An investigation of the contribution of proximal vibratory cues in tactile material perception

While the great majority of haptic research has focused on information
collected at the contact surface between object and skin, a recent line of
research has shown that mechanical information propagates way beyond the
contact surface (i.e., the fingertips) through, and even beyond, the human hand
during touch interactions. It has furthermore been demonstrated that these
remote vibrational signals (i.e., measured at the dorsal side of the hand)
contain rich mechanical information about the properties of manipulated objects
or different gestures.

Importantly, and using a very similar approach as the one proposed here,
Libouton et al. (2012) showed how the tactile discrimination of roughness of
fine surfaces (i.e., fine sandpapers) remains intact when innervation of the
finger is compromised. This holds true both for pathological conditions
affecting the innervation of the finger and during ring-block anaesthesia of
the index in healthy subjects; indicating the significance of remote
vibrotaction in one crucial area of material perception, namely the perception
of fine roughness. However, whether this can be generalized to more natural or
complex surfaces than sandpapers and the extent to which these remote
vibrations are picked up by the human somatosensory system and used in the
perception of other material dimensions, remains unexplored.

This study aims to investigate the contribution of remote propagation waves in
the tactile perception of two important material properties, namely roughness
and hardness.

While it is known that propagation waves arise and information is available
proximal, very little is known about the behavioural relevance of it; that is,
whether and when the somatosensory system uses this information in order to
derive at a percept during behavioural tasks like stroking or tapping a
surface. The main objective is thus to disentangle the role of local
information (at the contact surface) from information propagating to proximal
sites during surface property discrimination, rating and material
identification tasks and to determine the relative contribution of remote
mechanoreceptors in material perception.

A 2AFC discrimination procedure and a repeated measures design are used. We
will directly compare the percepts elicited during exploration of material
probes with an anaesthetised index finger and in a control condition without
anaesthesia. Potential confounding factors (such as the temperature difference
between skin and material and the skin moisture level) are monitored and the
vibrational signal is measured using sensors (accelerometer placed on the
dorsal side of the hand and a tribometer under the stimulus). All participants
will receive both conditions (randomized balanced order). No placebo is
administered.

Participants will receive Ropivacaïne injections in the index finger. The risks
and burden of this procedure have been mitigated by keeping the experimental
time short (maximum 2 1/2 hrs) for each experiment and thus minimising the
quantity of the local anaesthetic agent and avoiding other agents with a longer
duration of effect like bupivacaine or marcaine. Vulnerable groups (such as
children or pregnant women) and participants with any condition indicating an
increased risk, burden, or disability to give informed consent will be excluded
from this study (cf. Article 4 for more detail). Furthermore, participants will
be introduced to the risks and discomfort of the procedure, they will be given
sufficient time for consideration of their participation, can ask questions and
withdraw their consent or end participation at any time. Beyond the ones
associated with the Ropivacaïne injections in one of the two sessions, no risks
nor burdens beyond standard (everyday) risks are expected for participation in
this study. Beyond financial reimbursement there is no direct benefit for the
participants.