The dual-stream model of speech processing describes a cortical network involved in speech processing. However, it is not yet known if the dual-stream model represents actual intrinsic functional brain networks. Furthermore, it is unclear how disruptions after a stroke to the functional connectivity of the dual-stream model's regions are related to speech production and comprehension impairments seen in aphasia. To address these questions, in the present study, we examined two independent resting-state fMRI data sets: (1) 28 neurotypical matched controls and (2) 28 chronic left-hemisphere stroke survivors collected at another site. We successfully identified an intrinsic functional network among the dual-stream model's regions in the control group using functional connectivity. We then used both standard functional connectivity analyses and graph theory approaches to determine how this connectivity may predict performance on clinical aphasia assessments. Our findings provide evidence that the dual-stream model of speech processing is an intrinsic network as measured via resting-state MRI and that functional connectivity of the hub nodes of the dual-stream network defined by graph theory methods, but not overall average network connectivity, is weaker in the stroke group than in the control participants. In addition, the functional connectivity of the hub nodes predicted linguistic impairments on clinical assessments. In particular, the relative strength of connectivity of the right hemisphere's homologues of the left dorsal stream hubs to the left dorsal hubs, versus to the right ventral stream hubs, is a particularly strong predictor of poststroke aphasia severity and symptomology.

We investigated the ability of 40 left-hemisphere brain-lesioned individuals with various diagnoses of aphasia to temporally synchronize the audio of a spoken word to its congruent video using a maximum-likelihood adaptive psychophysical procedure. We found a statistically significant effect of aphasia type, not explained by lesion volume, on measures of audiovisual (AV) synchrony. Brain-lesioned individuals with no symptoms of aphasia, and those with conduction aphasia performed on the synchrony task more similarly to age-matched neurotypical controls, whereas those with anomic aphasia performed the poorest. In addition, we examined the correlation between this ability and AV integration (fusion) and observed a significant correlation between measures of AV synchrony and fusion. An ROI analysis of stroke lesion maps showed that damage to the left posterior temporal regions adversely affected AV processing, although whole-brain univariate lesion-symptom mapping analyses did not yield any significant results. These findings contribute to a better understanding of the functional relationship between different AV processes in multimodal integration and their underlying cortical networks in the human brain.

The neural basis of language has been studied for centuries, yet the networks critically involved in simply identifying or understanding a spoken word remain elusive. Several functional–anatomical models of critical neural substrates of receptive speech have been proposed, including (1) auditory-related regions in the left mid-posterior superior temporal lobe, (2) motor-related regions in the left frontal lobe (in normal and/or noisy conditions), (3) the left anterior superior temporal lobe, or (4) bilateral mid-posterior superior temporal areas. One difficulty in comparing these models is that they often focus on different aspects of the sound-to-meaning pathway and are supported by different types of stimuli and tasks. Two auditory tasks that are typically used in separate studies—syllable discrimination and word comprehension—often yield different conclusions. We assessed syllable discrimination (words and nonwords) and word comprehension (clear speech and with a noise masker) in 158 individuals with focal brain damage: left ( n = 113) or right ( n = 19) hemisphere stroke, left ( n = 18) or right ( n = 8) anterior temporal lobectomy, and 26 neurologically intact controls. Discrimination and comprehension tasks are doubly dissociable both behaviorally and neurologically. In support of a bilateral model, clear speech comprehension was near ceiling in 95% of left stroke cases and right temporal damage impaired syllable discrimination. Lesion-symptom mapping analyses for the syllable discrimination and noisy word comprehension tasks each implicated most of the left superior temporal gyrus. Comprehension but not discrimination tasks also implicated the left posterior middle temporal gyrus, whereas discrimination but not comprehension tasks also implicated more dorsal sensorimotor regions in posterior perisylvian cortex.

Broca's area has long been implicated in sentence comprehension. Damage to this region is thought to be the central source of “agrammatic comprehension” in which performance is substantially worse (and near chance) on sentences with noncanonical word orders compared with canonical word order sentences (in English). This claim is supported by functional neuroimaging studies demonstrating greater activation in Broca's area for noncanonical versus canonical sentences. However, functional neuroimaging studies also have frequently implicated the anterior temporal lobe (ATL) in sentence processing more broadly, and recent lesion–symptom mapping studies have implicated the ATL and mid temporal regions in agrammatic comprehension. This study investigates these seemingly conflicting findings in 66 left-hemisphere patients with chronic focal cerebral damage. Patients completed two sentence comprehension measures, sentence–picture matching and plausibility judgments. Patients with damage including Broca's area (but excluding the temporal lobe; n = 11) on average did not exhibit the expected agrammatic comprehension pattern—for example, their performance was >80% on noncanonical sentences in the sentence–picture matching task. Patients with ATL damage ( n = 18) also did not exhibit an agrammatic comprehension pattern. Across our entire patient sample, the lesions of patients with agrammatic comprehension patterns in either task had maximal overlap in posterior superior temporal and inferior parietal regions. Using voxel-based lesion–symptom mapping, we find that lower performances on canonical and noncanonical sentences in each task are both associated with damage to a large left superior temporal–inferior parietal network including portions of the ATL, but not Broca's area. Notably, however, response bias in plausibility judgments was significantly associated with damage to inferior frontal cortex, including gray and white matter in Broca's area, suggesting that the contribution of Broca's area to sentence comprehension may be related to task-related cognitive demands.

The role of Broca's area in sentence processing has been debated for the last 30 years. A central and still unresolved issue is whether Broca's area plays a specific role in some aspect of syntactic processing (e.g., syntactic movement, hierarchical structure building) or whether it serves a more general function on which sentence processing relies (e.g., working memory). This review examines the functional organization of Broca's area in regard to its contributions to sentence comprehension, verbal working memory, and other multimodal cognitive processes. We suggest that the data are consistent with the view that at least a portion of the contribution of Broca's area to sentence comprehension can be attributed to its role as a phonological short-term memory resource. Furthermore, our review leads us to conclude that there is no compelling evidence that there are sentence-specific processing regions within Broca's area.