Introduction:We report a very unique clinical presentation of a patient who complained, after a left parietal brain damage, about feeling tactile stimulations on his right upper limb without being able to localize them.Methods:Using a single case study approach, we report three experiments relying on several custom-made tasks to explore the different levels of somatosensory information processing, ranging from somato-sensation to somato-representation.Results:Our results showed a preserved ability to localize tactile stimuli applied on the right upper limb when using pointing responses while the ability to localize was less efficient when having to name the stimulated part (akin Numbsense). When the stimuli were applied on more distal locations (i.e., on the hand and on fingers), the number of correct responses decreased significantly independently of the modality of response. Finally, when visually presented with a stimulus delivered on the hand of an examiner in synchrony with the stimulation on the hidden hand of the patient, responses were largely influenced by the visual information available. Altogether, the convergence of these different customized tasks revealed an absence of autotopagnosia for motor responses for the right upper limb, associated with altered abilities to discriminate stimulus applied on distal and restricted/closer zones in the hand.Discussion:The somato-representation of our patient seemed to significantly rely on visual information, leading to striking deficits to localize tactile stimuli when vision and somesthesic afferences are discordant. This case report offers a clinical illustration of pathological imbalance between vision and somesthesia. Implications of these troubles in somato-representation on higher cognitive level processes are discussed.

Unilateral spatial neglect has been extensively described for visual and representational manifestations but tactile and motor manifestations as well as body neglect point to bodily manifestations of this neurological condition. This chapter reviews the perceptual, motoric and high-level representational symptoms manifested in neglect patients and attempt to classify them according to the body image/schema framework. One puzzling aspect of the wide spectrum of body neglect symptoms is that physiological bottom-up maneuvers, such as prism adaptation, which act at the level of body schema, do also efficiently improve body image manifestations of neglect. This relationship allows us to elaborate on the dialectical relationships between body image and body schema. Thus, understanding body neglect in terms of diagnosis, evaluation, physiopathology and therapeutics through the dynamical interactions between body schema and body neglect, provide perspectives to manage other lateralized body troubles, neglect-like manifestations of bodily attention or distorted representations.

Fast, online control of movement is an essential component of human motor skills, as it allows automatic correction of inaccurate planning. The present study explores the role of two types of concurrent signals in error correction: predicted visual reafferences coming from an internal representation of the hand, and actual visual feedback from the hand. While the role of sensory feedback in these corrections is well-established, much less is known about sensory prediction. The relative contributions of these two types of signals remain a subject of debate, as they are naturally interconnected. We address the issue in a study that compares online correction of an artificially induced, undetected planning error. Two conditions are tested, which only differ with respect to the accuracy of predicted visual reafferences. In the first, "Prism" experiment, a planning error is introduced by prisms that laterally displace the seen hand prior to hand movement onset. The prism-induced conflict between visual and proprioceptive inputs of the hand also generates an erroneous prediction of visual reafferences of the moving hand. In the second, "Jump" experiment, a planning error is introduced by a jump in the target position, during the orienting saccade, prior to hand movement onset. In the latter condition, predicted reafferences of the hand remained intact. In both experiments, after hand movement onset, the hand was either visible or hidden, which enabled us to manipulate the presence (or absence) of visual feedback during movement execution. The Prism experiment highlighted late and reduced correction of the planning error, even when natural visual feedback of the moving hand was available. In the Jump experiment, early and automatic corrections of the planning error were observed, even in the absence of visual feedback from the moving hand. Therefore, when predicted reafferences were accurate (the Jump experiment), visual feedback was processed rapidly and automatically. When they were erroneous (the Prism experiment), the same visual feedback was less efficient, and required voluntary, and late, control. Our study clearly demonstrates that in natural environments, reliable prediction is critical in the preprocessing of visual feedback, for fast and accurate movement.

L’homme ne perçoit pas la réalité, mais ce que son cerveau arrive à en reconstituer à partir des informations qu’il reçoit de ses différents sens. Or ses entrées sensorielles sont ambiguës et reposent sur des capteurs aux capacités limitées. Malgré tout, il arrive généralement à se représenter correctement l’espace autour de lui. Mais comment parvient-il à construire une perception unique et cohérente de celui-ci ?

El hombre no percibe la realidad, sino lo que su cerebro reconstruye de ella a partir de la informacion que recibe de sus distintos sentidos. Ahora bien, sus percepciones sensoriales son ambiguas y dependen de sensores con capacidades limitadas. Aun asi, suele ser capaz de representarse correctamente el espacio que le rodea. Pero, ?como consigue construir una percepcion unica y coherente de si mismo?

Sensorimotor plasticity allows the nervous system to set up appropriate motor and sensory compensations when individuals face changing demands in a given motor task. A much-debated question in neuroscience research is the identification of processes that encompass this capacity of plasticity. Prism adaptation is the oldest experimental paradigm that has been used to achieve this goal (Helmholtz, 1867). Since 1990's, other paradigms have emerged such as visuomotor rotations or dynamical perturbations (inertial Coriolis forces, velocity-dependent force-field).

Sensorimotor plasticity allows the nervous system to set up appropriate motor and sensory compensations when individuals face changing demands in a given motor task. A much-debated question in neuroscience research is the identification of processes that encompass this capacity of plasticity. Prism adaptation is the oldest experimental paradigm that has been used to achieve this goal (Helmholtz, 1867). Since 1990's, other paradigms have emerged such as visuomotor rotations or dynamical perturbations (inertial Coriolis forces, velocity-dependent force-field). We compared these paradigms with respect to three specific methodological features: application of the perturbation, after-effects, and generalization. This work aimed to shed light on the following central issue: Do all these paradigms involve similar processes? We used generalization properties—a relevant feature associated with the credit assignment problem—to emphasize the involvement of different processes in “adaptation” paradigms. We therefore classified these processes based on the context specificity of elicited transformations. This review reveals that the processes involved are closely linked to paradigm-related experimental conditions. Context-independent processes appear to be favored when errors are attributed to our own sensorimotor performance (prism, Coriolis) whereas context-dependent processes appear to be mostly mediated by attribution of errors to a specific external interface (visuomotor rotation, force-field). This work encourages researchers to consider the methodological aspects specific to each paradigm for future investigations of sensorimotor plasticity.

Visuo-motor adaptation has been classically studied using movements aimed at visual targets with visual feedback. In this type of experimental design, the respective roles of the different error signals cannot be fully disentangled. Here, we show that visuo-motor adaptation occurs despite the terminal success of the action and the compensation of the external error by a jump of the visual target. By using three grasping task conditions we manipulated the retinal error signal between the seen hand and the target (external error) and the conflict between the hand's visual reafference and either the proprioceptive or the efference copy signal (internal error), in order to estimate their respective roles in prism adaptation. In all conditions, subjects were asked to rapidly grasp an object. In the classical 'Prism' condition the object was stationary, which provided both external and internal errors. In the 'Prism & Jump' condition, at movement onset the object was suddenly displaced (jump) toward its virtual image location (visually displaced by the prism) which also corresponded to the location where the movement was planned to and executed through prisms. This jump therefore cancelled the external error (between the seen target and the seen hand), whereas the internal error (between the seen hand and the expected visual reafference of the hand, or between the seen hand and the hand felt by proprioception) was unchanged (because it is independent of the presence of the goal). In the 'Jump' condition, the movement was planned and executed without prismatic goggles and consequently with no internal error (no difference between where the hand visual reafference is expected to be and where it actually is), but the object was suddenly displaced at movement onset by a displacement equivalent to a prism shift which provided an external error. The 'Prism' and 'Prism & Jump' conditions exhibited similar aftereffects, whereas no aftereffect was observed in the 'Jump' condition. These results suggest that successful actions can be subjected to adaptation and that internal error is the only signal necessary to elicit true visuomotor adaptation characterized by context-independent generalization.

When looking at objects at various distances in the physical space, the accommodation and vergence systems adjust their parameters to provide a single and clear vision of the world. Subtended muscle activity provides oculomotor cues that can contribute to the perception of depth and distance. While several studies have outlined the role of vergence in distance perception, little is known about the contribution of its concommitant accommodation component. It is possible to unravel the role of each of these physiological systems by placing observers in a situation where there is a conflict between accommodation and vergence distances. We thus sought to determine the contribution of each response system to perceived depth by simultaneously measuring vergence and accommodation while participants judged the depth of 3D stimuli. The distance conflict decreased depth constancy for stimulus displayed with negative disparity steps (divergence). Although vergence was unaffected by the stimulus distance, accommodation responses were significantly reduced when the stimulus was displayed with negative disparities. Our results show that biases in perceived depth follow undershoots in the disparity-driven accommodation response. These findings suggest that accommodation responses (i.e., from oculomotor information) can contribute to perceived depth.

In see-through helmet-mounted displays (HMDs), image-intensifier tubes are located on the sides of the helmet. This HMD design increases the effective interpupillary distance and exposes the observer to hyperstereopsis. The modification of three-dimensional space perception by hyperstereopsis in laboratory studies is well known. Little is known, however, about its effects in real, operational conditions. The present study investigated the effects of hyperstereopsis on visual perception in helicopter pilots during the first flight of a night-flight training program using a see-through HMD. Fifteen pilots' percepts of distance and height under hyperstereoscopic viewing were assessed using a double-distance estimation task in standard flight circuits. In addition, two questionnaires were used to characterize the perceptual effects induced by hyperstereoscopic viewing during the first night flight, and to determine whether any aftereffects were still present during the 24-h period following the flight. Height and distance estimation errors were observed. On average, pilots tended to underestimate the near plane relative to the farther plane in the double-distance estimation task; this is the inverse of what is commonly observed under natural viewing. We ascribed this effect to a non-linear decrease in the hyperstereopsis effect with viewing distance. Six pilots experienced sensory illusions resembling a distortion of apparent visual space: the impression of "sinking into a hole" when landing, of objects appearing closer than they actually were, and reduced in size. Four pilots cited landing as the most difficult situation encountered during the flight. Only minor complaints were reported for the 24-h period following the flight. The results of this field study represent a step towards characterizing the perceptual and operational impact of this display technology in helicopter pilots during initial exposure.

Vertical binocular disparity is a source of distance information allowing the portrayal of the layout and 3D metrics of the visual space. The role of vertical disparity in the perception of depth, size, curvature, or slant of surfaces was revealed in several previous studies using cue conflict paradigms. In this study, we varied the configuration of stereo-cameras to investigate how changes in the horizontal and vertical disparity fields, conflicting with the vergence cue, affect perceived distance and depth. In four experiments, observers judged the distance of a cylinder displayed in front of a large fronto-parallel surface. Experiment 1 revealed that the presence of a background surface decreases the uncertainty in judgments of distance, suggesting that observers use the relative horizontal disparity between the target and the background as a cue to distance. Two other experiments showed that manipulating the pattern of vertical disparity affected both distance and depth perception. When vertical disparity specified a nearer distance than vergence (convergent cameras), perceived distance and depth were underestimated as compared with the condition where vertical disparity was congruent with vergence cues (parallel cameras). When vertical disparity specified a further distance than vergence, namely an infinite distance, distance and depth were overestimated. The removal of the vertical distortion lessened the effect on perceived distance. Overall, the results suggest that the vertical disparity introduced by the specific camera configuration is mainly responsible for the effect. These findings outline the role of vertical disparity in distance and depth perception and support the use of parallel cameras for designing stereograms.

PURPOSE:Stereoscopic displays challenge the neural cross-coupling between accommodation and vergence by inducing a constant accommodative demand and a varying vergence demand. Stereoscopic viewing calls for a decrease in the gain of vergence accommodation, which is the accommodation caused by vergence, quantified by using the convergence-accommodation to convergence (CA/C) ratio. However, its adaptability is still a subject of debate.METHODS:Cross-coupling (CA/C and AC/A ratios) and tonic components of vergence and accommodation were assessed in 12 participants (27.5 ± 5 years, stereoacuity better than 60 arc seconds, 6/6 acuity with corrected refractive error) before and after a 20-minute exposure to stereoscopic viewing. During stimulation, vergence demand oscillated from 1 to 3 meter angles along a virtual sagittal line in sinusoidal movements, while accommodative demand was fixed at 1.5 diopters.RESULTS:Results showed a decreased CA/C ratio (-10.36%, df = 10, t = 2.835, P = 0.018), with no change in the AC/A ratio (P = 0.090), tonic vergence (P = 0.708), and tonic accommodation (P = 0.493).CONCLUSIONS:These findings demonstrated that the CA/C ratio can exhibit adaptive adjustments. The observed nature and amount of the oculomotor modification failed to compensate for the stereoscopic constraint.

BACKGROUND:Several tests are available to optometrists for investigating accommodation and vergence. This study sought to investigate the agreement between clinical and laboratory methods and to clarify which components are actually measured when tonic and cross-link of accommodation and vergence are assessed.METHODS:Tonic vergence, tonic accommodation, accommodative vergence (AC/A) and vergence accommodation (CA/C) were measured using several tests. Clinical tests were compared to the laboratory assessment, the latter being regarded as an absolute reference. The repeatability of each test and the degree of agreement between the tests were quantified using Bland-Altman analysis.RESULTS:The values obtained for each test were found to be stable across repetitions; however, in most cases, significant differences were observed between tests supposed to measure the same oculomotor component. Tonic and cross-link components cannot be easily assessed because proximal and instrumental responses interfere with the assessment. Other components interfere with oculomotor assessment. Specifically, accommodative divergence interferes with tonic vergence estimation and the type of accommodation considered in the AC/A ratio affects its magnitude. Results on clinical tonic accommodation and clinical CA/C show that further investigation is needed to clarify the limitations associated with the use of difference of Gaussian as visual targets to open the accommodative loop.CONCLUSIONS:Although different optometric tests of accommodation and vergence rely on the same basic principles, the results of this study indicate that clinical and laboratory methods actually involve distinct components. These differences, which are induced by methodological choices, must be taken into account, when comparing studies or when selecting a test to investigate a particular oculomotor component.

von Hofsten proposed a model for explaining how the perception of egocentric distance is affected by vergence angle. According to his model, binocular distance estimation relies on the difference between vergence response and rest vergence, rather than on absolute vergence angle. We applied this model to the data obtained in a study on adaptation to telestereoscopic viewing. Such an optical distortion, found in some novel stereo displays, calls for a change in the relationship between vergence and perceived distance, as well as in the level of rest vergence. We sought to investigate any changes in the model coefficients during and after exposure. The model testing corroborated the observation of distinct visual adaptive mechanisms, for the two different exposure conditions of the study.