Visual bias of unseen hand position with a mirror: spatial and temporal factors

Two experiments examined the integration of visual and proprioceptive information concerning the location of an unseen hand, using a mirror positioned along the midsagittal plane. In experiment 1, participants tapped the fingers of both hands in synchrony, while viewing the mirror-reflection of their left hand. After 6 s, participants made reaching movements to a target with their unseen right hand behind the mirror. Reaches were accurate when visually and proprioceptively specified hand positions were congruent prior to the reach, but significantly biased by vision when the visual location conflicted with the real location. This effect was independent of the target location and depended strongly upon the relative position of the mirror-reflected hand. In experiment 2, participants made reaching movements following 4, 8, or 12 s active visuomotor or passive visual exposure to the mirror, or following passive exposure without the mirror. Reaching was biased more by the visual location following active visuomotor compared to passive visual exposure, and this bias increased with the duration of visual exposure. These results suggest that the felt position of the hand depends upon an integrated, weighted sum of visual and proprioceptive information. Visual information is weighted more strongly under active visuomotor than passive visual exposure, and with increasing exposure duration to the mirror reflected hand.     

Evidence from unilateral visual neglect

Abstract

We report the case of a patient, JR, who manifests left neglect in reading single words but right neglect in copying and drawing tasks. We show that left neglect is not confined to reading, but is also found in picture naming and in line-bisection tasks. In each of these tasks, JR neglected the right side of multiple stimulus displays. We show that neglect is manifest on the left or right side as a function of whether visual stimuli are encoded as parts of a single perceptual object or as separate perceptual objects. JR's left neglect in reading words can be switched to right neglect by having him read aloud all the letters in the string. He also showed left neglect in a search task in which the visual elements configure into a coherent object, but right neglect in displays using similar elements which did not so configure. JR's case demonstrates that the visual system employs separate representations to encode the spatial relations between the parts of single objects (within-object spatial coding) and between separate perceptual objects (between-object spatial coding). These two forms of spatial representation are encoded independently and in parallel.     

Unilateral neglect: a theory of proprioceptive space of a stimulus as determined by the cerebellar component of motor efference copy (and is autism a special case of neglect)

Unilateral neglect is a devastating condition, which manifests as a loss of a person's spatial awareness opposite the damaged side of the brain. It challenges our conception of the seat of the soul and its explanation is at the heart of the mind-body problem. A heuristic definition of the dorsal stream of a modality is here based on the categorization of parietal networks by the cerebellar component of motor efference copy. Taking this premise, the proprioceptive space of a stimulus is established as a concept in this paper. It is proposed that unilateral neglect is typically a dysfunction of proprioceptive space of a stimulus associated with lesions of the dorsal stream. Furthermore, most experimental findings of unilateral visual neglect (and by extrapolation, other sensory modalities), can be explained by two developmental mechanisms by which the proprioceptive space of a stimulus is encoded in the parietal cortex. Its right and left hemisphere representation can be dissociated from the hemifield of presentation of perceptual information, such that the left hemifield can have a left hemisphere representation through callosal connections and likewise, the right hemifield can have a right hemifield representation. The processing of a sensory stimulus in either parietal hemisphere is dynamically determined as shown by experimental modulation of performance. A theory of historical precedence will provide a developmental background to the organization of proprioceptive space and will invoke separate models according to specified terms of engagement. A model based on the expansion and contraction of the proprioceptive space of a stimulus as a gradient across both hemispheres and modulated by concurrent proprioceptive state will be differentiated from a model that is non-graduating but competitive and lacks such modulation. In other words, the dorsal representation of a sensory stimulus in the former case is shared to a varying degree by the two parietal hemispheres, whereas in the latter case the representation of left and right aspects of the same object stimulus is strictly divided between the two hemispheres. A further hypothesis of a dorsoventral gradient of the peripheral and foveal components of proprioceptive space characterizing dorsal stream networks will predict the double dissociation revealed by experimental paradigms. It will explain why some patients show neglect only in foveal while others only in peripheral vision. The paper proposes to unify neglect, extinction and optic ataxia on the one hand, and spatial and object-based neglect on the other hand, under a singularly proficient paradigmatic structure. A binding model as described is a component theory that acknowledges how the involved pathways or transitional zones in a pathway may contribute to a differential clinical picture as one progresses from posterior to anterior parietal cortex. Finally, a brief discussion is given on how autistic subjects neglect spatial cues and the inability of a spatial cognitive transformation underlies the impairments postulated for 'a theory of mind'.     

The mental number line modulates visual cortical excitability

The mental number line has been shown to exert an influence on the visuo-spatial allocation of attention, with presentation of numbers from the low and high ends of the mental number line inducing covert shifts of spatial attention to the left and right side of visual space, respectively. In the present study we used transcranial magnetic stimulation (TMS) to investigate whether this attentional modulation influences excitability of the early visual cortex. By using the phosphene threshold as a measure of visual cortical excitability, we show (in 10 subjects) that number priming modulates excitability of the early visual cortex in a topographic fashion: low numbers, associated with left side of space, increase the excitability of the right early visual cortex (the stimulation of which induces phosphenes in the left hemifield) and decrease the excitability of the left early visual cortex (the stimulation of which induces phosphenes in the right hemifield). The opposite pattern of results was observed for high numbers. Our results suggest that the attentional shifts induced by the mental number line are manifested at the earliest cortical stages of visual processing.     

Disorders of visuospatial orientation in the frontal plane in patients with visual neglect following right or left parietal lesions

Current models of spatial neglect focus on deficits in the patients’ horizontal or midsagittal plane. However, other evidence suggests that patients with temporoparietal lesions centered on the parieto-insular-vestibular cortex show disturbed spatial perception of the subjective visual vertical and oblique orientation discrimination in another spatial plane, the frontal plane. As the relationship between neglect and spatial orientation deficits is unclear, we examined how patients with and without visual neglect perform visuospatial tasks in the roll plane and how their performance is related to neglect. Thirteen patients with predominantly right parietal lesions and left-sided neglect, 14 control patients without neglect after right-hemispheric cerebral lesions (RBD-controls), 11 patients without neglect after left-hemispheric lesions (LBD-controls), 3 patients with right-sided neglect after left parietal lesions, and 12 normal subjects were investigated. Constant errors and difference thresholds were measured with a PC-based system when the subjects had to adjust a luminous line to their subjective visual vertical, subjective visual horizontal, and in relation to an obliquely oriented reference line. Subjects were oriented with their head and body earth-vertical while sitting in a chair in total darkness. Patients with left-sided as well as those with right-sided neglect showed a significant, in most cases contraversive, tilt of the three spatial orientations (about 5° counterclockwise in the left neglect group and 5.5°–8.5° clockwise in the right neglect group). In contrast, the two patient groups without neglect as well as the normal subjects showed nearly perfect visuospatial judgements with constant errors of less than 0.8°. Difference thresholds were significantly elevated in patients with left neglect and in two of three patients with right-sided neglect, whereas normal control subjects and both control patient groups without neglect performed indistinguishably, having thresholds of one-tenth of those of the neglect patients. Tilt of all three spatial axes was significantly related to the severity of neglect (mean r for unsigned errors, 0.74; for difference thresholds, 0.40), indicating a significant contribution to the symptomatology of left and right spatial neglect. These results indicate a close although not necessarily causal link between spatial orientation deficits in the frontal plane and hemispatial neglect in patients with left or right parietal lesions, surpassing the well-documented impairments of these patients in the horizontal plane. Received: 1 August 1997 / Accepted: 1 April 1998     

Selective visual neglect in right brain damaged patients with splenial interhemispheric disconnection

Left unilateral neglect is frequently reported after right hemispheric lesions of the middle cerebral artery (MCA) damaging the parietal–frontal cortical–subcortical network subserving space representation and awareness. However, accumulating evidence shows that neglect can also follow lesions of the posterior cerebral artery (PCA) that do not directly affect this parietal–frontal network. Surgical studies in the monkeys have demonstrated that complete callosal resection combined with lesion of the right optic tract entirely deprives the right hemisphere of visual inputs from the left hemispace provoking severe left unilateral neglect. Here, through the detailed study of two patients we show, for the first time, that PCA lesions selectively affecting the splenium of the corpus callosum and the adjacent right primary visual cortex provoke severe neglect selectively restricted to the visual domain. No trace of personal, motor or representational-imagery neglect was found. Also at variance with previous case studies in which neglect followed lesion of the trunk or the genu of the corpus callosum, no restriction of neglect to tasks performed with the right hand, no left hemispatial limb akinesia, no tactile extinction for the left hand and no tactile anomia for stimuli explored with the left hand were observed. These findings demonstrate that brain lesions depriving intact parietal and frontal attentional areas from specific sensory inputs can yield spatial neglect limited to specific sensory modalities or sectors of space.     

Can mirrors alleviate visual hemineglect?

Summary Following right hemisphere stroke, many patients display an indifference to objects and events in the left side of the world (‘neglect’). Here, we describe a new technique that might help accelerate recovery from neglect. The patient sits at a table and a mirror is propped vertically on the patient's right side in the parasagittal plane, so that when the patient rotates his head rightward and looks into the mirror, he sees the neglected side of the world reflected in the mirror. Our question was: since the sensory information was now coming from the non-neglected left side, would this somehow make him overcome the neglect? In pilot experiments, two types of responses were seen: (a) In one subset of patients the presence of the mirror seemed to enhance the patients' awareness of the neglected field, so that they reached correctly for an object that was shown in the neglected field. Will repeated practice with this task accelerate recovery from neglect? (b) The second group of patients kept reaching into the mirror to grasp the reflection or kept groping behind the mirror (‘mirror agnosia’). If the mirror was placed in the coronal position and the object placed behind their head, then some of these patients (from group B) reached correctly for the object. Quite apart from its obvious theoretical implications, we believe this technique might provide a new approach for the treatment of visual hemineglect.     

Assessing the effect of verbal working memory load on visuo-spatial exogenous orienting

The aim of this study was to assess whether or not loading verbal working memory (WM) affects the exogenous orienting of visuo-spatial attention. Visuo-spatial exogenous orienting was measured under low/high WM load conditions by means of an orthogonal spatial cuing paradigm. Participants had to discriminate the elevation (up versus down) of a visual target preceded by a peripheral spatially-nonpredictive visual cue presented on either the left or right. This elevation discrimination task was carried out in-between a verbal WM test, in which a sequence of six digits (to be remembered for report at the end of the trial) was presented either in numerically ascending or random order (i.e., low and high load conditions, respectively). Participants made significantly more recall errors in the high load than in the low load condition, demonstrating that the verbal WM load manipulation had been effective. However, WM load did not interact with visuo-spatial exogenous orienting (i.e., comparable cuing effects were reported in both load conditions). These results are consistent with the view that peripheral visual onsets automatically capture spatial attention, regardless any concurrent increase of verbal WM load.     

Impaired orienting of attention in left unilateral neglect: a componential analysis

Twenty-six patients suffering from damage to the right side of the brain, 19 of whom exhibited signs of left neglect, as well as 32 matched controls, ran 3 spatial cuing tasks. Patients were also tested with 2 cancellation tests, a line-bisection test, the copy of a complex drawing, and a visual extinction procedure. Results first showed correlations between extinction and cancellation tests performance on one hand, and between line bisection and copy on the other hand. Second, results demonstrated that an engagement deficit toward contralesional targets appeared to be the most striking feature of neglect, and the engagement score was correlated with the cancellation score and extinction. Most patients with neglect also presented a deficit in disengagement, a deficit of inhibition of return, and probably a deficit of alertness. Deficits in engagement and in disengagement, as well as poor scores in cancellation tests, seemed to be related with posterior cortical and subcortical lesions. Most important, even if an endogenous deficit (frequently related with a thalamic lesion) could aggravate the neglect behavior, neglect syndrome was mainly explained by a deficit of exogenous attention.     

The role of the right temporoparietal junction in intersensory conflict: detection or resolution?

The right temporoparietal junction (rTPJ) is a polysensory cortical area that plays a key role in perception and awareness. Neuroimaging evidence shows activation of rTPJ in intersensory and sensorimotor conflict situations, but it remains unclear whether this activity reflects detection or resolution of such conflicts. To address this question, we manipulated the relationship between touch and vision using the so-called mirror-box illusion. Participants’ hands lay on either side of a mirror, which occluded their left hand and reflected their right hand, but created the illusion that they were looking directly at their left hand. The experimenter simultaneously touched either the middle (D3) or the ring finger (D4) of each hand. Participants judged, which finger was touched on their occluded left hand. The visual stimulus corresponding to the touch on the right hand was therefore either congruent (same finger as touch) or incongruent (different finger from touch) with the task-relevant touch on the left hand. Single-pulse transcranial magnetic stimulation (TMS) was delivered to the rTPJ immediately after touch. Accuracy in localizing the left touch was worse for D4 than for D3, particularly when visual stimulation was incongruent. However, following TMS, accuracy improved selectively for D4 in incongruent trials, suggesting that the effects of the conflicting visual information were reduced. These findings suggest a role of rTPJ in detecting, rather than resolving, intersensory conflict.     

Ocular exploration of space as a function of neck proprioceptive and vestibular input — observations in normal subjects and patients with spatial neglect after parietal lesions

We recently argued that the specific compensation of spatial neglect by manipulating neck proprioceptive and vestibular input is due to a central correction of the disturbed neural transformation process converting the afferent input coordinates from the peripheral sensory organs into a central representation of egocentric space. Both types of stimulation were proposed to induce a reorientation of the deviated or distorted egocentric spatial reference frame. The aim of the present study was to observe this process of reorientation under a condition in which no visual stimulus can attract the subject's attention and thus influence exploration behaviour from outside. We recorded eye movements of normal subjects and of three patients with spatial neglect after right parietal lesions while searching for a non-existent target in complete darkness. It was assumed that the area of the outer space that subjects spontaneously explore under this condition is a direct function of the subject's representation of egocentric space. Ocular space exploration was biased and confined almost entirely to the right side of the midsagittal plane in patients with neglect. This spatial distribution of exploratory eye movements changed remarkably with left-sided neck muscle vibration as well as with left-sided vestibular stimulation using ice water calorics. The spatial area of exploration was significantly enlarged to the contralesional side and the exploration maximum shifted in the same direction. Whereas with both types of stimulation space exploration of patients with neglect was similar to that of normal subjects when not being stimulated, neck proprioceptive and vestibular stimulation in normal subjects induced a quasi neglect-like exploration pattern, i.e. a bias to one side of the objective midsagittal plane. If ocular space exploration was, however, related to the subjectively perceived position of the midsagittal plane in space, eye movements were symmetrically distributed and carried out to both sides of subjective straight ahead in all experimental conditions, in normal subjects as well as in patients with neglect. The present results support the above hypothesis and indicate that neck proprioceptive as well as vestibular input directly contribute to the computation of the subject's central representation of egocentric space used for localizing body orientation and for guiding motor behaviour in space.     

Visual neglect: Is there a relationship between impaired spatial working memory and re-cancellation?

In visual search tasks, neglect patients tend to explore and repeatedly re-cancel stimuli on the ipsilesional side, as if they did not realize that they had previously examined the rightward locations favoured by their lateral bias. The aim of this study was to explore the hypothesis that a spatial working memory deficit explains these ipsilesional re-cancellation errors in neglect patients. For the first time, we evaluated spatial working memory and re-cancellation through separate and independent tasks in a group of patients with right hemisphere damage and a diagnosis of left neglect. Results showed impaired spatial working memory in neglect patients. Compared to the control group, neglect patients cancelled fewer targets and made more re-cancellations both on the left side and on the right side. The spatial working memory deficit appears to be related to re-cancellations, but only for some neglect patients. Alternative interpretations of re-exploration of space are discussed.     

Exploring the syndrome of spatial unilateral neglect through an illusion of length

Both a neuropsychological syndrome (unilateral spatial neglect) and a visual illusion of length (the Brentano version of the Müller-Lyer illusion) bring about a misjudgement of the subjective centre of a horizontal line, with a unilateral shift. In experiment 1 we investigated, in patients with left unilateral neglect, illusory effects of horizontal length, with the aim of exploring the functional and neural basis of horizontal space perception, and the role of visual processing in shaping the patients' bisection performance. Fourteen right-brain-damaged patients with left spatial unilateral neglect, seven with and seven without left visual half-field deficits (assessed by confrontation, perimetry, and visual event-related potentials), entered this study. Two conditions of manual line bisection were assessed: setting the mid-point of a horizontal line, and of the shaft of the Brentano-Müller-Lyer illusion, with either a left- or a right-sided expansion. Both groups of patients set the subjective midpoint to the right of the objective centre of the line, consistent with the presence of left neglect. Patients with neglect and left hemianopia showed no illusory effects and a greater bisection error. The effects of the illusion, by contrast, were fully present in neglect patients without hemianopia, in both illusory conditions, adding to, or subtracting from, the rightward bisection bias. Anatomoclinical correlations revealed an association of damage to the occipital regions with the lack of illusory effects. Conversely, more anterior damage, sparing these regions, did not disrupt the illusion, revealing a dissociation between visual and spatial processing of extension. These findings suggest that processing of the Müller-Lyer illusion of length is likely to occur in the occipital cortex, at a retinotopic level of representation. In neglect patients with left homonymous hemianopia the visual deficit adds to the spatial bias, yielding a greater error in line bisection, but not in other visual exploratory tasks, such as cancellation, where the contribution of retinotopic frames is likely to be comparatively minor. Experiment 2 showed preserved illusory effects in patients with homonymous visual field defects without spatial unilateral neglect, suggesting that preserved spatial processing may compensate for unilateral visual field defects.     

The precision of proprioceptive position sense

The purpose of this study was to determine the precision of proprioceptive localization of the hand in humans. We derived spatial probability distributions which describe the precision of localization on the basis of three different sources of information: proprioceptive information about the left hand, proprioceptive information about the right hand, and visual information. In the experiment subjects were seated at a table and had to perform three different position-matching tasks. In each task, the position of a target and the position of an indicator were available in a different combination of two of these three sources of information. From the spatial distributions of indicated positions in these three conditions, we derived spatial probability distributions for proprioceptive localization of the two hands and for visual localization. For proprioception we found that localization in the radial direction with respect to the shoulder is more precise than localization in the azimuthal direction. The distributions for proprioceptive localization also suggest that hand positions closer to the shoulder are localized more precisely than positions further away. These patterns can be understood from the geometry of the arm. In addition, the variability in the indicated positions suggests that the shoulder and elbow angles are known to the central nervous system with a precision of 0.6–1.1°. This is a considerably better precision than the values reported in studies on perception of these angles. This implies that joint angles, or quantities equivalent to them, are represented in the central nervous system more precisely than they are consciously perceived. For visual localization we found that localization in the azimuthal direction with respect to the cyclopean eye is more precise than localization in the radial direction. The precision of the perception of visual direction is of the order of 0.2–0.6°. Received: 3 July 1997 / Accepted: 27 March 1998     

Frequency tagging of steady‐state evoked potentials to explore the crossmodal links in spatial attention between vision and touch

The sustained periodic modulation of a stimulus induces an entrainment of cortical neurons responding to the stimulus, appearing as a steady‐state evoked potential (SS‐EP) in the EEG frequency spectrum. Here, we used frequency tagging of SS‐EPs to study the crossmodal links in spatial attention between touch and vision. We hypothesized that a visual stimulus approaching the left or right hand orients spatial attention toward the approached hand, and thereby enhances the processing of vibrotactile input originating from that hand. Twenty‐five subjects took part in the experiment: 16‐s trains of vibrotactile stimuli (4.2 and 7.2 Hz) were applied simultaneously to the left and right hand, concomitantly with a punctate visual stimulus blinking at 9.8 Hz. The visual stimulus was approached toward the left or right hand. The hands were either uncrossed (left and right hands to the left and right of the participant) or crossed (left and right hands to the right and left of the participant). The vibrotactile stimuli elicited two distinct SS‐EPs with scalp topographies compatible with activity in the contralateral primary somatosensory cortex. The visual stimulus elicited a third SS‐EP with a topography compatible with activity in visual areas. When the visual stimulus was over one of the hands, the amplitude of the vibrotactile SS‐EP elicited by stimulation of that hand was enhanced, regardless of whether the hands were uncrossed or crossed. This demonstrates a crossmodal effect of spatial attention between vision and touch, integrating proprioceptive and/or visual information to map the position of the limbs in external space.     

The Brentano illusion influences goal-directed movements of the left and right hand to the same extent

Recently, Gonzalez et al. (J Neurophys 95:3496–3501, 2006) reported that movements with the left hand are more susceptible to visual size illusions than are those with the right hand. We hypothesized that this might be because proprioceptive information about the position of the left hand is less precise. If so, the difference between the hands should become clearer when vision of the hand is removed so that subjects can only rely on proprioception to locate their hand. We tested whether this was so by letting right-handed subjects make open-loop pointing movements within an illusory context with and without vision of their hand. On average, the illusion influenced the left and the right hand to the same extent, irrespective of the visibility of the hand. There were some systematic differences between the hands within certain regions of space, but these were not consistent across subjects. We conclude that there is no fundamental difference between the hands in susceptibility to the Brentano illusion.     

Effects of illusory spatial anisometry in unilateral neglect

Patients with visuospatial neglect tend to underestimate horizontal magnitudes in contralesional space. It has been recently hypothesised that this behaviour might be due to anisometry of space perception, by which horizontal stimuli would be progressively underestimated proceeding from the ipsi towards the contralesional side of space. We investigated the effects of modulating space perception through the Oppel-Kundt illusion (i.e. a filled space is perceived as more expanded than an empty space) on the behaviour of 28 neglect patients and 28 normal subjects. The two groups bisected lines on backgrounds of vertical lines evenly spaced or unevenly spaced, with distances which decreased progressively from one side of the page to the other. On the same backgrounds, they extended segments to the left or to the right so as to double them. Patients also had to cancel targets, the density of which was evenly distributed or horizontally increased from one side of the page to the other. Both groups were prone to the illusion. Neglect bias was modulated by the illusion in the expected direction. It was reduced when the illusion induced a perceptual distortion opposite to that thought to underlie neglect. On bisection and cancellation tasks, illusory effects were greater in patients with higher values of response bias on the Milner Landmark task. These findings, taken together with patients anatomical data, suggest that a modulation of neglect through a visual illusion can normally be induced in patients with relatively intact visual input processes.     

Interaction between gaze and visual and proprioceptive position judgements

There is considerable evidence that targets for action are represented in a dynamic gaze-centered frame of reference, such that each gaze shift requires an internal updating of the target. Here, we investigated the effect of eye movements on the spatial representation of targets used for position judgements. Participants had their hand passively placed to a location, and then judged whether this location was left or right of a remembered visual or remembered proprioceptive target, while gaze direction was varied. Estimates of position of the remembered targets relative to the unseen position of the hand were assessed with an adaptive psychophysical procedure. These positional judgements significantly varied relative to gaze for both remembered visual and remembered proprioceptive targets. Our results suggest that relative target positions may also be represented in eye-centered coordinates. This implies similar spatial reference frames for action control and space perception when positions are coded relative to the hand.     

Interactions between perception and action programming: Evidence from visual extinction and optic ataxia

We report a series of 7 experiments examining the interaction between visual perception and action programming, contrasting 2 neuropsychological cases: a case of visual extinction and a case with extinction and optic ataxia. The patients had to make pointing responses to left and right locations, whilst identifying briefly presented shapes. Different patterns of performance emerged with the two cases. The patient with “pure” extinction (i.e., extinction without optic ataxia) showed dramatic effects of action programming on perceptual report. Programming an action to the ipsilesional side increased extinction (on 2-item trials) and tended to induce neglect (on 1-item trials); this was ameliorated when the action was programmed to the contralesional side. Separable effects of using the contralesional hand and pointing to the contralesional side were apparent. In contrast, the optic ataxic patient showed few effects of congruency between the visual stimulus and the action, but extinction when an action was programmed. This effect was particularly marked when actions had to be made to peripheral locations, suggesting that it reflected reduced resources to stimuli. These effects all occurred using stimulus exposures that were completed well before actions were effected. The data demonstrate interactions between action programming and visual perception. Programming an action to the affected side with the contralesional limb reduces “pure” extinction because attention is coupled to the end point of the action. However, in a patient with deficient visuo-motor coupling (optic ataxia), programming an action can increase a spatial deficit by recruiting resources away from perceptual processing. The implications for models of perception and action are discussed.     

Number magnitude to finger mapping is disembodied and topological

It has been shown that humans associate fingers with numbers because finger counting strategies interact with numerical judgements. At the same time, there is evidence that there is a relation between number magnitude and space as small to large numbers seem to be represented from left to right. In the present study, we investigated whether number magnitude to finger mapping is embodied (related to the order of fingers on the hand) or disembodied (spatial). We let healthy human volunteers name random numbers between 1 and 30, while simultaneously tapping a random finger. Either the hands were placed directly next to each other, 30 cm apart, or the hands were crossed such that the left hand was on the right side of the body mid-line. The results show that naming a smaller number than the previous one was associated with tapping a finger to the left of the previously tapped finger. This shows that there is a spatial (disembodied) mapping between number magnitude and fingers. Furthermore, we show that this mapping is topological rather than metrically scaled.