Touch on the forearm can be associated with specific fingers
Therefore, for future clinical use, the noise could be
an additional sense to enhance learning.
Clinical implication
The long-term aim of the study was to enable amputees
who do not experience a PHM to use non-invasive
methods of sensory feedback in hand prostheses, as
reported previously (23). The present study was perfor-
med with able-bodied subjects who all had continuous
afferent nerve signalling from the forearm and hand.
This is in contrast to a forearm amputee who only has
afferent signalling from the forearm. Furthermore,
previous studies have shown that plasticity following
a change in afferent patterns results in more nerve
cells in S1 supplying the forearm area (39). The lack
of (competing) afferent signals from the hand and
an increased neuronal supply to the forearm leads us
to suggest that a person with a forearm amputation
would learn to associate touch on specific points of the
residual forearm faster than able-bodied individuals.
The PHM is an ideal interface for transferring sensory
information from receptors in the hand prosthesis to the
amputee. However, some amputees lack a PHM, but the
results of this study suggest that it is possible to learn to
associate touch on predefined areas on the forearm with
specific fingers. For clinical use it might not be neces-
sary to receive stimulation from 5 sites. By applying
only 3 predefined areas for stimulation in the U-shape
(d1, d3 and d5), it might be even easier to discriminate
the stimulations because of the increased distance bet-
ween the stimulation points. D’Anna et al. (40) has also
argued that trying to remember an increased number of
received force levels is a cognitive burden, and that it
is easier for the subjects to distinguish 3 different force
levels than a larger number of levels. This argument
could be applied to our study; that it might be easier for
subjects to identify only 3 stimulation positions rather
than identifying 5 stimulation sites. In a scenario with
a myoelectric prosthesis only 3 stimulation actuators,
instead of 5, in combination with the wider distance,
could therefore make it easier to make adjustments of
the areas for sensory stimulation when positioning the
EMG electrodes for controlling the motor functions.
Future studies should assess the effects of the de-
scribed training protocol and the possibility to learn
to associate touch on predefined areas on the forearm
with specific fingers in amputees without a PHM and
in congenital amputees.
ACKNOWLEDGEMENTS
This study was supported by the Promobilia Foundation and
Skåne County Council Research and Development Foundation.
215
The authors thank Jan-Åke Nilsson for help with the statistical
analysis and the subjects who participated in the training period.
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