However, recent advances in end-to-end digital techniques minimized such problems. Stereoscopic TV broadcasts have been conducted. Novel advances in stereoscopy brought viewing without goggles; however, the viewer and the monitor must have a fixed location and orientation with respect to each other for most autostereoscopic images. Multi-view autostereoscopic displays allow some horizontal parallax within a limited viewing angle. There are experiments in head-tracking autostereoscopic displays, as well as, free-view point video by providing the right pair of images based on the location of the viewer. Holography is not based on human perception, but targets perfect recording and reconstruction of light with all its properties. If such a reconstruction is achieved, the viewer, embedded in the same light distributionas the original, will of course see the same scene as the original.
Open Access
Perceptually-driven computer graphics and visualization
(2016-10) Yıldız, Zeynep Çipiloğlu
Despite the rapid advances in computer graphics technology, enhancing the visual quality and lowering the rendering cost is still the essential goal for computer graphics researchers; since improvements in computational power raise the users' expectations too. Especially in interactive 3D games and cinema industry, very realistic graphical contents are desired in real-time. In the meantime, due to the increasing popularity of social networking systems and data sharing, there is a huge amount of data to be visualized effectively. When used carefully, 3D introduces a new data channel for information visualization applications. For that reason, improving the visual quality of 3D computer-generated scenes is still of great interest in the computer graphics and visualization community. In the last decade, utilization of visual perception findings in computer graphics has started to get popular since visual quality is actually judged by the human perception and there is no need to spend additional cost for the physical realism of the details that cannot be perceived by the observer. There is still room for employing the perceptual principles in computer graphics. We contribute to the perceptual computer graphics research in two main aspects: First we propose several perceptual error metrics for evaluating the visual quality of static or animated 3D meshes. Second, we develop a system for ameliorating the perceived depth quality and comprehensibility in 3D visualization applications. A measure for assessing the quality of a 3D mesh is necessary in order to determine whether an operation on the mesh, such as watermarking or compression, affects the perceived quality. The studies on this field are limited when compared to the studies for 2D. A bottom-up approach incorporating both the spatial and temporal components of the low-level human visual system processes is suggested to develop a general-purpose quality metric designed to measure the local distortion visibility on dynamic triangle meshes. In addition, application of crowdsourcing and machine learning methods to implement a novel data-driven error metric for 3D models is also demonstrated. During the visualization of 3D content, using the depth cues selectively to support the design goals and enabling a user to perceive the spatial relationships between the objects are important concerns. In this regard, a framework for selecting proper depth cues and rendering methods providing these cues for the given scene and visualization task is put forward. This framework benefits from fuzzy logic for determining the importance of depth cues and knapsack method for modeling the cost-profit tradeoff between the rendering costs of the methods and their contribution to depth perception. All the proposed methods in this study are validated through formal user experiments and we obtain encouraging results for further research. These results are made publicly available for the benefit of graphics community. In conclusion, we try to make the gap between visual perception and computer graphics fields narrower with the suggested methods in this work.
Open Access
Predictive processing in the cortical network of biological motion perception
(2024-07) Tunca, Murat Batu
The literature on biological motion processing argues that it occurs in occipitotemporal, parietal and frontal regions of the brain. Nevertheless, the literature is currently unable to explain how this processing is affected by expectations. Although models exist to explain how biological motion is perceived, they usually ignore top-down processes. To this end, the current fMRI study presented two point-light displays (embedded in noise) on either side of the screen to the participants (N=29). One of the displays was a biological motion (walking or kicking) whereas the other one was its scrambled version. The participants were asked to report the location of the biological motion. Importantly, before the presentation of motions, the participants were shown a cue about the action type (walking or kicking) which was congruent with the motion 75% of the time. There were also two additional conditions in which the cue was uninformative about the action (neutral condition) or there were no motion stimuli at all. As expected, the action observation network (consisting of pSTS, parietal cortex and IFG) showed a clear and strong activation during the conditions that a motion was present. However, these regions have failed to significantly discriminate between congruent and incongruent conditions. It should be acknowledged that this lack of significant result might be caused by the low number of trials. In order to better investigate the connections within action observation network, a DCM analysis was conducted. The winning DCM model has successfully shown the presence of feedback connections in the biological motion processing. More specifically, the model argues that both feedforward and feedback modulatory connections are present during congruent, incongruent and neutral conditions. In sum, the study highlights the importance of incorporating top-down signals such as expectations in the computational models of biological motion perception.
Open Access
Unmet expectations delay sensory processes
(Elsevier, 2021-01-02) Ürgen, Buse Merve; Boyacı, Hüseyin
Expectations strongly affect and shape our perceptual decision-making processes. Specifically, valid expectations speed up perceptual decisions, and determine what we see in a noisy stimulus. Despite the well-established effects of expectations on decision-making, whether and how they affect low-level sensory processes remain elusive. To address this problem, we investigated the effect of expectation on temporal thresholds in an individuation task (detection of the position of an intact image, a house or face). We found that compared to a neutral baseline, thresholds increase when the intact images are of the unexpected category, but remain unchanged when they are of the expected category. Using a recursive Bayesian model with dynamic priors we show that delay in sensory processes is the result of further processing, consequently longer time, required in case of violated expectations. Expectations, however, do not alter internal parameters of the system. These results reveal that sensory processes are delayed when expectations are not met, and a simple parsimonious computational model can successfully explain this effect.
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Yalnızım : Görüyorum
(1998) Nesimi, Necati