Location coding by the human visual system: multiple topological adaptations in a case of strabismic amblyopia

Amblyopia, a major cause of vision loss, is a developmental disorder of visual perception commonly associated with strabismus (squint). Although defined by a reduction in visual acuity, severe distortions of perceived visual location are common in strabismic amblyopia. These distortions can help us understand the cortical coding of visual location and its development in normal vision, as well as in amblyopia. The history of retinotopic mapping in the visual cortex highlights the potential impact of amblyopia. Theories of amblyopia include topological disarray of receptors in primary visual cortex, undersampling from the amblyopic eye compared with normal eyes, and the presence of anomalous retinal correspondence or multiple cortical representations of the strabismic fovea. We examined the distortions in a strabismic amblyope, using a pop-out localization task, in which normal observers made errors dependent on the visual context of the stimulus. The localization errors of the strabismic amblyope were abnormal. We found that none of the available theories could fully explain this one patient's localization performance. Instead, the observed behavior suggests that multiple adaptations of the underlying cortical topology are possible simultaneously in different parts of the visual field.     

Localization of targets by strabismic subjects: Contrasting patterns in constant and alternating suppressors

Strabismic subjects in whom vision in one eye is constantly suppressed localized targets at different points in space when using each eye. Whether they were esotropes or exotropes determined the relation between the two locations and the angle of their vergence error correlated with the magnitude of the interocular discrepancy. These relationships suggest that constant suppressors utilize information about posture of the dominant eye in reaching toward targets presented to the normally suppressed eye. Neither orthotropes nor strabismics in whom vision in each eye is alternately suppressed showed this anomaly. These results were attributed to differences in the way in which visual space is represented, an outcome of the pattern of use of the eyes during early development.     

Functional and dynamic properties of visual peripersonal space

In order to code visual peripersonal space, human and non-human primates need an integrated system that controls both visual and tactile inputs within peripersonal space around the face and the hand, based on visual experience of body parts. The existence of such a system in humans has been demonstrated, and there is evidence showing that visual peripersonal space relating to the hand has important dynamic properties, for example, it can be expanded and contracted depending on tool use. There is also evidence for a high degree of functional similarity between the characteristics of the visual peripersonal space in humans and in monkeys.     

Visual space as physical geometry

The geometrical incongruence between patterns in visual space and structures and patterns of activity in the visual cortex, long known to investigators, serves as a criterion for evaluating physical theories of visual space. The problem of determining the geometry of the visual world (visual geometry) is compared with that of determining the geometry of the physical world (physical geometry). Theories as to the possible physical locus of visual space, whether in the brain or elsewhere, are reviewed, analyzed, and criticized accordingly. It is concluded that on the basis of congruence alone it would be predicted that visual space is not to be found in the brain, even though it is seemingly linked to it causally, as experimental neurology and neurophysiology demonstrate. Alternative theories as to the nature of visual space are considered, but are also found to be inadequate in explaining visual space in terms acceptable to contemporary science.     

Embodied infant attention

Does real time coupling between mental and physical activity early in development have functional significance? To address this question, we examined the habituation of visual attention and the subsequent response to change in two groups of 3‐month‐olds with different patterns of movement–attention coupling. In suppressors, the typical decrease in body movement at the onset of looks persists into the looks. In rebounders, the initial decrease is more transient and movement quickly returns above baseline. Suppressors and rebounders did not differ on measures of looking during habituation, but when the stimulus changed rebounders looked more than suppressors. When it did not change, they looked less. In addition, during habituation rebounders spent more time looking away from the stimulus. Rapid motor reactivation soon after gaze locks onto a target, characteristic of rebounders, may influence visual foraging and the response to change by keeping attention near a threshold of engagement.     

Fragmentation of monocular afterimages in individuals with and without normal binocular vision

Monocular afterimages of a vertical line were generated in each eye of binocularly normal individuals and of those lacking binocular single vision or stereopsis as a consequence of childhood strabismus. Fragmentations and disappearances of the afterimages occurred less frequently in the strabismic than in the normal subjects; they were also less frequent in the dominant (nonamblyopic) eyes of the strabismics than in their amblyopic eyes. The afterimages remained visible for longer when presented to the dominant eyes of the strabismic subjects, and a similar effect was found between the sighting dominant and nondominant eyes of normal subjects.     

Plasticity of the association between visual space and action space in a blind-walking task

Many experiments have shown that a brief visual preview provides sufficient information to complete certain kinds of movements (reaching, grasping, and walking) with high precision. This suggests that participants must possess a calibration between visual target location and the kinaesthetic, proprioceptive, and/or vestibular stimulation generated during movement towards the target. We investigated the properties of this calibration using a cue-conflict paradigm in which participants were trained with mismatched locomotor and visual input. After training, participants were presented with visual targets and were asked to either walk to them or locate them in a spatial updating task. Our results showed that the training was sufficient to produce significant, systematic miscalibrations of the association between visual space and action space. These findings suggest that the association between action space and visual space is modifiable by experience. This plasticity could be either due to modification of a simple, task-specific sensory motor association or it could reflect a change in the gain of a path integration signal or a reorganisation of the relationship between perceived space and action space. We suggest further experiments that might help to distinguish between these possibilities.     

Visual perception of collinearity

In a metrical space, there exists an intimate relation between collinearity and parallelity. In particular, in a Riemannian space collinearity is just a special case of parallelity. Is this true for visual space as well? We investigated the visual perception of collinearity by having subjects align two bars in the horizontal plane at eye height. The distances of the bars from the subject and the angles at which they were placed were varied. We found deviations of up to 22 degrees. The deviations of the left and right bars could be split into two independent components: namely, the sum and the difference of the deviations of the left and right bars. We found that the former depended only on the ratio between the distances of each bar from the subject, whereas the latter was largely independent of the positions of the bars. The difference in deviations corresponded to the deviation from parallelity. Compared with the results in the parallelity task (Cuijpers, Kappers, & Koenderink, 2000b), the deviations from parallel were much smaller. As a consequence, the results of the two experiments cannot be described by the same Riemannian geometry. This indicates that the intrinsic geometry of visual space differs across tasks. This is conceivable if the intrinsic geometry of visual space is operationally defined.     

Visual stimuli for strabismic suppression

The effects of orientation and spatial frequency of grating stimuli upon suppression were examined with a binocular rivalry paradigm in a group of ten strabismic patients and in a control normal group. Duration, frequency, and period of rivalry were examined as functions of differences in orientation and spatial frequency of dichoptic achromatic sinusoidal gratings. Records were made of responses by the sighting and by the nonsighting eye as well as responses during periods of combined binocular vision. Strabismic subjects reported normal binocular rivalry when presented with gratings of dissimilar orientation. Suppression of the deviating eye in strabismic subjects occurred with stimuli of similar orientation and was unaffected by spatial-frequency differences between dichoptic stimuli. Suppression was most intense under conditions that normally stimulate stereopsis and sensory fusion.     

Manual localization of lateralized visual targets

The effects of visual field, responding limb and extrapersonal space on the ability to localize visual targets using slow positioning movements of the arm were examined. Special contact lenses were used to lateralize visual information and to make comparisons with localization under monocular control conditions. Subjects made slow positioning movements to place a cursor directly beneath target lights. They saw target lights but not the moving limb during the trial. For directional error, results indicated that subjects were more accurate localizing targets lateralized to the right hemisphere than targets lateralized to the left hemisphere, indicating right hemisphere superiority for localization of visual targets in grasping space. Localization performance was significantly better with the right hand than the left hand. the left hand demonstrated a directional bias to the right of the targets. Responding hand and visual field did not interact. Finally, contrary to subjects' awareness and verbal reports, target localization was not less accurate in lens than in monocular control conditions. This was true for both amplitude and directional error. This is consistent with other studies where visual information about limb position is not available.     

Geometrical structures of photographic and stereoscopic spaces

Two experiments were conducted to investigate the geometrical structures of photographic and stereoscopic spaces. In Experiment 1, it was investigated how accurately photographic space reproduces real physical space, and the geometrical structure of photographic space was compared with that of visual space. As a result, the mapping function of distance between photographic and physical spaces (delta = ad(b)) shows that a and b range from 0.96-1.1 and 0.69-0.78. The mapping function of angle between photographic and physical spaces (phi = g phi(h)) shows that g and h range from 2.37-5.29 and 0.74-0.97. Further, photographic space has larger anisotropic property than visual space and photographic space may be hyperbolic. In Experiment 2, the geometrical structure of stereoscopic space was compared with that of visual space. It was found that stereoscopic space was almost the same as visual space.     

Alleys in visual space

Geometry of frameless visual space is dealt with. First, parallel and equidistant alleys, horopters in the horizontal plane of eyes' level are discussed within the framework of the Luneburg's model that the frameless visual space is a Riemannian space of constant curvature. That basic postulate and the specific mapping functions assumed by Luneburg between the euclidean map of visual space and the physical space are kept separate, and efforts are directed to make the model applicable to more natural conditions of our visual space. A possibility is pointed out to remove the constraint “frameless” in the sense that perceptual geometrical properties are primarily determined by the convergence of optic axes. So far, only alleys in the horizontal plane extending from us toward infinity have been studied, but more often we perceive parallel lines, horizontal or vertical, in front of us like shelves of a bookcase. Hence, equations are derived for horopter plane appearing fronto-parallel in the three-dimensional visual space and alleys running horizontally or vertically on the horopter plane. It is shown that parallel and equidistant alleys are not the same in the horopter plane as in the horizontal plane, if the visual space is not euclidean. A method to evaluate the discrepancy between the two alleys without using any mapping functions is stated with some numerical examples.     

The power of the imagination to affect peripersonal space representations

It is known that visual processing is altered for objects near the hands as well as for objects near the imagined position of the hands, indicating that the imagination can be used to remap peri-hand space. Little is known, however, about the physical conditions that allow for this remapping. In the present study, participants in one experiment performed visual searches through displays that were beyond reach while imagining that their hands were near the display. This imagined impossible posture slowed visual search rates, showing that imagination effectively remapped peri-hand space to be near the display. In another experiment, participants searched displays they were holding, but sometimes imagined their hands to be far away. This produced faster search rates, revealing a remapping of peri-hand space away from the monitor. The results provide new insights into the mechanisms involved in the representation of peri-hand space and about the power of imagined actions to influence body representations.     

Understanding the structural determinants of object confusion in memory: an assessment of psychophysical approaches to estimating visual similarity

Computer-generated shapes varying on visual dimensions such as curvature, tapering, and thickness have been used to investigate identification deficits in the category-specific visual agnosia (CSVA) Patient E.L.M.. However, whether the implemented variations on each of these dimensions were perceived by novice observers as "similar amounts of change" is unknown. To estimate distance in psychophysical shape space, sets of shapes were developed using two different scaling methods--an objective method based on visual search, and a subjective method based on judgments of similarity--and a third approach that did not involve scaling. How well each method estimated psychophysical shape space was assessed by measuring the confusions within memory among the shapes. The results suggested that, although neither of the approaches perfectly reflected psychophysical shape space, subjective scaling was a better estimator of distance in psychophysical shape space than were other approaches. The number of confusions produced on each set of shapes was used to develop a new set of shapes that accurately estimated distance in psychophysical shape space. These results suggest that a combination of approaches is preferable in order to accurately estimate distance in psychophysical shape space.     

Effects of Limitations on the Use of Some Visual and Kinaesthetic Information in Spatial Mapping during Exploration in the Rat

The purpose of this experiment was to study the effects of limiting visual and/or locomotor access to a part of the environment in the building up of a spatial representation of the whole space. During five sessions, rats were allowed to explore separately and successively the two halves (subspaces) of a circular open field containing four objects. During exploration of each half, continuous or discontinuous locomotor and/or visual access to the other half was provided by using opaque or transparent partitions, with or without doors. Once habituation was complete, the partition was removed for some subjects but remained for others. The locomotor and exploratory reactions to this removal were recorded. Whatever their locomotor experience (continuous or discontinuous), rats that had a discontinuous visual experience between the subspaces displayed a renewal of exploratory activity, whereas the rats that had received a continuous visual experience did not re-explore the objects. This result suggests that continuous visual access to the whole space is necessary for the construction of an overall representation. Furthermore, continuous locomotor activity does not seem to compensate for the discontinuity of visual information.     

Manifold embedding for zero-shot recognition

Zero-Shot Recognition (ZSR) has gained its popularity recently owing to its promising characteristic that extends the classifiers to the unseen classes. It is typically addressed by resorting to a class semantic space to transfer the knowledge from the seen classes to unseen ones. Therefore, constructing the effective interactions between the visual space and the class semantic space is the key for ZSR. In this paper, under the assumption that the distribution of the semantic categories in the semantic space has an intrinsic manifold structure, we propose two manifold embedding-based ZSR approaches to capture the intrinsic structures of both the visual space and the class semantic space, i.e., ME-ZSR and MCCA-ZSR. Specifically, ME-ZSR builds embedding from visual space to semantic space, while MCCA-ZSR explores to embed both visual and semantic features into a common space. The linear, closed-form solutions make both methods efficient to optimize. Extensive experiments on three popular datasets AwA, CUB and NAB validate the effectiveness of both methods.     

Visual field and empty space

In a paper titled “Seeing Empty Space,” Louise Richardson argues for the thesis that seeing empty space involves a certain “structural feature,” namely, “it [s] seeming to one as if some region of space is one in which if some visible object were there, one would see it” (SF; Richardson, 2010, p. 237). I will argue that there is a reason to question whether a structural feature such as SF is needed in order to visually experience empty space. I will also propose that the visual experience of empty space does involve a quite different structural feature, underscoring the mediating role of empty space in visual experience.