Max Planck Institute of Cognitive Neuroscience, Leipzig, Germany
Address correspondence to Angela D. Friederici, Max Planck Institute of Cognitive Neuroscience, PO Box 500 355, 04303 Leipzig, Germany. Email: angelafr{at}cns.mpg.de.
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Abstract |
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Introduction |
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A distinction between semantic processing and rule-based syntactic processing during sentence comprehension has also been reported in a number of studies using neurophysiological methods, such as event-related brain potentials (ERPs) and event-related magnetic fields. Semantic processes are reflected in a centro-parietal negativity around 400 ms (N400). This component of the ERP has been shown to vary as a function of lexical status (word versus non-word), lexical-semantic information (selectional restriction), thematic information (verb argument information), as well as pragmatic information (Kutas and Van Petten, 1994; Kutas and Federmeier, 2000
). Syntactic processes have been correlated with an early and a late ERP component, namely a left anterior negativity (E/LAN) present between 150 and 400 ms (Neville et al., 1991
; Friederici et al., 1993
; Hahne and Friederici, 2002
) and a late centro-parietal positivity (P600) present around 600 ms (Hagoort et al., 1993
; Osterhout et al., 1994
; Friederici, 2002
). The two syntax-related ERP components have been attributed to two functionally different stages of syntax processing, i.e. an initial, automatized structure-building process and late, controlled processes of syntactic reanalysis and repair (Friederici et al., 1996
; Hahne and Friederici, 1999
).
Attempts have been made to localize the neural generators underlying these different ERP components. It has been proposed that the N400 arises from a number of functionally and spatially distinct generators (Nobre and McCarthy, 1994, 1995
). This suggestion is mainly based on data from intra-cranial depth recordings of ERPs during word reading. These data specify medial temporal structures close to the hippocampus as a possible location of the N400 generator. Data from intra-cranial recordings from less deep structures, however, suggest that cortical areas along the superior temporal sulcus are involved in the generation of the N400 (Halgren et al., 1994
). There have also been attempts to localize the sources of the N400 by means of magnetoencephalography or MEG (Papanicolaou et al., 1998
). Simos and collaborators (Simos et al., 1997
), measuring neuro- magnetic signals over the left side of the scalp, identified the neural generator of the N400 in the left temporal lobe. Helenius and colleagues (Helenius et al., 1998
) used whole-head MEG recordings to identify the generators of the N400 and found structures in the immediate vicinity of the left auditory cortex bilaterally to be implicated in semantic aspects of sentence comprehension. Also using MEG, the neural generators of the early syntax-related ERP component (ELAN) were found to be localized in inferior frontal and anterior temporal cortices bilaterally with, however, a clear dominance in the left hemisphere (Friederici et al., 2000b
). The question remains as to which brain areas are responsible for the processes reflected in the P600, as attempts to localize P600-generators have so far failed to yield reliable source models. Additional information concerning the cerebral representation of on-line syntactic processing can be won from studies investigating language processing in neurological patients. Patients with circumscribed left anterior cortical lesions, who have difficulties in processing syntactic structures, do not show the early left anterior negativity seen in healthy adults (Friederici et al., 1999
). Patients with impaired basal ganglia function (i.e. patients suffering from subcortical lesions or degeneration caused by Parkinsons disease), on the other hand, do show an early negativity but only a reduced, if any, P600 (Friederici et al., 1999
; Friederici et al., 2003
). These latter results suggest that basal ganglia structures, in particular the caudate nucleus, the putamen and the globus pallidus, play an important role in the controlled syntactic processes underlying the P600.
Most recent studies using advanced brain imaging techniques to specify the functional significance of different brain areas for syntactic and lexical-semantic processes during sentence comprehension suggest that sentence processing is supported by a fronto-temporal network, with semantic and syntactic aspects specifically employing the following subregions. Semantic processes are assumed to be dependent upon posterior temporal areas (Caplan et al., 1998; Kuperberg et al., 2000
; Ni et al., 2000
) as well as Brodmanns area (BA) 45/47 in the inferior frontal gyrus (IFG) (Dapretto and Bookheimer, 1999
). Syntactic processing has been shown to activate frontal as well as temporal areas. With respect to the frontal cortex, a few studies (Ni et al., 2000
; Newman et al., 2001
) have reported an involvement of the superior frontal gyrus, while the majority (Just et al., 1996
; Stromswold et al., 1996
; Caplan et al., 1998
, 1999
; Dapretto and Bookheimer, 1999
; Embick et al., 2000
; Friederici et al., 2000a
) reported BA 44/45 in the left IFG as relevant areas supporting syntactic processing. With respect to the temporal cortex, it is in particular the anterior superior temporal gyrus (STG) which has been seen activated as a function of syntactic structure (Friederici et al., 2000a
; Meyer et al., 2000
; Friederici, 2002
). There is tentative evidence that within the left IFG, a further functional separation can be made with respect to syntactic processes. The anterior portion of the IFG [i.e. BA 44 on the border to BA 45 (Fiebach et al., 2001
) and BA 47 (Cooke et al., 2001
)] seems to support aspects of syntactic memory as necessary in the processing of long antecedent-gap dependencies, whereas the posterior-inferior portion of BA 44, i.e. the inferior tip of the pars opercularis and deep frontal operculum on the border to ventral premotor cortex, is involved in on-line syntactic structure building processes (Friederici et al., 2000a
).
The functional description of the superior temporal region, which is implicated in both semantic and syntactic processing, is still a matter of debate. Scott and collaborators (Scott et al., 2000) suggest that the processing of spoken language might be organized in the form of two separable pathways through the superior temporal lobe, starting from primary auditory cortex. These authors propose the presence of an anterolateral pathway specific for the comprehension of speech. This pathway projects to the anterior STG, which is activated only by intelligible speech stimuli (Scott et al., 2000
). As, however, other studies have demonstrated that the left anterior temporal region was not activated during the perception of auditorily presented word and pseudoword lists, but only for auditory stimuli with a syntactic structure (Friederici et al., 2000a
), the notion of intelligibility with respect to the anterior STG may not be as general as initially assumed. Friederici and colleagues (Friederici et al., 2000a
; Meyer et al., 2000
) proposed that the left anterior STG, together with the frontal operculum, is responsible for on-line syntactic processes. With respect to the posterior portion of the superior temporal lobe Wise et al. (Wise et al., 2001
) proposed that this region might be involved in the transient representation of phonetic sequences, independent of whether or not these sequences constitute intelligible speech. Based on this assumption, the posterior superior temporal lobe should be activated whenever words or sentences are processed. Despite these relatively specific assumptions regarding the function of STG areas, the specific functional description of the anterior and posterior portions of the superior temporal region is still a matter of debate.
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The Present Study |
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Materials and Methods |
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Eight axial slices (5 mm thickness, 2 mm inter-slice distance, FOV 19.2 cm, data matrix of 64 x 64 voxels, in-plane resolution of 3 x 3 mm) were acquired every 2 s during functional measurements [BOLD (blood oxygen level dependent) sensitive gradient EPI sequence, TR = 2 s, TE = 30 ms, flip angle = 90°, acquisition bandwidth = 100 kHz] with a 3 T Bruker Medspec 30/100 system. Prior to functional imaging, T1-weighted MDEFT images (data matrix 256 x 256, TR 1.3 s, TE 10 ms) were obtained with a non-slice-selective inversion pulse followed by a single excitation of each slice (Norris, 2000). These were used to coregister functional scans with previously obtained high-resolution whole-head 3D brain scans 128 sagittal slices, 1.5 mm thickness, FOV 25.0 x 25.0 x 19.2 cm, data matrix of 256 x 256 voxels (Lee et al., 1995
).
Participants
Fifteen native speakers of German (seven male, aged 2330 years, mean age 24.8 years) participated in the study after giving informed consent. No participant had any history of neurological or psychiatric disorders. All participants had normal or corrected to normal vision and were right handed with laterality quotients of 90100% according to the Edinburgh handedness scale (Oldfield, 1971).
Materials
The experimental material consisted of short sentences containing transitive verbs in the imperfect passive form. Participial forms of 96 different transitive verbs, all of which started with the regular German participial morpheme ge, were used to create the experimental sentences. For each participle, three different critical sentences and one filler sentence were constructed (see Table 1).
In the syntactically incorrect sentences, the participle immediately followed a preposition, thus yielding a phrase structure error. In semantically incongruous sentences, the meaning of the participle could not be satisfactorily incorporated into the preceding context of the sentence. The correct filler condition, which was not included in the final fMRI analysis, contained a completed prepositional phrase as well as the participle construction and was included to ensure that participants could not predict a syntactic violation based purely on the presence of a preposition. The sentences were spoken by a trained female native speaker, recorded and digitized, and presented auditorily to the participants. Sentence conditions differed slightly in average length (correct condition = 1747 ms; semantically incorrect condition = 1740 ms; syntactically incorrect condition = 1937 ms; filler condition = 2339 ms). The complete set of materials is available from the authors.
Experimental Procedure
Two differently randomized stimulus sequences were designed for the experiment. The 96 sentences from each of the four conditions were systematically distributed between two lists, so that each verb occurred in only two out of four conditions in the same list. Forty-eight null events, in which no stimulus was presented, were also added to each list. The lists were then pseudo-randomized with the constraints that (i) repetitions of the same participle were separated by at least 20 intervening trials, (ii) no more than three consecutive sentences belonged to the same condition and (iii) no more than four consecutive trials contained either correct or incorrect sentences. Furthermore, the regularity with which two conditions followed one another was matched for all combinations. The order of stimuli in each of the two randomized stimulus sequences was then reversed, yielding four different lists. These were distributed randomly across participants.
An experimental session consisted of three 11 min blocks. Blocks consisted of an equal number of trials and a matched number of items from each condition. Each session contained 240 critical trials, made up of 48 items from each of the four experimental conditions plus an equal number of null trials, in which no stimulus was presented and the BOLD response was allowed to return to a baseline state (Burock et al., 1998).
The 240 presented trials lasted 8 s each (i.e. four scans of TR = 2 s). The onset of each stimulus presentation relative to the beginning of the first of the four scans was randomly varied between 0, 400, 800 and 1200 ms. The purpose of this jitter was to allow for measurements to be taken at numerous time points along the BOLD signal curve, thus providing a higher resolution of the BOLD response (Miezin et al., 2000). After the initial jittering time a fixation cue, consisting of an asterisk in the center of the screen, was presented for 400 ms before presentation of the sentence began. Immediately after hearing the sentence, the asterisk was replaced by three question marks, which cued participants to make a judgment on the correctness of the sentence. Maximal response time allowed was 2000 ms. Identifying the type of error was irrelevant. Participants indicated their responses by pressing buttons on a response box. After the response, the screen was cleared. Incorrect responses and unanswered trials elicited a visual feedback. These trials, as well as two dummy trials at the beginning of each block, were not included in the data analysis.
Data Analysis
The functional imaging data processing was performed using the software package LIPSIA (Lohmann et al., 2001). Functional data were corrected first for motion artifacts and then for slicetime acquisition differences using sinc-interpolation. Low-frequency signal changes and baseline drifts were removed by applying a temporal highpass filter to remove frequencies <1/60 Hz. A spatial filter of 5.65 mm FWHM was applied.
The anatomical images acquired during the functional session were co-registered with the high-resolution full-brain scan and then transformed by linear scaling to a standard size (Talairach and Tournoux, 1988). The transformation parameters obtained from this step were subsequently applied to the preprocessed functional images.
The statistical evaluation was based on a least-squares estimation using the general linear model for serially autocorrelated observations (Friston, 1994; Friston et al., 1995a
,b
; Worsley and Friston, 1995
). The design matrix was generated with a synthetic hemodynamic response function (Friston et al., 1998
; Josephs et al., 1997
). The model equation, made up of the observed data, the design matrix and the error term, was convolved with a Gaussian kernel of dispersion of 4 s FWHM. For each participant, three contrast images were generated, which represented the main effects of (i) correct sentences, (ii) syntactically violated sentences and (iii) semantically violated sentences. Subsequent group analysis consisted of a one-sample t-test across the contrast images of all participants that indicated whether observed effects were significantly distinct from zero (Holmes and Friston, 1998
). The resulting t-statistics were transformed to standard normalized distribution. Statistical parametric maps [SPM{Z}] were thresholded at Z > 3.09 (P < 0.001, uncorrected). Only clusters of at least five connected voxels (i.e. 225 mm3) are reported.
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Results |
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Semantically anomalous sentences also brought on increased activation along the STG bilaterally. This activation extended, as in the syntactic condition, into more posterior regions than seen for the correct condition; however, it did not extend into the anterior STG regions observed for the syntactic condition (Table 2C and Fig. 1
). Additional increases in activation specific to the semantic condition were observed in the anterior insula bilaterally, as well as in the right inferior premotor cortex (see Fig. 1
).
We conducted direct statistical comparisons between the experimental conditions, in order to determine whether violation-specific activity in the regions described above did indeed differ reliably from activity elicited during the processing of correct sentences. To this end, spherical regions of interest (ROIs; radius 3 mm) were defined around the local maxima of each activation site, as reported in Table 2. For these ROIs, average contrast values were extracted for each participant and subjected to a repeated measures ANOVA (Bosch, 2000
). A significantly greater increase in activation for the syntactic condition in comparison with the correct condition could be observed throughout the length of the left STG: posterior portion, F(1,14) = 7.53, P < 0.05; middle portion, F(1,14) = 4.88, P < 0.05; anterior portion, F(1,14) = 9.37, P < 0.01. The processing of syntactically violated sentences showed further tendencies towards greater activation increases than during processing of correct sentences in the left posterior frontal operculum [F(1,14) = 3.26, P < 0.1] and left basal ganglia [F(1,14) = 3.69, P < 0.1]. For the semantic condition in comparison to the correct condition, a significantly greater increase in activation was present in the mid-portion of the STG, bilaterally left, F(1,14) = 34.75, P < 0.01; right, F(1,14) = 20.19, P < 0.01 as well as in the anterior insula bilaterally left, F(1,14) = 15.62, P < 0.01; right, F(1,14) = 4.70, P < 0.05.
The direct comparison of the two anomalous conditions showed greater levels of activation for the processing of syntactic errors over semantic errors in the left basal ganglia [F(1,14) = 7.73, P < 0.05]. The processing of semantically anomalous sentences brought on significantly increased levels of activation in comparison to the processing of syntactic errors in the mid-portions of the STG, bilaterally [left, F(1,14) = 11.07, P < 0.01; right, F(1,14) = 10.76, P < 0.01].
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Discussion |
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Semantic Processes
The results concerning semantic processing are, in general, in accordance with previous studies. Both the analysis of the semantically anomalous sentences as well as the comparison between the two violation conditions revealed higher activation in the STG bilaterally, suggesting a specialization of this area for semantic processes. The bilateral activation of the STG for semantically anomalous sentences in this study is in line with previous studies looking at the processing of semantic anomalies (Kuperberg et al., 2000; Newman et al., 2001
; Ni et al., 2000
). A few studies have shown additional increased activation of inferior frontal cortex (Dapretto and Bookheimer, 1999
; Newman et al., 2001
), which was not evident in our results. However, when trying to integrate our data into existing findings on language processing we should keep in mind that different studies have relied upon a large variety of different types of stimuli modalities, tasks and languages. In particular, the majority of studies on semantic processing have investigated this issue at the word level (Démonet et al., 1992
; Fiez, 1997
; Poldrack et al., 1999
), whereas only a few have looked at semantic processes at the sentence level. IFG activation for semantic processes at the word level was reported for tasks which included strategic aspects of processing (Fiez, 1997
; Thompson-Schill et al., 1997
). Activation in the IFG for sentence level processes was reported by Dapretto and Bookheimer (Dapretto and Bookheimer, 1999
) in a sentence-comparison task including aspects of working memory and by Newman et al. (Newman et al., 2001
) in a sentence-well-formed-judgment task, both using written stimulus material.
The present semantic violation condition, moreover, revealed activation of the insular cortex bilaterally. A similar insular activation in the left hemisphere related to semantic processing was, for example, reported for a positron emission tomography (PET) study focusing on automatic semantic mechanisms during semantic word priming (Mummery et al., 1999).
Syntactic Processes
The analysis of the syntactic violation condition revealed increased activation in the posterior and most anterior portion of the STG, as well as in the frontal operculum and the left basal ganglia. The comparison between the two violation conditions only showed higher activation in the left basal ganglia for the syntactic violation over the semantic violation, supporting the notion of a special role of this structure during syntactic processing.
With respect to the processing of syntactic violations, two of the activation sites, namely the left frontal operculum and the left anterior portion of the STG, are similar to those reported in earlier studies. On-line syntactic phrase structure building processes during auditory comprehension have been reported to involve the left frontal operculum as well as the temporal pole (Mazoyer et al., 1993), or the anterior STG (Friederici et al., 2000b
; Meyer et al., 2000
); Humphries et al. (Humphries et al., 2001
) report this latter area to play an important role in sentence-level comprehension. In particular, the left frontal operculum in the inferior frontal lobe was found to be activated in previous studies investigating the processing of syntactic information (Stromswold et al., 1996
; Friederici et al., 2000a
). The present data are in complete agreement with these last findings.
Additional activation for the processing of syntactically anomalous sentences was seen in the putamen of the left basal ganglia. There is evidence in the literature for the notion that some basal ganglia structures play a role in on-line syntactic processing. Ullman (Ullman, 2001) points to the involvement of basal ganglia structures in a so-called procedural memory system a system which has been implicated in controlling well-established cognitive skills and which is thought to be involved in rule-based syntactic procedures. The involvement of left basal ganglia structures in syntax processing was predicted on the basis of the finding that patients with Parkinsons disease have problems in the application of grammatical rule processes in verb inflection (Ullman et al., 1997
; Ullman, 2001
) and on the basis of earlier ERP studies with brain-lesioned patients, suggesting an involvement of these structures in controlled syntactic processes (Friederici et al. 1999
, 2003
). In these latter studies, in which the same sentence material as in the present study was used, impaired function of the basal ganglia affected the late syntactic processes, as evidenced by a reduction or absence of the P600. Although the present data can not speak to the issue of syntactic on-line procedural versus late syntactic processes, they clearly indicate an involvement of the putamen in the left basal ganglia in syntactic processes. A recent fMRI study comparing the processing of syntactic versus morpho- syntactic violations (Moro et al., 2001
) also found structures within the left basal ganglia to be particularly involved in syntactic processing. Taken together, the data discussed here and the present results suggest that areas within the basal ganglia are involved in the processing of syntax during language comprehension. Structures of the basal ganglia obviously play an important role in syntactic processing. Moreover, their specific role appears to lie in the support of late controlled processes rather than early syntactic processes of phrase-structure building.
Posterior STG
One area in particular, namely the posterior STG, brought on a greater increase in activation for both anomalous conditions in comparison to correct sentences. This finding suggests that the functionality of the posterior STG is not domain-specific, but may rather be related to processes of sentence evaluation or processes of sentential integration. But what is the particular function of this brain area during language comprehension as realized in the present study? Sentence acceptability judgments, which participants had to make in all experimental conditions, may be more difficult in anomalous than in correct sentences, leading to a higher activation for incorrect than for correct conditions. However, as there was no behavioral difference between correct and incorrect conditions, this judgment-related interpretation is unlikely. Rather, it appears that increased activation in the posterior STG is a result of the increased effort involved in integrating an anomalous structure into a sentence. This presumably unsuccessful integration process is the only shared delineating feature between incorrect and correct conditions, leading us to believe that the shared posterior STG activation observed for both violation conditions in some way reflects the additional costs of attempted integration. Thus we propose that the posterior STG supports a processing stage during which different types of information, e.g. semantic, syntactic and pragmatic, are mapped onto each other to achieve a final interpretation.
Left Inferior Frontal Gyrus
It is interesting to note that the present study does not indicate any increased levels of activation in Brocas area (BA 44) in the left IFG, an area classically thought to support several general aspects of language processing. We argue, however, that this is a result of differences in task and material presentation between our study and previous studies. Specifically, we propose that activation in Brocas area may reflect a greater involvement of language-related working memory rather than on-line language processes. It appears that the pars opercularis of the left IFG (i.e. BA 44) may not be a necessary part of the network supporting on-line, sentence-level semantic and syntactic processes, but may only come into play under particular task demands. We will discuss this in more detail below.
With respect to semantic processing, activation in anterior inferior frontal cortex has previously been reported for sentence-level semantic aspects in combination with tasks requiring the comparison of two consecutively presented sentences, thus involving aspects of working memory (Dapretto and Bookheimer, 1999). Various studies have located specific subprocesses of verbal working memory in structures of the left IFG (Paulesu et al., 1993
; Gabrieli et al., 1998
), whereas others have described the left IFG to be involved in strategic semantic processes (Fiez, 1997
; Thompson-Schill et al., 1997
; Gabrieli et al., 1998
). While the present study did not reveal specific involvement of the IFG for semantic processing, bilateral activation of insular cortex was observed. Similar activation has been reported for studies focusing on automatic semantic aspects of word priming paradigms (Mummery et al., 1999
). It is possible that insular cortex activity in the present study reflects automatic aspects of semantic processing, while antero-lateral IFG activation reflects strategic aspects of semantic processing.
A similar distributional difference emerges from a comparison of studies within the syntactic domain. Inferior frontal activation in Brocas area has often been tied to syntactic processing. However, such activation was mostly elicited in studies examining the processing of complex sentences with long-distance syntactic dependencies (Just et al., 1996; Stromswold et al., 1996
; Inui et al., 1998
; Caplan et al., 1998
, 1999
, 2000
; Cooke et al., 2001
), whereas studies investigating on-line syntactic processes of phrase-structure building have reported fronto- opercular activation (Friederici et al., 2000a
). Thus, it can be concluded with respect to the results of the present study that the activation observed in the posterior portion of the left fronto-opercular cortex most likely is related to the on-line detection of the word category mismatch in syntactically violated sentences during the initial syntactic analysis (Friederici et al., 2000b
).
Recent studies have demonstrated that the involvement of Brocas area is not a function of syntactic complexity as such, but seems to be related more specifically to syntactic working memory necessary to maintain a displaced element in working memory over a prolonged distance while processing a syntactically complex sentence (Cooke et al., 2001; Fiebach et al., 2001
). Note that natural languages allow the displacement of an element from its original sentential position to another and that when encountering such a displaced element (e.g. a sentence initial object), the processing system keeps this element in working memory until its original sentential position is reached (Fiebach et al., 2002
). The manipulation of the sentences undertaken in the present experiment did not cause an increased load for working memory processes. Therefore, if increased IFG activation is indeed a product of increased utilization of working memory resources, it should not be expected in the present study.
The combined data from the various studies suggest that the deep left frontal operculum is involved in local on-line processes of syntactic structure building, whereas the more laterally located pars opercularis of the IFG appears to support the working memory required during processing of long-distance syntactic dependencies. It may be interesting to note that the latter process is reflected in the ERP in a sustained frontal negativity, with a maximum over the left hemisphere and spanning the time from the perception of the displaced element to its original position (King and Kutas, 1995; Kluender et al., 1998
; Fiebach et al., 2002
). The former process, i.e. on-line syntactic structure building, is correlated with the observation of a local, short-lived early left anterior negativity (Neville et al., 1991
; Friederici et al., 1993
; Kluender et al., 1998
; Hahne and Friederici, 2002
). Taken together, it appears that the two functionally distinct processes of local syntactic structure building and syntactic working memory also have a distinct neural basis.
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Conclusion |
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Acknowledgments |
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References |
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