Although monitoring and subsequent control have
received quite some attention for cognitive systems
other than language, few studies have probed the neural
mechanisms underlying monitoring and control in overt
speech production. Recently, it has been hypothesized
that conflict signals within the language production
system might serve as cues to increase monitoring
and control (Nozari, Dell & Schwartz, 2011; Cognitive
Psychology). This hypothesis was linked directly to the
conflict monitoring hypothesis in non-linguistic action
control, which hypothesizes that one of the critical
cues to self-monitoring is the co-activation of multiple
response candidates (Yeung, Botvinick & Cohen, 2004;
Psychological Review). A region of the medial prefrontal
cortex (mPFC), the dorsal anterior cingulate cortex
(dACC), as well as the basal ganglia have consistently
been observed in both errors of commission and high
conflict.. Hence these regions serve as an important
testing ground for whether comparable monitoring
mechanisms are at play in language production. The
current study tests whether these regions are also
implicated in response to speech errors and high conflict
situations that precede the response. 32 native Dutch
subjects performed a tongue twister task and a factorial
combination of the Simon and Flanker task. In the tongue
twister task, participants overtly produced a string of
4 nonwords 3 times. In tongue twister trials (TT), the
onset phonemes followed a pattern of A-B-B-A, whereas
rhymes followed an A-B-A-B pattern (e.g. wep ruust
rep wuust). In non-tongue twister trials (nonTT), the
nonwords contained minimal phonological overlap
(e.g. jots brauk woelp zieg). These two conditions
correspond to a high conflict and a low conflict condition
respectively. In an arrow version of the the Simon-
Flanker task, subjects responded to the direction of a
middle arrow while flanking arrows faced in the same
(i.e., congruent; >>>>>) or different (i.e., incongruent;
>><>>) directions. These stimuli were presented either
on the right side or the left side of the screen, potentially
creating a spatial incongruency with their response
as well. Behavioral results demonstrated sensitivity
to conflict in both tasks, as subjects generated more
speech errors in tongue twister trials than non-tongue
twister trials, and were slower to incongruent relative
to congruent flanker trials. No difference between
spatial incongruency was observed. Neuroimaging
results showed that activation in the ACC significantly
increased in response to the high conflict flanker trials.
In addition, regions of interest analyses in the basal
ganglia showed a significant difference between correct
high and low conflict flanker trials in the left putamen
and right caudate nucleus. For the tongue twister task,
a large region in the mPFC - overlapping with the ACC
region from the flanker task - was significantly more
active in response to errors than correct trials. Significant
differences were also found in the left and right caudate
nuclei and left putamen. No differences were found
between correct TT and nonTT trials. The study therefore
provides evidence for overlap in monitoring between
language production and non-linguistic action at the
time of response (i.e. errors), but little evidence for preresponse
conflict engaging the same system.