The perception of weight and the primary somatosensory cortex

The present project aims at finalizing and extending the findings from a study
conducted by Nikola Valchev and Alessio Avenanti (Valchev et.al., in prep)in
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 S1)
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
(transcranic 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. To investigate this question we will record motor evoked
potential while subjects watch objects being lifted, after a cTBS (continue
theta burst stimulation) perturbation of SI, of the premotor cortex or sham.
The results of the study will shed light on the relationship between SI,
premotor cortex and M1 during the observation and perception of the weight of
objects being lifted.

Investigate how S1 and the premotor cortex modulate the cortico-spinal
excitability as measures by single pulse TMS stimulation of M1.

The magnitude of the MEPs, elicited by single pulse TMS while subjects watch
different types of video clip showing objects being lifted, will be compared
after cTBS stimulation over the SI, premotor cortex or sham. The study will be
composed of four sessions, collected in four separate days, for a total
duration of max.6,5 hours. Between the different sessions there will be at
least one day.
The detailed schedule for a single subject is the following:

FIRST session - day 1:
To optimize the localization of the cTBS stimulation through the use of a
neuronavigation system, an fMRI session is needed. During this scanning is
required:
a) An anatomical scan (~8 minutes) to localize the putative mirror neuron
system.
b) An action execution task, to localize S1 and the premotor cortex of the
sectoins involved in a hand-hand-object interactions, which are supposed to be
more involved in conveying information relative to the objects to M1.
c) An action observation task, to localize the regions in SI and the premotor
cortex which are involved in perceiving the weight of a box seen to be lifted
by others. Participants will observe actors lift objects of different weight
(~8min). The total length of the MRI session will be about 30 minutes including
preparation.

SECOND session - day 2
The total duration of the second session will be approximately 2 hours and it
will be composed of:
1) a preparation phase of about 45 minutes (skin preparation, electrode
placements, optimal scalp position, resting motor threshold and registration of
12 MEPs at rest)
2) ~15 min of relaxation for the subjects before the beginning of the cTBS
stimulation.
3) a stimulation phase of about 40 min which will include
i) 40 seconds of cTBS stimulation on areas A
ii) 5 minutes of rest
iii) ~20-25 minutes of MEPs registration during the presentation of video clips
showing objects being lifted
iv) 12 new MEPs registration at rest
4)debriefing

THIRD session - day 3
The same as day 2, but with cTBS on area B.

FOURTH session - day 4:
The same as day 2 and 3, but with cTBS on area C.

fMRI and rTMS are both non-invasive techniques, so there is no need of special
preparation of the subject.
fMRI:
There are no risks that have been associated with the fMRI acquisition.
Subjects will be exposed to a magnetic field of 3 Tesla and rapidly alternating
gradients and radio frequency fields. This field is used on a routinely basis
in fMRI and MRI research. No harmful side effects have been reported. On rare
occasions, a peripheral nerve (abdomen) is stimulated by the changing magnet
gradients. This might cause an etching feeling but it is not harmful. The data
collected during the fMRI and MRI scans will be used for research purposes
only. However, if severe abnormalities are noticed a specialist (radiologist or
psychiatrist) will be asked for advice, upon decision of the research team. If
it is confirmed by the specialist that medical treatment is needed, then the
General Practitioner indicated by the subject will be notified.

TMS:
The safety of the rTMS 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 fields used by
both fMRI and rTMS 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.
The effect of TMS stimulation lasts for around an hour is easily canceled at
the moment that the subject moves freely and receives enough stimulation from
the outer world.