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FIG. 9. Adjusted activity from a voxel in the posterior superior temporal region is shown as a function of word-production rate under the two conditions of extrinsic and intrinsic word generation. The change in the slope of these response functions under the two contexts is obvious. The voxel from which these data are derived is shown on the SPM(t) (Upper). These data come from a PET study of six normal subjects.

letter). In other words, we could also construe this regionally specific effect as a modulation of letter string-specific responses by implicit phonological retrieval. In the absence of phonological retrieval, there may be no differences in the response of this area to words or nonword letter strings. This distinction between word-specific responses in an extrastriate region and word recognition-dependent modulation of extrastriate responses to any letter string is not a specious one. The two interpretations are that there are (i) “receptive fields” for words in extrastriate regions or (ii) a selective modulation of “receptive fields” for any letter string by higher cortical areas with “receptive fields” for the phonology of the stimulus. If we were able to inhibit the activity of these higher-order areas (using, for example, magnetostimulation), the extrastriate responses might no longer differentiate between word-like and non-word-like letter strings. If this were the case, should the extrastriate area be designated a word-form area? Clearly not in a simple way; however, it would constitute a necessary component of a distributed system involved in the perception of visually presented words. From a psychological perspective, one could posit a psychological component that was responsible for the integration of phonological retrieval and visual analysis. The interaction (e.g., that between phonological retrieval and the visual analysis of letter strings) would then represent an activation on comparing brain activity in states that did and did not have this integration. Factorial designs represent one way of identifying these integrative or context-sensitive activations. In summary, this example highlights the importance of regionally specific interactions and factorial designs. One interpretation of interactions is that they represent the integration of different processes (e.g., visual processing of letter stings and phonological retrieval) in a dynamic and context-sensitive fashion.

In conclusion, we have reviewed the importance of context-sensitive effects in neuroimaging both from the perspective of functional integration and effective connectivity and from the perspective of functional specialization and the integration of componential cognitive and sensorimotor processing. Advances in the design and analysis of brain imaging experiments are revealing the nature and role of these effects.

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