Cognitive control and its impact on recovery from aphasic stroke

Abstract

Aphasic deficits are usually only interpreted in terms of domain-specific language processes. However, effective human communication and tests that probe this complex cognitive skill are also dependent on domain-general processes. In the clinical context, it is a pragmatic observation that impaired attention and executive functions interfere with the rehabilitation of aphasia. One system that is important in cognitive control is the salience network, which includes dorsal anterior cingulate cortex and adjacent cortex in the superior frontal gyrus (midline frontal cortex). This functional imaging study assessed domain-general activity in the midline frontal cortex, which was remote from the infarct, in relation to performance on a standard test of spoken language in 16 chronic aphasic patients both before and after a rehabilitation programme. During scanning, participants heard simple sentences, with each listening trial followed immediately by a trial in which they repeated back the previous sentence. Listening to sentences in the context of a listen-repeat task was expected to activate regions involved in both language-specific processes (speech perception and comprehension, verbal working memory and pre-articulatory rehearsal) and a number of task-specific processes (including attention to utterances and attempts to overcome pre-response conflict and decision uncertainty during impaired speech perception). To visualize the same system in healthy participants, sentences were presented to them as three-channel noise-vocoded speech, thereby impairing speech perception and assessing whether this evokes domain general cognitive systems. As expected, contrasting the more difficult task of perceiving and preparing to repeat noise-vocoded speech with the same task on clear speech demonstrated increased activity in the midline frontal cortex in the healthy participants. The same region was activated in the aphasic patients as they listened to standard (undistorted) sentences. Using a region of interest defined from the data on the healthy participants, data from the midline frontal cortex was obtained from the patients. Across the group and across different scanning sessions, activity correlated significantly with the patients' communicative abilities. This correlation was not influenced by the sizes of the lesion or the patients' chronological ages. This is the first study that has directly correlated activity in a domain general system, specifically the salience network, with residual language performance in post-stroke aphasia. It provides direct evidence in support of the clinical intuition that domain-general cognitive control is an essential factor contributing to the potential for recovery from aphasic stroke.

Keywords: aphasia; cingulate; executive; functional MRI; salience.

Figure 1

Overlay of the lesion distribution in the 16 patients with post-stroke aphasia. Projections are rendered onto a single-subject brain template. The colour code represents the absolute number of participants with a lesion in a given voxel (range: 1 shown in purple to 16 shown in red).

Figure 2

Scanning paradigm in functional MRI for healthy volunteers.

Figure 3

Scanning paradigm in functional MRI for subjects with aphasia.

Figure 4

Bar chart showing the mean percentage accuracy (with standard error) on repetition accuracy during scanning across both intelligibility conditions in the healthy volunteer (HV) group and in the patient group on normal speech.

Figure 5

Thresholded Z-statistic images for the contrasts of the top panel: listening to normal stimuli versus repeating normal stimuli in participants with aphasia (mean of both scanning sessions). Bottom panel: listening to vocoded stimuli versus listening to normal stimuli in healthy volunteers (mean of both sessions). All images are thresholded using clusters determined by Z > 2.3 and a (corrected) cluster significance threshold of P = 0.05. Numbers identify activity within (1) the dACC/SFG, (2) inferior frontal gyrus and adjacent anterior insular cortex, (3) dorsolateral prefrontal cortex, (4) dorsal inferior parietal cortex and adjacent lateral intraparietal sulcus (dorsal inferior parietal cortex and adjacent lateral intraparietal sulcus) and (5) middle frontal gyrus.

Figure 6

Thresholded Z statistic images for the Task × Intelligability interaction found in healthy volunteers. All images are thresholded using clusters determined by Z > 2.3 and a (corrected) cluster significance threshold of P = 0.05. Numbers identify activity within (1) the dACC/SFG and (2) inferior frontal gyrus and adjacent anterior insular cortex. R = right.

Figure 7

Thresholded Z statistic images for the contrasts of listening to vocoded stimuli versus listening to normal stimuli in healthy volunteers (mean of both sessions) multiplied by the contrast of listening to normal stimuli versus listening to white noise in patients (mean of sessions 2 and 3). All images are thresholded using clusters determined by Z > 2.3 and a (corrected) cluster significance threshold of P = 0.05. Numbers identify activity within (1) the dACC/SFG and (6) inferior frontal gyrus and adjacent anterior insular cortex, (8) dorsal inferior parietal cortex and adjacent lateral intraparietal sulcus.

Figure 8

Correlation between patients’ mean picture description scores and mean dACC/SFG percent signal change across all three sessions. BOLD = blood oxygen level-dependent.

Figure 9

Bar chart, with standard error bars, showing the mean dACC/SFG activation during trials where (left) healthy volunteers were listening to ListVoc trials (light grey), listening to ListNorm trials (mid-grey) and patients listening to ListNorm trials (black). Right: Also during trials where healthy volunteers were repeating the ListVoc trials (light grey), ListNorm trials (mid- grey) and patients were repeating ListNorm trials (Black).