**1. Introduction**

94 Neuroimaging – Cognitive and Clinical Neuroscience

Yoo, S.S.; Freeman, D.K.; McCarthy, J.J. 3rd; Jolesz, F.A. (2003). Neural Substrates of Tactile

Zwaan, R.A. (2004). The Immersed Experiencer: Toward an Embodied Theory of Language

(Ed.), 35-62, Academic Press, ISBN 0-12-543344-1, San Diego, USA.

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Comprehension. In: *The Psychology of Learning and Motivation, Vol. 44,* B.H. Ross

In the nineteenth century, two studies in aphasiology comprise a turning point for research of brain-language relationships: Broca, 1861 and Wernicke, 1874. Based on these two studies, it was claimed that Broca's area (i.e., the pars triangularis and pars opercularis of the left inferior frontal gyrus) and Wernicke's area (i.e., the posterior part of the left superior/middle temporal gyrus, but in some situations including a part of the inferior parietal lobule) were involved in language production and comprehension, respectively (Geschwind, 1970). Recently, due to the development of functional brain imaging techniques (e.g., PET and fMRI), normal brains have been measured to examine the neuro-cognitive architecture of language processing. In particular, both Broca's and Wernicke's areas have been shown to be responsible for several language functions, such as single word processing and sentence processing (Fig. 1).

However, these two important regions are also activated for working memory-related processes, at least, including executive functions and short term memory processes of linguistic information, and the processes of storage and access to long term memory of linguistic information. This memory system could be assumed essential for language comprehension. For example, in order to comprehend a word, we have to first identify a series of sounds or letters as a certain word and to access its semantic information from long term memory. For sentence comprehension, we have to tentatively memorize several words comprising the sentence to compute the syntactic and semantic structure of the sentence. For example, it is clear that if we do not tentatively memorize words comprising the sentence, we cannot comprehend the sentence, since we have to compute the syntactic/semantic information of the sentence by using these words. Hence, in order to understand a language expression, we need the involvement of both the short and long term memory systems. In previous studies, there were essentially two types of standpoints regarding the involvement of the memory system in language comprehension. The first is that of the "specialist", who assumes that the syntactic processing system of the language processing system exists in our brain and is independent from other congnitive functions. The second is that of the "generalist", who assumes that the syntactic processing system has neural substrates in common with other cognitive functions, mainly the working memory system.

In this chapter, recent neuroimaging studies of the neuro-cognitive architecture of single word and sentence processing will be briefly reviewed and the relationships between language and memory in the human brain will be discussed in the context of functional neuroimaging evidence.

Neuro-Anatomical Overlap Between Language and Memory Functions in the Human Brain 97

the inferior frontal gyrus/Brodomann area 45) and the left inferior parietal region (i.e., the supramarginal gyrus) comprise the verbal working memory circuit (for a recent metaanalysis see Vigneau et al., 2006). The former area is thought to be involved in articulatory rehearsal and the latter in phonological storage (e.g., Poldrack et al., 1999; Warburton et al., 1996; McGuire et al., 1996; Paulesu et al., 2000; Jessen et al., 1999; Zattore et al., 1996; Price et al., 1996). These two areas have often been reported to be active during single word processing (e.g., Hautzel et al., 2002; Jonides et al., 1998; Rypma et al., 1999; Cohen et al., 1997). The neuroimaging results are compatible with the working memory theory proposed by Baddeley, since the correlation between the sub-functions and locations of the involved brain regions reported in these neuroimaging studies is in line with the assumption of this

The left inferior frontal region, the left lateral and ventral middle/inferior temporal regions, and the left inferior parietal region are activated during semantic processing tasks. It is still unclear whether the left inferior frontal region is actived by single word semantic processing per se. Demb et al. (1995) have reported that brain activity in this region is greater for more difficult semantic processing tasks than for corresponding less difficult semantic processing tasks. Similarly, the left inferior frontal region was modulated by the frequency of words (Fiebach et al., 2002). It is common knowledge that low frequency words are more difficult to process than high frequency ones. Hence, in single word semantic processing, there exists the possibility that modulation of the left inferior frontal region by word frequency is explained by access to lexico-semantic information stored in long term memory. In contrast, it has been claimed that only the orbital part of the left inferior frontal gyrus is associated with the processing of semantic information retrieval. Several meta-analysis results in particular have supported this claim (Fiez, 1997; Bookheimer, 2002; Binder et al., 2009). A meta-analysis (Vigneau et al., 2006) has also supported the report that the left parietal lobe contributes to semantic processing regardless of the difference between pictures and words

While the temporal lobe plays a role in storing long term memory, the role of the left posterior part of superior/middle temporal gyri is still unclear. As evidence, most neuroimaging studies using comparisons between real word and pseudoword comprehension have reported that this region is more active for real word comprehension than for pseudoword comprehension (e.g., Pugh et al., 1996; Price et al., 1997; Friederici et al., 2000; Booth et al., 2002; Fiebach et al., 2002; Perani et al., 1999; Yokoyama et al., 2006b, and others). In contrast, Fiebach et al. (2002) showed that the left inferior frontal region is modulated by word frequency while the left posterior part of the middle temporal gyrus is not. Hence, at least the role of the left posterior part of the middle (and/or superior) temporal gyrus differs from that of the left inferior frontal region in lexico-semantic

It has been made clear that the left inferior temporal region contributes to semantic processing. The inferior temporal region is commonly known to be involved in the storage or the long term memory of word information. Lesion studies have reported that damage to the temporal lobe cause category-related deficits (Kapur et al., 1994; Gitelman et al., 2001; Lambon Ralph et al., 2007; Noppeney et al., 2007; Warrington, 1975; Hodges et al., 1992, 1995; Mummery et al., 2000). Patients with anterior temporal damage show more difficulty processing the concept of living things than that of artifacts, while patients with posterior

model (e.g., Baddeley, 2003).

(Vandenberghe et al., 1996).

processing.

**2.3 Lexico-semantic processing** 

Fig. 1. Broca's area and Wernicke's area.
