Single word reading in developmental stutterers and fluent speakers

Ten fluent speakers and nine developmental stutterers read isolated nouns a loud in a delayed reading paradigm. Cortical activation sequences were mapp ed with a whole-head magnetoencephalography system. The stutterers were mos tly fluent in this task. Although the overt performance was essentially ide ntical in the two groups, the cortical activation patterns showed clear dif ferences, both in the evoked responses, time-locked to word presentation an d mouth movement onset, and in task-related suppression of 20-Hz oscillatio ns. Within the first 400 ms after seeing the word, processing in fluent spe akers advanced from the left inferior frontal cortex (articulatory programm ing) to the left lateral central sulcus and dorsal premotor cortex (motor p reparation). This sequence was reversed in the stutterers, who showed an ea rly left motor cortex activation followed by a delayed left inferior fronta l signal. Stutterers thus appeared to initiate motor programmes before prep aration of the articulatory code. During speech production, the right motor /premotor cortex generated consistent evoked activation in fluent speakers but was silent in stutterers. On the other hand, suppression of motor corti cal 20-Hz rhythm, reflecting task-related neuronal processing, occurred bil aterally in both groups. Moreover, the suppression was right-hemisphere dom inant in stutterers, as opposed to left-hemisphere dominant in fluent speak ers. Accordingly, the right frontal cortex of stutterers was highly active during speech production but did not generate synchronous time-locked respo nses. The speech-related 20-Hz suppression concentrated in the mouth area i n fluent speakers, but was evident in both the hand and mouth areas in stut terers. These findings may reflect imprecise functional connectivity within the right frontal cortex and incomplete segregation between the adjacent h and and mouth motor representations in stutterers during speech production. A network including the left inferior frontal cortex and the right motor/p remotor cortex, likely to be relevant in merging linguistic and affective p rosody with articulation during fluent speech, thus appears to be partly dy sfunctional in developmental stutterers.