Christian J. Fiebach

fMRI Evidence for Dual Routes to the Mental Lexicon in Visual Word Recognition

Event-related fMRI was used to investigate lexical decisions to words of high and low frequency of occurrence and to pseudowords. The results obtained strongly support dual-route models of visual word processing. By contrasting words with pseudowords, bilateral occipito-temporal brain areas and posterior left middle temporal gyrus (MTG) were identified as contributing to the successful mapping of orthographic percepts onto visual word form representations. Low-frequency words and pseudowords elicited greater activations than high-frequency words in the superior pars opercularis [Brodmanns area (BA) 44] of the left inferior frontal gyrus (IFG), in the anterior insula, and in the thalamus and caudate nucleus. As processing of these stimuli during lexical search is known to rely on phonological information, it is concluded that these brain regions are involved in grapheme-to-phoneme conversion. Activation in the pars triangularis (BA 45) of the left IFG was observed only for low-frequency words. It is proposed that this region is involved in processes of lexical selection.

Revisiting the role of Broca's area in sentence processing: syntactic integration versus syntactic working memory.

Most previous neuroimaging studies of sentence processing have associated Brocas area with syntactic processing; however, the exact nature of the processes subserved by this brain region is yet not well understood. Although some authors suggest that Brodmann area (BA) 44 of the left inferior frontal gyrus (i.e., Brocas area) is relevant for syntactic integration processes, others claim that it is associated with working memory mechanisms relevant for language processing. To dissociate these two possible functions, the present study investigated hemodynamic responses elicited while participants processed German indirect wh‐questions. Activation increases were observed in left BA 44 together with superior temporal areas and right hemispheric homologues for sentences with noncanonical word order, in which a verb argument was dislocated from its canonical position over a relatively long distance. In these sentences, syntactic working memory load was assumed to be greatest. In contrast, no activation increase was elicited by object–initial as opposed to subject–initial sentences that did not differ with respect to working memory costs but with respect to syntactic integration costs. These data strongly suggest that Brocas area plays a critical role in syntactic working memory during online sentence comprehension. Hum. Brain Mapping 24:79–91, 2005.

How the brain integrates costs and benefits during decision making

When we make decisions, the benefits of an option often need to be weighed against accompanying costs. Little is known, however, about the neural systems underlying such cost–benefit computations. Using functional magnetic resonance imaging and choice modeling, we show that decision making based on cost–benefit comparison can be explained as a stochastic accumulation of cost–benefit difference. Model-driven functional MRI shows that ventromedial and left dorsolateral prefrontal cortex compare costs and benefits by computing the difference between neural signatures of anticipated benefits and costs from the ventral striatum and amygdala, respectively. Moreover, changes in blood oxygen level dependent (BOLD) signal in the bilateral middle intraparietal sulcus reflect the accumulation of the difference signal from ventromedial prefrontal cortex. In sum, we show that a neurophysiological mechanism previously established for perceptual decision making, that is, the difference-based accumulation of evidence, is fundamental also in value-based decisions. The brain, thus, weighs costs against benefits by combining neural benefit and cost signals into a single, difference-based neural representation of net value, which is accumulated over time until the individual decides to accept or reject an option.

Separating syntactic memory costs and syntactic integration costs during parsing: The processing of German WH-questions.

Event-related brain potentials (ERPs) were recorded while participants processed case-unambiguous German subject and object WH-questions with either a long or a short distance between the WH-filler and its gap. A sustained left anterior negativity was observed for object questions with long filler-gap distance but not for short object questions. This negativity was modulated by individual differences in working memory capacity. No comparable negativity was elicited by WHETHER-questions which did not contain a filler-gap dependency. A positive-going ERP effect was observed for short and long object WH-questions at the position of the second noun phrase. We interpret the sustained negativity as reflecting working memory processes required for maintaining the dislocated object in memory. Processing costs associated with integrating the stored element into the phrase structure representation are indicated by the local positivity. These results support the notion of separable syntactic working memory and syntactic integration cost components as causes of processing difficulty in complex sentences.

Syntactic working memory and the establishment of filler-gap dependencies: insights from ERPs and fMRI.

In this contribution, we review an ERP experiment and an fMRI experiment which investigated the processing of German wh-questions. On the basis of the ERP results, we will discuss current models of sentence processing and resource distribution during sentence comprehension. We argue that there exists a separate cognitive or neural resource that supports syntactic working memory processes necessary for the temporary maintenance of syntactic information for the parser. In the context of wh-movement, such a memory component is necessary for establishing filler-gap dependencies. The data obtained from the fMRI experiment will be used to discuss the results of previous neuroimaging studies of sentence processing. It is claimed that syntactic working memory, rather than syntactic processing per se, is supported by Brocas Area.

Processing concrete words: fMRI evidence against a specific right-hemisphere involvement

Behavioral, patient, and electrophysiological studies have been taken as support for the assumption that processing of abstract words is confined to the left hemisphere, whereas concrete words are processed also by right-hemispheric brain areas. These are thought to provide additional information from an imaginal representational system, as postulated in the dual-coding theory of memory and cognition. Here we report new event-related fMRI data on the processing of concrete and abstract words in a lexical decision task. While abstract words activated a subregion of the left inferior frontal gyrus (BA 45) more strongly than concrete words, specific activity for concrete words was observed in the left basal temporal cortex. These data as well as data from other neuroimaging studies reviewed here are not compatible with the assumption of a specific right-hemispheric involvement for concrete words. The combined findings rather suggest a revised view of the neuroanatomical bases of the imaginal representational system assumed in the dual-coding theory, at least with respect to word recognition.

Neuronal Mechanisms of Repetition Priming in Occipitotemporal Cortex: Spatiotemporal Evidence from Functional Magnetic Resonance Imaging and Electroencephalography

Repeated stimulus presentation (priming) is generally associated with a reduction in neuronal firing, macroscopically mirrored by a decrease in oscillatory electrophysiological markers as well as reduced hemodynamic responses. However, these repetition effects seem to be dependent on stimulus familiarity. We investigate the spatiotemporal correlates of repetition priming in cortical word-recognition networks and their modulation by stimulus familiarity (i.e., words vs pseudowords). Event-related functional magnetic resonance imaging results show reduced activation for repeated words in occipitotemporal cortical regions. Electroencephalogram recordings reveal a significant reduction of induced gamma-band responses (GBRs) between 200 and 350 ms after stimulus onset, accompanied by a decrease in phase synchrony between electrode positions. Pseudoword repetition, in contrast, leads to an activation increase in the same areas, to increased GBRs, and to an increased phase coupling. This spatiotemporal repetition by stimulus type interaction suggests that qualitatively distinct mechanisms are recruited during repetition priming of familiar and unfamiliar stimuli. Repetition of familiar stimuli leads to a “sharpening” of extrastriate object representations, whereas the repetition of unfamiliar stimuli results in the “formation” of a novel cortical network by means of synchronized oscillatory activity. In addition to isolating these mechanisms, the present study provides the first evidence for a possible link between induced electrophysiological and hemodynamic measures of brain activity.

Processing lexical semantic and syntactic information in first and second language: fMRI evidence from German and Russian.

We introduce two experiments that explored syntactic and semantic processing of spoken sentences by native and non‐native speakers. In the first experiment, the neural substrates corresponding to detection of syntactic and semantic violations were determined in native speakers of two typologically different languages using functional magnetic resonance imaging (fMRI). The results show that the underlying neural response of participants to stimuli across different native languages is quite similar. In the second experiment, we investigated how non‐native speakers of a language process the same stimuli presented in the first experiment. First, the results show a more similar pattern of increased activation between native and non‐native speakers in response to semantic violations than to syntactic violations. Second, the non‐native speakers were observed to employ specific portions of the frontotemporal language network differently from those employed by native speakers. These regions included the inferior frontal gyrus (IFG), superior temporal gyrus (STG), and subcortical structures of the basal ganglia. Hum Brain Mapp 25:266–286, 2005.

Dynamic Anticipatory Processing of Hierarchical Sequential Events: a Common Role for Broca's Area and Ventral Premotor Cortex Across Domains?

This paper proposes a domain-general model for the functional contribution of ventral premotor cortex (PMv) and adjacent Brocas area to perceptual, cognitive, and motor processing. We propose to understand this frontal region as a highly flexible sequence processor, with the PMv mapping sequential events onto stored structural templates and Brocas Area involved in more complex, hierarchical or hypersequential processing. This proposal is supported by reference to previous functional neuroimaging studies investigating abstract sequence processing and syntactic processing.

Modulation of inferotemporal cortex activation during verbal working memory maintenance

Regions of the left inferotemporal cortex are involved in visual word recognition and semantics. We utilized functional magnetic resonance imaging to localize an inferotemporal language area and to demonstrate that this area is involved in the active maintenance of visually presented words in working memory. Maintenance activity in this inferotemporal area showed an effect of memory load for words, but not pseudowords. In the absence of visual input, the selective modulation of this language-related inferotemporal area for the maintenance of words is accompanied by an increased functional connectivity with left prefrontal cortex. These results demonstrate an involvement of inferotemporal cortex in verbal working memory and provide neurophysiological support for the notion that nonphonological language representations can be recruited in the service of verbal working memory. More generally, they suggest that verbal working memory should be conceptualized as the frontally guided, sustained activation of pre-existing cortical language representations.