Visual Scene Perception Research Articles

Interactions between top-down and stimulus-driven processes in visual feature integration

Event Abstract Back to Event Interactions between top-down and stimulus-driven processes in visual feature integration Marc Schipper1*, Udo Ernst2, Klaus Pawelzik2 and Manfred Fahle1 1 Bremen University, Department for Human Neurobiology, Center for Cognitive Sciences, Germany 2 Bremen University, Department for Theoretical Physics, Center for Cognitive Sciences, Germany Perception of visual scenes requires the brain to link local image features into global contexts. Contour integration is such an example grouping colinearily aligned edge elements to form coherent percepts. Theoretical and modeling studies demonstrated that purely stimulus-driven mechanisms, as implemented by feedforward or recurrent network architectures, are well suited to explain this cognitive function. However, recent empirical work showed that top-down attention can strongly modulate contour integration. By combining psychophysical with electrophysiological methods, we studied how strongly prior expectations shape contour integration. These empirical techniques were complemented by model simulations to uncover the putative neural substrates and mechanisms underlying contour integration.Subjects participated in two experiments with identical visual stimuli but different behavioural tasks: a detection task (A) and a discrimination task (B). Stimuli consisted of vertical or horizontal ellipses formed by colinearily aligned Gabor elements embedded in a field of Gabors with random orientations and positions. Each hemifield could contain either (i) one vertical, (ii) one horizontal, or (iii) no ellipse. All combinations of these three basic configurations were possible, resulting in nine stimulus categories. In experiment A participants replied ‘yes’ whenever one stimulus contained at least one ellipse, in experiment B observers replied ‘yes’ only when a target was present (either a horizontal or vertical ellipse).The psychophysical data demonstrate a pronounced influence of higher cognitive processes on contour integration: In the discrimination task, reaction times (RT) are consistently shorter for targets than for distractors. The presence of redundant targets (e.g. two horizontal ellipses instead of only one horizontal ellipse) also shortens RTs. These first two effects were consistent with our expectations. Moreover we discovered an additional bias in RT for horizontal ellipses (~70 ms shorter than for vertical ellipses).In EEG recordings, we find pronounced differences in event-related potentials (ERPs) between stimulations with versus without the presence of contours. These differences appear at about 110-160 ms after stimulus onset in the occipital regions of the cortex. In the same regions the evoked potentials were substantially modulated by the number of contours present (~140 ms after stimulus onset) and depending on the behavioural task (~230 ms after stimulus onset). Psychophysical and electrophysiological results are qualitatively consistent: The larger the RT differences, the more dissimilar are ERPs in occipital regions. Moreover, phenomenological modeling reveals that the horizontal bias and task-induced effects either constructively or destructively combine in a multiplicative way. This may lead to much lower RTs when e.g. a horizontal bias combines with a horizontal target, or to a mutual cancellation of the different RT effects when e.g. a horizontal bias combines with a vertical target. Acknowledgments:This work was supported by the BMBF as part of the National Bernstein Network for Computational Neuroscience. Conference: Bernstein Conference on Computational Neuroscience, Frankfurt am Main, Germany, 30 Sep - 2 Oct, 2009. Presentation Type: Oral Presentation Topic: Sensory processing Citation: Schipper M, Ernst U, Pawelzik K and Fahle M (2009). Interactions between top-down and stimulus-driven processes in visual feature integration. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference on Computational Neuroscience. doi: 10.3389/conf.neuro.10.2009.14.158 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 28 Aug 2009; Published Online: 28 Aug 2009. * Correspondence: Marc Schipper, Bremen University, Department for Human Neurobiology, Center for Cognitive Sciences, Bremen, Germany, schipper.marc@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Marc Schipper Udo Ernst Klaus Pawelzik Manfred Fahle Google Marc Schipper Udo Ernst Klaus Pawelzik Manfred Fahle Google Scholar Marc Schipper Udo Ernst Klaus Pawelzik Manfred Fahle PubMed Marc Schipper Udo Ernst Klaus Pawelzik Manfred Fahle Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Human non-phase-locked gamma oscillations in experience-based perception of visual scenes

Perception results from ongoing interactions between stimulus-driven visual processes and cognitive context. These reciprocal relations are emphasized when a visual stimulus is degraded, forcing the perceiver to fill the missing information in, based on internal representations. The neural mechanisms by which internal representations facilitate visual perception are still unclear. Here we investigated the role of EEG oscillations in the gamma band, thought to reflect the elaboration of internal visual representations, in the experience-based perception of visual scenes. Twelve subjects were trained with degraded images of natural scenes. EEG was recorded while they performed a detection task on trained and untrained degraded stimuli. Non-phase-locked gamma band responses in a large frequency spectrum (55–85 Hz) were observed around 200 ms post-stimulus onset at posterior sites, and were larger when subjects reported an accurate perception based on previous experience. These results suggest that mid-latency gamma oscillations in the visual cortex underlie the experience-based perception of visual scenes.

Contour integration in striate cortex. Classic cell responses or cooperative selection?

Psychophysics has established various rules of contour integration in gestalt perception. We tested the rule of good continuation by stimulating behaving monkeys with simple figures composed of Gabor patches, while recording from upper layer cells in visual cortex (V1). By decomposing these figures into their components and stimulating receptive field centers and surrounds separately with this stimulus set, we tested center-surround interaction for linearity. In pure-fixation tasks, the interaction was negative for the early strong evoked response, i.e., in this phase figural context rather inhibited the cells. However, in the following tonic response phase, a subgroup of neurons showed positive interaction during the whole stimulus presentation period of at least 1,000 ms. Attention to the figure in discrimination tasks only slightly improved this positive interaction between 150 and 300 ms. We interpret these results as selective cooperation and mutual facilitation of cortical V1 cells, thereby supporting the saliency of borders and contours in perception of visual scenes.

Contextual influences on visual processing.

The visual image formed on the retina represents an amalgam of visual scene properties, including the reflectances of surfaces, their relative positions, and the type of illumination. The challenge facing the visual system is to extract the "meaning" of the image by decomposing it into its environmental causes. For each local region of the image, that extraction of meaning is only possible if information from other regions is taken into account. Of particular importance is a set of image cues revealing surface occlusion and/or lighting conditions. These information-rich cues direct the perceptual interpretation of other more ambiguous image regions. This context-dependent transformation from image to perception has profound-but frequently under-appreciated-implications for neurophysiological studies of visual processing: To demonstrate that neuronal responses are correlated with perception of visual scene properties, rather than visual image features, neuronal sensitivity must be assessed in varied contexts that differentially influence perceptual interpretation. We review a number of recent studies that have used this context-based approach to explore the neuronal bases of visual scene perception.

Concurrent processing of spatial relations and objects in two cerebral hemispheres.

This study examined hemispheric asymmetry for concurrent processing of object and spatial information. Participants viewed two successive stimuli, each of which consisted of two digits and two pictures that were randomly located and judged them as identical or different. A sample stimulus was presented in a central visual field, followed by a matching stimulus presented briefly in a left or right visual field. The matching stimuli were different from the sample stimuli with respect to the object (digit or picture) or spatial (locations or distances of items) aspect. No visual field asymmetry was found in the detection of object change. However, a left visual field advantage was found in the detection of spatial change. This result can be explained by the double filtering by frequency theory of Ivry and Robertson, who asserted that the left hemisphere has a bias for processing information contained in relatively high spatial frequencies whereas the right hemisphere has a bias for processing information contained in relatively low spatial frequencies. Based upon this evidence, the importance of interhemispheric integration for visual scene perception is discussed.

Electrophysiological measurement of rapid shifts of attention during visual search.

The perception of natural visual scenes that contain many objects poses computational problems that are absent when objects are perceived in isolation. Vision researchers have captured this attribute of real-world perception in the laboratory by using visual search tasks, in which subjects search for a target object in arrays containing varying numbers of non-target distractor objects. Under many conditions, the amount of time required to detect a visual search target increases as the number of objects in the stimulus array increases, and some investigators have proposed that this reflects the serial application of attention to the individual objects in the array. However, other investigators have argued that this pattern of results may instead be due to limitations in the processing capacity of a parallel processing system that identifies multiple objects concurrently. Here we attempt to address this longstanding controversy by using an electrophysiological marker of the moment-by-moment direction of attention-the N2pc component of the event-related potential waveform--to show that attention shifts rapidly among objects during visual search.

Stereoscopic transparency: a test for binocular vision's disambiguating power.

It has been suggested that to resolve ambiguities implicit in binocular perception of complex visual scenes, the brain adopts a continuity constraint assuming that disparities change smoothly with eccentricity. Stereoscopic transparency is characterized by abrupt changes of binocular disparity across retinal locations. The focus of the present study is how the brain uses the continuity constraint in the perception of stereoscopic transparency despite the presence of abrupt disparity changes. Observers viewed random-dot stereograms of overlapping transparent plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strictly controlled depth fixation. Surprisingly, maximal dot density of the transparent plane at which perception is still veridical dramatically decreases as depth separation between the surfaces grows. Persistence of this relationship, when binocular matching processes at each surface are separated to on and off brightness channels, suggests at least two stages in the underlying computation binocular matching and inter-surface interactions. We show that these phenomena cannot be accounted for by either higher severity of matching with high dot densities or the ability of the denser surface to pull vergence to its depth. We also measure contrast sensitivity and near-far symmetry of the underlying mechanism and propose a model of competitive interactions between dissimilar disparities.

Hyperglobal Interactions in Perception of Stereoscopic Transparency

A method has been developed (1) to measure limitations of human observers in perception of visual scenes containing a transparent surface, and (2) to study computational stages underlying stereoscopic transparency perception. Observers viewed random-dot stereograms of overlapping plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strict depth fixation control. Dot density of the transparent plane was increased at various intersurface depth separations until identification of cylinder orientation became random (limiting density). Limiting density dramatically decreased as depth separation between the surfaces grew, and this basic relationship could not be accounted for either by higher severity of matching with larger dot densities or by the ability of a denser surface to pull vergence to its depth. The basic relationship persisted with opposite contrast polarities of plane and cylinder dots, which suggests that competitive interactions between the surfaces occur as a separate process from binocular matching at each surface [Harris and Parker, 1995 Nature (London)374 808 – 811]. Thus, successive computational stages of matching and intersurface interaction characterising stereoscopic hyperglobality may be distinguished. We also found a facilitating role of intersurface brightness difference and a near — far asymmetry of the basic limiting density versus depth separation relationship with reversed surface positioning. We suggest a scheme of cortical inhibitory connections that agrees with the concept of disparity gradient limit and predicts performance similar to that reported. In the scheme, each activated unit produces a suppression zone in the disparity projection field so that lateral extension of the inhibition grows with binocular disparity.

Contributions to the subject of binocular vision

It has been suggested that to resolve ambiguities implicit in binocular perception of complex visual scenes, the brain adopts a continuity constraint assuming that disparities change smoothly with eccentricity. Stereoscopic transparency is characterized by abrupt changes of binocular disparity across retinal locations. The focus of the present study is how the brain uses the continuity constraint in the perception of stereoscopic transparency despite the presence of abrupt disparity changes. Observers viewed random-dot stereograms of overlapping transparent plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strictly controlled depth fixation. Surprisingly, maximal dot density of the transparent plane at which perception is still veridical dramatically decreases as depth separation between the surfaces grows. Persistence of this relationship, when binocular matching processes at each surface are separated to on and off brightness channels, suggests at least two stages in the underlying computation binocular matching and inter-surface interactions. We show that these phenomena cannot be accounted for by either higher severity of matching with high dot densities or the ability of the denser surface to pull vergence to its depth. We also measure contrast sensitivity and near–far symmetry of the underlying mechanism and propose a model of competitive interactions between dissimilar disparities.