mMTG and Lemma Access in Comprehension and Production

According to a prominent theory (Levelt, Roelofs, & Meyer, 1999), a particular
type of mental representation, called a lemma, helps speakers and listeners to
link a word*s meaning to its sound. A meta-analysis of neuroimaging studies on
spoken-word production by Indefrey and Levelt (2004; Indefrey, 2011a) localized
lemmas to the middle section of the left middle temporal gyrus (mMTG; MNI y =
-6 to -39 mm). However, evidence for the localization of lemmas for
comprehension is lacking. Moreover, evidence is also lacking on when the mMTG
is engaged during the course of comprehension and production.
In this study we will conduct 2 tasks (1 production and 1 comprehension).
According to the theory by Levelt et al. (1999), both of these tasks require
lemma access. During the task the mMTG or Vertex of the subject will be
stimulated during varying time-windows in which we expect lexical access to
happen. If the lemma is indeed localised to mMTG, then stimulating it in the
correct time window should result in reaction times that differ from the
condition in which the mMTG is not stimulated (i.e., Vertex).

Our main objective is to probe for the functional time-windows of the
mid-portion of the left middle temporal gyrus (mMTG) during word comprehension
and word production to determine whether the left mMTG is common to both
language comprehension and production. Triple-pulse TMS (tpTMS) will be applied
to temporarily disrupt function in this region during one of three different
time-windows after stimulus onset. Naming latencies and reaction times will be
used to infer the effects of TMS on left mMTG (as compared to sham-vertex
stimulation).

The study takes place at the Donders Centre for Cognitive Neuroimaging (DCCN)
and is designed as a within-subject experiment. Subjects will participate in a
picture naming and semantic classification task, with participants blind to the
relevance of the time and site of stimulation. The study will span one intake
session and two experimental sessions, however the intake session and first
experimental session will take place on the same day.
The first experimental session is an MRI structural scan and the second
experimental session is a TMS session. Triple pulse transcranial magnetic
stimulation (tpTMS) will be applied to either the mid portion of the left mMTG
or the Vertex. During the tasks participants will receive a tpTMS stimulation
to either of the 2 sites, at one of three time-windows following stimulus
onset.

Each participant will receive no direct benefit from participating in the
study, but will receive a compensatory (financial) incentive. Transcranial
magnetic stimulation (TMS) is a widely used non-invasive brain stimulation
technique, based on the principle of electromagnetic induction. During
stimulations, the participant will likely hear the clicks of the TMS pulses and
experience stimulation of nerves and muscles of the head. The most common side
effect is a light transient headache (2-4% occurrence). A severe headache is
uncommon (0.3-0.5% occurrence). In TMS studies of patient populations (e.g.
epilepsy) or that exceeded the standard protocols (e.g. in intensity or
frequency) epileptic seizures have been reported in rare cases. In the current
study all participants will be stimulated with protocols that fall within the
safety guidelines. All subjects are screened for their relevant medical history
and other TMS safety aspects (e.g. presence of metal parts in the head). In
summary, because the risk and burden associated with participation can be
considered negligible-to-minimal, we do not expect serious adverse events
during the project.
The noise in the MRI scanner, and lying in a small space, may lead to
discomfort in some subjects.