Over time, both the functional and anatomical boundaries of 'Wernicke's area' have become so broad as to be meaningless. We have re-analysed four functional neuroimaging (PET) studies, three previously published and one unpublished, to identify anatomically separable, functional subsystems in the left superior temporal cortex posterior to primary auditory cortex. From the results we identified a posterior stream of auditory processing. One part, directed along the supratemporal cortical plane, responded to both non-speech and speech sounds, including the sound of the speaker's own voice. Activity in its most posterior and medial part, at the junction with the inferior parietal lobe, was linked to speech production rather than perception. The second, more lateral and ventral part lay in the posterior left superior temporal sulcus, a region that responded to an external source of speech. In addition, this region was activated by the recall of lists of words during verbal fluency tasks. The results are compatible with an hypothesis that the posterior superior temporal cortex is specialized for processes involved in the mimicry of sounds, including repetition, the specific role of the posterior left superior temporal sulcus being to transiently represent phonetic sequences, whether heard or internally generated and rehearsed. These processes are central to the acquisition of long- term lexical memories of novel words.

We investigated the brain systems engaged during propositional speech (PrSp) and two forms of non- propositional speech (NPrSp): counting and reciting overlearned nursery rhymes. Bilateral cerebral and cerebellar regions were involved in the motor act of articulation, irrespective of the type of speech. Three additional, left-lateralized regions, adjacent to the Sylvian sulcus, were activated in common: the most posterior part of the supratemporal plane, the lateral part of the pars opercularis in the posterior inferior frontal gyrus and the anterior insula. Therefore, both NPrSp and PrSp were dependent on the same discrete subregions of the anatomically ill-defined areas of Wernicke and Broca. PrSp was also dependent on a predominantly left-lateralized neural system distributed between multi-modal and amodal regions in posterior inferior parietal, anterolateral and medial temporal and medial prefrontal cortex. The lateral prefrontal and paracingulate cortical activity observed in previous studies of cued word retrieval was not seen with either NPrSp or PrSp, demonstrating that normal brain- language representations cannot be inferred from explicit metalinguistic tasks. The evidence from this study indicates that normal communicative speech is dependent on a number of left hemisphere regions remote from the classic language areas of Wernicke and Broca. Destruction or disconnection of discrete left extrasylvian and perisylvian cortical regions, rather than the total extent of damage to perisylvian cortex, will account for the qualitative and quantitative differences in the impaired speech production observed in aphasic stroke patients.

Recovery of speech after infarction of the left pars opercularis (POp) may be due to recruitment of homotopic cortex in the right hemisphere. Using positron emission tomography, we investigated activity within left and right POp during everyday propositional speech. We studied seven aphasic patients with left anterior perisylvian infarction which included the POp. We compared their data with two control groups: 12 normal subjects and 7 anterior aphasic patients whose infarcts spared the left POp. During propositional speech, normal subjects activated the left POp, left posterior perisylvian cortex, and a distributed, predominantly left-lateralized, extrasylvian neural network. Importantly, activity in the right POp was reduced relative to a baseline nonspeech condition. In patients with infarction of the left POp, activity in the right POp was reversed: speech activated the right POp above baseline. Patient controls activated the left POp but did not show the normal relative reduction in activity in the right POp. In both patient groups, posterior perisylvian and extrasylvian activations remained unchanged from normal. This result demonstrates that infarction of the left POp is associated with a chronic change in function of the contralateral homotopic cortex during speech.