Syntactic Computation in the Human Brain: The Degree of Merger as a Key Factor

Our goal of this study is to characterize the functions of language areas in most precise terms. Previous neuroimaging studies have reported that more complex sentences elicit larger activations in the left inferior frontal gyrus (L. F3op/F3t), although the most critical factor still remains to be identified. We hypothesize that pseudowords with grammatical particles and morphosyntactic information alone impose a construction of syntactic structures, just like normal sentences, and that “the Degree of Merger” (DoM) in recursively merged sentences parametrically modulates neural activations. Using jabberwocky sentences with distinct constructions, we fitted various parametric models of syntactic, other linguistic, and nonlinguistic factors to activations measured with functional magnetic resonance imaging. We demonstrated that the models of DoM and “DoM+number of Search (searching syntactic features)” were the best to explain activations in the L. F3op/F3t and supramarginal gyrus (L. SMG), respectively. We further introduced letter strings, which had neither lexical associations nor grammatical particles, but retained both matching orders and symbol orders of sentences. By directly contrasting jabberwocky sentences with letter strings, localized activations in L. F3op/F3t and L. SMG were indeed independent of matching orders and symbol orders. Moreover, by using dynamic causal modeling, we found that the model with a inhibitory modulatory effect for the bottom-up connectivity from L. SMG to L. F3op/F3t was the best one. For this best model, the top-down connection from L. F3op/F3t to L. SMG was significantly positive. By using diffusion-tensor imaging, we confirmed that the left dorsal pathway of the superior longitudinal and arcuate fasciculi consistently connected these regions. Lastly, we established that nonlinguistic order-related and error-related factors significantly activated the right (R.) lateral premotor cortex and R. F3op/F3t, respectively. These results indicate that the identified network of L. F3op/F3t and L. SMG subserves the calculation of DoM in recursively merged sentences.

Syntactic Computation in the Human Brain: The Degree of Merger as a Key Factor Shinri Ohta, Naoki Fukui, Kuniyoshi L. Sakai

Task Instructions
Before the experiments, all participants were fully informed about the stimuli and task. We instructed and trained the sentence conditions in the order of Simple (S) , Conjoined (S) , Nested (S) , Simple (L) , Conjoined (L) , and Nested (L) , as the number of vowel extraction increased in this order. The following is a translation of task instructions in Japanese.
You will perform a task under three types of conditions during scanning of your head images inside the MRI scanner. At the beginning of each trial, a cue ("G", "M", or "R") denoting a condition will appear. By noting these cues, please judge which condition will be presented. During the task, a small red cross remains at the center of the monitor. Fixate this red cross as much as you can, but you may blink. Please do not speak or read aloud during the task.
During the task, please respond by pressing a button. Hold a switch box by both hands, with a red button to your right. When you press one, use your right thumb without looking at the buttons. Please note the following crucial points: • Press a button only once, when necessary.
• Please press the button as fast as you can, while the stimuli are presented.
• Every stimulus is consisted of a cue denoting a condition and of four or six phrases or strings.
• Do not press a button before the last stimulus.

Grammar conditions
There are six patterns under the Grammar conditions. The task is to judge whether or not the vowel of a subject is matched with the last vowel of the corresponding verb root. At the beginning of each trial, the cue "G" denoting a condition will appear. Do not overlook this cue, so that you can answer correctly. For every pattern shown below, "G" will appear.

Pattern 1 (Simple (S) )
Pattern 1 is similar to "Taro-no ani-ga sinobi aruita" ("Taro's brother sneaked around"). The subject is "ani", and the verb is "aruita". Please judge whether or not the vowel of the second phrase (subject) is matched with that of the fourth phrase (corresponding verb). Please press the rightmost button if correct, and press the second one from the right if wrong. Press the button quickly with your right thumb, while the fourth stimulus is presented.

Pattern 2 (Conjoined (S) )
Pattern 2 is similar to "Taro-ga utatte Hanako-ga odoru" ("Taro sings, and Hanako dances"). Please judge whether or not the vowel of the first phrase is matched with that of the second phrase, and the vowel of the third phrase with that of the fourth phrase.

Pattern 3 (Nested (S) )
Pattern 3 is similar to "Taro-ga Hanako-ga utau-to omotta" ("Taro thought that Hanako would sing"). Please judge whether or not the vowel of the first phrase is matched with that of the fourth phrase, and the vowel of the second phrase with that of the third phrase.
2. There are sentences that have multiple errors; please check them up to the last phrase. 3. Please press the button as fast as you can, while the last phrase is presented. 4. When the last phrase disappears, please quit the button press, and concentrate on the next trial.

Memory condition (Same (S) )
The task is to memorize the presented letter strings. At the beginning of each trial, the cue "M" denoting a condition will appear. Do not overlook this cue, so that you can answer correctly.
Four strings will appear one by one on the monitor. Please memorize these four strings, and judge whether or not the first and third strings, and the second and fourth ones, are exactly the same.

Reverse condition (Reverse (S) )
The task is to memorize the presented letter strings. At the beginning of each trial, the cue "R" denoting a condition will appear. Do not overlook this cue, so that you can answer correctly.
Four strings will appear one by one on the monitor. Please memorize these four strings, and judge whether or not the first and fourth strings, and the second and third ones, are exactly the same.
Correct examples: "yogayo tarute tarute yogayo" "hiruhu garara garara hiruhu" Wrong examples: "yonoyo tettata tatetta yonoyo" "hihhata nogugu nogugu settaso" Pattern 4 is similar to "Taro-no ani-no yujin-ga hasiri mawatte utatta" ("Taro's brother's friend run around and sang"). The subject is "yujin", and the verbs are "mawatte" and "utatta". Please judge whether or not the vowel of the third phrase is matched with that of the fourth or fifth phrase, and the vowel of the third phrase with that of the sixth phrase. Please press the rightmost button if correct, and press the second one from the right if wrong. Press the button quickly with your right thumb, while the sixth stimulus is presented.

Correct examples:
"zaza-no yoyo-no mumu-ga tete kikutte hihuru" "zaza-no yoyo-no mumu-ga tetutte kiki hihutta" × Wrong examples: "zaza-no yoyo-no mumu-ga tetutte kiki hiharu" × × "zaza-no yoyo-no mumu-ga tete kikatte hihotta" 1.2 Pattern 5 (Conjoined (L) ) Pattern 5 is similar to "Taro-ga odotte Hanako-ga utatte Jiro-ga asobu" ("Taro dances, Hanako sings, and Jiro plays"). Please judge whether or not the vowel of the first phrase is matched with that of the second phrase, the vowel of the third phrase with that of the fourth phrase, and the vowel of the fifth phrase with that of the sixth phrase.
Correct examples: "zaza-ga tetatte yoyo-ga kikotte mumu-ga hihuru" "zaza-ga tetatte yoyo-ga kikotte mumu-ga hihutta" Wrong examples: × "zaza-ga tetatte yoyo-ga kikotte mumu-ga hiharu" × × "zaza-ga tetutte yoyo-ga kikotte mumu-ga hihatta" 1.3 Pattern 6 (Nested (L) ) Pattern 6 is similar to "Taro-ga Hanako-ga Jiro-ga utau-to omou-to kangaeta" ("Taro supposed that Hanako would think that Jiro would sing"). Please judge whether or not the vowel of the first phrase is matched with that of the sixth phrase, the vowel of the second phrase with that of the fifth phrase, and the vowel of the third phrase with that of the fourth phrase.

Memory condition (Same (L) )
Six strings will appear one by one on the monitor. Please memorize these six strings, and judge whether or not the first and fourth strings, the second and fifth ones, and the third and sixth ones, are exactly the same.

Training Procedures
Experiments with short or long stimuli were performed on separate days in the order short, then long. Before scanning, the participants were trained until they scored 80 % at each of the following stages. Each of the Simple, Conjoined, and Nested were separately tested with self-paced reading in 10 to 30 trials with this order. Only one participant took 50 trials for the Nested (L) . When