Dynamic size-change of peri-hand space following tool-use:
determinants and spatial characteristics revealed through
cross-modal extinction
A. Farnè*, A. Serino & E. Làdavas
Dipartimento di Psicologia, Università degli Studi di Bologna, Italy
Centro Studi e Ricerche in Neuroscienze Cognitive, Cesena, Italy
* now at Espace et Action, UMR-S 534, INSERM and Université Claude Bernard, 16 avenue
Lépine, Case 13, 69676 Bron France
CORRESPONDENCE to Dr. Alessandro Farnè
UMR-S 534 - 16, av. Doyen Lépine
69676 Bron cedex, France
Phone: +33 (4) 7291 3412
Fax: +33 (4) 7291 3401
E-mail: farne@lyon.inserm.fr
Running head: Tool-use shapes multisensory space
1
Abstract
In human and non human primates, evidence has been reported supporting the idea that near
peripersonal space is represented through integrated multisensory processing. In humans, the
interaction between near peripersonal space representation and action execution can be
revealed in brain damaged patients through the use of tools that, by extending the reachable
space, modify the strength of visual-tactile extinction, thus showing that tool-mediated actions
modify the multisensory coding of near peripersonal space. For example, following the use of
a rake to retrieve distant, otherwise non reachable objects, the peri-hand multisensory area has
been documented to extend to include the distal part of a rake (Farnè and Làdavas 2000). The
re-sizing of peri-hand space seems to be selective for tool-use, as directional motor activity
alone (i.e., pointing without the tool) and visual/proprioceptive experience alone (protracted
passive exposure to the tool) does not vary the extent of the visual-tactile peri-hand space
(Farnè et al., 2005a). Moreover, the amount of dynamic re-sizing varies with the length of the
used tool, and is specifically centred on the functionally relevant part of the tool (Farnè et al,
2005b). Here, besides reviewing and discussing these results, we report new evidence, based
on a single-case study, supporting the idea that dynamic re-sizing of peri-hand space consists
of a real spatial extension of the visual-tactile integrative area along the tool axis.
2
Introduction
In the last decade, the physiological and behavioural study of multisensory integration
processes and their neural bases has seen an exponential growth (Spence and Driver, 2004;
Calvert et al., 2004). Since Iriki and colleagues’ seminal paper (Iriki et al., 1996), this growth
also propelled several investigations of the physiological and behavioural effects exerted by
the use of tools upon the multisensory representation of near peripersonal space (Làdavas,
2002; Maravita and Iriki, 2004). Iriki and colleagues reported that visual receptive fields
(RF) of monkey’s parietal visual-tactile neurons enlarged along the axis of a rake soon after
its use for retrieving distant food pellets. The same visual RFs shrunk backwards following
passive tool-wielding, recorded immediately after tool-use, thus showing a tool-usedependent extension of the visual-tactile space immediately surrounding the animal hand
(Iriki et al., 1996).
This peri-hand space (Rizzolatti et al., 1997) is represented in human and non human
primates by multisensory systems that appear to share several functional commonalties
(Bremmer et al., 2001a,b; Làdavas, 2002; Macaluso et al., 2003; Rizzolatti et al., 1983). In
monkeys, visuo-tactile neurons display a gradient of firing dependent upon the distance of
the visual stimulus from the tactile RF; the firing rate increases when the stimulus comes
closer, but decreases when it goes away from the body (Duhamel et al., 1991, 1998; Fogassi
et al., 1996; Fogassi et al., 1999; Graziano and Gross, 1995; Rizzolatti et al., 1981). In
humans, evidence of similar visual-tactile processing of peri-hand space comes from studies
on cross-modal extinction (Bender, 1952; Mattingley et al., 1997), whereby contralesional
tactile perception can be modulated by the distance from the body at which ipsilesional visual
stimuli are presented (di Pellegrino et al., 1997; Làdavas et al., 1998a,b). In the case of the
hand, nearby visual stimuli (~5 cm) are more efficient than farther ones (~35 cm) in
3
extinguishing contralesional tactile stimuli, this near-far modulation representing the
behavioural hallmark of multisensory coding for peri-hand space (see, for review, Làdavas
and Farnè, 2004ab).
In this context, the successful use of a tool, as an extension of the corporeal
boundaries, requires integration of (at least) visual, tactile, proprioceptive and motor aspects
(Napier, 1956). Under normal circumstances, such a polymodal merging of sensorimotor
information from different locations (hand & tool) ensures that the appropriate action (e.g.,
retrieval) is performed with an appropriately oriented tool (e.g., a rake), in the position where
the target object is located (Beck, 1980; Bradshaw, 1997; Farnè and Làdavas, 2000; JohnsonFrey, 2003). Accordingly, some studies have reported evidence that various types of toolrelated experience may modify the spatial extent of the peri-hand area within which visualtactile integration occurs. For example, adapting to humans the task originally introduced for
monkeys (Iriki et al., 1996), we showed that the weak visual-tactile integration usually
observed far from the subject’s hand can be significantly increased, at the same far location,
following tool-use (Farnè and Làdavas, 2000). By investigating left cross-modal extinction in
right brain-damaged (RBD) patients, we found that ipsilesional visual stimuli presented at the
distal edge of a 38 cm long rake induced more left tactile extinction immediately after tooluse (retrieving far objects with the rake for 5 minutes) than before tool-use. Moreover, when
tool-use was impeded, the severity of cross-modal extinction decreased to pre-tool-use levels.
Thus, stronger cross-modal extinction after tool-use, as measured at the same location far
from the hand, has been taken as evidence for an extension of peri-hand space along the tool
axis, whereas its reduction following tool-inactivity has been considered as the behavioural
counterpart of backward contraction of the formerly extended peri-hand space (Farnè and
Làdavas, 2000). In a similar vein, Maravita and colleagues (Maravita et al., 2001) also found
4
stronger visual-tactile extinction at the tip of a stick wielded by a patient, as compared to
when the stick was absent, or present but not connected to the patient’s hand. Further
evidence of tool-related far/near space re-mapping has been documented in neglect patients
(Ackroyd et al., 2002; Berti and Frassinetti, 2000; Maravita et al., 2002b; but see, Pegna et
al., 2001; Humprheys et al., 2004). Convergent evidence comes from healthy participants
investigated in tasks involving the displacement and/or crossing of hand-held ‘tools’ (Riggio
et al., 1986; Yamamoto and Kitazawa, 2001). Maravita and colleagues, for example, have
shown that significant changes in the spatial distribution of cross-modal effects may emerge
when subjects repeatedly cross-over two hand-held tools, and that the phenomenon develops
with increased practice in crossing the tools (Maravita et al., 2002a).
By assessing visual-tactile extinction in RBD patients, we also investigated the crucial
determinants of peri-hand space extension. In a single-case study (Farnè et al., 2005a) we
verified the role played by passive vs. active experience with tools in re-sizing peri-hand
space. In particular, we investigated whether a prolonged passive experience with a rake (60
cm long) was sufficient to elongate the peri-hand space, or whether active tool-use was
necessary. The results showed that the severity of visual-tactile extinction, as assessed at the
distal edge of the tool (60 cm away from the patient’s hand) after a prolonged passive
exposure to the proprioceptive and visual experience of wielding a rake, did not differ from
that obtained when the tool was absent. In contrast, cross-modal extinction was significantly
increased when assessed equally far in space, but following an equally long period of active
use of the same tool. Therefore, in agreement with both neurophysiological and
neuropsychological findings (Iriki et al., 1996; Maravita et al., 2002ab) these results
suggested that plastic modifications of the body schema (Head and Holmes, 1991) require the
tool to be actively involved in a task.
5
In a recent group study (Farnè et al., 2005b) we investigated the relationships between
the physical and functional properties of an active tool-use and the spatial extent of the
subsequent peri-hand space elongation. In particular, by assessing visual-tactile extinction far
from the patients’ hand (60 cm) after use of a 60, or 30 cm long rake, we found that perihand space elongation varied according to the tool length. When assessed at the same 60 cm
far location, cross-modal extinction was stronger after use of a long tool (60 cm) as
compared to the use of a short tool (30 cm). Remarkably, even the use of the short tool (30
cm) produced a significant increase of cross-modal extinction at the same far location (60
cm), although weaker than that induced by the use of the long tool (60 cm). This indicates
that the elongated area is not sharply limited to the tool tip, but extends beyond it including a
peri-tool space whereby visual-tactile integration fades. In the same study we also showed
that the extent of tool embodiment tightly depends upon the functional, not the physical,
length of the used tool, when these were dissociated through a hybrid rake that was
physically long (60 cm), but operationally short (30 cm). Indeed, the severity of cross-modal
extinction observed at the same far location ( 60 cm) after the use of the hybrid tool was
significantly less severe than that found after the use of the 60 cm long tool. In contrast,
comparable cross-modal extinction was observed after use of the short tool (30 cm) and the
hybrid tool, whose absolute length was the same as the 60 cm long tool, but whose functional
length was the same of the 30 cm long tool. As the two rakes were identical, except for the
spatial location of the tines, and required similar motor activity, the findings showed that the
functionally effective length of the tool is the crucial determinant of the spatial extension of
peri-hand space (Farnè et al., 2005b).
But is this a real elongation of the visual-tactile integrative area along the tool axis?
The question concerning the shape of the peri-hand space representation and its tool-related
6
plastic changes (Làdavas et al., 1998b; Farnè et al., 2005a) has been recently addressed by
Holmes and colleagues (Holmes et al., 2004) in normal subjects by investigating visualtactile congruency effects in three positions along a tool (handle, middle, tip). In this context,
though, the nature of the cross-modal task was quite different from the typical confrontation
method used to assess extinction in brain damaged patients, as it was the task to be
performed with the tools. In particular, cross-modal congruency refers to a behavioural
paradigm of visual-tactile interference (Spence et al, 1998; 2004) that measures the subjects’
ability in discriminating the spatial location of a tactile stimulus on his/her body (e.g., thumb
vs. index finger) in the presence of visual distractors that can be spatially congruent or
incongruent with the tactile target location on the skin. Despite the fact that visual
disctractors are totally irrelevant for the tactile discrimination task, the incongruent visualtactile mapping (e.g., a light aligned with the index with simultaneous tactile stimulation of
the thumb) produces an interference, as compared to the congruent mapping, worsening the
subjects’ performance. Holmes and colleagues asked subjects to perform this cross-modal
congruency task interleaved between series of ‘tool use’ trials (c.f. Holmes et al, 2004, page
64). Tool-use consisted in having the subjects to use the tip, the shaft, or the handle of one of
the two hand-held tools to push a button located at different distances form the subject’s
body. They found that, when the task involved the use of the shaft (or the tip) of the tool,
visual-tactile interaction were stronger at the tips of the tools than in the middle of the shaft.
They suggested their findings were more compatible with selective incorporation of the tooltip, rather than with peri-hand space elongation encompassing the whole tool. Besides the
obvious differences, which may surely be responsible for the different results, Holmes et al.’s
findings may appear at variance with neurophysiological evidence showing that, after tool-
7
use, visual RFs of bimodal parietal neurons actually enlarge along the tool axis, or extend to
the whole space that can be reached by the tool (Iriki et al., 1996).
To shed light on this issue, we assessed cross-modal visual-tactile extinction in a
RBD patient while she was wielding a 60 cm long rake, before and immediately after its use
to retrieve distant objects. At variance with previous patients studies, visual-tactile extinction
was assessed near the ipsilesional hand (holding the rake handle), near the distal edge of the
rake, as well as in a middle position between the hand and the distal end of the rake. In the
light of the previously reported neurophysiological findings, we expected that tool-use would
increase cross-modal extinction both at the distal and middle positions, whereas no change
was expected near the patient’s hand.
Methods
Case report
The patient (T.R.) is an 72-years-old right-handed woman (5 years of education) who
suffered an haemorrhagic stroke in the right hemisphere 3 months before the test. She gave
her informed consent to participate in the study, whose experimental procedures were
approved by the local ethics committee. The investigation was carried out according to the
Declaration of Helsinki. At the time of the investigation she did not present left hemiparesis,
on neurological examination. A CT scan revealed that the lesion involved the inferior and
superior parietal lobules (BA 39, 40, 5, 7), the paraventricular parieto-occipital area, the
posterior part of the middle temporal gyrus (BA 37) and the auditory area (BA 22, see Figure
1). She was alert, well oriented in time and space and very collaborative. Signs of moderate
left visuospatial neglect were present when she was submitted to the Behavioural Inattention
8
Test (BIT) battery (Wilson et al., 1987), whereas personal neglect was absent (Cocchini et
al., 2001).
She was enrolled in the present study because of her severe tactile extinction. When
light touches (20 trials for left and right single stimulation and 20 trials for double
simultaneous stimulations) were manually delivered (through Semmes-Weinstein-like
monofilaments) to the dorsal surface of the second phalanx of her index fingers she reported
correctly all left unilateral stimuli (100% of accuracy), but was able to detect only 30% of
left touches under double simultaneous stimulation. Then she was screened for cross-modal
visual-tactile extinction, which turned out to be also present in a quite severe form. Crossmodal extinction was investigated by presenting tactile stimuli on the contralesional screened
left hand (as described above) and visual stimuli near (~5 cm) the visible ipsilesional hand.
Visual stimuli were brief movements of the experimenter’s left index finger. With this
combination of stimuli (20 trials for left and right single stimulation and 20 trials for double
simultaneous stimulations), she consistently perceived all the left single touches (100%
correct), as well as all the ipsilesional single visual stimuli (100% correct), but reported only
a minority of left touches presented concurrently with proximal visual stimulation (30% of
accuracy).
(Figure 1 and 2 about here)
Cross-modal extinction was also assessed by presenting ipsilesional visual stimuli
further away (~35 cm) from the visible right hand, in two locations (see Figure 2), one on the
9
vertical and the other in the para-sagittal plane. The same number of trials as before was used
to assess cross-modal extinction in both far positions (20 trials for left and right single
stimulation and 20 trials for double simultaneous stimulations). Again, her performance in
reporting single touches of the left hand was errorless (100% accuracy). In addition, under
simultaneous visual-tactile stimulation, cross-modal extinction was much weaker as
compared to stimulation near the hand, as she was able to report 75% of left touches, both in
the vertical and para-sagittal plane (~35 cm). Therefore, the distance-dependent modulation
of left tactile detection under cross-modal stimulation induced a change in performance
corresponding to 45% of accuracy.
Apparatus and procedure
For the experimental investigation, the patient had to fixate a red dot, aligned with her
sagittal axis and located on the table surface (45 cm from the proximal border of the table)
while passively keeping her hands on the table surface (separated by 45 cm).
At the beginning of each trial, the experimenter checked that the subject was keeping
fixation. A green plastic shield (width 18 cm, height 18 cm, depth 40 cm) was used to
prevent vision of tactile stimuli delivered to the left hand. Tactile stimulation was silently
applied by means of a pair of synthetic monofilaments, analogous to Semmes-Weinstein
probes, which provide near identical indenting stimulation across trials and conditions (see,
for details, Farnè et al., 2003). Visual stimuli consisted of a rapid flexion-extension of the
examiner’s left index finger (~5 cm of excursion). The efficacy of this clinical approach to
visual-tactile stimulation paradigms has been confirmed by studies from other laboratories
that, by employing computer-controlled stimuli, found very similar results in both patients
and neurologically intact subjects (Maravita et al., 2001; Kitagawa and Spence, 2005).
10
Both before and after tool-use (see below), cross-modal extinction was assessed as
follows (see Figure 3). The patient wielded the wooden ergonomic handle (14 cm long) of a
wooden rake (60 cm long) that was oriented along the para-sagittal plane and whose distal
tines were laying on the table surface (total tool length = 74 cm). The handle was gently
grasped by the patient, with the right hand passively laying on the table surface. While
passively grasping the tool, the most distal part of the patient’s right hand was located near
the distal border of the handle (~2 cm). Three different locations along the tool axis, called
hereinafter near (N), middle (M) and far (F), were probed for assessing visual-tactile
extinction. In separate blocks of trials, presented in a prefixed pseudo-random order, visual
stimuli could be presented immediately above (~5 cm) the patient’s right hand holding the
tool (N), or immediately above (~5 cm) the middle location (M) along the tool shaft (i.e., ~32
cm from the patient’s hand), or immediately above (~5 cm) the distal edge (F) of the tool
(i.e., ~62 cm from the patient’s hand). In each block, four types of stimulation were
delivered: unilateral left or right stimulation, bilateral simultaneous stimulation, or no
stimulation at all (catch trials, CT). The patient was instructed to verbally indicate the side of
the stimulation by saying ‘left’, ‘right’, ‘both’, or ‘none’, independent of stimulus modality.
She was informed that in some occasion she would not receive any stimulus (Catch Trials).
For each type of stimulation, 10 trials were presented according to a fixed pseudo-random
sequence. Each block was run four times in separate sessions, whereby cross-modal
extinction was assessed both before and after tool use. Across four sessions, the pre-tool use
blocks of trials for each location of the visual stimulus along the tool (arranged in the
following order: NMF-FNM-MFN-NMF) always preceded the post tool-use blocks (order:
FMN-MNF-NFM-NMF), which were run immediately afterwards.
11
Tool-use consisted of 50 tool-mediated retrieval movements, lasting about 5 minutes.
The patient was asked to use the rake to reach and retrieve an object located on the table, out
of the hand-reaching space. Objects were constituted by plastic disks (3 cm of diameter, 0,5
cm thick) and were presented one at a time. They were randomly presented in
correspondence with patients’ midsagittal axis, or 10 and 20 degrees to the left and to the
right of the central position.
Results
T.R. made no false alarm on catch trials throughout the experiment. Across all the blocks of
trials, she performed errorless when responding to single touches delivered to her left hand
(100% of accuracy). However, her performance was poor under bilateral stimulation
conditions. In the next sections, her performance before tool-use will be reported first,
followed by the performance obtained after tool-use.
(Figure 3 about here)
Before using the tool, as expected, the amount of visual-tactile extinction was
significantly modulated by the distance at which the visual stimulus was presented from the
patient’s hand (see Figure 3), as statistically assessed by using the Fisher Exact Test, after
alpha-correction for multiple comparisons. Cross-modal extinction was more severe near the
patient’s right hand holding the rake (35% of accuracy), as compared to both the middle
(73% of accuracy; p<0.001) and the distal location (78% of accuracy; p<0.001) along the
tool. Cross-modal extinction did not differ between the two latter positions (p=0.4).
12
After tool-use, the amount of visual-tactile extinction was no longer significantly
modulated by the distance of the visual stimulus from the patient’s hand (see Figure 3).
Cross-modal extinction assessed near the patient’s right hand holding the rake (35% of
accuracy) was comparable to that obtained both at the middle (45% of accuracy; p=0.3) and
the distal location (28% of accuracy; p=0.3). The extinction level for the middle and far
locations was not significantly different, whether or not alpha-correction for multiple
comparisons were applied (p=0.08).
To verify whether tool-use induced a real elongation of peri-hand space, the severity
of cross-modal extinction obtained before and after tool-use was compared per each position.
No differences were found at the level of the hand; actually, the same amount of visualtactile extinction was observed before and after tool-use (35% of accuracy). As expected on
the basis of previous studies, cross-modal extinction was more severe after tool-use when
measured at the distal edge of the rake (28% of accuracy), as compared to before tool-use
78% of accuracy; p<0.001 by Fisher Exact test). Critically, cross-modal extinction following
tool-use was more severe also when measured at the middle location (45% of accuracy), as
compared to before tool-use (73% of accuracy; p<0.01 by Fisher Exact test). Therefore, the
reason why visual-tactile extinction was no longer modulated by the distance of the visual
stimulus from the patient’s hand was due to the fact that its severity increased, after tool-use,
both at the middle and distal location along the tool axis, but not at the proximal position.
Discussion
The present study was aimed at investigating the spatial characteristics of the increase in
cross-modal effects, documented in cross-modal extinction patients after training in using
tools. In particular, we verified whether this increase is due to an actual elongation of the
13
visual-tactile peri-hand area along the tool axis, or to the shift or the creation of a new
integrative area at the distal edge of the used tool. These alternative possibilities were
verified by probing different tool locations for visual-tactile extinction in a RBD patient,
T.R., before and after the active use of the same tool. Three main findings were obtained and
will be summarised and discussed below.
First, before tool-use, cross-modal extinction was more severe when assessed close to
the patient’s hand (holding the rake) than at farther distances. This finding is consistent with
previous reports and further confirms the well known near-far modulation (di Pellegrino et
al., 1997; Làdavas, 2002; Maravita et al., 2001). The new result here consisted in showing
that outside the area of maximal visual-tactile integration (near the hand) cross-modal effects
dropped off without showing any further significant decrement. Cross-modal extinction,
which was quite strong at the hand location (only 35% of accuracy), was comparably weak
both at the middle (30 cm) and distal (60 cm) location (73% and 78% of accuracy,
respectively). This shows that, once the borders of the peri-hand integrative area are
exceeded, the relationships between the distance of the visual stimulus and the severity of
cross-modal effects is no longer present. This finding is in agreement with
neurophysiological data showing a limited extension in depth of the visual RFs of monkeys
visual-tactile neurons (Duhamel et al., 1998; Graziano and Gross, 1998; Rizzolatti et al.,
1981). Although one needs to be cautious in generalizing single-case based evidence, this
finding may suggest that the distance of about 30 cm, which has been repeatedly documented
to show a significant near-far modulation of cross-modal extinction in group-based studies,
may represent a reliable estimate of the extent of the near peripersonal space in humans.
Second, after tool-use, there was no change in the severity of cross-modal extinction
measured at the level of the hand (35% before and after tool-use). This result is consistent
14
with the view that the visual-tactile peri-hand area did not shift, but rather extended from the
hand to a farther location. Indeed, if the peri-hand space would have shifted, we should have
expected to find a better performance at the level of the hand after tool-use. Instead, the lack
of any change following tool-use suggests that the integrative strength at the level of the hand
was kept constant, which is compatible with an expansion of the peri-hand area to a farther
location. This conclusion was supported by further results, as described below.
Third, and most relevant for the purpose of the present study, a significant worsening
of cross-modal extinction was present after tool-use as compared to before tool-use. The
worsening was present both at the distal edge (28% vs. 78% of accuracy), as well as midway
(45% vs. 73% of accuracy) between the hand and the tool-tip. The worsening of the patient’s
performance at the middle location sharply contrasts with what should be expected if the
peri-hand space would have shifted from the hand towards the tool-tip following tool-use. It
also contrasts with the possibility that tool-use would generate a new visual-tactile integrative
area in correspondence with the distal edge of the tool. Both these hypotheses, in fact, predict
that no change should be observed at the middle location. In contrast, the significant
worsening of cross-modal extinction observed after tool-use at the middle location clearly
supports the notion that tool-use induces a real extension of the peri-hand space (Farnè and
Làdavas, 2000; Farnè et al., 2005a,b).
In this respect, it should be noted that in absolute terms, but not statistically, crossmodal extinction after tool-use was somewhat more severe at the distal location (28% of
accuracy) compared to the middle one (45% of accuracy). Bearing in mind the caveat for
overgeneralization from single-case evidence, this seems to suggest that the way in which
tool-use extends the peri-hand space along the tool-axis might be non homogeneous, certain
portions of the tool (e.g., the handle and the tines) being allocated with more integrative
15
strength than others (e.g., the shaft). Although preliminary findings from two additional
patients, examined with a similar procedure, seem to support this result, more complete
investigations are needed to address this issue (Serino et al., in preparation), as it may be
relevant to solve the apparent discrepancy between the results of the present study and those
recently reported in normal subjects (Holmes et al., 2004). By assessing visual-tactile
interference effects at different locations along hand-held tools, Holmes and colleagues did
not find significant effects at a middle location, but only at the proximal (handle) and distal
(tool-tip) position. The null effect at the middle location was observed despite the fact that
subjects had to use each part of the tool to perform an active task (pushing a button). On the
basis of their results, these authors suggested that tool-use may highlight only certain
portions of a tool, as the handle and the tip. However, taking into consideration previous tooluse studies on humans, as well as the neurophysiological findings, they also acknowledged
that some forms of tool-use may result in a genuine extension of peri-hand space (Holmes et
al., 2004), as we report here for the first time in a single case. Other account have been
proposed and deserve further investigations in the future (see Holmes et al., 2004;
Humphreys et al., 2004).
The present study, by showing that tool-use literally extends the visual-tactile
integrative area surrounding the hand along the tool axis, thus encompassing both the distal
and the middle location along the shaft of the rake, may both solve the apparent discrepancy
between normal and pathological evidence in humans, and reconcile the behavioural
evidence in humans with the neurophysiological findings in monkeys.
Indeed, the present results are fully consistent with the work reported by Iriki and
colleagues, documenting a real enlargement of visual RFs of bimodal neurons following tool-
16
use, rather than a shift, or a split of visual integrative areas (Iriki et al., 1996; Iriki et al.,
2001; Obayashi et al., 2001).
Furthermore, these findings add to the previous results from our laboratory in
showing that plastic modifications of peri-hand space representation may be highly flexible.
For example, we previously showed that the multisensory peri-hand area can be extended
differentially by using tools of different length, and that the external border of the area
elongated through the use of tools is not sharply limited to the tool tip, but extends (fading)
beyond it (Farnè et al., 2005a). The present results further support this view, in that the
integrative strength, as measured at the level of the hand and the tool-tip through cross-modal
extinction, was absolutely comparable. Indeed, this pattern of results is what should be
expected in the case of a real elongation of the hand-centred peripersonal space to include the
distal edge of the rake, as if it were the hand. Finally, the strong increase in cross-modal
extinction obtained after tool-use at the distal edge of the rake is also consistent with
previously reported evidence showing that the distal border of the elongated peri-hand space
is tightly linked to the functionally effective part of the tool (Farnè et al., 2005b).
Overall, the reviewed findings together with the new evidence reported here favour
the notion that cross-modal effects documented in extinction patients reflect a genuine
extension of the peri-hand space that is specifically dependent upon the functionally effective
use of a tool. While it remains entirely possible in humans, as it has been demonstrated in the
monkey, that a new visual-tactile integrative area can originate in a remote location
physically disconnected with the body (Iriki et al., 2001), the active use of a tool to
physically and effectively interact with objects in the distant space appears to produce a
spatial extension of the multisensory peri-hand space that encompasses the whole length of
the tool.
17
Acknowledgements
We wish to thank T.R. for her enthusiastic collaboration. We also thank S. Bonifazi for
helping in data collection and F. Frassinetti for lesion reconstruction. This work was
supported by grants from MIUR and RFO.
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Figure 1.
CT scan showing the damage in the right hemisphere (upper row). The lesion reconstruction
with the damaged areas (grey shaded areas) is also reported (lower row), according to the
Damasio and Damasio’s (1989) coding system (lesion details in the text).
Figure 2.
Schematic illustration (viewed from the right side) of the locations (black symbols) used for
assessing visual-tactile extinction as a function of the distance of the visual stimulus from the
patient’s right ipsilesional hand. Close to each location (near the hand, far on the sagittal and
vertical planes) the accuracy in reporting the tactile stimulus simultaneously delivered to the
contralesional hand (not shown) is reported (in percentage). The grey shaded sector is a
tentative representation of the visual-tactile area within which the strength of cross-modal
integration is maximal towards the hand, as plotted on the basis of the experimental results
(see text for details).
Figure 3
Schematic illustration of the experimental setting for assessing visual-tactile extinction by
probing three different locations (black symbols) along the tool axis (handle, middle, tip)
before (left panel) and after (right panel) the use of the rake to retrieve distant objects. Tactile
stimuli (black arrow) were delivered to the left screened (grey rectangles) hand. The mean
accuracy in reporting the tactile stimulus in bilateral visual-tactile trials is reported (in
percentage) for each position (on the right of each panel). Before tool-use (left panel):
significant differences between accuracy in the handle location as compared to the middle and
tool-tip locations are reported on the left and indicated by grey arrows. After tool-use (right
24
panel): no significant differences among positions were observed. The significant
comparisons between accuracy before and after tool-use are reported (outside the right panel)
for each position. Cross-modal extinction increased both at the middle and the tool-tip
locations (surrounded by circles).
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