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U. Wijk et al.
METHODS
the central nervous system to interpret the new affe-
rent signals (23). An important issue when designing
The study was conducted during 5 weeks for each participant,
sensory feedback systems in hand prostheses is how
on 18 learning occasions, including follow-up at occasions 11,
17 and 18. Each occasion comprised 4 sessions (Table I). Each
the feedback should be presented to the user in order to
subject was provided with a silicone cuff to be placed on the
be easy to interpret. The most optimal way to present
forearm with 5 servo motors representing the 5 fingers, and
sensory feedback is a combination of modality as well
constituting a tactile display that gave pressure stimuli in a
as somatotopically matched solutions (19). Modality
pseudo random order during the learning sessions. The subject
matched sensory feedback is when the feedback is
was seated in front of a laptop with the forearm resting on the
table during the sessions (Fig. 1). While given stimulations on
analogous to the external stimulation of the prosthesis,
the forearm, the subject was provided with feedback on a screen
and therefore logical in the interpretation for the user.
with a photo of a hand with 5 fingers. The user application that
For example, if the fingertips of the prosthesis receive
was developed for the purpose of this study was used to control
pressure the user should experience the stimulation as
the tactile display (Fig. 1), provide the user with visual feedback,
pressure (19). Mechanotactile stimulation (pressure)
and log performance. The main menu of the user application
can be seen in Fig. 2a.
has been proven to be easier to discriminate, compared
with vibrotactile feedback (21). Ideally the feedback
should also be somatotopically matched, meaning that
Subjects
the individual experiences the feedback as if it was
Able-bodied adults were included in the study and the exclusion
applied to the corresponding location on the lost limb
criterion was regular medication with drugs that might inhibit
(19). To achieve somatotopically matched sensory
concentration and learning. Thirty-five individuals enrolled in
the study. The subjects were students recruited from the Faculty
feedback non-invasively, the PHM can be used as a
of Medicine, Lund University and staff at the Department of
target for the actuators of the sensory feedback (21,
Hand Surgery, Skåne University Hospital, Malmö, Sweden.
24). Some amputees and all congenital amputees lack
The study was approved by the regional ethics review board
the PHM on the amputation stump and therefore also
in Lund (Dnr 2012/778) and all subjects gave their written in-
lack the possibility to use the PHM as an interface
formed consent. The study was conducted in accordance with
the Declaration of Helsinki.
for transferring sensory feedback from a prosthesis
(25). Thus, it is interesting to explore if it is possible
to learn to associate stimulation on areas on the skin
Learning protocol and follow-up
on the forearm with specific fingers of the hand, i.e. to
All subjects had a personal introduction to the programme and
induce an association of touching the fingers when the
learning by one of the authors (UW), who instructed all sub-
forearm is touched.
jects. The subjects were given a computer with a programme
and the associated hardware, which they used at each learning
The ability in localizing stimuli in the PHM has
occasion. The learning occasions were unsupervised during a
been investigated using vibrotactile or mechanotactile
2-week period (Fig. 3) and the participant chose the location for
(pressure) feedback and pressure stimulation surpassed
training. Following the 2 weeks there were additional follow-up
vibrotactile stimulation in multi-site sensory feedback
occasions. During the 2 weeks, there were 15 learning occasions
discrimination (26). A study of 7 amputees has reported
in total. In the first week the training was completed twice a day
during 5 days chosen by the subject (occasions 1–10). A mini-
that electrotactile feedback in somatotopically matched
areas was better than non-somatotopically
matched feedback for both accuracy and re-
sponse time (26). In another study of 11 subjects
(9 able-bodied and 2 amputees) electrotactile
stimulation was used to compare somatotopi-
cally matched areas with non-somatotopically
matched areas concerning correct identification
rate and response time. Results indicate that
areas on the skin without referred sensation
(non-somatotopically matched area) of the
phantom hand can be learned to be associated
with predefined stimulation areas (27).
The aim of this study was to explore whether
touch on predefined areas on the forearm can be
associated with specific fingers, using mechano-
tactile stimuli. A further aim was to investigate
Fig. 1. Left: The training set-up. Right: The cuff that was used on the left
if the associated sensory learning is influenced
forearm, with the servo motors in the black boxes. d1 – Thumb, d2 – Index,
by age and sex.
d3 – Middle, d4 – Ring and d5 - little.
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