High contrast, moving targets in an emerging target paradigm promote fast visuomotor responses during visually guided reaching

Humans have a remarkable capacity to rapidly interact with the surrounding environment, often by transforming visual input into motor output on a moment-to-moment basis. But what visual features promote the shortest-latency reach responses? To address this question, we had human subjects perform visually guided reaches to moving targets varied by speed (experiment 1), or speed and contrast (experiment 2) in an emerging target paradigm, which has recently been shown to robustly elicit fast visuomotor responses. Our analysis focused on stimulus-locked responses (SLRs) on upper limb muscles. SLRs represent the first wave of muscle recruitment tied to visual target onset, appearing within <100 ms. Across 32 subjects studied in both experiments, 97% expressed SLRs in the emerging target paradigm. In comparison, 69% of these subjects expressed SLRs in a visually-guided reach paradigm. Within the emerging target paradigm, we found that target speed impacted SLR magnitude, whereas target contrast impacted SLR latency and magnitude. Thus, high contrast, faster-moving targets in the emerging target paradigm robustly recruited the circuitry mediating the most rapid visuomotor transformations for reaching, and such responses were associated with shorter latency RTs. Our results support the hypothesis that a subcortical pathway originating in the superior colliculus may be involved in the earliest wave of muscle recruitment following visual stimulus presentation. In scenarios requiring expedited responses, cortical areas may serve to prime this subcortical pathway, and elaborate subsequent phases of muscle recruitment following the SLR.Significance StatementHumans have a remarkable capacity, when necessary, to rapidly transform vision into action. But how does the brain do this? Here, by studying human subjects reaching to suddenly-appearing targets, we find that the earliest visually-guided actions are produced in response to high-contrast, moving targets. A millisecond-resolution examination of upper limb muscle recruitment shows that motor output can begin within less than 100 ms of target presentation. We surmise that this earliest recruitment arises from a phylogenetically-conserved brainstem circuit originating in the superior colliculus. Rather than being directly involved in the earliest phase of visuomotor actions, cortical areas may prime this brainstem circuit to produce initial muscle recruitment, and then elaborate subsequent phases of recruitment when time is of the essence.

Deciphering the frontostriatal circuitry through the fiber dissection technique: direct structural evidence on the morphology and axonal connectivity of the fronto-caudate tract

OBJECTIVEThe authors sought to investigate the very existence and map the topography, morphology, and axonal connectivity of a thus far ill-defined subcortical pathway known as the fronto-caudate tract (FCT) since there is a paucity of direct structural evidence regarding this pathway in the relevant literature.METHODSTwenty normal adult cadaveric formalin-fixed cerebral hemispheres (10 left and 10 right) were explored through the fiber microdissection technique. Lateral to medial and medial to lateral dissections were carried out in a tandem manner in all hemispheres. Attention was focused on the prefrontal area and central core since previous diffusion tensor imaging studies have recorded the tract to reside in this territory.RESULTSIn all cases, the authors readily identified the FCT as a fan-shaped pathway lying in the most medial layer of the corona radiata and traveling across the subependymal plane before terminating on the superolateral margin of the head and anterior part of the body of the caudate nucleus. The FCT could be adequately differentiated from adjacent fiber tracts and was consistently recorded to terminate in Brodmann areas 8, 9, 10, and 11 (anterior pre–supplementary motor area and the dorsolateral, frontopolar, and fronto-orbital prefrontal cortices). The authors were also able to divide the tract into a ventral and a dorsal segment according to the respective topography and connectivity observed. Hemispheric asymmetries were not observed, but instead the authors disclosed asymmetry within the FCT, with the ventral segment always being thicker and bulkier than the dorsal one.CONCLUSIONSBy using the fiber microdissection technique, the authors provide sound structural evidence on the topography, morphology, and connectional anatomy of the FCT as a distinct part of a wider frontostriatal circuitry. The findings are in line with the tract’s putative functional implications in high-order motor and behavioral processes and can potentially inform current surgical practice in the fields of neuro-oncology and functional neurosurgery.

Fast responses to images of animate and inanimate objects in the nonhuman primate amygdala

Visual information reaches the amygdala through the various stages of the ventral visual stream. There is, however, evidence that a fast subcortical pathway for the processing of emotional visual input exists. To explore the presence of this pathway in primates, we recorded local field potentials in the amygdala of four rhesus monkeys during a passive fixation task showing images of ten object categories. Additionally, in one of the monkeys we also obtained multi-unit spiking activity during the same task. We observed remarkably fast medium and high gamma responses in the amygdala of the four monkeys. These responses were selective for the different stimulus categories, showed within-category selectivity, and peaked as early as 60 ms after stimulus onset. Multi-unit responses in the amygdala were lagging the gamma responses by about 40 ms. Thus, these observations add further evidence that selective visual information reaches the amygdala of nonhuman primates through a very fast route.

Rapid and coarse face detection: With a lack of evidence for a nasal-temporal asymmetry

Humans have structures dedicated to the processing of faces, which include cortical components (e.g., areas in occipital and temporal lobes) and subcortical components (e.g., superior colliculus and amygdala). Although faces are processed more quickly than stimuli from other categories, there is a lack of consensus regarding whether subcortical structures are responsible for rapid face processing. In order to probe this, we exploited the asymmetry in the strength of projections to subcortical structures between the nasal and temporal hemiretina. Participants detected faces from unrecognizable control stimuli and performed the same task for houses. In Experiments 1 and 3, at the fastest reaction times, participants detected faces more accurately than houses. However, there was no benefit of presenting to the subcortical pathway. In Experiment 2, we probed the coarseness of the rapid pathway, making the foil stimuli more similar to faces and houses. This eliminated the rapid detection advantage, suggesting that rapid face processing is limited to coarse representations. In Experiment 4, we sought to determine whether the natural difference between spatial frequencies of faces and houses were driving the effects seen in Experiments 1 and 3. We spatially filtered the faces and houses so that they were matched. Better rapid detection was again found for faces relative to houses, but we found no benefit of preferentially presenting to the subcortical pathway. Taken together, the results of our experiments suggest a coarse rapid detection mechanism, which was not dependent on spatial frequency, with no advantage for presenting preferentially to subcortical structures.

A thalamocortical pathway for fast rerouting of tactile information to occipital cortex in congenital blindness

In congenitally blind individuals, the occipital cortex responds to various nonvisual inputs. Some animal studies raise the possibility that a subcortical pathway allows fast re-routing of tactile information to the occipital cortex, but this has not been shown in humans. Here we show using magnetoencephalography (MEG) that tactile stimulation produces occipital cortex activations, starting as early as 35 ms in congenitally blind individuals, but not in blindfolded sighted controls. Given our measured thalamic response latencies of 20 ms and a mean estimated lateral geniculate nucleus to primary visual cortex transfer time of 15 ms, we claim that this early occipital response is mediated by a direct thalamo-cortical pathway. We also observed stronger directed connectivity in the alpha band range from posterior thalamus to occipital cortex in congenitally blind participants. Our results strongly suggest the contribution of a fast thalamo-cortical pathway in the cross-modal activation of the occipital cortex in congenitally blind humans.

Disruption of the ascending arousal network in acute traumatic disorders of consciousness

ObjectiveTo determine whether ascending arousal network (AAn) connectivity is reduced in patients presenting with traumatic coma.MethodsWe performed high-angular-resolution diffusion imaging in 16 patients with acute severe traumatic brain injury who were comatose on admission and in 16 matched controls. We used probabilistic tractography to measure the connectivity probability (CP) of AAn axonal pathways linking the brainstem tegmentum to the hypothalamus, thalamus, and basal forebrain. To assess the spatial specificity of CP differences between patients and controls, we also measured CP within 4 subcortical pathways outside the AAn.ResultsCompared to controls, patients showed a reduction in AAn pathways connecting the brainstem tegmentum to a region of interest encompassing the hypothalamus, thalamus, and basal forebrain. When each pathway was examined individually, brainstem-hypothalamus and brainstem-thalamus CPs, but not brainstem-forebrain CP, were significantly reduced in patients. Only 1 subcortical pathway outside the AAn showed reduced CP in patients.ConclusionsWe provide initial evidence for the reduced integrity of axonal pathways linking the brainstem tegmentum to the hypothalamus and thalamus in patients presenting with traumatic coma. Our findings support current conceptual models of coma as being caused by subcortical AAn injury. AAn connectivity mapping provides an opportunity to advance the study of human coma and consciousness.

Fatigue in multiple sclerosis: The role of thalamus

Fatigue is very common in multiple sclerosis (MS) and is often considered as its most disabling symptom. Over the last 20 years, an increasing number of studies have evaluated the pathogenetic bases of MS-related fatigue. Converging evidence from neurophysiology and neuroimaging research suggests that a dysfunction in a cortico-subcortical pathway, centered on thalamus, is involved in the pathogenesis of fatigue. However, type and significance of such dysfunction remain unknown, and some studies reported an increase in the activity and connectivity within the thalamic network, whereas others suggested its reduction. Hereby, we review the results of neuroimaging studies supporting the different hypotheses about the role of thalamic network in the pathophysiology of MS-related fatigue and discuss limitations and shortcomings of available data, highlighting the key challenges in the field and the directions for future research.