Perception of weight through mental simulation

The present project aims at finalizing and extending the findings from a study
conducted by Nikola Valchev and Alessio Avenanti (Valchev et al., submitted) at
the Center for Cognitive Neurosciences at Cesena, University of Bologna during
the month of March 2009. In that experiment we explored the role of
somatosensory cortex (brain area SI) on the perception of weight. Subjects were
presented with videos of a hand lifting a box and were asked to judge the
weight of the box either after a TMS (transcranial magnetic stimulation)
perturbation of SI, primary motor cortex (M1) or sham (i.e. no stimulation).
Results show that a TMS disruption of SI but not of M1 weakened the precision
with which subjects could estimate the weight of the lifted box. These results
therefore show that the primary somatosensory cortex plays a role in our
perception of weight of an object we see lifted by somebody else. In addition,
Pobric et al (2006) found that a disruption of the premotor cortex leads to a
similar degree of impairment of the above mentioned task. Alaerts et al (2010)
found that the amplitude of the motor evoked potential (MEP), elicited by
single pulse TMS on M1, is modulated by the observation of an object being
lifted. With the present study we therefore wish to investigate whether the
modulation found by Alaerts et al is caused by the flow of information running
from SI to M1 and/or from the premotor cortex. In order to investigate if this
question we need first to replicate the findings of Alaerts et al (2010)
recording MEPs from the right hand muscles when participants view videos of a
right hand lifting an object of different weights. However we plan to use
videos in which the object is not visible (hidden behind a screen) and record
MEPs from muscles that are visible (ECR) and occluded (FDI, ADM) but clearly
involved in the action. If modulation of the MEPs is found for the non-visible
muscles and is proportional to the weight of the box, that would be an
indication that weight estimation could be done by mentally simulating not only
the kinematics of the action but also the effort of the action (somatosensory
simulation).

Investigate how is the motor cortex affected by the observation of a hand
lifting a box of different weights.

We will measure the magnitude of the MEPs, elicited by single pulse TMS while
subjects watch videos of a hand lifting a box of different weights. The box
will be hidden behind a screen, providing us with the possibility to measure
from visible and occluded muscles of the hand of the observer.

Single pulse TMS is a non-invasive technique, so there is no need of special
preparation of the subject.
The safety of the single pulse TMS has been demonstrated extensively (Gates,
1992; Pascual-Leone et al., 1993; Wassermann et al., 1996; Wassermann, 1998).
No harmful side effects have been reported when the international safety
guidelines are followed (Wassermann, 1998).

The strong magnetic field used by TMS can dislocate ferromagnetic particles
inside the brain and the eyes. In order to exclude subjects with metal
particles inside their brain, subjects will be required to complete a
questionnaire and only if none of the exclusion criteria is met the subject
will be allowed to participate in our experiment.