Dissociating the role of the inferior frontal gyrus and middle temporal gyrus in language comprehension with transcranial magnetic stimulation

Language comprehension requires people to retrieve long-term information about
words, and to integrate, or unify this information into a coherent message.
Previous neuroimaging evidence has associated the left middle temporal gyrus
(MTG) and inferior frontal gyrus (IFG) with the storage and unification of
lexical (i.e., word-based) information in sentence processing. Although this
research demonstrates that the importance of these two brain regions in
language comprehension, the brain behaviour relationship inferred through
functional neuroimaging is correlational in nature. The goal of the present
study is to investigate whether the MTG and IFG are causally involved
(respectively) in the representation and integration of lexical information in
sentence processing. In order to establish this causal relationship, repetitive
trancranial magnetic stimulation (rTMS) will be applied to the left MTG and IFG
as people perform a language comprehension task.

The aim of this study is to investigate whether the MTG and IFG serve
dissociable roles in the storage and integration of lexical information in
sentence comprehension. We will test this by applying rTMS to each of these
areas on separate days as people are engaged in reading sentences while we
measure their eye movements.

The study will consist of three experimental sessions, each separated by
approximately one week. During the first session, the motor threshold (MT) will
be determined and a structural MRI-scan will be made. In the other two
sessions, an off-line TMS-protocol will be used in which continuous Theta Burst
Stimulation (cTBS) is administered to each of the two brain regions of interest
(on different weeks) followed by performance of the sentence processing task.
Two different types manipulations on the sentence material will be used in the
present study. One set of sentences will vary according to the presence or
absence of word-class (i.e., noun/verb) ambiguous words. The second set of
sentences will vary both in comprehension difficulty (easy and difficult) and
in the frequency (high and low) with which words occur in Dutch.

Each participant will receive two different theta-burst stimulations of 600
pulses at 80% of active motor threshold for 40 seconds. Each session will
involve one TMS intervention.
1. Continuous TBS (inhibition) of the left MTG
2. Continuous TBS (inhibition) of the left IFG

TMS is not painful at the level of intensity used in this project (i.e. at 80%
of active motor threshold), but can induce muscle twitches and eye blinks that
some people find uncomfortable. According to previous literature, it might be
possible that, in rare cases, subjects could report a (light) headache. This
could be treated easily with paracetamol. On the basis of incidental epileptic
seizures triggered by TMS in early 90*s, safety-guidelines were established to
set up the maximum duration of TMS stimulation (Anderson et al., 2006;
Wassermann, 1998; see attached appendix for the Wasserman (1998) guidelines).
Therefore, our protocols will follow these safety TMS guidelines. Furthermore,
all subjects will be pre-screened for relevant medical history, epilepsy, drug
abuse, head trauma, neurological or psychiatric illness, pregnancy, heart
disease, cardiac pacemakers, medication pumps, tricyclic antidepressants,
neuroleptics and a family history of neurological illness, psychiatric illness
or epilepsy. Because the risk associated with participation can be considered
negligible and the burden can be considered minimal, we do not expect adverse
events during the project.