Not all reading is alike: Task modulation of magnetic evoked response to visual word.
Background. Previous studies have shown that brain response to a written word depends on the task: whether the word is a target in a version of lexical decision task or should be read silently. Although this effect has been interpreted as an evidence for an interaction between word recognition processes and task demands, it also may be caused by greater attention allocation to the target word.
Objective. We aimed to examine the task effect on brain response evoked by non- target written words.
Design. Using MEG and magnetic source imaging, we compared spatial-temporal pattern of brain response elicited by a noun cue when it was read silently either without additional task (SR) or with a requirement to produce an associated verb (VG).
Results.The task demands penetrated into early (200-300 ms) and late (500-800 ms) stages of a word processing by enhancing brain response under VG versus SR condition. The cortical sources of the early response were localized to bilateral inferior occipitotemporal and anterior temporal cortex suggesting that more demanding VG task required elaborated lexical-semantic analysis. The late effect was observed in the associative auditory areas in middle and superior temporal gyri and in motor representation of articulators. Our results suggest that a remote goal plays a pivotal role in enhanced recruitment of cortical structures underlying orthographic, semantic and sensorimotor dimensions of written word perception from the early processing stages. Surprisingly, we found that to fulfil a more challenging goal the brain progressively engaged resources of the right hemisphere throughout all stages of silent reading.
Conclusion. Our study demonstrates that a deeper processing of linguistic input amplifies activation of brain areas involved in integration of speech perception and production. This is consistent with theories that emphasize the role of sensorimotor integration in speech understanding.
Themes: Cognitive psychology
Keywords: visual word recognition, top-down modulations, sensorimotor transformation, speech lateralization, magnetoencephalography (MEG)