A fMRI study of word retrieval in aphasia
Introduction
The neurological mechanisms responsible for the recovery of aphasia due to acute vascular damage of the language-dominant hemisphere are largely unknown. In the early phase after a stroke, the functional impairment of brain networks not directly affected by the lesion (diaschisis), both in perilesional and contralateral brain areas, might contribute to the global severity of language impairment (Perani, Vallar, Cappa, Messa, & Fazio, 1987; Vallar et al., 1988). The recovery of these “distant effects” has been suggested to play a role in the “early,” partial recuperation stage, which takes place in many aphasic patients, in the weeks and months immediately following the stroke (Cappa et al., 1997).
Although recovery may be observed also many years after a stroke, the following stages of aphasia are usually characterised by a slower recovery rate, which has been suggested to be related to functional re-organisation. The hypothesis of a “take-over” of function by homotopic areas in the right hemisphere as the mechanism underlying aphasia recovery goes back to the last century (Gowers, 1895). On the basis of the results of positron emission tomography activation studies (Weiller et al., 1995), a functional re-organisation, involving in particular the regions homotopic to the language areas in the contralateral hemisphere, has been suggested to be responsible for long-term recovery. However, other recent neuroimaging investigations have reported more complex patterns of activation in recovered patients. In particular, several studies have indicated that the reactivation of ipsilateral, perilesional areas is often associated with good recovery (Belin et al., 1996; Cao, Vikingstad, George, Johnson, & Welch, 1999; Heiss, Kessler, Thiel, Ghaemi, & Karbe, 1999; Hund-Georgiadis, Lex, Morris, & von Cramon, 2000; Rosen et al., 2000; Warburton, Price, Swinburn, & Wise, 1999). Besides performance level, another crucial determinant to be considered to interpret the pattern of activation in aphasia is the type of task. As suggested by Gold and Kertesz (2000), it is plausible that tasks, which are known to involve the right hemisphere in normal controls, may be associated with more extensive right-sided activation in aphasics. These typically include semantic comprehension tasks, rather than speech production and word generation. Even within relatively simple tasks, modifications of the linguistic requirements may change the cognitive strategy and result in different pattern of cerebral reorganisation.
In the present study, we took advantage of the well-known difference in word retrieval to letter and to semantic cues. Different component processes are involved in the two tasks. In particular, letter fluency has been suggested to place greater demand on “frontal” strategic processes, while semantic fluency is more dependent from semantic memory, in which the temporal lobe plays a crucial role. There is both behavioral (Martin, Wiggs, Lalonde, & Mack, 1994) and imaging (Mummery, Patterson, Hodges, & Wise, 1996; Paulesu et al., 1997) evidence to support this claim. The two tasks can thus be considered as “probes” for two partially independent networks, involving different brain regions, which may have different potential for reorganisation. Here, we report the anatomo-functional correlates of cued lexical retrieval in five aphasic patients, who showed different degrees of recovery in the late stage of stroke after rehabilitation therapy. The patterns of brain activation were assessed with functional Magnetic Resonance (fMRI) during verbal fluency tasks.
Section snippets
Methods
The experimental protocol was approved by the local hospital Ethics Committee, and all the subjects signed the informed consent.
Normal subjects
Table 3, Table 4 report the results of the simple main effects of the phonemic and semantic verbal fluency tasks compared to the time-matched rest condition. For normal controls the pattern of activation was comparable to the one reported in the previous paper (see Paulesu et al., 1997). In particular, we found a selective activation of the left frontal operculum (Ba 44) during phonemic fluency and a larger activation of the retrosplenial cortex in the semantic fluency.
More in detail, during
Discussion
In agreement with previous findings (Poline, Vandenberghe, Holmes, Friston, & Frackowiak, 1996), in normal subjects phonemic and semantic fluency were associated with prevalent left prefrontal cortex activation. Some areas of task-specific activation were also present. In particular, the opercular portion of Broca’s area (Ba 44) and the basal ganglia and thalamus were engaged by phonemic fluency, whereas the left middle temporal gyrus (Ba 21) and the retrosplenial cortex/cuneus were activated
Acknowledgements
This study was partially supported by the V European Program for the project Connectivity in language rehabilitation in stroke in Europe (LSDE).
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