Mechanisms of recovery from aphasia: evidence from positron emission tomography studies

Abstract

Objectives-Language functions comprise a distributed neural system, largely lateralised to the left cerebral hemisphere. Late recovery from aphasia after a focal lesion, other than by behavioural strategies, has been attributed to one of two changes at a systems level: a laterality shift, with mirror region cortex in the contralateral cortex assuming the function(s) of the damaged region; or a partial lesion eVect, with recovery of perilesional tissue to support impaired language functions. Functional neuroimaging with PET allows direct observations of brain functions at systems level. This study used PET to compare regional brain activations in response to a word retrieval task in normal subjects and in aphasic patients who had shown at least some recovery and were able to attempt the task. Emphasis has been placed on single subject analysis of the results as there is no reason to assume that the mechanisms of recovery are necessarily uniform among aphasic patients. Methods-Six right handed aphasic patients, each with a left cerebral hemispheric lesion (five strokes and one glioma), were studied. Criteria for inclusion were symptomatic or formal test evidence of at least some recovery and an ability to attempt word retrieval in response to heard word cues. Each patient underwent 12 PET scans using oxygen-15 labelled water (H 2 15 O) as tracer to index regional cerebral blood flow (rCBF). The task, repeated six times, required the patient to think of verbs appropriate to diVerent lists of heard noun cues. The six scans obtained during word retrieval were contrasted with six made while the subject was "at rest". The patients' individual results were compared with those of nine right handed normal volunteers undergoing the same activation study. The data were analysed using statistical parametric mapping (SPM96, Wellcome Department of Cognitive Neurology, London, UK). Results-Perception of the noun cues would be expected to result in bilateral dorsolateral temporal cortical activations, but as the rate of presentation was only four per minute the auditory perceptual activations were not evident in all people. Anterior cingulate, medial premotor (supplementary speech area) and dorsolateral frontal activations were evident in all normal subjects and patients. There were limited right dorsolateral frontal activations in three of the six patients, but a similar pattern was also found in four of the nine normal subjects. In the left inferolateral temporal cortex, activation was found for the normal subjects and five of the six patients, including two of the three subjects with lesions involving the left temporal lobe. The only patient who showed subthreshold activation in the left inferolateral temporal activation had a very high error rate when performing the verb retrieval task. Conclusions-Thenormal subjects showed a left lateralised inferolateral temporal activation, reflecting retrieval of words appropriate in meaning to the cue from the semantic system. Lateralisation of frontal activations to the left wa...

Functional MRI-Based Strategy of Therapeutic rTMS Application: A Novel Approach forPost-Stroke Aphasic Patients

“…In fact, many researchers have published clinical papers in support of the important role of left hemisphere (especially the peri-lesional area of the hemisphere) in the process of language functional recovery (Heiss et al, 1999;Warburton et al, 1999;Fernandez et al, 2004). On the other hand, however, other reports have also emphasized the involvement of the right hemisphere in some language functional recovery in a subgroup of post-stroke patients.…”

Update on central function relevant to sex: remodeling the basis of drug treatments for sex and the brain

“…We know that a small volume stroke can produce language defects of astonishing subtlety, suggesting that function is localizedFbut we have also discovered that recovery of adjacent tissue can recapture some of that functionFsuggesting that function may also be somewhat arbitrarily regionalized. 3 So, in the representation of brain areas vital to sexual function some specificity is to be expected and it may be possible to identify specific pathways and neurochemicals. But, recognizing the necessity for multiple and overlapping systems, there will generally be areas involved (nuclei, tracts, bodies) rather than individual neurons.…”

Neurophysiological Markers of Language Recovery in Subacute Stroke

“…Evidence from patient data also supports this 'map extension' (Thompson, 2000a) with reports of renewed language impairments in people with aphasia with previously recovered language function, but who go on to experience a second left hemisphere stroke (Basso, Gardelli, Grassi, & Mariotti, 1989). Neuroimaging studies have also demonstrated activation of perilesional tissue adjacent to the lesioned area in response to language processing (Heiss, Kessler, Thiel, Ghaemi, & Karbe, 1999;Kurland et al, 2004;Warburton, Price, Swinburn, & Wise, 1999). Moreover, if the perilesional area surrounds an active language area it is more likely that it will be able to subsume some of language function previously carried out by the damaged regions (Marsh & Hillis, 2006) perhaps due to functional redundancy (Thompson & den Ouden, 2008).…”

“…During this period recovery is less rapid and consists primarily of neural reorganisation leading to the formation of new alternative networks and changes in the efficacy of synaptic connections (Lazar & Antoniello, 2008). Recovery in this phase is thought to be associated with the regression of diaschisis (that is the normalisation over time of dysfunctional brain activity in areas remote or distant to the lesion and which correlate with behaviour, see Carrera and Tononi (2014) for a full review), perilesional restitution (Thompson & den Ouden, 2008;Warburton et al, 1999) or a functional transfer to the right hemisphere homologue (Saur et al, 2006). The chronic phase of recovery is dominated by neural reorganisation and compensatory cognitive strategies which may continue for a number of years (Grafman & Litvan, 1999).…”

“…In the subacute phase, which begins at approximately two weeks post-stroke and lasts for weeks, the recovery process slows and the upregulation of intact, undamaged regions occurs as new alternate networks are reorganised and formed (Lazar & Antoniello, 2008). Mechanisms during this recovery phase include recruitment of perilesional tissue (Warburton et al, 1999), an engagement of homologous language regions (Saur et al, 2006;Turkeltaub et al, 2011) or resolution of diaschisis whereby a reduction in dysfunctional brain activity occurs in structurally unaffected regions, that are connected to but distant from the lesion (for a review see Carrera & Tononi, 2014). During the final chronic phase, after the language network has experienced significant reorganisation and language recovery has stabilised, neural reorganisation and compensatory cognitive strategies continue to contribute to recovery and this may continue for many years (Grafman & Litvan, 1999;Muñoz-Cespedes et al, 2005).…”