Transfer of Visuomotor Adaptation to Unpractised Hands and Sensory Modalities

doi:10.4236/psych.2013.412145

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Psychology

2013. Vol.4, No.12, 1004-1007

Published Online December 2013 in SciRes (http://www.scirp.org/journal/psych) http://dx.doi.org/10.4236/psych.2013.412145

Open Access

1004

Transfer of Visuomotor Adaptation to Unpractised Hands

and Sensory Modalities

Otmar Bock1, Gerd Schmitz2

1Institute of Physiology and Anatomy, German Sport University Cologne, Cologne, Germany

2Institute of Sports Science, Leibniz University Hannover, Hannover, Germany

Email: [email protected]

Received September 18th, 2013; revised October 23rd, 2013; accepted November 21st, 2013

Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,

provided the original work is properly cited. In accordance of the Creative Commons Attribution License all

A recent model (Bock, 2013) predicts that sensorimotor adaptation, achieved while pointing at visual tar-

gets, will transfer fully to acoustic targets. The model further predicts that visual-to-acoustic transfer is

not diminished even if the left and right arms have adapted to a different distortion. To scrutinize these

predictions, we asked subjects to point at visual targets with their right hands under a +30 deg rotation of

visual feedback (group “single”), or alternately, with their right hands under a +30 deg and with their left

hands under a −30 deg rotation of visual feedback. Aftereffects were registered for each hand and for vis-

ual as well as acoustic targets, in counterbalanced order. We found that acoustic aftereffects were only

about 66% of visual ones, which violates the first prediction and calls for an amendment of the model. We

further found that acoustic aftereffects were of similar magnitude in both groups, which supports the sec-

ond prediction. Finally, we observed an intermanual transfer of only about 29%. These findings suggest

that unpractised acoustic inputs are weighted somewhat lower than practised visual ones, and that outputs

to the unpractised left hand are weighted substantially lower than those to the practised right hand.

Keywords: Sensorimotor Adaptation; Motor Learning; Intermanual; Intersensory; Transfer

Introduction

Sensorimotor adaptation is not limited to the task by which it

was established, but rather can transfer to unpractised motor

systems (Cohen, 1973; Cotti, Guillaume, Alahyane, Pélisson, &

Vercher, 2007), new target locations (Bock, 1992; Malfait,

Gribble, & Ostry, 2005) and different distortions (Thomas &

Bock, 2010). These findings should not be taken as evidence

that adaptation is achieved by one common process, shared by

all motor systems, target locations and distortions. Rather, mul-

tiple adaptive processes seem to exist since subjects can con-

currently adapt to two distinct distortions, administered in de-

pendence on the arm used (Prablanc, Tzavaras, & Jeannerod,

1975; Thomas & Bock, 2012), on target location (Alahyane,

2004; Woolley, Tresilian, Carson, & Rick, 2007) or on contex-

tual cues (Wada et al., 2003). We are thus left with the puzzling

fact that adaptation can transfer from one arm to the other—as

if both arms shared a common adaptive process— whilst both

arms also can adapt differently—as if each had its own adaptive

process.

A model has recently been proposed to explain this apparent

discrepancy. As shown in Figure 1, it stipulates that multiple

sensory modalities serve as inputs to multiple adaptive mecha-

nisms which, in turn, send their outputs to multiple motor sys-

tems via a context-dependent switch; the outputs are differently

weighted for practised and for unpractised motor systems.

According to this model, single-limb practice doesn’t affect the

switch position and thus yields transfer of adaptation, while

two-limb practice changes the switch position in dependence on

the currently active motor system and thus yields limb-specific

adaptation.

Figure 1.

Conceptual model of sensorimotor adaptation. Sensory inputs from

different modalities (I1, I2, …) are transformed by multiple adaptive

mechanisms, and their outputs are connected by a context-dependent

switch to various motor outputs (O1, O2, …). The connections are

weighted differently (W1, W2, …) depending on whether the motor

system is practised or not. Reprinted with modifications from (Bock,

2013).

O. BOCK, G. SCHMITZ

The present study scrutinizes two predictions of the pub-

lished model. First, practised and unpractised motor systems

are assigned by different weights, while practised and unprac-

tised sensory modalities are not; if so, adaptation should trans-

fer partially between arms but fully between targets from dif-

ferent modalities. It has indeed been shown that adaptation with

visual targets transfers to acoustic targets (Zwiers, van Opstal,

& Paige, 2003) and that aftereffects are equal in both modalities

(Kagerer & Contreras-Vidal, 2009); however the latter study

always tested acoustic targets first and visual targets thereafter,

so that a possible visual advantage might have dissipated by the

time of testing. To overcome this problem, we now test both

arms and modalities in counterbalanced order.

According to the second prediction of the published model,

both arms can concurrently adapt to different distortions with-

out interference, and this dual adaptation should again transfer

fully between sensory modalities. The first part of this predict-

tion has been confirmed in literature (Bock, Worringham, &

Thomas, 2005; Prablanc et al., 1975), and we now address the

second part.

Methods

25 male and 23 female subjects aged 22.0  1.6 years parti-

cipated after signing their informed consent to this study, which

was approved in advance by the first author’s Ethics Committee.

Subjects were right-handed, healthy, and had no prior experi-

ence with adaptation research. As shown by the inset of Figure

1, subjects faced an opaque horizontal panel with a wooden

dowel protruding downwards underneath their chin. Targets

were presented on the upper surface of the panel, along a semi-

circle of 36 cm radius around the dowel. They were presented

in balanced order at 30, 18, and 6 deg about straight-ahead,

either as light dots of 1.5 cm radius, or as sounds (mix of 0.45,

1.35, 2.30 and 3.20 kHz) from loudspeakers of 1.5 cm radius;

the loudspeaker array was hidden from view by a fabric screen.

Subjects pointed with their index fingertip underneath the panel,

moving from the dowel towards each target where the radial

response component was stopped by a semicircular barrier.

Subjects then moved the finger along that barrier until it was

aligned with the target. They couldn’t see their arm and hand

because of the opaque panel, but index fingertip position was

registered by the Fastrak® system (resolution 1 mm and 17 ms),

and could be displayed as a cursor on the upper surface of the

panel to provide real-time visual feedback.

The experiment was subdivided into pointing episodes of 45

s duration (or about 25 responses), separated by rest breaks of 5

s. Each experiment started with six baseline episodes. In the

first two, subjects pointed with their right hand at visual targets

and received veridical visual feedback. In the remaining four

baseline episodes, they pointed without visual feedback at vis-

ual targets with their right hand (VR), at visual targets with

their left hand (VL), at acoustic targets with their right hand

(AR), and at acoustic targets with their left hand (AL); the order

of these episodes was counterbalanced across subjects. In the

subsequent adaptation episodes, subjects pointed at visual tar-

gets under rotated visual feedback. One half of them formed

group “single”, which used their right hand under a +30 deg

rotated feedback. The other half formed group “dual”, which

alternately used their right hand under a +30 deg rotated feed-

back, and their left hand under a −30 deg rotated feedback.

Group “single” performed 20 and group “dual” 40 adaptation

episodes, i.e., there were 20 adaptation episodes per hand in

both groups. The experiment concluded with four aftereffect

episodes, which replicated VR, VL, AR and AL, again in coun-

terbalanced order.

The registered finger position data were analysed by an in-

teractive computer routine which determined the directional

error of each response, defined as angle between target and

cursor direction 166 ms after movement onset. This quantifies

the feedforward component of motor control without confound-

ing it with feedback-based error corrections, which emerge later

during the response. For graphical presentation and statistics,

we calculated the mean error of each adaptation episode minus

that of the second baseline episode, and the mean errors of each

aftereffect episode minus that of the pertinent baseline episode.

To facilitate comparisons, data yielded with the left hand of

group “dual” were sign-reversed. The outcome was submitted

to analyses of variance (ANOVAs) with Greenhouse-Geyser

corrections if variances were unequal.

Results

Figure 2 illustrates the errors of group “single” and “dual”

throughout the adaptation block. For clarity, the left hand of

group “dual” is plotted in same episodes as the right hand al-

though the data actually came from separate, interleaved epi-

sodes. ANOVA with the between-factor Task (single, dual right,

dual left) and the within-factor Episode (1, 2, …, 20), yielded

significance only for Episode (F(19,1292) = 13.15, p < 0.001).

The aftereffects of both groups are summarized in Figure 3.

ANOVA with the between-factor Group (single, dual) and the

within-factors Hand (R, L) and Modality (V, A) yielded signi-

ficance for Group (F(1,46) = 5.83; p < 0.05), Hand (F(1,46) =

8.46; p < 0.01) and Group*Hand (F(1,46) = 64.80; p < 0.001):

aftereffects with the right hand reached a similar magnitude in

both groups, but only 29% of that magnitude with the unprac-

tised left hand of group “single”; the practised left hand of

group “dual” even showed slightly larger aftereffects than the

right hand. Furthermore, ANOVA yielded significance for

Modality (F(1,46) = 15.39; p < 0.001): aftereffects with acous-

tic targets were only 66% of those with visual targets. No other

interactions reached significance.

Discussion

The present study evaluates the transfer of sensorimotor ad-

aptation to an unpractised limb and to an unpractised sensory

modality, to scrutinize a recently published conceptual model

(Bock, 2013). Our discussion will focus on the observed after-

effects rather than on the time-course of adaptation, since after-

effects are thought to be a pure indicator of sensorimotor re-

calibration while the time-course of adaptation may also reflect

workaround strategies (McNay & Willingham, 1998; Redding,

1996; Werner & Bock, 2007).

The above model predicts that inputs from different sensory

modalities are weighted equally, which implies that adaptation

will fully transfer between modalities. This, however, was not

the case in our study: following adaptation with visual targets,

the magnitude of aftereffects with acoustic targets was only

66% of that with visual ones. The model therefore must be

amended; specifically, if the practised visual modality in our

study is given a weight of 1.00, then the unpractised acoustic

modality should obtain a weight of only 0.66. Further work is

Open Access 1005

O. BOCK, G. SCHMITZ

Open Access

1006

Figure 2.

Above-baseline errors during the adaptation block in groups “single” and “dual”. VR and VL stand for visual rotation while using the right and left

hand, respectively, with the type of rotation indicated in parentheses. Left-hand errors are inverted to facilitate comparisons. Symbols represent the

across-subject means for each episode, and bars the corresponding standard errors. The inset is a sketch of the experimental setup: subjects pointed

underneath an opaque panel, from a wooden dowel (D) towards visual (V) or acoustic targets (A) which were located along a semicircle around D; the

hardware for delivery of acoustic stimuli was concealed from view, but one stimulus location is made visible in the sketch.

Figure 3.

Aftereffects in group “single” and “dual”. Histograms represent across-

subject means, and bars the corresponding standard errors. Left-hand

data are inverted to facilitate comparisons. VR: visual targets and right

hand, VL: visual targets and left hand, AR: acoustic targets and right

hand, AL: acoustic targets and left hand.

needed to find out whether this lower weight is due to the lack

of practise, or rather to the poorer localization of acoustic com-

pared to visual targets.

Our finding, that input weights differ between sensory mo-

dalities, contrasts with the outcome of an earlier study, which

found visual and acoustic aftereffects of similar magnitude

following adaptation with visual targets (Kagerer & Contreras-

Vidal, 2009). We suggest that this discrepancy is due to an

artefact of fixed-order testing in the earlier study: acoustic af-

tereffects were always tested first and visual aftereffects second,

such that the latter might already have declined by the time of

testing.

The published model further predicts that there will be no

interference between sensory modalities when both arms adapt

to different distortions, which was indeed confirmed by our

data: the difference between visual and acoustic aftereffects

was comparable for both hands and groups. This might seem

trivial at a first glance; note, however, that the left-hand data of

group “dual” was sign-inversed for graphical presentation and

statistical analysis. Without that transformation, it would be

more obvious that visual and acoustic aftereffects of group

“dual” were positive with the right hand, but both were nega-

tive with the left hand. Thus, differential adaptation of the right

hand to +30 deg rotation and of the left hand to −30 deg rota-

tion, achieved with visual targets, led to an unabridged differ-

ential transfer to acoustic targets.

As an additional outcome of the present study, we now can

apply specific values to the output weights of the adaptation

model. Given the different magnitude of aftereffects with the

right and left hand of group “single”, a weight of 1.00 for the

practised right hand corresponds to a weight of 0.29 for the

unpractised left hand. However, this outcome can’t be general-

ized across all adaptation paradigms since different earlier

studies yielded substantially different magnitudes of interma-

nual transfer, depending e.g. on the distribution of practise

(Taub & Goldberg, 1973) and on the provision of continuous

versus terminal feedback (Cohen, 1967).

Summing up, the present study allowed us to confirm, modi-

fy and enumerate some aspects of a conceptual model of sen-

sorimotor adaptation. We plan to scrutinize other aspects in

future work.

Acknowledgements

This study was supported by the German Research Council

DFG (Bo 649/8) and by the German Federal Ministry for Re-

search and Technology (50WB9942). Responsibility for the

contents rests with the authors.

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