Background: The left inferior frontal gyrus (Broca's area) is generally believed to be critical for the motor act of speech. A lesion-based analysis has, however, shown that the left anterior insula is necessary for accurate articulation. We used functional imaging in normal people to show the neural systems involved in speech during different speech tasks.
Methods: 12 normal people underwent positron emission tomography with oxygen-15-labelled water as tracer. We measured cerebral activity while participants performed three different tasks: repetition of heard nouns at different rates; listening to single nouns at different rates; and anticipation of listening or repetition. We analysed the data with imaging software.
Findings: Repetition of single words did not activate Broca's area but activity in three left-lateralised regions was seen: the anterior insula, a localised region in the lateral premotor cortex, and the posterior pallidum. The left anterior insula and lateral premotor cortex showed a conjunction of activity for hearing and articulation. In addition, articulation modulated the response to hearing words in the left dorsolateral temporal cortex, the physiological expression of the speaker's auditory attention being directed towards the stimuli and not his or her articulated responses.
Interpretation: The formulation of an articulatory plan is a function of the left anterior insula and lateral premotor cortex and not of Broca's area. The left basal ganglia seem to be dominant for speech, although the axial muscles involved receive their motor output from both cerebral hemispheres.