Diffusion tensor imaging of dorsal stream language areas in patients with post-stroke aphasia

Background Aphasia complicating stroke occurs due to language deficits that decrease communication abilities and functional independence. Our study aims to assess fractional anisotropy (FA) and mean diffusivity (MD) parameters of diffusion tensor imaging (DTI) of the dorsal stream language areas in patients with post-stroke aphasia. It was conducted on 27 patients with post-stroke aphasia and 27 age- and sex-matched controls who underwent DTI of the brain. FA and MD values of Broca's area (BA), Wernick's area (WA), superior longitudinal fasciculus (SLF), and arcuate fasciculus (AF), and number of tract fibers (TF) of AF and SLF were calculated. Results were correlated with National Institutes of Health Stroke Scale (NIHSS), Arabic version of Comprehensive Aphasia Test (Arabic CAT), and Mansoura Arabic Screening Aphasia Test (MASAT). Results FA of AF and SLF in patients was significantly lower (P = 0.001) than controls. MD of AF and SLF in patients was significantly higher (P = 0.001) than controls. The mean volume TF of AF and SLF in patients was significantly (P = 0.001) lower than the mean volume in controls for AF and SLF. FA cutoff for AF was 0.34 and for SLF, it was 0.35 with sensitivity, specificity, and accuracy (85.2%, 62.1%, 73.2%) for AF, (74.1%, 69%, 71.4%) for SLF, respectively. MD cutoff value for AF was 0.87, and 0.84 for SLF with sensitivity, specificity, and accuracy (63%, 72.4%, 67.8%) for AF, (81.5%, 79.3%, 80.4%) for SLF, respectively. Cutoff TF of AF was 1728 and for SLF it was 601 with sensitivity, specificity, and accuracy (88.9%, 72.4%, 80.4%) for AF and (85.2%, 85.2%, 78.6%) for SLF, respectively. Conclusions DTI is a non-invasive promising method that can be used to assess language areas in patients with post-stroke aphasia.

Escaping the nocturnal bottleneck, and the evolution of the dorsal and ventral streams of visual processing in primates

Early mammals were small and nocturnal. Their visual systems had regressed and they had poor vision. After the extinction of the dinosaurs 66 mya, some but not all escaped the ‘nocturnal bottleneck’ by recovering high-acuity vision. By contrast, early primates escaped the bottleneck within the age of dinosaurs by having large forward-facing eyes and acute vision while remaining nocturnal. We propose that these primates differed from other mammals by changing the balance between two sources of visual information to cortex. Thus, cortical processing became less dependent on a relay of information from the superior colliculus (SC) to temporal cortex and more dependent on information distributed from primary visual cortex (V1). In addition, the two major classes of visual information from the retina became highly segregated into magnocellular (M cell) projections from V1 to the primate-specific temporal visual area (MT), and parvocellular-dominated projections to the dorsolateral visual area (DL or V4). The greatly expanded P cell inputs from V1 informed the ventral stream of cortical processing involving temporal and frontal cortex. The M cell pathways from V1 and the SC informed the dorsal stream of cortical processing involving MT, surrounding temporal cortex, and parietal–frontal sensorimotor domains. This article is part of the theme issue ‘Systems neuroscience through the lens of evolutionary theory’.

Multivariate FMRI Signatures of Learning in a Hebb Repetition Paradigm With Tone Sequences

Important information from the environment often arrives to the brain in temporally extended sequences. Language, music, actions, and complex events generally unfold over time. When such informational sequences exceed the limited capacity of working memory, the human brain relies on its ability to accumulate information in long-term memory over several encounters with a complex stimulus. A longstanding question in psychology and neuroscience is whether the neural structures associated with working memory storage—often viewed as capacity limited and temporary—have any builtin ability to store information across longer temporal delays. According to the classic Hebbian dual memory theory, temporally local “activity traces” underlie immediate perception and working memory, whereas “structural traces” undergird long-term learning. Here we examine whether brain structures known to be involved in working maintenance of auditory sequences, such as area Spt, also show evidence of memory persistence across trials. We used representational similarity analysis (RSA) and the Hebb repetition paradigm with supracapacity tonal sequences to test whether repeated sequences have distinguishable multivoxel activity patterns in the auditory-motor networks of the brain. We found that, indeed, area Spt and other nodes of the auditory dorsal stream show multivoxel patterns for tone sequences that become gradually more distinct with repetition during working memory for supracapacity tone-sequences. The findings suggest that the structures are important for working memory are not “blank slates,” wiped clean from moment to moment, but rather encode information in a way can lead to cross-trial persistence.

Dorsal and Ventral Stream Function in Children With Developmental Coordination Disorder

Dorsal stream cortical networks underpin a cluster of visuomotor, visuospatial, and visual attention functions. Sensitivity to global coherence of motion and static form is considered a signature of visual cortical processing in the dorsal stream (motion) relative to the ventral stream (form). Poorer sensitivity to global motion compared to global static form has been found across a diverse range of neurodevelopmental disorders, suggesting a “dorsal stream vulnerability.” However, previous studies of global coherence sensitivity in Developmental Coordination Disorder (DCD) have shown conflicting findings. We examined two groups totalling 102 children with DCD (age 5–12 years), using the “Ball in the Grass” psychophysical test to compare sensitivity to global motion and global static form. Motor impairment was measured using the Movement-ABC (M-ABC). Global coherence sensitivity was compared with a typically developing control group (N = 69) in the same age range. Children with DCD showed impaired sensitivity to global motion (p = 0.002), but not global form (p = 0.695), compared to controls. Within the DCD group, motor impairment showed a significant linear relationship with global form sensitivity (p < 0.001). There was also a significant quadratic relationship between motor impairment and global motion sensitivity (p = 0.046), where poorer global motion sensitivity was only apparent with greater motor impairment. We suggest that two distinct visually related components, associated with global form and global motion sensitivity, contribute to DCD differentially over the range of severity of the disorder. Possible neural circuitry underlying these relationships is discussed.

Neural Mechanisms of Visual Motion Anomalies in Autism: A Two-Decade Update and Novel Aetiology

The 21st century has seen dramatic changes in our understanding of the visual physio-perceptual anomalies of autism and also in the structure and development of the primate visual system. This review covers the past 20 years of research into motion perceptual/dorsal stream anomalies in autism, as well as new understanding of the development of primate vision. The convergence of this literature allows a novel developmental hypothesis to explain the physiological and perceptual differences of the broad autistic spectrum. Central to these observations is the development of motion areas MT+, the seat of the dorsal cortical stream, central area of pre-attentional processing as well as being an anchor of binocular vision for 3D action. Such development normally occurs via a transfer of thalamic drive from the inferior pulvinar → MT to the anatomically stronger but later-developing LGN → V1 → MT connection. We propose that autistic variation arises from a slowing in the normal developmental attenuation of the pulvinar → MT pathway. We suggest that this is caused by a hyperactive amygdala → thalamic reticular nucleus circuit increasing activity in the PIm → MT via response gain modulation of the pulvinar and hence altering synaptic competition in area MT. We explore the probable timing of transfer in dominance of human MT from pulvinar to LGN/V1 driving circuitry and discuss the implications of the main hypothesis.

The Visuospatial and Sensorimotor Functions of Posterior Parietal Cortex in Drawing Tasks: A Review

Drawing is a comprehensive skill that primarily involves visuospatial processing, eye-hand coordination, and other higher-order cognitive functions. Various drawing tasks are widely used to assess brain function. The neuropsychological basis of drawing is extremely sophisticated. Previous work has addressed the critical role of the posterior parietal cortex (PPC) in drawing, but the specific functions of the PPC in drawing remain unclear. Functional magnetic resonance imaging and electrophysiological studies found that drawing activates the PPC. Lesion-symptom mapping studies have shown an association between PPC injury and drawing deficits in patients with global and focal cerebral pathology. These findings depicted a core framework of the fronto-parietal network in drawing tasks. Here, we review neuroimaging and electrophysiological studies applying drawing paradigms and discuss the specific functions of the PPC in visuospatial and sensorimotor aspects. Ultimately, we proposed a hypothetical model based on the dorsal stream. It demonstrates the organization of a PPC-centered network for drawing and provides systematic insights into drawing for future neuropsychological research.

Individual Symptom Severity in Adult Autism Spectrum Disorder Predicts Reduction in Right Dorsal Stream Neural Activity During Live Eye-to-Eye Contact

Background: Social symptomatology quantified by clinical interview (Autism Diagnostic Observation Schedule, ADOS) and self-report (Social Responsiveness Scale, SRS) indicate symptom severity in autism spectrum disorder (ASD). Reluctance to engage in interpersonal eye contact is a frequently observed behavioral hallmark, though neural bases for these difficulties and relation to symptomatology are not understood. We test the hypothesis that eye contact in ASD activates atypical neural mechanisms that are related to individual differences in symptomatology. Methods: Neural activity represented by hemodynamic signals was acquired by functional near-infrared spectroscopy (fNIRS) during real person-to-person eye contact (confirmed by eye-tracking) for 17 adult ASD (3 female, 14 male) and 19 typically-developed (TD) participants (8 female, 11 male). Assessment of social function was based on ADOS scores for ASD participants and SRS scores for the combined group of ASD and TD participants. Results: Individual ADOS scores were negatively correlated (r = -0.69) with individual fNIRS beta-values (representing strength of hemodynamic signals) within clusters in the right dorsal stream regions: somatosensory cortices, angular gyrus, and supramarginal gyrus. Hemodynamic responses in the right dorsolateral prefrontal cortex (DLPFC) were also negatively correlated (r = -0.77) with ADOS scores. Similarly, SRS scores for the combined ASD and TD groups were also negatively correlated (r = -0.58) with somatosensory cortices and the supramarginal gyrus. Conclusions: These findings are consistent with the hypothesis that neural mechanisms in the dorsal stream and DLPFC are related to social symptomatology and implicate high-level interactive face and eye-processing systems as potential neurobiological markers of ASD.

Age-related deficits in rapid visuomotor decision-making

Many goal-directed actions that require rapid visuomotor planning and perceptual decision-making are affected in older adults, causing difficulties in execution of many functional activities of daily living. Visuomotor planning and perceptual decision-making are mediated by the dorsal and ventral visual streams, respectively, but it is unclear how age-induced changes in sensory processing in these streams contribute to declines in goal-directed actions. Previously, we have shown that in healthy adults, task demands influence movement strategies during visuomotor decision-making, reflecting differential integration of sensory information between the two streams. Here, we asked the question if older adults would exhibit larger declines in interactions between the two streams during demanding motor tasks. Older adults (n=15) and young controls (n=26) performed reaching or interception movements towards virtual objects. In some blocks of trials, participants also had to select an appropriate movement goal based on the shape of the object. Our results showed that older adults corrected fewer initial decision errors during both reaching and interception movements. During the interception decision task, older adults made more decision- and execution-related errors than young adults, which were related to early initiation of their movements. Together, these results suggest that older adults have a reduced ability to integrate new perceptual information to guide online action, which may reflect impaired ventral-dorsal stream interactions.

"Jittered" Oculomotor Sensing Concomitant with Hypoactive Dorsal Parietal Systems during Live Eye-Contact in Autism Spectrum Disorder

Reluctant eye contact and reduced social interactions characteristic of autism spectrum disorder (ASD) are consistent with deficits in oculomotor and face processing systems. We test the hypothesis that these deficits are interrelated ASD. Eye-tracking and hyperscanning with functional near-infrared spectroscopy (fNIRS) were used to acquire neuroimaging data during live, dynamic eye-to-eye contacts in 17 ASD and 19 typically-developed (TD) adults. Real eye contact conditions were contrasted with conditions where eye gaze was directed at a comparable dynamic face video. These findings were regressed with eye-contact dwell-time, i.e., times when gaze of both partners was in the eye-box of the other, to confirm the relationship between visual sensing and neural coding. Visual fixations and positional variance were also determined. Average gaze dwell-times in the "eye-box" did not vary between ASD and TD participants but were longer for the Real Eye than Video Eye condition for both groups. However, positional gaze variability, "jitter", was higher for ASD in both conditions. Neural findings for TD [Real Eye > Video Eye] were consistent with previous findings for interactive face-gaze with activity in right temporal and dorsal parietal regions. However, in ASD ventral temporal regions were observed for this contrast without evidence for dorsal parietal activity. This neural difference was enhanced when regressed by eye-contact dwell-times. Together findings are consistent with the hypothesis that unstable bottom-up oculomotor signals contribute to deficits in live face processing and reduced dorsal stream activity in ASD.

Lip movements enhance speech representations and effective connectivity in speech dorsal stream and its relationship with neurite architecture

Lip movements facilitate speech comprehension, especially under adverse listening conditions, but the neural mechanisms of this perceptual benefit at the phonemic and feature levels remain unclear. This fMRI study addresses this question by quantifying regional multivariate representation and network organization underlying audiovisual speech-in-noise perception. We found that valid lip movements enhanced neural representations of phoneme, place of articulation, or voicing feature of speech differentially in dorsal stream regions, including frontal speech motor areas and supramarginal gyrus. Such local changes were accompanied by strengthened dorsal stream effective connectivity. Moreover, the neurite orientation dispersion of left arcuate fasciculus, a structural basis of speech dorsal stream, predicted the visual enhancements of neural representations and effective connectivity. Our findings provide novel insight to speech science that lip movements promote both local phonemic and feature encoding and network connectivity in speech dorsal pathway and the functional enhancement is mediated by the microstructural architecture of the circuit.