Chromatic induction and the layout of colours within a complex scene

A target's apparent colour is influenced by the colours in its surrounding. If the surrounding consists of a single coloured surface, the influence is a shift 'away' from the surface's colour. If the surface is more than 1 degrees from the target area the shift is very small. If there are many surfaces, then not only the average luminance and chromaticity of the surfaces matters, but also the chromatic variability. It is not yet clear whether it makes any difference where the chromatic variability is within the scene, so we constructed stimuli in which the chromatic variability was restricted to certain regions. We found that it made very little difference where the chromatic variability was located. The extent to which the average colour of nearby surfaces influences the apparent colour of the target seems to depend on the average chromatic variability of the whole scene.     

Perceptual completion in a dynamic scene: an investigation with an ambiguous motion paradigm

In this study we employed the streaming-bouncing stimulus to investigate aspects of dynamic occlusion, e.g., of objects that temporarily move under occlusion while covertly being tracked. Two occluders, both either luminance-defined or invisible (virtual), were placed on the trajectories of the moving objects in the streaming-bouncing stimulus. We found that the bouncing percept was dominant when the objects moved under luminance-defined occluders but not when they moved under virtual occluders. Perceived motion direction thus varied with occluder visibility. The results seem to suggest that perceptual completion of a moving object interferes with constant motion processing of the same object.     

Assessing visual search performance using a novel dynamic naturalistic scene

Daily activities require the constant searching and tracking of visual targets in dynamic and complex scenes. Classic work assessing visual search performance has been dominated by the use of simple geometric shapes, patterns, and static backgrounds. Recently, there has been a shift toward investigating visual search in more naturalistic dynamic scenes using virtual reality (VR)-based paradigms. In this direction, we have developed a first-person perspective VR environment combined with eye tracking for the capture of a variety of objective measures. Participants were instructed to search for a preselected human target walking in a crowded hallway setting. Performance was quantified based on saccade and smooth pursuit ocular motor behavior. To assess the effect of task difficulty, we manipulated factors of the visual scene, including crowd density (i.e., number of surrounding distractors) and the presence of environmental clutter. In general, results showed a pattern of worsening performance with increasing crowd density. In contrast, the presence of visual clutter had no effect. These results demonstrate how visual search performance can be investigated using VR-based naturalistic dynamic scenes and with high behavioral relevance. This engaging platform may also have utility in assessing visual search in a variety of clinical populations of interest.     

Spatiotemporal characteristics of dynamic feature binding in visual working memory

It has been proposed that visual working memory can hold a set of four to five coherent object representations. As a test of this proposal, I devised a paradigm called multiple object permanence tracking (MOPT) that measures memory for feature-location binding in dynamic situations. Observers were asked to detect any feature switch in the middle of a regular rotation of a pattern with multiple objects behind an occluder. The feature switch detection performance dramatically declined as the pattern rotation velocity increased. The behavioral evidence for the use of multiple color-shape conjunction was observed only when the objects were stationary. These results cast doubt on the view that the functional unit of visual working memory involves coherent object representation, where object features are tightly bound and dynamically updated.     

Spatiotemporal adaptation through corticothalamic loops: a hypothesis

The thalamus is the major gate to the cortex and its control over cortical responses is well established. Cortical feedback to the thalamus is, in turn, the anatomically dominant input to relay cells, yet its influence on thalamic processing has been difficult to interpret. For an understanding of complex sensory processing, detailed concepts of the corticothalamic interplay need yet to be established. Drawing on various physiological and anatomical data, we elaborate the novel hypothesis that the visual cortex controls the spatiotemporal structure of cortical receptive fields via feedback to the lateral geniculate nucleus. Furthermore, we present and analyze a model of corticogeniculate loops that implements this control, and exhibit its ability of object segmentation by statistical motion analysis in the visual field.     

Sensitivity to luminance and chromaticity gradients in a complex scene

Image gradients--smooth changes in color and luminance--may be caused by intrinsic surface reflectance properties or extrinsic illumination phenomena, including shading, shadowing, and inter-reflections. In turn, image gradients may provide the visual system with information concerning the origin of these factors, such as the orientation of surfaces with respect to the light source. Color gradients induced by mutual illumination (MI) may play a similar role to that of luminance gradients in shape-from-shading algorithms; it has been shown that 3D shape perception modulates the influence of MI on surface color perception (M. G. Bloj, D. Kersten, & A. C. Hurlbert, 1999). In this study, we assess human sensitivity to changes in color and luminance gradients that arise from changes in the light source position, within a complex scene. In Experiment 1, we tested whether observers were able to discriminate between gradients due to different light source positions. We found that observers reliably detected a change in the gradient information when the light source position differed by only 4 deg from the reference scene. This sensitivity was mainly based on the luminance information in the gradient (Experiments 2 and 3). Some observers make use of the spatial distribution of chromaticity and luminance values within gradients when discriminating between them (Experiment 4). The high sensitivity to gradient differences supports the notion that gradients contain information that may assist in the recovery of 3D shape and scene configuration properties.     

Shared cognitive mechanisms involved in the processing of scene texture and scene shape

Recent research has demonstrated that the parahippocampal place area represents both the shape and texture features of scenes, with the importance of each feature varying according to perceived scene category. Namely, shape features are predominately more diagnostic to the processing of artificial human–made scenes, while shape and texture are equally diagnostic in natural scene processing. However, to date little is known regarding the degree of interactivity or independence observed in the processing of these scene features. Furthermore, manipulating the scope of visual attention (i.e., globally vs. locally) when processing ensembles of multiple objects—stimuli that share a functional neuroanatomical link with scenes—has been shown to affect their cognitive visual representation. It remains unknown whether manipulating the scope of attention impacts scene processing in a similar manner. Using the well-established Garner speeded-classification behavioral paradigm, we investigated the influence of both feature diagnosticity and the scope of visual attention on potential interactivity or independence in the shape and texture processing of artificial human–made scenes. The results revealed asymmetric interference between scene shape and texture processing, with the more diagnostic feature (i.e., shape) interfering with the less diagnostic feature (i.e., texture), but not vice versa. Furthermore, this interference was attenuated and enhanced with more local and global visual processing strategies, respectively. These findings suggest that the scene shape and texture processing are mediated by shared cognitive mechanisms and that, although these representations are governed primarily via feature diagnosticity, they can nevertheless be influenced by the scope of visual attention.     

The induction and suppression of the apoptotic response of HSV-1 in human corneal epithelial cells

PURPOSE: HSV-1 has been shown to block apoptosis in some cell lines when the cells are exposed to exogenous agents (e.g., sorbitol). The purpose of this study was to determine whether HSV-1 infection of human corneal epithelial (HCE) cells alone induces an early proapoptotic response and whether this response is subsequently downregulated during the infection. METHODS: HCE cells were infected with HSV-1 or subjected to osmotic shock (sorbitol). Fluorescent staining for annexin V binding, mitochondrial membrane potential, and DNA condensation and assays for caspase 8, 9, and 3 activity and cytokeratin 18 cleavage were performed, to assess the apoptotic pathway. RESULTS: HSV-1 infection of HCE cells induced a rapid proapoptotic response, characterized by translocation of phosphatidylserine to the external membrane, activation of caspases 8 and 3 within 2 hours, and cleavage of cytokeratin 18. However, the induced response was downregulated during the infection, and later stages of the apoptotic responses (e.g., DNA condensation) were not produced. Sorbitol treatment led to terminal apoptosis by 12 hours, as indicated by DNA condensation of treated cells and reduction in the number of viable cells. CONCLUSIONS: HSV-1 can induce and subsequently suppress the apoptotic pathway in HCE. Suppression of apoptosis occurred only during HSV-1 infection and not after treatment with sorbitol, suggesting that the suppression of apoptosis may be a mechanism of viral survival.     

Object speed derived from the integration of motion in the image plane and motion-in-depth signaled by stereomotion and looming

We investigate the influence of local motion in the retinal image plane on the perception of speed-in-depth. Observers judged the apparent speed-in-depth of a square plane of dynamic dots that moved towards the observer. Dots forming the surface of the plane underwent random-direction motion in the image plane. We examined the consequences of changing the dots’ image-plane speed on the apparent speed of the stimulus as it traversed depth, where depth is signaled by stereomotion or looming. Results for both the stereomotion and looming conditions indicate that as the speed of random-direction motion in the image plane increases, the apparent speed-in-depth of the stimulus also increases. When stereomotion was used to signal motion-in-depth, the speed judgment is adequately modeled by the resultant of a vector sum of dot-speed in the image plane and speed-in-depth. However, when looming was used to define motion-in-depth, a different pattern of results was found - the apparent speed-in-depth is lower than the actual speed-in-depth, and the results are best predicted by simple averaging. Our results demonstrate that the integration of speed in the image plane and speed-in-depth, to determine object speed, is dependent on the type of cue used to signal motion-in-depth, and this difference is a consequence of the ways in which looming and stereomotion cue motion-in-depth. Looming is derived not at a local stage of motion analysis, but is available only via global integration of local velocities, and consequently global speed conforms to the average speed. Stereomotion, on the other hand, provides an effective cue for individuating local velocities in depth.     

The dynamics of visual pattern masking in natural scene processing: a magnetoencephalography study

We investigated the dynamics of natural scene processing and mechanisms of pattern masking in a scene-recognition task. Psychophysical recognition performance and the magnetoencephalogram (MEG) were recorded simultaneously. Photographs of natural scenes were briefly displayed and in the masked condition immediately followed by a pattern mask. Viewing the scenes without masking elicited a transient occipital activation that started approximately 70 ms after the pattern onset, peaked at 110 ms, and ended after 170 ms. When a mask followed the target an additional transient could be reliably identified in the MEG traces. We assessed psychophysical performance levels at different latencies of this transient. Recognition rates were reduced only when the additional activation produced by the pattern mask overlapped with the initial 170 ms of occipital activation from the target. Our results are commensurate with an early cortical locus of pattern masking and indicate that 90 ms of undistorted cortical processing is necessary to reliably recognize a scene. Our data also indicate that as little as 20 ms of undistorted processing is sufficient for above-chance discrimination of a scene from a distracter.     

Processing scene context: fast categorization and object interference

The extent to which object identification is influenced by the background of the scene is still controversial. On the one hand, the global context of a scene might be considered as an ultimate representation, suggesting that object processing is performed almost systematically before scene context analysis. Alternatively, the gist of a scene could be extracted sufficiently early to be able to influence object categorization. It is thus essential to assess the processing time of scene context. In the present study, we used a go/no-go rapid visual categorization task in which subjects had to respond as fast as possible when they saw a "man-made environment", or a "natural environment", that was flashed for only 26 ms. "Man-made" and "natural" scenes were categorized with very high accuracy (both around 96%) and very short reaction times (median RT both around 390 ms). Compared with previous results from our group, these data demonstrate that global context categorization is remarkably fast: (1) it is as fast as object categorization [Fabre-Thorpe, M., Delorme, A., Marlot, C., & Thorpe, S. (2001). A limit to the speed of processing in ultra-rapid visual categorization of novel natural scenes. Journal of Cognitive Neuroscience, 13(2), 171-180]; (2) it is faster than contextual categorization at more detailed levels such as sea, mountain, indoor or urban contexts [Rousselet, G. A., Joubert, O. R., & Fabre-Thorpe, M. (2005). How long to get to the "gist" of real-world natural scenes? Visual Cognition, 12(6), 852-877]. Further analysis showed that the efficiency of contextual categorization was impaired by the presence of a salient object in the scene especially when the object was incongruent with the context. Processing of natural scenes might thus involve in parallel the extraction of the global gist of the scene and the concurrent object processing leading to categorization. These data also suggest early interactions between scene and object representations compatible with contextual influences on object categorization in a parallel network.     

Visual evoked potentials during suppression in exotropic and esotropic strabismics: strabismic suppression objectified

Background We performed an electrophysiological study in order to objectify suppression in strabismus. The extent of cortical involvement in the process of interocular suppression was also explored. Possible differences in the suppressive process of esotropic and exotropic strabismics were also studied.Methods An electroencephalographic recorder with eight leads was applied to the posterior one-third of the skull; three occipital, three parietal, and two temporal leads. We measured the activity of these visual cortical areas during stimulation of each eye under monocular as well as binocular viewing conditions with hemisinusoidal light pulses in a nature-like complex visual background. Recordings were made from six primary esotropic strabismic subjects and four primary exotropic and one consecutive exotropic strabismic subject. Also, five normal controls were studied.Results A characteristic, triphasic response complex was found at approximately 80 ms following the start of each light pulse under monocular viewing conditions in the dominant and the nondominant eye. However, under dichoptic viewing conditions in the nondominant eye of all esotropic cases as well as in the nondominant eye of three of five exotropic cases, this response complex was completely absent. They showed approximately 100% reduction of their cortical response activity.Conclusions These results show the vast extent of the cortex that is involved in the suppressive process, giving a good insight in the power of suppression.     

The horizontal effect in suppression: Anisotropic overlay and surround suppression at high and low speeds

When a pattern of broad spatial content is viewed by an observer, the multiple spatial components in the pattern stimulate detecting-mechanisms that suppress each other. This suppression is anisotropic, being relatively greater at horizontal, and least at obliques (the “horizontal effect”). Here, suppression of a grating by a naturalistic (1/f) broadband mask is shown to be larger when the broadband masks are temporally similar to the target’s temporal properties, and generally anisotropic, with the anisotropy present across all spatio-temporal parings tested. We also show that both suppression from within the region of the test pattern (overlay suppression) and from outside of this region (surround suppression) show the horizontal-effect anisotropy. We conclude that these suppression effects stem from locally-tuned and anisotropically-weighted gain-control pools.     

Clinical suppression and binocular rivalry suppression: the effects of stimulus strength on the depth of suppression

In observers with abnormal binocular vision (such as strabismics or anisometropes) one eye's view is often suppressed. This clinical suppression serves to eliminate binocular diplopia and confusion. Suppression may also occur in observers with normal binocular vision, when the two eyes view disparate retinal images, a phenomenon known as binocular rivalry. When the image in an eye is suppressed, it is possible to determine the amount by which that suppressed stimulus is below the visibility threshold, or the depth of suppression. In the experiments presented here, the depth of suppression in an eye was measured as the strength of the stimulus in the contralateral eye (the stimulus inducing suppression) was varied. This was done for both clinical suppressors and normal observers undergoing binocular rivalry suppression. Independent changes were made to the contrast, the luminance, and the spatial frequency of the inducing stimulus. For both clinical suppression and binocular rivalry suppression, the depth of suppression was constant, regardless of the changes to the inducing stimulus.     

Reversing ocular dominance and suppression in a single flash

An observer views a long (1-2 sec) monocular stimulus. Following stimulus offset there is a dark interstimulus interval (ISI). After the ISI, brief test flashes are presented to both eyes simultaneously. For ISIs less than 100-200 msec, only the test flash contralateral to the stimulated eye is seen. The test flash in the ipsilateral eye is not seen. For ISIs of 200-1000 msec, the contralateral test flash is seen more clearly and completely than the ipsilateral flash (Experiments 1 and 2). The initial and ipsilateral test flash need not be spatially similar. The initial stimulus can be dim but must be longer than 150 msec (Experiment 3). Experiment 3 precludes forward masking or light adaptation as explanation. The effect is relatively independent of the length of the test flash (Experiment 4). The results can be explained in terms of the known properties of binocular rivalry and reinforce the hypothesis (Fox and Check, 1968) that rivalry acts nonselectively, independent of factors such as spatial frequency, luminance, or orientation.     

Spatiotemporal limitations in bisection and separation discrimination

Exposure duration was found to have a different effect on bisection thresholds than on separation-discrimination thresholds. Bisection thresholds were higher than separation discrimination thresholds between 33 and 150 msec but equal to or lower than them at longer durations. Experiments in which stimulus contrast was manipulated showed that the effect of exposure duration on separation-discrimination and bisection thresholds could not be attributed primarily to temporal contrast integration. The data could be accounted for by a model in which bisection is done by encoding the two separations in bisection sequentially.     

The deafferented visual cortex and interhemispheric relationships: a physiological approach

A condition of asymmetrical activation in the visual system, induced by unilateral optic tract section (OTX) was examined in nine adult cats, four of which had been reared after early onset of monocular deprivation (MD-OTX) during the critical developmental period. Their results were compared to those of monocularly deprived (MD) and normal control cats. Extracellular single-unit recordings from areas 17, 18 and their border were performed at various intervals after OTX to study the effects of this isolation of one hemisphere from direct geniculo-cortical input. Electrophysiological results from the isolated hemispheres of both OTX and MD-OTX cats revealed that contralateral input via the corpus callosum was virtually undetectable in the majority of cats. Only in the cats in which the deprivation was ipsilateral to the OTX, 3.5% of the cells exhibited visual activity in the isolated hemisphere. In the intact hemisphere of the OTX group, binocularity was unaffected overall in comparison to the normal control animals, except for a slight decrease within the 0-4 degrees region from the vertical meridian. In the MD-OTX group, the cats with onset of deprivation prior to natural eye opening possessed an increase in deprived eye responsiveness compared to the control MD cats. No such an increase was seen in later onset of deprivation (3-4 weeks following natural eye opening) in the MD-OTX cats. Overall, visual responsiveness was reduced in all of the OTX and MD-OTX cats, with a return towards normal control values seen only in one animal with extended recovery (6 months). Orientation and direction selectivity were dramatically decreased in the OTX and MD-OTX cats. As the majority of receptive fields mapped from the OTX cats were in the ipsilateral visual field to the section, the remaining small number of receptive fields in the contralateral "blind" visual fields adds further support for a nasotemporal overlap in the retina of the cat. The conclusion from these results is that an asymmetrical level of direct geniculo-cortical input in the visual system of the adult cat yields a physiological bidirectional inactivation of the callosal pathway for the transfer of visual information. Thus, activation of the callosal pathway connecting the cortical visual areas has been postulated to be dependent upon simultaneous, reciprocal interaction between the two hemispheres.     

Spatiotemporal characterization of a transient mechanism in the human visual system

By sinusoidally flickering appropriate spatial patterns, the properties of a transient mechanism in the human visual system can be isolated. The data indicate that the spatiotemporal response of this mechanism may be decomposed into the product of separate spatial and temporal functions. The spatial line spread function has a broad summation region flanked by weak inhibitory regions and is in good agreement with the properties of the U-mechanism hypothesized by Wilson and Bergen (1979). The flicker sensitivity function displays a pronounced low frequency falloff indicative of strong temporal inhibition. Finally, it is shown that the visual system behaves like a single channel at low spatial and high temporal frequencies.