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a MRC
Cyclotron Unit, Imperial College School of Medicine, Hammersmith
Hospital, London W12 UK, b Wellcome Department of Cognitive
Neurology, Institute of Neurology, Queen Square, London WC1 UK
Correspondence to: Dr E Warburton, Department of Medicine for the Elderly, Box 135, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ, UK.
Received 12 November
1997 and in revised form 6 August 1998;
Accepted 25 August
1998
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 effect, 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.
METHODSSix 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
(H215O) as tracer to index regional cerebral
blood flow (rCBF). The task, repeated six times, required the patient
to think of verbs appropriate to different 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).
RESULTSPerception 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.
CONCLUSIONSThe normal
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
was only relative, with right prefrontal involvement in half of the
normal subjects. Frontal activations are associated with parallel
psychological processes involved in word retrieval, including task
initiation, short term (working) memory for the cue and responses, and
prearticulatory processes (even though no overt articulation was
required). There was little evidence of a laterality shift of word
retrieval functions to the right temporal lobe after a left hemispheric
lesion. In particular, left inferolateral temporal activation was seen
in all patients except one, and he proved to be very inefficient at the
task. The results provide indirect evidence that even limited salvage
of peri-infarct tissue with acute stroke treatments will have an
important impact on the rehabilitation of cognitive functions.
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