Trends in Cognitive Sciences
The functional anatomy of word comprehension and production
Section snippets
Neuroimaging studies
The full set of regions activated during language production can be revealed by contrasting brain activity during language production tasks with brain activity during non-linguistic tasks. Fig. 2 illustrates the brain regions where activity increased (relative to resting with eyes closed) during (A) self-generated and (B) stimulus-driven word production. During the self-generated task6, 7subjects heard a noun and then had to think of as many related verbs as possible. For example, on hearing
Neuroimaging studies
Neuroimaging studies have found that the area that responds most strongly during semantic tasks is the left prefrontal cortex including Brodmann's areas (BA) 44, 45, 46, 47 (Refs 10, 11, 12, 13, 14, 15). (See Fig. 3 for the anatomical locations of the Brodmann areas.) Activity in the prefrontal cortex has also been associated with phonology[16]and willed action to self-generate motor or linguistic responses[17], therefore its role during semantic tasks has been questioned6, 17, 18, 19, 20. When
Neuroimaging studies
Identifying the system that translates semantics to phonology requires a baseline task that controls for presemantic processing without involving phonological retrieval. This combination of tasks has proved difficult to find (see Appendix A) because even when subjects are not instructed to name in the baseline task, phonological processes are automatically activated by word-like or object-like stimuli24, 45thereby minimising activation differences. There are two alternative approaches. One is
From orthography to phonology
There are two principal ways that orthography can be encoded into phonology. The first involves direct connections (O–P) and is required when we read unfamiliar letter combinations such as `mavnava'. The second is via semantics and involves translating familiar orthography to semantics (O–S) and semantics to phonology (S–P). Because the S–P route is shared by pictures (see above), we should be able to isolate the O–P route by contrasting reading with picture naming.
Conclusions
Inferences that can be made about functional specialization from lesion studies are limited because the role of a damaged brain structure cannot be established beyond the fact that it was necessary for the lost function. In the same vein, although neuroimaging of normal subjects identifies the set of brain areas associated with a particular task difference, the fallacy is that these activations are necessary—some activations might be unrelated to the task[18]. This review has attempted to avoid
Acknowledgements
I would like to thank Karl Friston, Richard Wise, Richard Frackowiak, Caroline Moore, Rik Vandenberghe and Karalyn Patterson for facilitating the work reported in this review.
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