show me the skin! does seeing the back enhance tactile acuity at the back?
TRANSCRIPT
Accepted Manuscript
Show me the skin! Does seeing the back enhance tactile acuity at the back?
Mark J. Catley , BPhysio (Hons) Abby Tabor , BPhysio (Hons) Rohan G. Miegel ,Benedict M. Wand , PhD Charles Spence , PhD G. Lorimer Moseley , PhD
PII: S1356-689X(14)00080-0
DOI: 10.1016/j.math.2014.04.015
Reference: YMATH 1562
To appear in: Manual Therapy
Received Date: 27 October 2013
Revised Date: 18 April 2014
Accepted Date: 28 April 2014
Please cite this article as: Catley MJ, Tabor A, Miegel RG, Wand BM, Spence C, Lorimer Moseley G,Show me the skin! Does seeing the back enhance tactile acuity at the back?, Manual Therapy (2014),doi: 10.1016/j.math.2014.04.015.
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Show me the skin! Does seeing the back enhance tactile acuity at the back?
Mark J. Catley1 (BPhysio (Hons)), Abby Tabor1 (BPhysio (Hons)), Rohan G. Miegel1,
Benedict M. Wand2 (PhD), Charles Spence3 (PhD)& G. Lorimer Moseley1,4 (PhD)
1. Body in Mind Research Group, Sansom Institute for Health Research, University of South
Australia, Adelaide, Australia.
2. School of Health Sciences, The University of Notre Dame, Fremantle, Australia.
3. Crossmodal Research Laboratory, Department of Experimental Psychology, University of
Oxford, Oxford, UK.
4. Neuroscience Research Australia, Sydney, Australia.
Corresponding author:
Lorimer Moseley
The University of South Australia
GPO Box 2471 Adelaide 5001 Australia
T: +61 8 83021416
Funding:
This project supported by NHMRC Project Grant ID 1008017.
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ABSTRACT
A growing body of literature associates musculoskeletal disorders with cortical reorganisation. One condition in which reorganisation is established and treatments that 'train the brain' are being widely used is chronic back pain. Recent evidence suggests that treatments that involve tactile training are more effective if they incorporate multimodal mechanisms, most obviously vision. With regard to back pain however, we must first determine if tactile function is enhanced by incorporating other modalities. A series of three cross-over experiments were conducted in healthy pain-free subjects to determine whether tactile acuity is enhanced when participants can see the skin of their back during testing. An initial randomised cross-over experiment suggested tactile acuity was significantly enhanced when participants could see their backs (t(25) = -4.226, p<0.001, r = .65). However, a second replication experiment was not corroborative. Both the second (F(3,66) = 1.00, p = .398) and third (t(9) = .969, p = .358) experiments suggested that seeing the back did not significantly affect tactile acuity, confirming that our initial results were likely due to chance. The principle that visual feedback improves tactile acuity at the hand does not apply to the back. These results strongly suggest that attempts to enhance tactile training by incorporating vision will not offer the benefit to treatment of back pain that has been observed for treatment of hand pain.
Key words: tactile acuity, two-point discrimination, tactile discrimination training, visuo-tactile.
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INTRODUCTION
Altered tactile acuity and primary somatosensory cortex reorganisation are evident in chronic
musculoskeletal conditions, for example complex regional pain syndrome (CRPS) (Flor et al.,
1997; Flor et al., 1995; Pleger et al., 2006; Stanton et al., 2013). Moreover, the extent of these
aberrations are directly related to pain intensity (Flor et al., 1995; Pleger et al., 2005) and
chronicity (Flor et al., 1997). As chronic pain patients recover, tactile acuity and cortical
reorganisation appear to normalise (Maihöfner et al., 2004). While a causal link has yet to be
established, the substantial and consistent relationship between tactile acuity, cortical
reorganization and pain has driven the growing interest in tactile discrimination training
interventions. Tactile discrimination training has been shown to effectively improve tactile
acuity, reduce pain and normalise cortical representation in both phantom limb pain (Flor et
al., 2001) and CRPS (Moseley et al., 2008). More recently, there have been moves to extend
these interventions to other techniques and to the most common and burdensome chronic pain
condition - chronic low back pain (Morone et al., 2012; Wand et al., 2013; Wand et al., 2011)
(see Moseley et al. (2012) for review). This would seem sensible – tactile acuity at the back is
reduced in those suffering from back pain (Moseley, 2008; Wand et al., 2010) and
preliminary data suggest that tactile training with visual feedback may play the same
therapeutic role in the recovery from back pain (Wand et al., 2011) as it has been shown to do
in chronic hand pain (Moseley & Wiech, 2009). Indeed, this suggestion seems well endorsed
clinically.
There is, however, an important issue that has been overlooked. For the arm and hand, visual
input of the skin enhances tactile acuity for stimuli delivered to the arm (Taylor-Clarke et al.,
2002) and there is compelling evidence that tactile discrimination training in unilateral CRPS
is more effective when participants can see the arm being trained than when they cannot
(Moseley et al., 2009; Serino et al., 2007). The most likely mechanism underpinning this
effect involves bimodal visuo-tactile brain cells that modulate primary somatosensory cortex
cells in a precise way (Driver & Spence, 1998; Magosso et al., 2010). Such cells have been
identified in monkey brains (Zhou & Fuster, 2000) and almost certainly exist in humans.
However, there is no evidence that such cells exist for the back, an area that is difficult to
view ourselves, and, importantly, there is no evidence that vision of the back enhances tactile
acuity at the back. Here we describe a series of independent experiments that aimed to
determine whether visual feedback enhances tactile acuity of the back in healthy, pain-free
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participants. Our initial experiment hypothesized that tactile acuity of the back would be
greater when participants could view their back than when they could not. This hypothesis
was supported however the effect could not be replicated in a consequent second experiment
that aimed to further the initial results. A third study was conducted to expound the findings
of the two initial experiments.
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METHODS
A convenience sample of naïve healthy participants was sought for each of the three
experiments. Further detail regarding the participants, the assessment protocols and study
limitations is included as an appendix. Approval was granted by the University of South
Australia Human Research Ethics Committee.
Experiment 1
A randomized, single blinded cross-over design was used to interrogate whether providing
non-informative visual feedback of the back (i.e. participants could see the assessor’s hand
approaching their back but the assessment tool was obscured from sight) would enhance
tactile acuity in 26 (11 female) healthy pain-free participants (Table 1).
Eight physiotherapists assessed two-point discrimination (TPD) according to the method
described by Moberg (1990). Mechanical sliding callipers with a precision of 0.1mm were
applied with pressure sufficient to first blanch the skin. TPD was assessed laterally from the
midline at the level of the L3 spinous process. Assessment commenced with 0mm between
the two points and gradually increased until the participant discerned two points. A series of
five ascending and descending staircases, centred around the participant’s initial TPD
threshold was conducted and the average of these assessments was analysed. TPD was
assessed under two conditions: with and without visual feedback. The sequence in which the
participant received the two conditions was randomly assigned.
The participants laid prone, looking downward through the face hole of a plinth. A monitor
was mounted below the face hole and a camera was mounted over their lower back such that
the participant could see real-time footage of their back. Assessors were instructed to hold the
callipers in such a way that the participant could see the assessor’s hand approaching and
touching their back, but not see the distance between the points of the callipers. The assessors
were blinded to whether the participant could see the monitor or not.
A Kolmogorov–Smirnov test was used to assess whether the sampling distribution of the
differences was normally distributed. Data were consequently analysed using a paired t-test
(two-sided) with significance set at α=0.05.
Results
Data were normally distributed indicating a paired t-test could be used to compare the two
conditions. TPD threshold was lower when participants could see the skin of their back
(mean (SD): 49.9 (18.4) mm) than when they could not (57.9 (18.2) mm; mean difference =
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8.1 (9.4) mm, t(25) = -4.226, p<0.001, r = .65). Figure 2A shows the difference in TPD
between the two conditions with most (77%), but not all, participants displaying a lowered
TPD threshold.
Experiment 2
Experiment 1 demonstrated that providing visual feedback can significantly enhance tactile
acuity of the back therefore suggesting that the principle that visual feedback improves tactile
acuity at the hand also applies to the region of the back. The putative mechanism underlying
such a crossmodal enhancement effect could be hypothesised to involve bimodal visuo-tactile
cells but might simply also have reflected spatial attention. That is, real-time footage of the
back could have helped the participant to focus their attention on the area receiving the tactile
input (Tipper et al., 2001). Experiment 2 used a randomized, repeated-measures design with
four conditions (Figure 1) to determine whether the significant enhancement of tactile acuity
was due to visual feedback of the skin of the back or to the focusing of spatial attention on
that region of the skin surface. Furthermore, we sought to determine whether the
enhancement was due to visual feedback of the assessed region or visual feedback of touch.
Positioning of the participants, the delivery of visual feedback and the test conditions were
consistent with that described for Experiment 1. However, TPD was assessed by one trained
assessor using the method described by Kennett et al. (2001). This method uses a modified
PEST calculator (Taylor & Creelman, 1967) to centre on a participants TPD threshold and
thus removes the bias that may be introduced because of the need for the assessor to
subjectively decide when the threshold is reached.
To provide the illusion of being touched, but not seeing the back, a pre-recorded footage was
played. To assess the extent to which the illusion was maintained, participants were asked to
describe how they thought the illusion was achieved. Data pertaining to those who
specifically stated that the footage was pre-recorded were removed. Timing of the stimulus
with the footage appeared paramount in maintaining the illusion.
Mauchly’s test was used to assess whether the data violated the assumption of sphericity.
Data were analysed using a one-way repeated-measures ANOVA with significance set at
α=0.05.
Results
Twenty five (14 female) healthy pain-free participants were recruited for Experiment 2 but
two persons were excluded because they reported discrepancies between the timing of the
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stimuli and visual feedback in the pre-recorded condition. That is, the illusion that they could
see the assessor’s hand but not their back was not upheld. Table 1 displays the characteristics
of the remaining 23 participants.
The assumption of sphericity had not been violated, x2(5) = 4.48, p = .48 and no significant
difference was noted between any of the four conditions (F(3,66) = 1.00, p = .398). That is,
tactile acuity was not altered by visual feedback in any of the conditions. Furthermore,
Bonferroni post hoc tests revealed no difference between the two conditions (i.e., with and
without visual feedback) assessed in Experiment 1 (CI.95 = -.555 (lower), .451 (upper)) and
no trends were detected visually in the data (see Figure 2B).
Experiment 3
Rather than differentiating the enhancing effects of vision from spatial attention, the results of
Experiment 2 suggested that the TPD threshold was not significantly altered by vision.
Critically, Experiment 2 failed to replicate the findings of Experiment 1. To interrogate these
contradictory findings, Experiment 3 aimed to re-establish whether non-informative visual
feedback of the back enhanced tactile acuity using the protocol described for Experiment 2.
Experiment 3 used a randomized, cross-over design with two conditions (with and without
visual feedback). It was decided a priori that a further 10 naïve participants would be
assessed and the data would be analysed and visually inspected for trends.
Normality of the sampling distribution of the differences was assessed using Kolmogorov–
Smirnov tests. Assuming normality, data were analysed using a paired t-test (two-sided) with
significance set at α=0.05.
Results
Ten (5 female) healthy pain –free volunteers participated in Experiment 3 (Table 1). Data
were normally distributed indicating a paired t-test could be used to compare the two
conditions. TPD threshold was not significantly different (t(9) = .969, p = .358) when
participants could see the skin of their back (mean (SD): 50.0 (10.7) mm) than when they
could not (53.3 (14.3) mm. No trends were detected visually in the data (see Figure 2C).
Data from Experiment 2 was combined with the data from Experiment 3 (n = 33), allowing
further comparison between conditions with and without visual feedback. Again, no
significant differences were detected (t(32) = .359, p = .722, r = .13).
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DISCUSSION
The current series of experiments aimed to determine whether visual feedback enhances
tactile acuity of the back in healthy, pain-free participants. We hypothesized that tactile
acuity of the back would be greater when participants could view their back than when they
could not. The results of our first experiment supported the hypothesis, as evidenced by a
significant enhancement of acuity when vision of the back was present. Unlike the limbs, the
back is not viewed directly during normal function, yet it appeared that the same enhancing
effect was evident. We sought to further interrogate this phenomenon in Experiment 2 where
we hypothesised that the enhancing effect was due to visual feedback of the skin, as appears
to be the case for the hand, rather than spatial attention alone. Rather than corroborate and
further the findings of Experiment 1, our second experiment failed to support the first. We
found no significant difference between any of the four conditions, including those used in
the first experiment, nor did we identify any trends in the data. Our third experiment, while
arguably underpowered, confirmed the second as no clear trends were identified.
Furthermore, no significant effect was noted when the data from Experiments 2 and 3 were
combined, which removes the possibility that it was simply an issue of power. Given the
findings of the second two studies, we conclude that seeing the back does not enhance tactile
acuity in healthy controls.
That our initial results were not replicated points to a critical issue in research. As little as 1-
5% of the published replication studies in the behavioural sciences manage to replicate the
original published findings (Ioannidis, 2012). This remarkable and damning statistic points to
a clear publication bias in the literature, an issue which itself is still attracting editorial
coverage (Yong, 2012), and a problem that appears to be worse in psychology and psychiatry
than in other fields (Fanelli, 2010). Replication studies are difficult to publish, but our results
show how important they can be.
We hypothesized that visual feedback of the back would enhance tactile acuity of the back.
Non-informative visual feedback of the arm improves tactile performance (Press et al., 2004;
Ro et al., 2004) and enhances tactile spatial acuity (Haggard et al., 2007; Kennett et al., 2001;
Taylor-Clarke et al., 2002). That is, when a person can see the skin of the stimulated arm but
not the stimulator, their tactile detection threshold is reduced and their tactile spatial acuity is
enhanced. Indeed, magnifying the visual feedback further enhances performance (Kennett et
al., 2001). While spatial attention alone has been shown to enhance tactile acuity (Macaluso
et al., 2000), the enhancements due to vision are evident when any modulation of spatial
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attention has been controlled for (Cardini et al., 2012; Longo et al., 2011; Ro et al., 2004).
This visual enhancement of tactile acuity is thought to be due to modulation of primary
somatosensory cortex cells by specific bimodal visuo-tactile brain cells (Driver et al., 1998;
Macaluso & Driver, 2001; Magosso et al., 2010). These bimodal visuo-tactile cells have been
identified for haptic tasks in monkey brains (Zhou et al., 2000) and it is likely they exist in
humans.
To our knowledge, the current data represent the only study of this phenomenon in the back.
Perhaps it is not surprising that visual feedback of the back did not enhance tactile acuity of
the back. While evidence of bimodal visuo-tactile cells exists for upper limbs (Zhou et al.,
2000), it is plausible that these cells do not exist for the back. Intuitively this makes sense
because we explore the world with our hands and they are usually within our field of vision.
Conversely, we cannot see our backs and touch at the back serves a protective role rather than
an exploratory role. While our findings lend support to this theory, our study is clearly not
able to make conclusions about the presence or not of bimodal cortical cells subserving the
back.
Interestingly, that visual feedback did not alter tactile acuity of the back suggests that our
protocol either did not direct spatial attention toward the back, or that our protocol did, but it
had no effect on acuity. We provided visual feedback by means of a camera and monitor, a
method previously shown to improve tactile detection performance (Tipper et al., 1998) and
facilitate modulation of primary somatosensory cells (Schaefer et al., 2006). However, the
feedback of the back that the participants received in this study was not presented in full size.
Magnifying the visual input enhances tactile acuity (Kennett et al., 2001) and it is plausible
that shrinking the representation of the back diminished the effect of seeing the back. In
addition, studies of the arm have shown that viewing a neutral object, in place of the arm,
does not enhance tactile acuity (Cardini et al., 2012; Kennett et al., 2001). Perhaps the visual
feedback of the back, via the monitor, was perceived as a neutral object rather than one’s own
body part. That is, perhaps the participants did not take ownership of their back because they
are not used to visualising it. Indeed, Taylor-Clarke et al. (2002) suggest primary
somatosensory cortex cells respond to visual stimuli when they have been previously
associated with tactile information – the association with the forearm, in contrast with the
back, is well formed because it is seen in everyday life. Similarly, Tipper et al. (2001) found
visual feedback enhanced tactile detection performance for the face, a region that cannot be
viewed directly but that is frequently viewed indirectly (i.e., via mirrors), to a greater extent
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than the neck, a region that cannot be viewed directly and is infrequently viewed indirectly.
Finally, while Schaefer et al. (2006) provided visual feedback of the hand via monitors, they
noted modulation of primary sensory cortex correlated to the belief that the participant was
seeing his or her hand. We did not assess belief and it is plausible some people did not
believe they were looking at their own back. Alternatively, perhaps by placing the monitor
under the table, we directed spatial attention to the monitor rather than the back. These
explanations may explain our overall findings but do not account for our failure to replicate
our initial results, because the method of visual feedback was kept constant throughout the
experiments.
There was a difference in the assessment protocols between studies. Eight different assessors,
blinded to the hypothesis, were used in Experiment 1. Only two of these eight assessors had
previous experience testing TPD yet, despite the group’s minimal experience, a large effect
was detected. Experiments 2 and 3 found no effect using one assessor, with extensive
assessment experience and reliable performance, following the protocol described by Kennett
et al. (2001). Although it is feasible that Experiment 1 introduced a source of bias that was
not present in Experiments 2 and 3, it is not clear what that source of bias would be. We have
previously verified the protocol used in Experiment 1 and shown it is not influenced by
clinical experience (Catley et al., 2013), and others have previously verified the protocol used
in Experiments 2 and 3 (Kennett et al., 2001). We therefore contend that the most
parsimonious interpretation is that the findings of Experiment 1 were a false positive result
due to chance.
Tactile acuity is altered in many chronic pain conditions (Maihofner et al., 2003; Pleger et al.,
2006; Stanton et al., 2013; Tecchio et al., 2002) including chronic low back pain (Flor et al.,
1997; Luomajoki & Moseley, 2011; Moseley, 2008; Wand et al., 2010). As chronic pain
resolves, tactile acuity normalises (Pleger et al., 2005), highlighting this strong and consistent
relationship. Indeed, specifically training tactile acuity has been shown to decrease pain (Flor
et al., 2001) and there is a growing literature to support the use of tactile discrimination
training interventions in clinical rehabilitation (Morone et al., 2012; Moseley et al., 2008;
Wand et al., 2011). Visual feedback significantly enhances tactile acuity at the arm in patients
with impaired acuity (Serino et al., 2007) and tactile performance appears to be maintained
after the visual feedback has been removed, suggesting visual feedback may not only
modulate performance but also induce long-term change (Taylor-Clarke et al., 2004).
Viewing the skin of the arm enhanced the effect of tactile discrimination training (Moseley et
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al., 2009) in CRPS patients with reduced tactile acuity. As little as 30 minutes of tactile
discrimination training was sufficient to induce significant improvements with or without
visual feedback, but the enhancement was only maintained 48 hours later in those who had
received visual feedback. These findings prompted Wand et al. (2011) to include tactile
discrimination training with visual feedback, via mirrors, in a case series of three chronic
back pain patients. This preliminary study suggested the provision of visual feedback was
clinically feasible and supported the use of tactile discrimination training. While further
research to verify the efficacy of tactile discrimination training for back pain is required, our
findings suggest visual feedback is unlikely to induce the additional benefits that have been
shown people with CRPS of an arm.
The findings of the current series of experiments suggest seeing the back does not enhance
tactile acuity. However, Serino et al. (2007), in a sample of healthy individuals, demonstrated
that visual feedback enhanced tactile performance, only in those with poor acuity. While we
did not detect a trend toward people with poorer acuity showing larger improvements, it is
possible that people with low back pain, a condition associated with poor acuity (Luomajoki
et al., 2011), may respond differently to visual feedback. Further research may be warranted
to explore this hypothesis.
In summary, the principle that visual feedback improves tactile acuity at the hand is not
supported for the back. It is plausible that bimodal visuo-tactile cells, thought to be involved
in visuo-tactile performance in the hand, do not exist for the back - an area that is not usually
viewed. It is, however, interesting that tactile acuity was not enhanced by spatial attention.
While further research is needed to determine the efficacy of tactile discrimination training
interventions in the management of back pain, these findings suggest visual feedback will not
provide an additional enhancement.
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APPENDIX
Participants
For each of the experiments, a convenience sample of healthy pain-free participants was
sought via flyers and social media. All participants gave their written informed consent to
participate in this study. Participants were excluded if they had: (a) current pain; (b)
uncorrected impaired vision (c) neurological disease or overt neurological signs; (d) were
unable to detect light touch; (d) were aged over 35 years; or (e) were unable to read, write or
understand English. Participants were ineligible to participate in the second and third
experiment if they had participated in a previous experiment. That is, no individuals in this
study participated in more than one experiment.
For Experiment 1, the sample size was based on previously reported data (Catley et al.,
2013). We calculated that a sample size of 26 persons would demonstrate a statistically
significant effect size difference of 0.5 (approx. 6mm) between the two conditions (Cohen,
1988). For Experiment 2, the sample size calculation was based on the results of Experiment
1. Given an estimated correlation among repeated measures of r = .75 and an effect size of r
= .65, a minimum sample of 23 participants was required to detect a large effect with 80%
power and significance set at α= 0.05. All participants were naïve to the experimental
hypothesis.
In Experiment 1, eight naïve physiotherapists (6 male) were recruited to assess TPD.
Previous experience in the assessment of TPD was not required as we have previously
reported acceptable test-retest reliability (ICC = 0.81) for TPD assessment of the low back in
young healthy participants (Catley et al., 2013). The physiotherapists underwent a brief
training session on the evaluation of TPD (see below). Each participant was assessed under
both conditions by one of the eight assessors. The physiotherapists were blinded to the
hypothesis of the study.
Assessment of two-point discrimination
Mechanical sliding callipers (Duratech TA-2081) with a precision of 0.1mm were used to
assess two-point discrimination (TPD). They were applied with pressure sufficient to first
blanch the skin at the points. All participants nominated their dominant side according to
their response to the question “Would you judge yourself to be more right-handed or more
left-handed?”
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Assessors were instructed to locate the spinous process of L3 and assess TPD horizontally out
from the midline on the participant’s dominant side. In Experiment 1, assessment commenced
with 0mm between the two points and gradually increased until the participant discerned two
points (Moberg, 1990; Moseley, 2008). A series of five ascending and descending staircases,
centred around the subject’s initial TPD threshold, was conducted and the average of these
assessments was analysed. Each participant was instructed to report ‘one’ if they felt one
point or ‘two’ if they felt two points. If they were unsure, they were instructed to report one
point. The only feedback they were to give to the assessor was if they discerned two points
because of a temporal delay between each point. When this occurred, that report was rejected
and the trial was repeated. Participants were instructed to get up and move about for
approximately 2 minutes between the two assessment periods.
In Experiments 2 and 3, TPD was assessed by one trained therapist in accord with the method
described by Kennett et al. (2001). This method uses an online modified Pest calculator to
centre on a participants TPD threshold and therefore removes the need for the assessor to
subjectively decide when the threshold is reached. Participants received 30 stimuli, of which
5 were randomly allocated one-point catch trials. Staircase trials began with the points 60mm
apart and were subsequently made easier or harder depending on the participant’s response.
Subsequent trials were made more difficult (i.e., smaller distance between the points)
following a response indicating two points were applied. Conversely, they were made easier
(i.e., the distance between the points was increased) following a response indicating only one
point was felt. The size of the incremental steps was varied by the online software according
to a modified PEST staircase procedure (Taylor et al., 1967). The estimate of the participant’s
TPD threshold was defined as the measure calculated after 25 trials (i.e. the catch trials did
not affect the staircase).
CONFLICTS OF INTEREST
Nil.
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ACKNOWLEDGEMENTS
MJC supported by an Australian post-graduate award. AT supported by the University of
South Australia President’s Scholarship. GLM supported by the National Health & Medical
Research Council of Australia ID 571090.
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Table 1. Demographic characteristics for healthy participants
Experiment 1
(n = 26)
Experiment 2
(n=23)
Experiment 3
(n=10)
Gender, n (%)
Males 15 (57.7) 9 (39.1) 5 (50.0)
Females 11 (42.3) 14 (60.9) 5 (50.0)
Age (yrs), mean (SD) 24.2 (4.6) 22.2 (1.9) 21.1 (2.6)
Handedness, n (%)
Right 22 (84.6) 21 (91.3) 10 (100)
Left 4 (15.4) 2 (8.7) 0 (0)
Figure 1. Experimental conditions. In experiments 1 and 3, participants’ two-point
discrimination threshold was assessed under two conditions: with visual feedback (A) and
without (B). Experiment 2 included two additional conditions: visual feedback of the hand
reaching to touch them, but not their back (C) and visual feedback of their back, but not of
the touch (D). Note: while participants could see the hand reaching to touch them in some
conditions, at no time could they see the assessment callipers.
Figure 2. Comparison of two-point discrimination thresholds with and without visual
feedback LEGEND: (A) Experiment 1, (B) Experiment 2 and (C) Experiment 3.
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