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.     

Rôle des propriétés inertielles segmentaires sur la perception de l’étendue de l’espace péripersonnel

Previous studies have suggested that the perception of peripersonal space can hardly be achieved using only scene-based visual cues and requires combining visual information with motor representations. Motor representation can be viewed as a component of a predictive system, which includes a neural process that simulates through motor imagery the dynamic behaviour of the body in relation to the environment. In this study, we analysed whether modifying the force required to reach a visual target influences the perception of what is reachable. In a visuomotor task, the experimental group (n = 10) adapted to a 1.5 kg weight attached to the right wrist while performing a series of pointing movements. The control group (n = 10) performed the motor task without inertial perturbation. A perceptual judgement task of what is reachable was performed before and after the motor task. Results showed that inertial perturbation produced initially an undershoot of the target suggesting a lack of motor force to overcome the inertial perturbation, but spatial errors receded progressively through movement rehearsal. Perceptual estimates of what is reachable slightly overestimated action capacities but were not affected by motor adaptation. Thus, modifying motor force required to compensate for inertial perturbation had no direct effect on the perception of peripersonal space. When interpreted in regard to previous experimental work, this result suggests that motor representations may provide information about the sensory or spatial consequences of action rather than the sense of effort associated with motor production.     

Impairments in tactile search following superior parietal damage

The superior parietal cortex is critical for the control of visually guided actions. Research suggests that visual stimuli relevant to actions are preferentially processed when they are in peripersonal space. One recent study demonstrated that visually guided movements towards the body were more impaired in a patient with damage to superior parietal cortex. Whereas past studies have explored disordered movement in optic ataxic patients, there has been less exploration of space perception in terms of search capacity in this population. In addition, there is some debate concerning the relationship between deficits of visuomotor control and impaired attention/perception in optic ataxia. Given that the dorsal stream has been implicated in the spatial processing of stimuli in peripersonal space, and damage to this region is known to cause optic ataxia, we felt that further investigation was warranted. We examined tactile search behavior in the fronto-parallel and radial planes in a patient with right superior parietal damage and optic ataxia. We used a pegboard with removable cylindrical pegs that allowed for the reorganization of targets between trials. To better characterize three-dimensional search behavior, we included both horizontal and vertical search conditions. Results showed that the patient spent more time searching, was more accurate and revisited more targets in right versus left space. Interestingly, the patient spent the majority of her time specifically searching the lower right quadrant of the stimulus array. Further analysis revealed lower target detection rates along the outer borders of the pegboard on all sides. The search pattern observed here is unusual considering that all targets were within arm's reach. The present experiment demonstrates that damage to superior parietal cortex impairs tactile search and biases exploration towards lower right peripersonal space.     

Spatial biases in peripersonal space in sighted and blind individuals revealed by a haptic line bisection paradigm

Our representation of peripersonal space does not always accurately reflect the physical world. An example of this is pseudoneglect, a phenomenon in which neurologically normal individuals bisect to the left of the veridical midpoint, reflecting an overrepresentation of the left portion of space compared with the right one. Consistent biases have also been observed in the vertical and radial planes. It is an open question whether these biases depend on normal visual experience for their occurrence. Here we systematically investigated this issue by testing blindfolded sighted and early blind individuals in a haptic line bisection task. Critically, we found a robust leftward bias in all participants. In the vertical and radial planes, sighted participants showed a consistent downward and proximal bias. Conversely, the directional bias in blind participants was dependent on the final movement direction; thus, there was no general bias in either direction. These findings are discussed in terms of different reference frames adopted by sighted and blind participants when encoding space.     

Merging familiar and new senses to perceive and act in space

Our experience of the world seems to unfold seamlessly in a unitary 3D space. For this to be possible, the brain has to merge many disparate cognitive representations and sensory inputs. How does it do so? I discuss work on two key combination problems: coordinating multiple frames of reference (e.g. egocentric and allocentric), and coordinating multiple sensory signals (e.g. visual and proprioceptive). I focus on two populations whose spatial processing we can observe at a crucial stage of being configured and optimised: children, whose spatial abilities are still developing significantly, and naïve adults learning new spatial skills, such as sensing distance using auditory cues. The work uses a model-based approach to compare participants’ behaviour with the predictions of alternative information processing models. This lets us see when and how—during development, and with experience—the perceptual-cognitive computations underpinning our experiences in space change. I discuss progress on understanding the limits of effective spatial computation for perception and action, and how lessons from the developing spatial cognitive system can inform approaches to augmenting human abilities with new sensory signals provided by technology.

     

How tool use and arm position affect peripersonal space representation

Experiment 1 investigated whether tool use can expand the peripersonal space into the very far extrapersonal space. Healthy participants performed line bisection in peripersonal and extrapersonal space using wooden sticks up to a maximum of 240 cm. Participants misbisected to the left of the true midpoint, both for lines presented in peripersonal and for those presented in extrapersonal space, confirming a peripersonal space expansion up to a distance of 240 cm. Experiment 2 investigated whether arm position could influence the perception of peripersonal and extrapersonal space during tool use. Participants performed line bisection in the peripersonal and in the extrapersonal space (up to a maximum of 120 cm) using wooden sticks in two different conditions: either with the arm bent or with the arm stretched. Results showed stronger pseudoneglect in the stretched arm condition.     

Visual acquaintance,action and the explanatory gap

Much attention has recently been paid to the idea, which I label ‘External World Acquaintance’ (EWA), that the phenomenal character of perceptual experience is partially constituted by external features. One motivation for EWA which has received relatively little discussion is its alleged ability to help deal with the ‘Explanatory Gap’. I provide a reformulation of this general line of thought, which makes clearer how and when EWA could help to explain the specific phenomenal nature of visual experience. In particular, I argue that by focusing on the different kinds of perceptual actions that are available in the case of visual spatial vs. colour perception, we get a natural explanation for why we should expect the specific nature of colour phenomenology to remain less readily intelligible than the specific nature of visual spatial phenomenology.

     

Spatial localization of touch in the first year of life: early influence of a visual spatial code and the development of remapping across changes in limb position

Two experiments investigated infants' ability to localize tactile sensations in peripersonal space. Infants aged 10 months (Experiment 1) and 6.5 months (Experiment 2) were presented with vibrotactile stimuli unpredictably to either hand while they adopted either a crossed- or uncrossed-hands posture. At 6.5 months, infants' responses were predominantly manual, whereas at 10 months, visual orienting behavior was more evident. Analyses of the direction of the responses indicated that (a) both age groups were able to locate tactile stimuli, (b) the ability to remap visual and manual responses to tactile stimuli across postural changes develops between 6.5 and 10 months of age, and (c) the 6.5-month-olds were biased to respond manually in the direction appropriate to the more familiar uncrossed-hands posture across both postures. The authors argue that there is an early visual influence on tactile spatial perception and suggest that the ability to remap visual and manual directional responses across changes in posture develops between 6.5 and 10 months, most likely because of the experience of crossing the midline gained during this period.     

Introspection and Primacy of Perception: A Critical Reflection on Naïve Realism

A fundamental issue in philosophy of perception is to understand the nature of experience and the relation of the experience with objects or states of affairs that is experienced. A prominent philosophical issue here is posed by the possibility of hallucinatory experiences, which are subjectively indistinguishable from veridical perception for the experiencer. The philosophical views in this matter can be grouped into three major positions on the basis of the nature of the subjective experience and relation of the subjective experience with the object. These are the sense-data theories—which consider that the objects of perception are mental entities; the representative theories—according to which perception is a representation of the objects in the external world; and the naïve realist theories—which proclaim that the external objects are constitutive of the very perceptual experience and not a representation of it. Naïve realism claims it is the defence of common-sense notion regarding experience i.e. how experience seems to the experiencer upon introspective reflection on it. This position has a growing number of proponents in philosophy especially in the last two decades. But it also entails radical departure of established philosophical views regarding the nature of experience, the phenomenal character of experience, and the experiencer–object relation. In this paper, we critically examine naïve realism from two crucial aspects pertaining to it—the question of introspection being basis of naïve realist thesis, and the notion of primacy of perception over non-veridical forms of experiences. We find that there are significant problems which weaken the naïve realist thesis.

     

Synaesthetic perception of colour and visual space in a blind subject: an fMRI case study

In spatial sequence synaesthesia (SSS) ordinal stimuli are perceived as arranged in peripersonal space. Using fMRI, we examined the neural bases of SSS and colour synaesthesia for spoken words in a late-blind synaesthete, JF. He reported days of the week and months of the year as both coloured and spatially ordered in peripersonal space; parts of the days and festivities of the year were spatially ordered but uncoloured. Words that denote time-units and triggered no concurrents were used in a control condition. Both conditions inducing SSS activated the occipito-parietal, infero-frontal and insular cortex. The colour area hOC4v was engaged when the synaesthetic experience included colour. These results confirm the continued recruitment of visual colour cortex in this late-blind synaesthetes. Synaesthesia also involved activation in inferior frontal cortex, which may be related to spatial memory and detection, and in the insula, which might contribute to audiovisual integration related to the processing of inducers and concurrents.     

Horizontal and vertical pseudoneglect in peri- and extrapersonal space

The present study investigates the influence of depth on pseudoneglect in healthy young participants (n = 18) within three-dimensional virtual space, by presenting a variation of the greyscales task and a landmark task, which were specifically matched for stimulus–response compatibility, as well as perceptual factors within and across the tasks. Tasks were presented in different depth locations (peripersonal, extrapersonal) and different orientations (horizontal, vertical) within three-dimensional virtual space, using virtual reality technique. A horizontal leftward bias (pseudoneglect) for both tasks was found, which was stronger in peripersonal than in extrapersonal space. For the vertical condition, an upward bias was observed in the greyscales task, but not in the landmark task. These results support the hypotheses of right hemispheric dominance for visual spatial attention and our study is the first to examine horizontal and vertical orienting biases with the greyscales task in peri- and extrapersonal space. Furthermore, the differences in attentional asymmetries with respect to depth suggest dissociable neural mechanisms for visual attentional processing in near and far space and the lack of significant correlations implies independence of horizontal and vertical stimulus processing.     

Visual Feeling of Presence

Everyday visual experience constantly confronts us with things we can interact with in the real world. We literally feel the outside presence of physical objects in our environment via visual perceptual experience. The visual feeling of presence is a crucial feature of vision that is largely unexplored in the philosophy of perception, and poorly debated in vision neuroscience. The aim of this article is to investigate the feeling of presence. I suggest that visual feeling of presence depends on the visual representation of a very particular spatial relation with the object we interact with: the visual representation of absolute egocentric depth, which is due to stereoscopic vision.     

Serial dependence in position occurs at the time of perception

Observers perceive objects in the world as stable over space and time, even though the visual experience of those objects is often discontinuous and distorted due to masking, occlusion, camouflage, or noise. How are we able to easily and quickly achieve stable perception in spite of this constantly changing visual input? It was previously shown that observers experience serial dependence in the perception of features and objects, an effect that extends up to 15 seconds back in time. Here, we asked whether the visual system utilizes an object’s prior physical location to inform future position assignments in order to maximize location stability of an object over time. To test this, we presented subjects with small targets at random angular locations relative to central fixation in the peripheral visual field. Subjects reported the perceived location of the target on each trial by adjusting a cursor’s position to match its location. Subjects made consistent errors when reporting the perceived position of the target on the current trial, mislocalizing it toward the position of the target in the preceding two trials (Experiment 1). This pull in position perception occurred even when a response was not required on the previous trial (Experiment 2). In addition, we show that serial dependence in perceived position occurs immediately after stimulus presentation, and it is a fast stabilization mechanism that does not require a delay (Experiment 3). This indicates that serial dependence occurs for position representations and facilitates the stable perception of objects in space. Taken together with previous work, our results show that serial dependence occurs at many stages of visual processing, from initial position assignment to object categorization.     

Extended multisensory space in blind cane users

In the present work, we investigated whether an auditory peripersonal space exists around the hand and whether such a space might be extended by a brief tool-use experience or by long-term experience using a tool in everyday life. To this end, we studied audio-tactile integration in the space around the hand and in far space, in blind subjects who regularly used a cane to navigate and in sighted subjects, before and after brief training with the cane. In sighted subjects, auditory peripersonal space was limited to around the hand before tool use, then expanded after tool use, and contracted backward after a resting period. In contrast, in blind subjects, peri-hand space was immediately expanded when they held the cane but was limited to around the hand when they held a short handle. These results suggest that long-term experience with the cane induces a durable extension of the peripersonal space.     

Visuomotor cuing through tool use in unilateral visual neglect

In the present study, the authors examined the effect of tool use in a patient, MP, with neglect of peripersonal space. They found that target detection improved when the patient searched with his arm outstretched, when both visual and motor cues were present. Motor cues (arm outstretched but hidden from view) and visual cues alone (shining a torch on the objects) were less effective. In a final experiment, the authors reported that MP established a better memory for the objects that were searched for when a combined visual and motor cue was present. The authors argue that search was improved by combined visuomotor cuing, which was effective when the action could affect the objects present. Visuomotor cuing also led to stronger memories for searched locations, which reduced any tendency to reexamine positions that had been searched previously. The data are discussed in terms of the interaction between perception and action.     

Seeing Empty Space

Abstract: In this paper I offer an account of a particular variety of perception of absence, namely, visual perception of empty space. In so doing, I aim to make explicit the role that seeing empty space has, implicitly, in Mike Martin's account of the visual field. I suggest we should make sense of the claim that vision has a field—in Martin'ss sense—in terms of our being aware of its limitations or boundaries. I argue that the limits of the visual field are our own sensory limitations, and that we are aware of them as such. Seeing empty space, I argue, involves a structural feature of experience that constitutes our awareness of our visual sensory limitations, and thus, in virtue of which vision has a field.     

Depth perception in virtual reality: distance estimations in peri- and extrapersonal space.

The present study investigated depth perception in virtual environments. Twenty-three participants verbally estimated ten distances between 40 cm and 500 cm in three different virtual environments in two conditions: (1) only one target was presented or (2) ten targets were presented at the same time. Additionally, the presence of a metric aid was varied. A questionnaire assessed subjective ratings about physical complaints (e.g., headache), the experience in the virtual world (e.g., presence), and the experiment itself (self-evaluation of the estimations). Results show that participants underestimate the virtual distances but are able to perceive the distances in the right metric order even when only very simple virtual environments are presented. Furthermore, interindividual differences and intraindividual stabilities can be found among participants, and neither the three different virtual environments nor the metric aid improved depth estimations. Estimation performance is better in peripersonal than in extrapersonal space. In contrast, subjective ratings provide a preferred space: a closed room with visible floor, ceiling, and walls.