4.2. fNIRS
Overall, the fNIRS analysis revealed a limited prefrontal activity in
SRC and more widespread prefrontal activity in ORC condition. Moreover,
direct ORC>SRC comparison revealed significant activity
both in the left and right DLPFC. Given that the ORC processing was
expected to be more difficult than SRC processing in Turkish, this
finding was consistent with the initial hypotheses.
In the SRC condition, the middle frontal gyrus (i.e., BA 9) showed
significant hemodynamic activity (Fig. 3A). It was found that left BA 9
was active during maintaining or remembering the piece of information
just received (Raye et al., 2002). Therefore, activity in these regions
can be due to the subjects’ need to recall the relationship between the
object and subject of the main and relative clauses to answer the
follow-up questions correctly. Furthermore, Jörgens et al. (2007) showed
that BA 9 is also active during a sentence completion task. The other
regions showing significant activity were in the lateral portions of the
left prefrontal cortex including BA 46. It was shown that left BA 46
shows activity during semantic processing (Ni et al., 2000; Wang et al.,
2008) and syntax processing (Ni et al., 2000) in sentence level. Even
though the focus of the present study was to investigate syntax
processing at the sentence level, subjects had to process the semantics
as well to answer the questions. Furthermore, it was shown that the left
DLPFC plays role in various aspects of sentence processing, such as
parsing or plausibility (Hertrich et al., 2021).
As can be seen in Figure 3B, the strongest hemodynamic activity was
mostly localized in the left DLPFC (i.e., BA 9, 45, and 46) in ORC
condition. It was shown that the left DLPFC involves in language
processing (Hertrich et al., 2021; Klaus & Schutter, 2018) and syntax
processing (Chen et al., 2006; Indefrey et al., 2001; Just et al., 1996;
Ni et al., 2000). Therefore, it can be said that the activity in the
lateral portions of the left prefrontal cortex occurs due to sentence
processing. The other active region was the right DLPFC, covering mostly
BA 46 (Fig. 3B), which was also reported for relative clause
> noise processing (Hassanpour et al., 2015) and
ORC>SRC processing (Kovelman, Baker, et al., 2008). It was
suggested that the right DLPFC could be an indicator of greater
cognitive control required for processing more difficult sentences (Ding
et al., 2021). It is known that both left and right DLPFC are part of
the executive control network (Hertrich et al., 2021) and the right
DLPFC, together with the anterior cingulate cortex, plays role in
error/conflict monitoring and resolution (Badre & Wagner, 2004; Jackson
et al., 2022). It was also shown that the right DLPFC is active while
processing more complex garden-path sentences (den Ouden et al., 2016).
Several explanations have been put forward for RC processing asymmetry
in Turkish (Özge et al., 2010). The authors’ first claim was that SRCs
are more common in Turkish compared to ORCs, which can lead to SRC
processing advantage. The other account was based on the word-order
similarity. In Turkish, the canonical word order is as SOV, while ORCs
are produced with SVO, and SRCs are produced with OVS word order. As can
be seen, the word order of SRC shows higher similarity to the canonical
word order of SOV, which can lead to SRC processing advantage in
Turkish. The perspective shift was proposed as another explanation for
SRC processing advantage. Relative clauses are prenominal structures in
Turkish and the focus in SRC is on the agent (subject), while the focus
in ORC is on the patient (object). Since the canonical sentence starts
with a subject in Turkish, it was expected that starting of a sentence
should have the agent role. If it does not coincide with this role, as
in ORC containing sentence, then a conflict arises (i.e., shift in
perspective is required) and leads to a processing disadvantage. As
explained above, SRC and ORC are constructed with different suffixes.
Özge et al. (2009) stated that the suffix used in ORCs (i.e., “-dık”)
could also be seen in other structures in Turkish, such as noun
complement clause (Kahraman et al., 2010), while the suffix used in SRCs
not. Therefore, it could be said that a direct form-function mapping
leads to a processing advantage in SRCs in Turkish, while participants
should employ further cognitive resources such as error monitoring to
correctly process ORCs.
Direct ORC>SRC contrast revealed a significantly greater
hemodynamic activity in three frontal regions (Fig. 4). The first region
corresponds to the left DLPFC (i.e., BA 45 and 46). The left BA 45
corresponds to Broca’s Area, known to be involved in language
processing. Furthermore, multiple imaging studies on English relative
clause processing have reported that the activity in BA 45 and BA 46
increases in more difficult condition (Caplan et al., 2002; Chen et al.,
2006; Just et al., 1996; Kovelman, Baker, et al., 2008). An EEG study
carried out in Basque, in which SRC was expected to be the more
difficult condition, revealed that SRCs induced greater P600 amplitudes
compared to ORCs, especially in the left dorsolateral regions (Carreiras
et al., 2010). Another study showed that the left DLPFC activity could
be associated with checking the thematic roles in the sentence as well
as with response selection among alternatives (Kovelman, Baker, et al.,
2008). The left DLPFC was also associated with working memory (Hertrich
et al., 2021), which was also crucial process in the present task as the
subjects had to identify the agent and patient in both main and relative
clauses to answer the question.
The other active channel collects signal from both left frontopolar
cortex and orbitofrontal cortex (i.e., BA 10 and 11, respectively). BA
10 and 11 have not been reported in RC literature, except for one study
(Caplan et al., 2008). The authors reported that BA 11 shows significant
activity for ORC>SRC comparison. However, the locus of this
activity was more in the lateral part of the prefrontal cortex. On the
other hand, the active channel in this report was located in the medial
parts of the prefrontal cortex. Therefore, it is more likely that the
source of this activity was the frontopolar cortex. Even though this
region is not specifically associated with language processing, it was
shown that it involves many cognitive processes including
problem-solving and decision-making (Koechlin & Hyafil, 2007). They
proposed that the frontopolar cortex involves in cognitive branching
processes, such as computing a complicated math problem by first
computing smaller fragments and then combining the results of these
fragments. This might also be the case in relative clause processing,
during which the participants should identify the role of each noun
phrase in both main and relative clauses and then compare them to answer
the question correctly. Another study also stated that thematic role
control is mostly performed by frontal areas of the left hemisphere
(Kovelman, Baker, et al., 2008). Even though we made changes in the
stimuli to prevent subjects to take advantage of thematic roles, it is
hard to rule out the possibility that subjects employed this strategy in
syntactically more difficult sentences.
Lastly, we found a significant activity difference for
ORC>SRC in the right DLPFC (i.e., BA 45 and BA 46). The
activity of this region was associated with processing more complex
sentences (Ding et al., 2021; Kovelman, Baker, et al., 2008). As stated
above, the right DLPFC is part of the domain-general executive control
network and plays role in many cognitive processes including
error/conflict monitoring. It could be the case that the activity in
right DLPFC was associated with the perspective-based explanation. To
recall, Turkish speakers expect an agent at the beginning of a canonical
sentence. On the other hand, encountering an ORC at the beginning of the
sentence creates a conflict to be resolved, resulting in greater
activity in this region compared to the SRC condition. The other
possibility could be that the activity within this region may reflect
the evaluation of the alternatives induced by the suffix “-dık” used
in ORCs. It was claimed that the right DLPFC shows activity in response
selection (Badre & Wagner, 2004). As stated above, the suffix “-dık”
is used in other structures in Turkish and the participant should
eliminate the alternatives to choose the correct representation, which
might require further cognitive control and conflict
monitoring/resolution.