Visual Cortical Area MT Is Required for Development of the Dorsal Stream and Associated Visuomotor Behaviors.

The middle temporal (MT) area of the extrastriate visual cortex has long been studied in adulthood for its distinctive physiological properties and function as a part of the dorsal stream, yet interestingly it possesses a similar maturation profile as the primary visual cortex (V1). Here, we examined whether an early-life lesion in MT of marmoset monkeys (six female, two male) altered the dorsal stream development and the behavioral precision of reaching-to-grasp sequences. We observed permanent changes in the anatomy of cortices associated with both reaching (parietal and medial intraparietal areas) and grasping (anterior intraparietal area), as well as in reaching-and-grasping behaviors. In addition, we observed a significant impact on the anatomy of V1 and the direction sensitivity of V1 neurons in the lesion projection zone. These findings indicate that area MT is a crucial node in the development of primate vision, affecting both V1 and areas in the dorsal visual pathway known to mediate visually guided manual behaviors.SIGNIFICANCE STATEMENT Previous studies have identified a role for the MT area of the visual cortex in perceiving motion, yet none have examined its central role in the development of the visual cortex and in the establishment of visuomotor behaviors. To address this, we used a unilateral MT lesion model in neonatal marmosets before examining the anatomic, physiological, and behavioral consequences. In adulthood, we observed perturbations in goal-orientated reach-and-grasp behavior, altered direction selectivity of V1 neurons, and changes in the cytoarchitecture throughout dorsal stream areas. This study highlights the importance of MT as a central node in visual system development and consequential visuomotor activity.

Retinotopic specializations of cortical and thalamic inputs to area MT.

Retinotopic specializations in the ventral visual stream, especially foveal adaptations, provide primates with high-acuity vision in the central visual field. However, visual field specializations have not been studied in the dorsal visual stream, dedicated to processing visual motion and visually guided behaviors. To investigate this, we injected retrograde neuronal tracers occupying the whole visuotopic representation of the middle temporal (MT) visual area in marmoset monkeys and studied the distribution and morphology of the afferent primary visual cortex (V1) projections. Contrary to previous reports, we found a heterogeneous population of V1-MT projecting neurons distributed in layers 3C and 6. In layer 3C, spiny stellate neurons were distributed mainly in foveal representations, while pyramidal morphologies were characteristic of peripheral eccentricities. This primate adaptation of the V1 to MT pathway is arranged in a way that we had not previously understood, with abundant stellate projection neurons in the high-resolution foveal portions, suggesting rapid relay of motion information to visual area MT. We also describe that the medial portion of the inferior pulvinar (PIm), which is the main thalamic input to area MT, shows a retinotopic organization, likely reflecting the importance of this pathway during development and the establishment of area MT topography.

Extensive Connectivity Between the Medial Pulvinar and the Cortex Revealed in the Marmoset Monkey.

The medial pulvinar (PM) is a multimodal associative thalamic nucleus, recently evolved in primates. PM participates in integrative and modulatory functions, including directed attention, and consistently exhibits alterations in disorders such as schizophrenia and autism. Despite essential cognitive functions, the cortical inputs to the PM have not been systematically investigated. To date, less than 20 cortices have been demonstrated to project to PM. The goal of this study was to establish a comprehensive map of the cortical afferents to PM in the marmoset monkey. Using a magnetic resonance imaging-guided injection approach, we reveal 62 discrete cortices projecting to the adult marmoset PM. We confirmed previously reported connections and identified further projections from discrete cortices across the temporal, parietal, retrosplenial-cingulate, prefrontal, and orbital lobes. These regions encompass areas recipient of PM efferents, demonstrating the reciprocity of the PM-cortical connectivity. Moreover, our results indicate that PM neurones projecting to distinct cortices are intermingled and form multimodal cell clusters. This microunit organization, believed to facilitate cross-modal integration, contrasts with the large functional subdivisions usually observed in thalamic nuclei. Altogether, we provide the first comprehensive map of PM cortical afferents, an essential stepping stone in expanding our knowledge of PM and its function.

Transient visual pathway critical for normal development of primate grasping behavior.

An evolutionary hallmark of anthropoid primates, including humans, is the use of vision to guide precise manual movements. These behaviors are reliant on a specialized visual input to the posterior parietal cortex. Here, we show that normal primate reaching-and-grasping behavior depends critically on a visual pathway through the thalamic pulvinar, which is thought to relay information to the middle temporal (MT) area during early life and then swiftly withdraws. Small MRI-guided lesions to a subdivision of the inferior pulvinar subnucleus (PIm) in the infant marmoset monkey led to permanent deficits in reaching-and-grasping behavior in the adult. This functional loss coincided with the abnormal anatomical development of multiple cortical areas responsible for the guidance of actions. Our study reveals that the transient retino-pulvinar-MT pathway underpins the development of visually guided manual behaviors in primates that are crucial for interacting with complex features in the environment.

Thalamocortical Afferents Innervate the Cortical Subplate much Earlier in Development in Primate than in Rodent.

The current model, based on rodent data, proposes that thalamocortical afferents (TCA) innervate the subplate towards the end of cortical neurogenesis. This implies that the laminar identity of cortical neurons is specified by intrinsic instructions rather than information of thalamic origin. In order to determine whether this mechanism is conserved in the primates, we examined the growth of thalamocortical (TCA) and corticofugal afferents in early human and monkey fetal development. In the human, TCA, identified by secretagogin, calbindin, and ROBO1 immunoreactivity, were observed in the internal capsule of the ventral telencephalon as early as 7-7.5 PCW, crossing the pallial/subpallial boundary (PSB) by 8 PCW before the calretinin immunoreactive corticofugal fibers do. Furthermore, TCA were observed to be passing through the intermediate zone and innervating the presubplate of the dorsolateral cortex, and already by 10-12 PCW TCAs were occupying much of the cortex. Observations at equivalent stages in the marmoset confirmed that this pattern is conserved across primates. Therefore, our results demonstrate that in primates, TCAs innervate the cortical presubplate at earlier stages than previously demonstrated by acetylcholinesterase histochemistry, suggesting that pioneer thalamic afferents may contribute to early cortical circuitry that can participate in defining cortical neuron phenotypes.

The medial pulvinar: function, origin and association with neurodevelopmental disorders.

The pulvinar is primarily referred to for its role in visual processing. However, the 'visual pulvinar' only encompasses the inferior and lateral regions of this complex thalamic nucleus. The remaining medial portion (medial pulvinar, PM) establishes distinct cortical connectivity and has been associated with directed attention, executive functions and working memory. These functions are particularly impaired in neurodevelopmental disorders, including schizophrenia and attention deficit and hyperactivity disorder (ADHD), both of which have been associated with abnormal PM architecture and connectivity. With these disorders becoming more prevalent in modern societies, we review the literature to better understand how the PM can participate in the pathophysiology of cognitive disorders and how a better understanding of the development and function of this thalamic nucleus, which is most likely exclusive to the primate brain, can advance clinical research and treatments.

Acute or Delayed Systemic Administration of Human Amnion Epithelial Cells Improves Outcomes in Experimental Stroke.

BACKGROUND AND PURPOSE: Human amnion epithelial cells (hAECs) are nonimmunogenic, nontumorigenic, anti-inflammatory cells normally discarded with placental tissue. We reasoned that their profile of biological features, wide availability, and the lack of ethical barriers to their use could make these cells useful as a therapy in ischemic stroke. METHODS: We tested the efficacy of acute (1.5 hours) or delayed (1-3 days) poststroke intravenous injection of hAECs in 4 established animal models of cerebral ischemia. Animals included young (7-14 weeks) and aged mice (20-22 months) of both sexes, as well as adult marmosets of either sex. RESULTS: We found that hAECs administered 1.5 hours after stroke in mice migrated to the ischemic brain via a CXC chemokine receptor type 4-dependent mechanism and reduced brain inflammation, infarct development, and functional deficits. Furthermore, if hAECs administration was delayed until 1 or 3 days poststroke, long-term functional recovery was still augmented in young and aged mice of both sexes. We also showed proof-of-principle evidence in marmosets that acute intravenous injection of hAECs prevented infarct development from day 1 to day 10 after stroke. CONCLUSIONS: Systemic poststroke administration of hAECs elicits marked neuroprotection and facilitates mechanisms of repair and recovery.

Reduced post-stroke glial scarring in the infant primate brain reflects age-related differences in the regulation of astrogliosis.

Ischemic stroke remains a leading cause of disability worldwide. Surviving patients often suffer permanent neurological impairments, and spontaneous recovery rarely occurs. However, observations that early-life brain injuries, including strokes, elicit less severe long-term functional impairments, compared to adults, continue to intrigue. While much research has focussed on neuronal changes and plasticity, less is known regarding the regulation of astrogliosis and glial scar formation after a stroke at different stages of life. Therefore, we investigated the cellular, molecular and temporal differences in chronic scar development in the infant and adult nonhuman primate (NHP) post-stroke as it bears greater clinical relevance in the close temporal and pathophysiological homology with humans. This project utilized the endothelin-1 model of focal ischemic stroke in the infant and adult primary visual cortex and investigated differences in the subacute and chronic period. We report here that the post-stroke infant neocortex generates a smaller, more discrete chronic scar, correlating to greater neuronal sparing. Reactive astrocytes that comprise the chronic scar are generated earlier in infants compared to adults, and the expression of critical markers of astrocyte reactivity differs in the subacute period between post-stroke infants and adults. Most importantly, we report that unlike adults, infant astrocyte reactivity is not dependent on several crucial regulators: signal transducer and activator of transcription 3, lipocalin2 and collagen I. Our results demonstrate that infant reactive astrocytes are not regulated by the same intrinsic and extrinsic factors that control these processes in adults, resulting in a more discrete chronic glial scar that is more permissible to neuronal sparing.

The guidance molecule Semaphorin3A is differentially involved in the arealization of the mouse and primate neocortex.

The visual cortex is organized into discrete domains characterized by their specific function, connectivity, chemoarchitecture, and cytoarchitecture. Gradients of transcription factors across the anteroposterior and mediolateral axes of the neocortex have previously been demonstrated to specify the main sensory regions. However, they do not account for the establishment of multiple areas in the primate visual cortex, which occupies approximately 50% of the neocortical surface. We demonstrate that the guidance molecule Semaphorin3A (Sema3A) is initially secreted in the cortical plate of the embryonic marmoset monkey and acts as an intrinsic cue to control the migration of subpopulations of neuronal progenitors and projection neurons expressing the receptor Neuropilin 1 (Npn1). During the first 2 postnatal weeks, Sema3A expression becomes primarily associated with ventral visual cortical areas, leading to the specific migration of Npn1+ neurons in the late maturing visual areas. In the mouse, Sema3A distribution is not arealized, but Npn1 expression becomes restricted to the posterior neocortex at embryonic day 16.5. The selective reduction in the striate cortex we observe in Sema3A-/- animals potentially results from the differential distribution of Npn1+ cells. Therefore, the Sema3A/Npn1 pathway participates to the parcellation of the visual neocortex in both the mouse and the marmoset, however, through different regulatory processes.

Over-expression of RCAN1 causes Down syndrome-like hippocampal deficits that alter learning and memory.

People with Down syndrome (DS) exhibit abnormal brain structure. Alterations affecting neurotransmission and signalling pathways that govern brain function are also evident. A large number of genes are simultaneously expressed at abnormal levels in DS; therefore, it is a challenge to determine which gene(s) contribute to specific abnormalities, and then identify the key molecular pathways involved. We generated RCAN1-TG mice to study the consequences of RCAN1 over-expression and investigate the contribution of RCAN1 to the brain phenotype of DS. RCAN1-TG mice exhibit structural brain abnormalities in those areas affected in DS. The volume and number of neurons within the hippocampus is reduced and this correlates with a defect in adult neurogenesis. The density of dendritic spines on RCAN1-TG hippocampal pyramidal neurons is also reduced. Deficits in hippocampal-dependent learning and short- and long-term memory are accompanied by a failure to maintain long-term potentiation (LTP) in hippocampal slices. In response to LTP induction, we observed diminished calcium transients and decreased phosphorylation of CaMKII and ERK1/2-proteins that are essential for the maintenance of LTP and formation of memory. Our data strongly suggest that RCAN1 plays an important role in normal brain development and function and its up-regulation likely contributes to the neural deficits associated with DS.

EphA4 is associated with multiple cell types in the marmoset primary visual cortex throughout the lifespan.

Ephs form the largest family of receptor tyrosine kinases. They interact with the membrane-bound ligands - ephrins - to control crucial aspects of brain development. EphA4 is the most prominent member of the family in terms of versatility and ability to bind most ephrin ligands. EphA4 regulates brain development by modulating neuronal migration and connectivity. In the present study, we address the involvement of EphA4 in patterning the primary visual cortex (V1) of the marmoset monkey by characterizing the cellular expression profile of EphA4 from late embryonic stages to adulthood. We identified continuous expression on neurons in the cortical plate and mature neocortical layers, similar to that described in the mouse, excluding a role for EphA4 in the formation of borders between visual areas in the marmoset neocortex. In addition to neurons, we also report expression of EphA4 on glial populations, including radial glia and astrocytes. In contrast to what is seen in the mouse, EphA4 expression on astrocytes persists in the adult marmoset V1, including around blood vessels and in the white matter. Robust expression by glial populations, which retain neurogenic properties in the postnatal marmoset, indicates that EphA4 may have acquired additional roles during evolution, with important implications for the benefits of EphA4-blocking therapies following brain injury.

The early maturation of visual cortical area MT is dependent on input from the retinorecipient medial portion of the inferior pulvinar.

The hierarchical development of the primate visual cortex and associated streams remains somewhat of a mystery. While anatomical, physiological, and psychological studies have demonstrated the early maturation of the dorsal "where"/"how" or motion cortical stream, little is known about the circuitry responsible. The influence of the retinogeniculostriate pathway has been investigated, but little attention has been paid to the role of two more recently described disynaptic retinothalamic projections to the middle temporal (MT) area, an early maturing dorsal stream cortical field, and which bypass the primary visual cortex (V1). These pathways are via the koniocellular layers of the lateral geniculate nucleus (LGN) and the medial portion of the inferior pulvinar (PIm). Both have been demonstrated in the adult nonhuman primate, but their influence during the maturation of the visual cortex is unknown. We used a combination of neural tracing and immunohistochemistry to follow the development of LGN and PIm inputs to area MT in the marmoset monkey. Our results revealed that the early maturation of area MT is likely due to the disynaptic retinopulvinar input and not the retinogeniculate input or the direct projection from V1. Furthermore, from soon after birth to adulthood, there was a dynamic shift in the ratio of input from these three structures to area MT, with an increasing dominance of the direct V1 afference.

Compartmentalization of cerebral cortical germinal zones in a lissencephalic primate and gyrencephalic rodent.

Previous studies of macaque and human cortices identified cytoarchitectonically distinct germinal zones; the ventricular zone inner subventricular zone (ISVZ), and outer subventricular zone (OSVZ). To date, the OSVZ has only been described in gyrencephalic brains, separated from the ISVZ by an inner fiber layer and considered a milestone that triggered increased neocortical neurogenesis. However, this observation has only been assessed in a handful of species without the identification of the different progenitor populations. We examined the Amazonian rodent agouti (Dasyprocta agouti) and the marmoset monkey (Callithrix jacchus) to further understand relationships among progenitor compartmentalization, proportions of various cortical progenitors, and degree of cortical folding. We identified a similar cytoarchitectonic distinction between the OSVZ and ISVZ at midgestation in both species. In the marmoset, we quantified the ventricular and abventricular divisions and observed similar proportions as previously described for the human and ferret brains. The proportions of radial glia, intermediate progenitors, and outer radial glial cell (oRG) populations were similar in midgestation lissencephalic marmoset as in gyrencephalic human or ferret. Our findings suggest that cytoarchitectonic subdivisions of SVZ are an evolutionary trend and not a primate specific feature, and a large population of oRG can be seen regardless of cortical folding.

Breaking camouflage: responses of neurons in the middle temporal area to stimuli defined by coherent motion.

Camouflaged animals remain inconspicuous only insofar as they remain static. This demonstrates that motion is a powerful cue for figure-ground segregation, allowing detection of moving objects even when their luminance and texture characteristics are matched to the background. We investigated the neural processes underlying this phenomenon by testing the responses of neurons in the middle temporal area (MT) to 'camouflaged' bars, which were rendered visible by motion. These responses were compared with those elicited by 'solid' bars, which also differed from background in terms of their mean luminance. Most MT neurons responded strongly to camouflaged bars, and signaled their direction of motion with precision, with direction-tuning curves being only slightly wider than those measured with solid bars. However, the tuning of most MT cells to stimulus length and speed depended on the type of stimulus - in comparison with solid bars, responses to camouflaged bars typically showed more extensive length summation, weak end-inhibition, and stronger attenuation at high speeds. Moreover, the emergence of direction selectivity was delayed in trials involving camouflaged bars, relative to solid bars. Comparison with results obtained in the first (V1) and second (V2) visual areas, using similar stimuli, indicates that neural computations performed in MT result in significantly stronger and more accurate signals about camouflaged objects, particularly in situations in which these are relatively large and slow moving. These computations are likely to represent an important step in enabling cue-invariant perception of moving objects, particularly in biologically relevant situations.

Modelling behaviors relevant to brain disorders in the nonhuman primate: Are we there yet?

Recent years have seen a profound resurgence of activity with nonhuman primates (NHPs) to model human brain disorders. From marmosets to macaques, the study of NHP species offers a unique window into the function of primate-specific neural circuits that are impossible to examine in other models. Examining how these circuits manifest into the complex behaviors of primates, such as advanced cognitive and social functions, has provided enormous insights to date into the mechanisms underlying symptoms of numerous neurological and neuropsychiatric illnesses. With the recent optimization of modern techniques to manipulate and measure neural activity in vivo, such as optogenetics and calcium imaging, NHP research is more well-equipped than ever to probe the neural mechanisms underlying pathological behavior. However, methods for behavioral experimentation and analysis in NHPs have noticeably failed to keep pace with these advances. As behavior ultimately lies at the junction between preclinical findings and its translation to clinical outcomes for brain disorders, approaches to improve the integrity, reproducibility, and translatability of behavioral experiments in NHPs requires critical evaluation. In this review, we provide a unifying account of existing brain disorder models using NHPs, and provide insights into the present and emerging contributions of behavioral studies to the field.

Visual motion integration by neurons in the middle temporal area of a New World monkey, the marmoset.

The middle temporal area (MT/V5) is an anatomically distinct region of primate visual cortex that is specialized for the processing of image motion. It is generally thought that some neurons in area MT are capable of signalling the motion of complex patterns, but this has only been established in the macaque monkey. We made extracellular recordings from single units in area MT of anaesthetized marmosets, a New World monkey. We show through quantitative analyses that some neurons (35 of 185; 19%) are capable of signalling pattern motion ('pattern cells'). Across several dimensions, the visual response of pattern cells in marmosets is indistinguishable from that of pattern cells in macaques. Other neurons respond to the motion of oriented contours in a pattern ('component cells') or show intermediate properties. In addition, we encountered a subset of neurons (22 of 185; 12%) insensitive to sinusoidal gratings but very responsive to plaids and other two-dimensional patterns and otherwise indistinguishable from pattern cells. We compared the response of each cell class to drifting gratings and dot fields. In pattern cells, directional selectivity was similar for gratings and dot fields; in component cells, directional selectivity was weaker for dot fields than gratings. Pattern cells were more likely to have stronger suppressive surrounds, prefer lower spatial frequencies and prefer higher speeds than component cells. We conclude that pattern motion sensitivity is a feature of some neurons in area MT of both New and Old World monkeys, suggesting that this functional property is an important stage in motion analysis and is likely to be conserved in humans.

Replicating infant-specific reactive astrocyte functions in the injured adult brain.

Infants and adults respond differently to brain injuries. Specifically, improved neuronal sparing along with reduced astrogliosis and glial scarring often observed earlier in life, likely contributes to improved long-term outcomes. Understanding the underlying mechanisms could enable the recapitulation of neuroprotective effects, observed in infants, to benefit adults after brain injuries. We reveal that in primates, Eph/ ephrin signaling contributes to age-dependent reactive astrocyte behavior. Ephrin-A5 expression on astrocytes was more protracted in adults, whereas ephrin-A1 was only expressed on infant astrocytes. Furthermore, ephrin-A5 exacerbated major hallmarks of astrocyte reactivity via EphA2 and EphA4 receptors, which was subsequently alleviated by ephrin-A1. Rather than suppressing reactivity, ephrin-A1 signaling shifted astrocytes towards GAP43+ neuroprotection, accounting for improved neuronal sparing in infants. Reintroducing ephrin-A1 after middle-aged focal ischemic injury significantly attenuated glial scarring, improved neuronal sparing and preserved circuitry. Therefore, beneficial infant mechanisms can be recapitulated in adults to improve outcomes after CNS injuries.

Retinal ganglion cells projecting to superior colliculus and pulvinar in marmoset.

We determined the retinal ganglion cell types projecting to the medial subdivision of inferior pulvinar (PIm) and the superior colliculus (SC) in the common marmoset monkey, Callithrix jacchus. Adult marmosets received a bidirectional tracer cocktail into the PIm (conjugated to Alexa fluor 488), and the SC (conjugated to Alexa fluor 594) using an MRI-guided approach. One SC injection included the pretectum. The large majority of retrogradely labelled cells were obtained from SC injections, with only a small proportion obtained after PIm injections. Retrogradely labelled cells were injected intracellularly in vitro using lipophilic dyes (DiI, DiO). The SC and PIm both received input from a variety of ganglion cell types. Input to the PIm was dominated by broad thorny (41%), narrow thorny (24%) and large bistratified (25%) ganglion cells. Input to the SC was dominated by parasol (37%), broad thorny (24%) and narrow thorny (17%) cells. Midget ganglion cells (which make up the large majority of primate retinal ganglion cells) and small bistratified (blue-ON/yellow OFF) cells were never observed to project to SC or PIm. Small numbers of other wide-field ganglion cell types were also encountered. Giant sparse (presumed melanopsin-expressing) cells were only seen following the tracer injection which included the pretectum. We note that despite the location of pulvinar complex in dorsal thalamus, and its increased size and functional importance in primate evolution, the retinal projections to pulvinar have more in common with SC projections than they do with projections to the dorsal lateral geniculate nucleus.

Mapping the neural circuitry of predator fear in the nonhuman primate.

In rodents, innate and learned fear of predators depends on the medial hypothalamic defensive system, a conserved brain network that lies downstream of the amygdala and promotes avoidance via projections to the periaqueductal gray. Whether this network is involved in primate fear remains unknown. To address this, we provoked flight responses to a predator (moving snake) in the marmoset monkey under laboratory conditions. We combined c-Fos immunolabeling and anterograde/retrograde tracing to map the functional connectivity of the ventromedial hypothalamus, a core node in the medial hypothalamic defensive system. Our findings demonstrate that the ventromedial hypothalamus is recruited by predator exposure in primates and that anatomical connectivity of the rodent and primate medial hypothalamic defensive system are highly conserved.