Overt sentence production in event-related fMRI
Introduction
Fundamental aspects of language include syntax, i.e. grammatically well formed words and sentences, and semantics, i.e. the meaning of a word or phrase. However, syntax and semantics are inseparable in natural language. Compare as an example the headlines “Dog bites man” and “Man bites dog”. Regarding the neural processes underlying human language, a further distinction has to be made between production and comprehension. During speech production, a mental concept is expressed (Kircher et al., 2004). Words with appropriate semantic meaning are retrieved from a store and an expression is created according to syntactic rules (Levelt, 1989, Levelt, 1995). During speech perception, the inverse process is performed. The final result of the comprehension process is the idea or the concept of a given expression. There is evidence from brain lesion studies involving patients with aphasia that distinct neural networks compute these two processes (Ferstl, Guthke, & von Cramon, 1999; Munhall, 2001). In particular, it has been shown that aphasic patients are unable to produce syntactically correct sentences, while being able to judge the syntactical correctness of presented sentences (Linebarger, Schwartz, & Saffran, 1983).
Most previous neuroimaging studies on language processing on a sentence level focused on the investigation of comprehension (Cabeza & Nyberg, 2000; Indefrey & Levelt, 2000; Kaan & Swaab, 2002). Many of these studies used sentences with spelling errors or wrong word order (Sakai, Hashimoto, & Homae, 2001) to test for different aspects of syntactic processing. Subjects usually performed plausibility judgments, i.e. they had to decide whether a sentence was correct or not (Embick, Marantz, Miyashita, O’Neil, & Sakai, 2000). In other studies, subjects had to decide as to whether or not two sentences were similar (Dapretto & Bookheimer, 1999). Language production should ideally be investigated by analyzing natural speech (Kircher, Brammer, Williams, & McGuire, 2000). However, it is difficult to control for the underlying cognitive processes involved in the generation of longer utterances. Therefore, the majority of previous functional neuroimaging studies on language production reduced verbal output to single words. Particularly well investigated are verbal fluency tasks, where subjects are instructed to produce words starting with a particular letter or of a semantic category (Cabeza & Nyberg, 2000). However, higher language properties such as syntax or the meaning of a phrase cannot be covered by these paradigms. Consequently, little is known about the cerebral substrates of sentence-level production processes (Indefrey & Levelt, 2000). In the only systematic imaging study (using positron emission tomography, PET) on syntax during language production on a sentence level (Indefrey, Brown, et al., 2001), subjects had to generate utterances of different syntactic complexity that described the motion of visually presented objects. The authors provided evidence that a region caudally adjacent to Broca's area is involved in both sentence-level and local (phrase-level) syntactic encoding during speaking.
In the present study, we investigated syntax production on a sentence level. Subjects had to produce simple subject verb object (SVO) sentences like e.g. “The child throws the ball”. We systematically varied the demand on syntax production in three conditions, while subjects spoke overtly. We hypothesed increasing signal changes in the left inferior frontal gyrus (IFG) with increased syntactic production demand.
Section snippets
Subjects
Fifteen right-handed (score ranging from 14 to 20 in the online version of the Edinburgh Handedness Inventory (Oldfield, 1971), http://airto.loni.ucla.edu [scores range from −20 (left handed) to +20 (right handed)]) male subjects (aged 19–44, mean 26.2, S.D. 6.5 years) gave their written informed consent prior to inclusion in the study. Subjects had no history of medical, neurological or psychiatric disorders. All subjects were native German speakers. The study has been approved by the local
Behavioral data
The correct response was given in 99.2, 100 and 95.8% of trials for conditions WR, SR and SG, respectively. The most common incorrect response was neglect of the second article (e.g. ‘The woman cooks noodles’ instead of ‘The woman cooks the noodles’), which occurred in 3.3% of trials in condition SG. There was a significant difference in the group comparison (p < 0.05 one-way ANOVA). Pair-wise analysis revealed significant difference between SR and SG (p < 0.05 Bonferroni's Multiple Comparison
Discussion
In the present study, subjects had to produce simple SVO sentences. The demand on syntactic production load was varied between three different conditions. In line with our predictions, activation was mainly observed in left inferior frontal gyrus/Broca's area for the higher syntactic demand condition.
In particular, a region of interest analysis revealed, that within the IFG, the maximum BOLD signal change of the active condition was significantly stronger in BA 45 compared to BA 44. Further,
Conclusions
In conclusion, we could demonstrate that language production on a sentence level can be investigated in fMRI using overt speech. In particular, activation associated with syntactic encoding during overt sentence production was predominant in the left inferior frontal gyrus and other left hemispheric areas. We emphasize the need to further investigate speech production directly, despite difficulties of controlling conceptual processing underlying the generation of sentences.
Acknowledgement
We thank Peter Indefrey for helpful comments on earlier versions of the manuscript.
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