Everyday use of the computer mouse extends peripersonal space representation

Auditory and tactile stimuli are integrated within a limited space around the body to form an auditory peripersonal space (APPS). Here we investigate whether the APPS representation around the hand can be extended through the use of a common technological tool such as the computer mouse. When using a mouse, an action occurring in the space around the hand has a distal effect in the space defined by the computer screen; thus, the mouse virtually links near and far space. Does prolonged experience with the mouse durably extend APPS representation to the far space? We examined 16 habitual mouse users to determine whether a sound presented near the right hand or near the computer screen affected reaction times to a tactile target at the hand. When subjects sat in front of the computer, without holding the mouse, they responded faster to tactile stimuli when sounds were presented near the hand rather than near the screen, consistent with a normal segregation of APPS around the hand. In contrast, when subjects either actively used or even passively held the mouse, the difference between the effects of near and far sounds disappeared, thus showing an extension of the APPS toward the far space. This effect was selective for the effector used to operate the mouse: if tactile stimuli were presented on the left hand, rarely used to act upon the mouse, a sound presented near the hand speeded up reactions times when subjects both held and did not hold the mouse in their left hand.     

A supramodal representation of the body surface

The ability to accurately localize both tactile and painful sensations on the body is one of the most important functions of the somatosensory system. Most accounts of localization refer to the systematic spatial relation between skin receptors and cortical neurons. The topographic organization of somatosensory neurons in the brain provides a map of the sensory surface. However, systematic distortions in perceptual localization tasks suggest that localizing a somatosensory stimulus involves more than simply identifying specific active neural populations within a somatotopic map. Thus, perceptual localization may depend on both afferent inputs and other unknown factors. In four experiments, we investigated whether localization biases vary according to the specific skin regions and subset of afferent fibers stimulated. We represented localization errors as a ‘perceptual map’ of skin locations. We compared the perceptual maps of stimuli that activate Aβ (innocuous touch), Aδ (pinprick pain), and C fibers (non-painful heat) on both the hairy and glabrous skin of the left hand. Perceptual maps exhibited systematic distortions that strongly depended on the skin region stimulated. We found systematic distal and radial (i.e., towards the thumb) biases in localization of touch, pain, and heat on the hand dorsum. A less consistent proximal bias was found on the palm. These distortions were independent of the population of afferent fibers stimulated, and also independent of the response modality used to report localization. We argue that these biases are likely to have a central origin, and result from a supramodal representation of the body surface.     

On the nature of near space: effects of tool use and the transition to far space

Many researchers have proposed that the near space immediately surrounding the body is represented differently than more distant space. Indeed, it has often been suggested that near space encompasses that within arm's reach. The present study used a line bisection task in healthy adults to investigate the effects of tool use on space perception, and the nature of the transition between near and far space. Subjects bisected lines at four distances controlled for both veridical and angular size using a laser pointer and a set of sticks. When the laser pointer was used, a left to right shift in bias was observed as stimuli were moved from near to far space. When a tool was used, however, a leftward bias was observed at all distances, similar to that observed with the laser pointer in near space. These results suggest that the tool expanded the range of near space. Additionally, the transition from near to far space was gradual, with no abrupt shift at arm's length (or at any other distance). In contrast to theories describing near space as that within arm's reach, these findings suggest that the representation of near space is less rigid, extending with tool use and gradually transitioning into far space.     

Viewing and feeling touch modulates hand position for reaching

Action requires knowledge of our body location in space. Here we asked if interactions with the external world prior to a reaching action influence how visual location information is used. We investigated if the temporal synchrony between viewing and feeling touch modulates the integration of visual and proprioceptive body location information for action. We manipulated the synchrony between viewing and feeling touch in the Rubber Hand Illusion paradigm prior to participants performing a ballistic reaching task to a visually specified target. When synchronous touch was given, reaching trajectories were significantly shifted compared to asynchronous touch. The direction of this shift suggests that touch influences the encoding of hand position for action. On the basis of this data and previous findings, we propose that the brain uses correlated cues from passive touch and vision to update its own position for action and experience of self-location.     

Local-global processing in Alzheimer's disease: an examination of interference, inhibition and priming

Impairments of memory, praxis, gnosis, language and executive functioning are well documented in Alzheimer's disease (AD). Functions, such as attention, however, have only recently been systematically investigated. We used Navon-type stimuli (large "global" digits composed of smaller "local" digits) to assess 12 AD participants' plus age-matched controls' ability to focus and alter the scale of their spatial attention. In the first experiment, participants responded to either the global or local characters within a block, ignoring characters at the other spatial scale. Healthy young adults (n=12) demonstrated the normal 'global precedence' effect on this task. In contrast, participants with AD and their age-matched controls were significantly faster on the local task than on the global task, suggesting in these groups a 'local precedence' effect. This consisted of both a local advantage and a local-on-global interference effect. In a second experiment, participants searched for designated targets which occurred unpredictably at either the local or global spatial scale. Participants with AD were significantly slower and more error-prone than older controls. In addition, participants with AD showed a greater cost in reaction time (RT) when required to switch spatial scales on consecutive trials, compared to no switch responses at the same spatial scale on consecutive trials. Thus, AD may impair the ability to process global figures, due perhaps to involvement of posterior parietal areas. Further, participants with AD were poor at inhibiting irrelevant stimuli and at inhibiting attentional allocation to an irrelevant spatial scale, which may relate to prefrontal pathology.     

Bare hands and attention: evidence for a tactile representation of the human body

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or “wand”, one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand.

To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB’s left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC’s ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.     

From maps to form to space: Touch and the body schema

Evidence from patients has shown that primary somatosensory representations are plastic, dynamically changing in response to central or peripheral alterations, as well as experience. Furthermore, recent research has also demonstrated that altering body posture results in changes in the perceived sensation and localization of tactile stimuli. Using evidence from behavioral studies with brain-damaged and healthy subjects, as well as functional imaging, we propose that the traditional concept of the body schema should be divided into three components. First are primary somatosensory representations, which are representations of the skin surface that are typically somatotopically organized, and have been shown to change dynamically due to peripheral (usage, amputation, deafferentation) or central (lesion) modifications. Second, we argue for a mapping from a primary somatosensory representation to a secondary representation of body size and shape (body form representation). Finally, we review evidence for a third set of representations that encodes limb position and is used to represent the location of tactile stimuli relative to the subject using external, non-somatotopic reference frames (postural representations).     

Action-specific remapping of peripersonal space

Peripersonal space processing in monkeys’ brain relies on visuo-tactile neurons activated by objects near, not touching, the animal's skin. Multisensory interplay in peripersonal space is now well documented also in humans, in brain damaged patients presenting cross-modal extinction as well as in healthy subjects and typically takes the form of stronger visuo-tactile interactions in peripersonal than far space. We recently showed in healthy humans the existence of a functional link between voluntary object-oriented actions (Grasping) and the multisensory coding of the space around us (as indexed by visual-tactile interaction). Here, we investigated whether performing different actions towards the same object implies differential modulations of peripersonal space. Healthy subjects were asked to either grasp or point towards a target object. In addition, they discriminated whether tactile stimuli were delivered on their right index finger (up), or thumb (down), while ignoring visual distractors. Visuo-tactile interaction was probed in baseline Static conditions (before the movement) and in dynamic conditions (action onset and execution). Results showed that, compared to the Static baseline both actions similarly strengthened visuo-tactile interaction at the action onset, when Grasping and Pointing were kinematically indistinguishable. Crucially, Grasping induced further enhancement than Pointing in the execution phase, i.e., when the two actions kinematically diverged. These findings reveal that performing actions induce a continuous remapping of the multisensory peripersonal space as a function of on-line sensory-motor requirements, thus supporting the hypothesis of a role for peripersonal space in the motor control of voluntary actions.     

Altered visual perception near the hands: A critical review of attentional and neurophysiological models

Visual perception changes as a function of hand proximity. While various theoretical accounts have been offered for this alteration (attentional prioritisation, bimodal cell involvement, detailed evaluation, and magnocellular neuron input enhancement), the current literature lacks consensus on these mechanisms. The purpose of this review, therefore, is to critically review the existing body of literature in light of these distinct theoretical accounts. We find that a growing number of results support the magnocellular (M-cell) enhancement account, and are difficult to reconcile with general attention-based explanations. Despite this key theoretical development in the field, there has been some ambiguity with interpretations offered in recent papers, for example, equating the existing attentional and M-cell based explanations, when in fact they make contrasting predictions. We therefore highlight the differential predictions arising from the distinct theoretical accounts. Importantly, however, we also offer novel perspectives that synthesises the role of attention and neurophysiological mechanisms in understanding altered visual perception near the hands. We envisage that this theoretical development will ensure that the field can progress from documenting behavioural differences, to a consensus on the underlying visual and neurophysiological mechanisms.     

A functional MRI study of preparatory signals for spatial location and objects

We investigated preparatory signals for spatial location and objects in normal observers using functional magnetic resonance imaging (fMRI). Activity for attention-directing cues was separated from activity for subsequent test arrays containing the target stimulus. Subjects were more accurate in discriminating a target face among distracters when they knew in advance its location (spatial directional cue), as compared to when the target could randomly appear at one of two locations (spatial neutral cue), indicating that the spatial cue was used. Spatially specific activations occurred in a region at the intersection of the ventral intraparietal sulcus and transverse occipital sulcus (vIPS-TOS), which showed significantly stronger activation for rightward- than leftward-directing cues, while other fronto-parietal areas were activated by the cue but did not show spatial specificity. In visual cortex, activity was weak or absent in retinotopic occipital regions following attention-directing cues and this activity was not spatially specific.

In a separate task, subject discriminated a target outdoor scene among distracters after the presentation of spatial neutral cues. There was no significant difference in dorsal frontoparietal activity during the face versus scene discrimination task. Also, there was only weak evidence for selective preparatory activity in ventral object-selective regions, although the activation of these regions to the subsequent test array did depend upon which discrimination (face or place) was performed. We conclude first that under certain circumstances, spatial cues that produce strong behavioral effects may modulate parietal-occipital regions in a spatially specific manner without producing similar modulations in retinotopic occipital regions. Second, attentional modulations of object-selective regions in temporal-occipital cortex can occur even though preparatory object-selective modulations of those regions are absent or weak.     

Transcranial electric stimulation optimizes the balance of visual attention across space

ObjectiveTranscranial direct current stimulation (tDCS) provides a way to modulate spatial attention by enhancing the ratio of neural activity between the left and right hemispheres, with a potential benefit for the rehabilitation of visual neglect.MethodsWe tested the effect of bilateral tDCS in healthy individuals performing a visual detection task. This protocol consists in the positioning of the anode and cathode on mirror positions over the left and right parietal areas. The stimulation was repeated over three days to maximize the chance to observe a bias to the hemispace controlateral to the anode.ResultsCompared to a sham treatment, left anodal – right cathodal stimulation enhanced attention across the full range of space, since the first day with no build-up effect on the next days, and modified the balance of left-right omissions when stimuli appeared at the same time.ConclusionBilateral tDCS improved detection in both visual fields, with no privileged processing of one side, except when concurrent stimuli were presented. The results provide partial support to the hemispheric rivalry hypothesis.SignificanceThe technique has the potential to boost attention in neglect patients but should be used as an adjuvant rather than as an alternative to functional rehabilitation.     

The neural representation of face space dimensions

Functional neural imaging studies have identified a network of brain areas that are more active to faces than to other objects. However, it remains largely unclear how these areas encode individual facial identity. To investigate the neural representations of facial identity, we constructed a multidimensional face space structure, whose dimensions were derived from geometric information of faces using the Principal Component Analysis (PCA). Using fMRI, we recorded participants' neural responses when viewing blocks of faces that differed only on one dimension within a block. Although the response magnitudes to different blocks of faces did not differ in a univariate analysis, multi-voxel pattern analysis revealed distinct patterns related to different face space dimensions in brain areas that have a higher response magnitude to faces than to other objects. The results indicate that dimensions of the face space are encoded in the face-selective brain areas in a spatially distributed way.     

The effect of a cognitive task on the postural control of dyslexic children

We explore the influence of a secondary cognitive task on concurrent postural control in dyslexic children. Seventeen children with dyslexia (DYS) were compared with thirteen non-dyslexic children (NDYS). Postural control was recorded in Standard Romberg (SR) and Tandem Romberg (TR) conditions while children, in separate sessions, have to fixate on a target and name simple objects appearing consecutively on a computer screen. The surface, the length and the mean speed of the center of pressure were analyzed; the percentage of correct responses to the cognitive task was also measured. DYS are significantly more unstable than NDYS. The secondary cognitive task significantly decreases the postural stability in DYS only. For both children postural performances in the TR condition is significantly worse than in the SR condition. The percentage of wrong responses to the cognitive task is significantly higher in DYS. Postural instability observed in DYS supports the hypothesis that there is a deficit of automatic integration of visual information and postural control in these children. This result is in line with the U-shaped non linear model showing that a secondary task performed during a postural task leads to an impaired postural stability probably due to focus attention on the cognitive task.     

Crossmodal congruency measures of lateral distance effects on the rubber hand illusion

Body ownership for an artificial hand and the perceived position of one's own hand can be manipulated in the so-called rubber hand illusion. To induce this illusion, typically an artificial hand is placed next to the participant's body and stroked in synchrony with the real hand, which is hidden from view. Our first aim was to test if the crossmodal congruency task could be used to obtain a measure for the strength of body ownership in the rubber hand illusion. In this speeded location discrimination task participants responded to tactile targets presented to their index or middle finger, while trying to ignore irrelevant visual distracters placed on the artificial hand either on the congruent finger or on the incongruent finger. The difference between performance on congruent and incongruent trials (crossmodal congruency effect, CCE) indicates the amount of multisensory interactions between tactile targets and visual distracters. In order to investigate if changes in body ownership influence the CCE, we manipulated ownership for an artificial hand by synchronous and asynchronous stroking before the crossmodal congruency task (blocked design) in Experiment 1 and during the crossmodal congruency task (interleaved trial-by-trial design) in Experiment 2. Modulations of the CCE by ownership for an artificial hand were apparent in the interleaved trial-by-trial design. These findings suggest that the CCE can be used as an objective measure for body ownership. Secondly, we tested the hypothesis that the lateral spatial distance between the real hand and artificial hand limits the rubber hand illusion. We found no lateral spatial limits for the rubber hand illusion created by synchronous stroking within reaching distances. In conclusion, the sense of ownership seems to be related to modulations of multisensory interactions possibly through peripersonal space mechanisms, and these modulations do not appear to be limited by an increase in distance between artificial hand and real hand.     

Visual search patterns in neglect: Comparison of peripersonal and extrapersonal space

Previous studies of visual search patterns in visuospatial neglect have analyzed shifts of attention during search tasks using eye tracking technology and verbal reports. The purpose of the present study was to replicate and extend upon reported parameters of visual scanning patterns of neglect patients in peripersonal space (within arms reach) and to examine whether similar patterns of visual search are also apparent in extrapersonal space (beyond arms reach). Using a simple verbal visual search and target detection paradigm right-hemisphere stroke participants, with and without neglect, and healthy older volunteers named targets on scanning sheets placed in peripersonal and extrapersonal space. The healthy controls and right-hemisphere stroke group without neglect showed similar ‘reading’ type strategies, while the neglect group displayed an unsystematic search pattern, during search in both peripersonal and extrapersonal space. Group comparisons of search parameters support the presence of multiple cognitive deficits affecting the complex visual search patterns of neglect patients, including a rightward attentional bias, a reduced spatial scale of attention (local processing bias), and a deficit of working memory affecting both near and far space search. Ventral visual stream damage and neglect, however, were related to slower target report rate and more misidentification errors in extrapersonal space. The ease of administration of this verbal target detection task in both peripersonal and extrapersonal space, and the relationship of the measures produced to theorized attentional and executive deficits in neglect, provide impetus for further research on the severity and independence of individual scanning deficits in neglect.     

Attention, motor control and motor imagery in schizophrenia: implications for the role of the parietal cortex

Many recent models of schizophrenia have attempted to explain the so-called first-rank symptoms in terms of a breakdown in the self-monitoring of thoughts and behaviours. These models have focused on the most common symptom of schizophrenia auditory hallucinations—suggesting that they may represent disordered self-monitoring of internal speech. As such, much attention has been given to the role of the temporal and frontal cortices in the clinical presentation of patients with schizophrenia. In this review, we examine the role of the posterior parietal cortex (PPC) in schizophrenia within the context of recent models of self-monitoring deficits in these patients. Attentional dysfunctions and certain impairments of motor control and motor imagery all point towards the involvement of the parietal cortex in the disorder. In particular, we suggest that patients experiencing passivity phenomena (e.g., delusions of control) may have particular impairments of parietal function related to poor utilisation of forward models of intended actions. We also present a novel hypothesis that suggests differential impairments of the left and right parietal cortices in schizophrenia may help explain many of the first-rank symptoms of the disorder.     

Neural bases of peri-hand space plasticity through tool-use: Insights from a combined computational-experimental approach

Visual peripersonal space (i.e., the space immediately surrounding the body) is represented by multimodal neurons integrating tactile stimuli applied on a body part with visual stimuli delivered near the same body part, e.g., the hand. Tool use may modify the boundaries of the peri-hand area, where vision and touch are integrated. The neural mechanisms underlying such plasticity have not been yet identified. To this aim, neural network modelling may be integrated with experimental research. In the present work, we pursued two main objectives: (i) using an artificial neural network to postulate some physiological mechanisms for peri-hand space plasticity in order to account for in-vivo data; (ii) validating model predictions with an ad-hoc behavioural experiment on an extinction patient.The model assumes that the modification of peri-hand space arises from a Hebbian growing of visual synapses converging into the multimodal area, which extends the visual receptive field (RF) of the peripersonal bimodal neurons. Under this hypothesis, the model is able to interpret and explain controversial results in the current literature, showing how different tool-use tasks during the learning phase result in different re-sizing effects of the peri-hand space. Importantly, the model also implies that, after tool-use, a far visual stimulus acts as a near one, independently of whether the tool is present or absent in the subject's hand. This prediction has been validated by an in-vivo experiment on a right brain-damaged patient suffering from visual-tactile extinction. This study demonstrates how neural network modelling may integrate with experimental studies, by generating new predictions and suggesting novel experiments to investigate cognitive processes.     

Tri-modal integration of visual, tactile and auditory signals for the perception of sequences of events

We investigated the interactions between visual, tactile and auditory sensory signals for the perception of sequences of events. Sequences of flashes, taps and beeps were presented simultaneously. For each session, subjects were instructed to count the number of events presented in one modality (Target) and to ignore the stimuli presented in the other modalities (Background). The number of events presented in the background sequence could differ from the number of events in the target sequence. For each session, we quantified the Background-evoked bias by comparing subjects' responses with and without Background (Target presented alone). Nine combinations between vision, touch and audition were tested. In each session but two, the Background significantly biased the Target. Vision was the most susceptible to Background-evoked bias and the least efficient in biasing the other two modalities. By contrast, audition was the least susceptible to Background-evoked bias and the most efficient in biasing the other two modalities. These differences were strongly correlated to the relative reliability of each modality. In line with this, the evoked biases were larger when the Background consisted of two instead of only one modality. These results show that for the perception of sequences of events: (1) vision, touch and audition are automatically integrated; (2) the respective contributions of the three modalities to the integrated percept differ; (3) the relative contribution of each modality depends on its relative reliability (1/variability); (4) task-irrelevant stimuli have more weight when presented in two rather than only one modality.