Showing papers in "Journal of Vision in 2003"

Visual field representations and locations of visual areas V1/2/3 in human visual cortex.

Abstract: The position, surface area and visual field representation of human visual areas V1, V2 and V3 were measured using fMRI in 7 subjects (14 hemispheres). Cortical visual field maps of the central 12 deg were measured using rotating wedge and expanding ring stimuli. The boundaries between areas were identified using an automated procedure to fit an atlas of the expected visual field map to the data. All position and surface area measurements were made along the boundary between white matter and gray matter. The representation of the central 2 deg of visual field in areas V1, V2, V3 and hV4 spans about 2100 mm2 and is centered on the lateral-ventral aspect of the occipital lobes at Talairach coordinates -29, -78, -11 and 25, -80, -9. The mean area between the 2-deg and 12-deg eccentricities for the primary visual areas was: V1: 1470 mm2; V2: 1115 mm2; and V3: 819 mm2. The sizes of areas V1, V2 and V3 varied by about a factor of 2.5 across individuals; the sizes of V1 and V2 are significantly correlated within individuals, but there is a very low correlation between V1 and V3. These in vivo measurements of normal human retinotopic visual areas can be used as a reference for comparison to unusual cases involving developmental plasticity, recovery from injury, identifying homology with animal models, or analyzing the computational resources available within the visual pathways.

The pattern of visual deficits in amblyopia.

Abstract: Amblyopia is usually defined as a deficit in optotype (Snellen) acuity with no detectable organic cause. We asked whether this visual abnormality is completely characterized by the deficit in optotype acuity, or whether it has distinct forms that are determined by the conditions associated with the acuity loss, such as strabismus or anisometropia. To decide this issue, we measured optotype acuity, Vernier acuity, grating acuity, contrast sensitivity, and binocular function in 427 adults with amblyopia or with risk factors for amblyopia and in a comparison group of 68 normal observers. Optotype acuity accounts for much of the variance in Vernier and grating acuity, and somewhat less of the variance in contrast sensitivity. Nevertheless, there are differences in the patterns of visual loss among the clinically defined categories, particularly between strabismic and anisometropic categories. We used factor analysis to create a succinct representation of our measurement space. This analysis revealed two main dimensions of variation in the visual performance of our abnormal sample, one related to the visual acuity measures (optotype, Vernier, and grating acuity) and the other related to the contrast sensitivity measures (Pelli-Robson and edge contrast sensitivity). Representing our data in this space reveals distinctive distributions of visual loss for different patient categories, and suggests that two consequences of the associated conditions--reduced resolution and loss of binocularity--determine the pattern of visual deficit. Non-binocular observers with mild-to-moderate acuity deficits have, on average, better monocular contrast sensitivity than do binocular observers with the same acuity loss. Despite their superior contrast sensitivity, non-binocular observers typically have poorer optotype acuity and Vernier acuity, at a given level of grating acuity, than those with residual binocular function.

Visual memory and motor planning in a natural task.

Abstract: This paper investigates the temporal dependencies of natural vision by measuring eye and hand movements while subjects made a sandwich. The phenomenon of change blindness suggests these temporal dependencies might be limited. Our observations are largely consistent with this, suggesting that much natural vision can be accomplished with "just-in-time" representations. However, we also observe several aspects of performance that point to the need for some representation of the spatial structure of the scene that is built up over different fixations. Patterns of eye-hand coordination and fixation sequences suggest the need for planning and coordinating movements over a period of a few seconds. This planning must be in a coordinate frame that is independent of eye position, and thus requires a representation of the spatial structure in a scene that is built up over different fixations.

Real world illumination and the perception of surface reflectance properties

Abstract: Under typical viewing conditions, we find it easy to distinguish between different materials, such as metal, plastic, and paper. Recognizing materials from their surface reflectance properties (such as lightness and gloss) is a nontrivial accomplishment because of confounding effects of illumination. However, if subjects have tacit knowledge of the statistics of illumination encountered in the real world, then it is possible to reject unlikely image interpretations, and thus to estimate surface reflectance even when the precise illumination is unknown. A surface reflectance matching task was used to measure the accuracy of human surface reflectance estimation. The results of the matching task demonstrate that subjects can match surface reflectance properties reliably and accurately in the absence of context, as long as the illumination is realistic. Matching performance declines when the illumination statistics are not representative of the real world. Together these findings suggest that subjects do use stored assumptions about the statistics of real-world illumination to estimate surface reflectance. Systematic manipulations of pixel and wavelet properties of illuminations reveal that the visual system's assumptions about illumination are of intermediate complexity (e.g., presence of edges and bright light sources), rather than of high complexity (e.g., presence of recognizable objects in the environment).

What you see is what you need.

Abstract: We studied the role of attention and task demands for implicit change detection. Subjects engaged in an object sorting task performed in a virtual reality environment, where we changed the properties of an object while the subject was manipulating it. The task assures that subjects are looking at the changed object immediately before and after the change. Our results demonstrate that in this situation subjects' ability to notice changes to the object strongly depends on momentary task demands. Surprisingly, frequent noticing is not guaranteed by task relevance of the changed object attribute per se, but the changed object attribute needs to be task relevant at exactly the right times. Also, the simplicity of the used objects indicates that change blindness occurs in situations where the visual short term memory load is minimal, suggesting a potential dissociation between short term memory limitations and change blindness. Finally, we found that changes may even go unnoticed if subjects are visually tracking the object at the moment of change. Our experiments suggest a highly purposive and task specific nature of human vision, where information extracted from the fixation point is used for certain computations only "just in time" when needed to solve the current goal.

Timecourse of neural signatures of object recognition

Abstract: How long does it take for the human visual system to recognize objects? This issue is important for understanding visual cortical function as it places constraints on models of the information processing underlying recognition. We designed a series of event-related potential (ERP) experiments to measure the timecourse of electrophysiological correlates of object recognition. We find two distinct types of components in the ERP recorded during categorization of natural images. One is an early presentation-locked signal arising around 135 ms that is present when there are low-level feature differences between images. The other is a later, recognition-related component arising between 150-300 ms. Unlike the early component, the latency of the later component covaries with the subsequent reaction time. In contrast to previous studies suggesting that the early, presentation-locked component of neural activity is correlated to recognition, these results imply that the neural signatures of recognition have a substantially later and variable time of onset.

Is it an animal? Is it a human face? Fast processing in upright and inverted natural scenes.

Abstract: Object categorization can be extremely fast. But among all objects, human faces might hold a special status that could depend on a specialized module. Visual processing could thus be faster for faces than for any other kind of object. Moreover, because face processing might rely on facial configuration, it could be more disrupted by stimulus inversion. Here we report two experiments that compared the rapid categorization of human faces and animals or animal faces in the context of upright and inverted natural scenes. In Experiment 1, the natural scenes contained human faces and animals in a full range of scales from close-up to far views. In Experiment 2, targets were restricted to close-ups of human faces and animal faces. Both experiments revealed the remarkable object processing efficiency of our visual system and further showed (1) virtually no advantage for faces over animals; (2) very little performance impairment with inversion; and (3) greater sensitivity of faces to inversion. These results are interpreted within the framework of a unique system for object processing in the ventral pathway. In this system, evidence would accumulate very quickly and efficiently to categorize visual objects, without involving a face module or a mental rotation mechanism. It is further suggested that rapid object categorization in natural scenes might not rely on high-level features but rather on features of intermediate complexity.

Maximum likelihood difference scaling.

Abstract: We present a stochastic model of suprathreshold perceptual differences based on difference measurement. We develop a maximum likelihood difference scaling (MLDS) method for estimating its parameters and evaluate the reliability and distributional robustness of the fitting method. We also describe a method for testing whether the difference measurement model is appropriate as a description of human judgment of perceptual differences in any specific experimental context.

Eye-movements aid the control of locomotion

Abstract: Eye-movements have long been considered a problem when trying to understand the visual control of locomotion. They transform the retinal image from a simple expanding pattern of moving texture elements (pure optic flow), into a complex combination of translation and rotation components (retinal flow). In this article we investigate whether there are measurable advantages to having an active free gaze, over a static gaze or tracking gaze, when steering along a winding path. We also examine patterns of free gaze behavior to determine preferred gaze strategies during active locomotion. Participants were asked to steer along a computer-simulated textured roadway with free gaze, fixed gaze, or gaze tracking the center of the roadway. Deviation of position from the center of the road was recorded along with their point of gaze. It was found that visually tracking the middle of the road produced smaller steering errors than for fixed gaze. Participants performed best at the steering task when allowed to sample naturally from the road ahead with free gaze. There was some variation in the gaze strategies used, but sampling was predominantly of areas proximal to the center of the road. These results diverge from traditional models of flow analysis.

Shared motion signals for human perceptual decisions and oculomotor actions.

Abstract: A fundamental question in primate neurobiology is to understand to what extent motor behaviors are driven by shared neural signals that also support conscious perception or by independent subconscious neural signals dedicated to motor control. Although it has clearly been established that cortical areas involved in processing visual motion support both perception and smooth pursuit eye movements, it remains unknown whether the same or different sets of neurons within these structures perform these two functions. Examination of the trial-by-trial variation in human perceptual and pursuit responses during a simultaneous psychophysical and oculomotor task reveals that the direction signals for pursuit and perception are not only similar on average but also co-vary on a trial-by-trial basis, even when performance is at or near chance and the decisions are determined largely by neural noise. We conclude that the neural signal encoding the direction of target motion that drives steady-state pursuit and supports concurrent perceptual judgments emanates from a shared ensemble of cortical neurons.

The effect of perceived surface orientation on perceived surface albedo in binocularly viewed scenes

Abstract: We examined how observers discount perceived surface orientation in estimating perceived albedo (lightness). Observers viewed complex rendered scenes binocularly. The orientation of a test patch was defined by depth cues of binocular disparity and linear perspective. On each trial, observers first estimated the orientation of the test patch in the scene by means of a gradient probe and then matched its perceived albedo to a reference scale. We found that observers' perception of orientation was nearly veridical and that they substantially discounted perceived orientation in estimating perceived albedo.

Interactions between color and luminance in the perception of orientation.

Abstract: At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.

Extraocular connective tissue architecture.

Abstract: Extraocular muscle pulleys, now well known to be kinematically significant extraocular structures, have been noted in passing and described in fragments several times over the past two centuries. They were late to be fully appreciated because biomechanical modeling of the orbit was not available to derive their kinematic consequences, and because pulleys are distributed condensations of collagen, elastin and smooth muscle (SM) that are not sharply delineated. Might other mechanically significant distributed extraocular structures still be awaiting description?An imaging approach is useful for describing distributed structures, but does not seem suitable for assessing mechanical properties. However, an image that distinguished types and densities of constituent tissues could give strong hints about mechanical properties. Thus, we have developed methods for producing three dimensional (3D) images of extraocular tissues based on thin histochemically processed slices, which distinguish collagen, elastin, striated muscle and SM. Overall tissue distortions caused by embedding for sectioning, and individual-slice distortions caused by thin sectioning and subsequent histologic processing were corrected by ordered image warping with intrinsic fiducials. We describe an extraocular structure, partly included in Lockwood's ligament, which contains dense elastin and SM bands, and which might refine horizontal eye alignment as a function of vertical gaze, and torsion in down-gaze. This active structure might therefore be a factor in strabismus and a target of therapeutic intervention.

Orienting of attention without awareness is affected by measurement-induced attentional control settings.

Abstract: McCormick (1997) concluded that peripheral cues presented below a threshold of awareness could nevertheless attract attention because they facilitated target processing near the cue shortly after its presentation. Yet, whereas an exogenous shift of attention typically exhibits a biphasic pattern (initial facilitation followed by inhibition of return [IOR]), at late cue-target onset asynchronies, IOR was not observed by McCormick. In our study, targets requiring a detection response were preceded by masked and nonmasked, uninformative cues presented under two conditions: one in which the cue was ignored (no report) and one in which the cue was detected and localized following the response to the target (cue report). When participants were required to make cue judgments at the end of each trial, we replicated McCormick's pattern, finding facilitation (but not IOR) following both masked and nonmasked cues. When there was no requirement to judge the presence or location of the cues, IOR was present with and without masks, whereas facilitation was observed only when the cues were not masked. That the assessment of cue awareness increases attentional facilitation and prevents (or delays) the onset of IOR is attributed to attentional control settings put in place to perform the cue-awareness assessments in the cue-report condition.

Effects of scene inversion on change detection of targets matched for visual salience.

Abstract: This work examines how context may influence the detection of changes in flickering scenes. Each scene contained two changes that were matched for low-level visual salience. One of the changes was of high interest to the meaning of the scene, and the other was of lower interest. High-interest changes were more readily detected. To further examine the effects of contextual significance, we inverted the scene orientation to disrupt top-down effects of global context while controlling for contributions of visual salience. In other studies, inverting scene orientation has had inconsistent effects on detection of high-interest changes. However, this experiment demonstrated that inverting scene orientation significantly reduced the advantage for high-interest changes in comparison to lower-interest changes. Thus, scene context influences the deployment of attention and change-detection performance, and this top-down influence may be disrupted by scene inversion.

A comparison of pursuit eye movement and perceptual performance in speed discrimination

Abstract: Currently there is considerable debate as to the nature of the pathways that are responsible for the perception and motor performance. We have studied the relationship between perceived speed, which is the experiential representation of a moving stimulus, and the speed of smooth pursuit eye movements, the motor action. We determined psychophysical thresholds for detecting small perturbations in the speed of moving patterns, and then by an ideal observer analysis computed analogous "oculometric" thresholds from the eye movement traces elicited by the same stimuli on the same trials. Our results confirm those of previous studies that show a remarkable agreement between perceptual judgments for speed discrimination and the fine gradations in eye movement speed. We analyzed the initial pursuit period of long duration (1000 ms) and short (200 ms) duration perturbations. When we compared the errors for perception and pursuit on a trial-by-trial basis there was no correlation between perceptual errors and eye movement errors. The observation that both oculometric and psychometric performance were similar, with Weber fractions in the normal range, but that there is no correlation in the errors suggests that the motor system and perception share the same constraints in their analysis of motion signals, but act independently and have different noise sources. We simulated noise in two models of perceptual and eye movement performance. In the first model we postulate an initial common source for the perceptual and eye movement signals. In that case about ten times the observed noise is required to produce no correlation in trial-by-trial performance. In the second model we postulate that the perceptual signal is a combination of a reafferent eye velocity signal plus the perturbation signal while the pursuit signal is derived from the oculomotor plant plus the perturbation signal. In this model about three times the noise level in the independent signals will mask any correlation due to the common perturbation signal.

Feature binding in object-file representations of multiple moving items.

Abstract: Maintenance of episodic representations by feature-location binding is important for visual cognition. It has been proposed that we can hold and update coherent episodic representations of up to four objects. This study investigated the dynamic maintenance of feature-location bindings with multiple objects. In a series of seven experiments, participants judged whether a sequence of rotating patterns of three or four colored disks contains any color switch between two disks. Color-switch detection is in general difficult, even when tracking of objects' motions is successful, suggesting that our ability for dynamic maintenance is limited. The performance improved when the interframe rotation angle became sufficiently small. Moreover, spatiotemporal predictability was necessary for this improvement, suggesting that the maintenance of multiple episodic representations is an interactive process between our prediction and sensory mechanisms.

Perceptual and oculomotor evidence of limitations on processing accelerating motion.

Abstract: Psychophysical studies have demonstrated that humans are less sensitive to image acceleration than to image speed (e.g., Gottsdanker, 1956; Werkhoven, Snippe, & Toet, 1992). Because there is evidence that a common motion-processing stage subserves perception and pursuit (e.g., Watamaniuk & Heinen, 1999), either pursuit should be similarly impaired in discriminating acceleration or it must receive input from a system different from the one that processes visual motion for perception. We assessed the sensitivity of pursuit to acceleration or speed, and compared the results with those obtained in perceptual experiments done with similar stimuli and tasks. Specifically, observers pursued or made psychophysical judgments of targets that moved at randomly selected base speeds and subsequent accelerations. Oculomotor and psychophysical discrimination were compared by analyzing performance for the entire stimulus set sorted by either target acceleration or speed. Thresholds for pursuit and perception were higher for target acceleration than speed, further evidence that a common motion-processing stage limits the performance of both systems.

Change in corneal shape and corneal wave-front aberrations with accommodation

Abstract: This study investigated the change in corneal curvature and corneal wave-front aberrations with accommodation. The corneal curvature of the right eyes of 12 young adults was measured using a corneal topography system, while subjects fixated far (4.0 m) and near (0.2 m) targets with their left eyes. Convergence was controlled. Both the mean corneal radius at the vertex and the shape parameter significantly increased from the far to the near viewing condition. No significant change in root mean square of wave-front aberrations with accommodation was observed for the group, but there was individual variation in the change of wave-front aberration. A significant mean change for the group in both x-axis coma and spherical aberration was found. The change in corneal surface with accommodation suggests an increase in peripheral curvature with flattening at the vertex.

Saccade target selection in visual search: Accuracy improves when more distractors are present

Abstract: We report four experiments with search displays of Gabor patches. Our aim was to study the accuracy of gaze control in search tasks. In Experiment 1, a target was presented with a single distractor Gabor of a different spatial frequency on the same axis. Subjects could locate the target with the first saccade if the distractor was more distant, but when the distractor was between the fixation point and the target, the first saccade landed much closer to the distractor. In Experiment 2, the number of display items was increased to 16 in a double ring configuration. With this configuration, first saccades were accurately directed to the target, even when there was an intervening distractor in exactly the same configuration as in Experiment 1. Experiment 3 suggested that the improvement in accuracy was not due to distractor homogeneity but rather may be attributable to the increased first saccade latency with the ring configuration. In the final experiment, latency was shown to covary with saccade accuracy. The results are related to a general framework whereby the presence of distractors operates to hold fixation for a longer period of time, thus allowing a greater period of visual processing and more accurate eye movements.

Contour interpolation by vector-field combination

Abstract: We model the visual interpolation of missing contours by extending contour fragments under a smoothness constraint Interpolated trajectories result from an algorithm that computes the vector sum of two fields corresponding to different unification factors: the good continuation (GC) field and the minimal path (MP) field As the distance from terminators increases, the GC field decreases and the MP field increases Viewer-independent and viewer-dependent variables modulate GC-MP contrast (ie, the relative strength of GC and MP maximum vector magnitudes) Viewer-independent variables include the local geometry as well as more global properties such as contour support ratio and shape regularity Viewer-dependent variables include the retinal gap between contour endpoints and the retinal orientation of their stems GC-MP contrast is the only free parameter of our field model In the case of partially occluded angles, interpolated trajectories become flatter as GC-MP contrast decreases Once GC-MP contrast is set to a specific value, derived from empirical measures on a given configuration, the model predicts all interpolation trajectories corresponding to different types of occlusion of the same angle Model predictions fit psychophysical data on the effects of viewer-independent and viewer-dependent variables

Competition and selection during visual processing of natural scenes and objects.

Abstract: When a visual scene, containing many discrete objects, is presented to our retinae, only a subset of these objects will be explicitly represented in visual awareness. The number of objects accessing short-term visual memory might be even smaller. Finally, it is not known to what extent “ignored” objects (those that do not enter visual awareness) will be processed –or recognized. By combining free recall, forced-choice recognition and visual priming paradigms for the same natural visual scenes and subjects, we were able to estimate these numbers, and provide insights as to the fate of objects that are not explicitly recognized in a single fixation. When presented for 250 ms with a scene containing 10 distinct objects, human observers can remember up to 4 objects with full confidence, and between 2 and 3 more when forced to guess. Importantly, the objects that the subjects consistently failed to report elicited a significant negative priming effect when presented in a subsequent task, suggesting that their identity was represented in high-level cortical areas of the visual system, before the corresponding neural activity was suppressed during attentional selection. These results shed light on neural mechanisms of attentional competition, and representational capacity at different levels of the human visual system.

Biological motion as a cue for the perception of size.

Abstract: Animals as well as humans adjust their gait patterns in order to minimize energy required for their locomotion. A particularly important factor is the constant force of earth's gravity. In many dynamic systems, gravity defines a relation between temporal and spatial parameters. The stride frequency of an animal that moves efficiently in terms of energy consumption depends on its size. In two psychophysical experiments, we investigated whether human observers can employ this relation in order to retrieve size information from point-light displays of dogs moving with varying stride frequencies across the screen. In Experiment 1, observers had to adjust the apparent size of a walking point-light dog by placing it at different depths in a three-dimensional depiction of a complex landscape. In Experiment 2, the size of the dog could be adjusted directly. Results show that displays with high stride frequencies are perceived to be smaller than displays with low stride frequencies and that this correlation perfectly reflects the predicted inverse quadratic relation between stride frequency and size. We conclude that biological motion can serve as a cue to retrieve the size of an animal and, therefore, to scale the visual environment.

Detectability of onsets versus offsets in the change detection paradigm

Abstract: The human visual system is particularly sensitive to abrupt onset of new objects that appear in the visual field. Onsets have been shown to capture attention even when other transients simultaneously occur. This has led some authors to argue for the special role that object onset plays in attentional capture. However, evidence from the change detection paradigm appears contradictory to such findings. Studies of change blindness demonstrate that the onset of new objects can often go unnoticed. Assessing the relative detectability of onsets compared with other visual transients in a change detection procedure may help resolve this contradiction. We report the results of four experiments investigating the efficacy with which onsets capture attention compared with offsets. In Experiment 1, we employed a standard flicker procedure and assessed whether participants were more likely to detect the change following a frame containing an onset or following a frame containing an offset. In Experiment 2, we employed the one-shot method and investigated whether participants detected more onsets or offsets. Experiment 3 used the same method but assessed whether onsets would be detected more rapidly than offsets. In Experiment 4, we investigated whether the effect obtained in Experiments 1-3 using simple shapes would replicate when images of real-world objects were used. Results showed that onsets were less susceptible to change blindness than were offsets. We argue that the preservation of information is greater in onsets than in offsets.

Bootstrapped learning of novel objects.

Abstract: Recognition of familiar objects in cluttered backgrounds is a challenging computational problem. Camouflage provides a particularly striking case, where an object is difficult to detect, recognize, and segment even when in "plain view." Current computational approaches combine low-level features with high-level models to recognize objects. But what if the object is unfamiliar? A novel camouflaged object poses a paradox: A visual system would seem to require a model of an object's shape in order to detect, recognize, and segment it when camouflaged. But, how is the visual system to build such a model of the object without easily segmentable samples? One possibility is that learning to identify and segment is opportunistic in the sense that learning of novel objects takes place only when distinctive clues permit object segmentation from background, such as when target color or motion enables segmentation on single presentations. We tested this idea and discovered that, on the contrary, human observers can learn to identify and segment a novel target shape, even when for any given training image the target object is camouflaged. Further, perfect recognition can be achieved without accurate segmentation. We call the ability to build a shape model from high-ambiguity presentations bootstrapped learning.

Binocular coordination of saccades at far and at near in children and in adults.

Abstract: The goal of the study was to test the quality of binocular coordination of saccades in children and adults, and its dependency upon the viewing distance. Fourteen normal children (4.5-12 years old) and 10 normal adults (22-44 years old) participated. Two distances were studied: far (150 cm) and close (20 cm). Horizontal saccades from both eyes were recorded simultaneously by a photoelectric device. The results show (i) poor binocular coordination of saccades in children is distance dependent: coordination is particularly poor at near and could compromise single binocular vision; and (ii) post-saccadic drift analysis indicates that stability of the eyes after the saccade as well as the quality of binocular alignment during fixation are poor in children, regardless of the viewing distance. The results provide a reference for the normal development of binocular motor control and have important implications for reading in young children. Language: en

The reentry hypothesis: linking eye movements to visual perception.

Abstract: Cortical organization of vision appears to be divided into perception and action. Models of vision have generally assumed that eye movements serve to select a scene for perception, so action and perception are sequential processes. We suggest a less distinct separation. According to our model, occulomotor areas responsible for planning an eye movement, such as the frontal eye field, influence perception prior to the eye movement. The activity reflecting the planning of an eye movement reenters the ventral pathway and sensitizes all cells within the movement field so the planned action determines perception. We demonstrate the performance of the computational model in a visual search task that demands an eye movement toward a target.

Contributions of fixational eye movements to the discrimination of briefly presented stimuli.

Abstract: Although it is known that images tend to disappear when they are stabilized on the retina for tens of seconds or minutes, the possible functions of fixational movements during the brief periods of visual fixation that occur during natural viewing remain controversial. Studies that investigated the retinal stabilization of stimuli presented for less than a few seconds have observed neither decrement in contrast sensitivity nor image fading. In this study, we analyzed the effect of retinal stabilization on discriminating the orientation of a low-contrast and noisy small bar that was displayed for either 500 ms or 2 s. The bar was randomly tilted by 45 degrees either clockwise or counterclockwise. For both exposure durations, percentages of correct discrimination were significantly lower under conditions of visual stabilization than in the presence of the normally moving retinal image. These results are consistent with the predictions of recent computational models that simulated neuronal responses in the early visual system during oculomotor activity and support the hypothesis that visual processes deteriorate rapidly in the absence of retinal image motion.

Sensitivity for global shape detection.

Abstract: In order to understand the nature of the mechanisms responsible for global shape detection, we measured coherence thresholds in a 2IFC task where subjects judged which of two arrays of Gabors contained global circular structure. The stimulus was composed of an array of oriented Gabor patches positioned on a polar grid. Thresholds were obtained for different array parameters (e.g. different area, density, number and positions of elements) as well as for different element parameters (e.g. different carrier spatial frequencies, contrasts, polarities and orientations). Global structure was detected when ~10% of the elements were coherently oriented. Neither the properties of the array (density, area, number or position of elements), nor those of the individual elements (carrier spatial frequency, contrast, polarity) altered coherence thresholds. Varying contrast or carrier spatial frequency within individual arrays also did not alter performance. Sensitivity was invariant to positional perturbations of the array grid. Only jittering the local orientation of elements decreased sensitivity. The underlying mechanisms are broadly tuned for contrast, spatial frequency and the spatial positioning of image samples. Detecting circular structure is a robust process and, in this case, a purely global one. Sensitivity was highest for circular as opposed to radial or spiral shapes.

Comparison of two weighted integration models for the cueing task: linear and likelihood.

Abstract: In a task in which the observer must detect a signal at two locations, presenting a precue that predicts the location of a signal leads to improved performance with a valid cue (signal location matches the cue), compared to an invalid cue (signal location does not match the cue) The cue validity effect has often been explained with a limited capacity attentional mechanism improving the perceptual quality at the cued location Alternatively, the cueing effect can also be explained by unlimited capacity models that assume a weighted combination of noisy responses across the two locations We compare two weighted integration models, a linear model and a sum of weighted likelihoods model based on a Bayesian observer While qualitatively these models are similar, quantitatively they predict different cue validity effects as the signal-to-noise ratios (SNR) increase To test these models, 3 observers performed in a cued discrimination task of Gaussian targets with an 80% valid precue across a broad range of SNR's Analysis of a limited capacity attentional switching model was also included and rejected The sum of weighted likelihoods model best described the psychophysical results, suggesting that human observers approximate a weighted combination of likelihoods, and not a weighted linear combination