Functional organization of motor cortex of adult macaque monkeys is altered by sensory loss in infancy

Brainwide mesoscale functional networks revealed by focal infrared neural stimulation of the amygdala

The primate amygdala serves to evaluate the emotional content of sensory inputs and modulate emotional and social behaviors; it modulates cognitive, multisensory and autonomic circuits predominantly via the basal, lateral and central nuclei, respectively. Recent evidence has suggested the mesoscale (millimeter-scale) nature of intra-amygdala functional organization. However, the connectivity patterns by which these mesoscale regions interact with brainwide networks remain unclear. Using infrared neural stimulation of single mesoscale sites coupled with mapping in ultrahigh field 7-T functional magnetic resonance imaging, we have discovered that these mesoscale sites exert influence over a surprisingly extensive scope of the brain. Our findings strongly indicate that mesoscale sites within the amygdala modulate brainwide networks through a 'one-to-many' (integral) way. Meanwhile, these connections exhibit a point-to-point (focal) topography. Our work provides new insights into the functional architecture underlying emotional and social behavioral networks, thereby opening up possibilities for individualized modulation of psychological disorders.

Cortical reorganization following dorsal spinal injuries in newborn monkeys reveals a critical period in the development of the somatosensory cortex

Lesions of the dorsal columns of the spinal cord in adult macaque monkeys lead to the loss of hand inputs and large-scale expansion of the face inputs in the hand region of the somatosensory cortex. Inputs from alternate spinal pathways do not reactivate the deafferented regions of area 3b. Here, we determined how transections of the dorsal columns done within a few days after birth affect the developing somatosensory cortex. Dorsal columns were transected between the 3rd and 12th postnatal day (PND), and the somatosensory cortex was mapped when the macaques were over 3 y old. There were two distinct outcomes depending on the age at the time of the lesion. In monkeys lesioned between the 3rd and 5th PND, neurons in the entire hand region of area 3b and the adjacent somatosensory cortex responded to touch on the hand. An alternate spinal pathway must have replaced the lost pathway. In monkeys lesioned between the 9th and 12th PND, neurons in the deafferented hand region did not respond to touch on the hand. There was medialward expansion of the face representation into the deafferented cortex and a lateral expansion of the arm representation as in lesioned adults. Thus, different mechanisms underlie the reorganization of area 3b and the adjacent somatosensory cortex following identical spinal cord injuries sustained as early or late newborns. The results suggest that alternate spinal cord pathways can develop within a critical period before the 9th PND, but not later.

Brainwide mesoscale functional networks revealed by focal infrared neural stimulation of the amygdala

The primate amygdala serves to evaluate emotional content of sensory inputs and modulate emotional and social behaviors; it modulates cognitive, multisensory and autonomic circuits predominantly via the basal (BA), lateral (LA), and central (CeA) nuclei, respectively. Based on recent electrophysiological evidence suggesting mesoscale (millimeters-scale) nature of intra-amygdala functional organization, we have investigated the connectivity of these nuclei using Infrared Neural Stimulation of single mesoscale sites coupled with mapping in ultrahigh field 7T functional Magnetic Resonance Imaging (INS-fMRI). Stimulation of multiple sites within amygdala of single individuals evoked 'mesoscale functional connectivity maps', allowing comparison of BA, LA and CeA connected brainwide networks. This revealed a mesoscale nature of connected sites, complementary spatial patterns of functional connectivity, and topographic relationships of nucleus-specific connections. Our data reveal a functional architecture of systematically organized brainwide networks mediating sensory, cognitive, and autonomic influences from the amygdala.

Structural plasticity in the entorhinal and perirhinal cortices following hippocampal lesions in rhesus monkeys

Immature neurons expressing the Bcl2 protein are present in various regions of the mammalian brain, including the amygdala and the entorhinal and perirhinal cortices. Their functional role is unknown but we have previously shown that neonatal and adult hippocampal lesions increase their differentiation in the monkey amygdala. Here, we assessed whether hippocampal lesions similarly affect immature neurons in the entorhinal and perirhinal cortices. Since Bcl2-positive cells were found mainly in areas Eo, Er, and Elr of the entorhinal cortex and in layer II of the perirhinal cortex, we also used Nissl-stained sections to determine the number and soma size of immature and mature neurons in layer III of area Er and layer II of area 36 of the perirhinal cortex. We found different structural changes in these regions following hippocampal lesions, which were influenced by the time of the lesion. In neonate-lesioned monkeys, the number of immature neurons in the entorhinal and perirhinal cortices was generally higher than in controls. The number of mature neurons was also higher in layer III of area Er of neonate-lesioned monkeys but no differences were found in layer II of area 36. In adult-lesioned monkeys, the number of immature neurons in the entorhinal cortex was lower than in controls but did not differ from controls in the perirhinal cortex. The number of mature neurons in layer III of area Er did not differ from controls, but the number of small, mature neurons in layer II of area 36 was lower than in controls. In sum, hippocampal lesions impacted populations of mature and immature neurons in discrete regions and layers of the entorhinal and perirhinal cortices, which are interconnected with the amygdala and provide major cortical inputs to the hippocampus. These structural changes may contribute to some functional recovery following hippocampal injury in an age-dependent manner.

Representations of Fine Digit Movements in Posterior and Anterior Parietal Cortex Revealed Using Long-Train Intracortical Microstimulation in Macaque Monkeys

The current investigation in macaque monkeys utilized long-train intracortical microstimulation to determine the extent of cortex from which movements could be evoked. Not only were movements evoked from motor areas (PMC and M1), but they were also evoked from posterior parietal (5, 7a, 7b) and anterior parietal areas (3b, 1, 2). Large representations of digit movements involving only the index finger (D2) and thumb (D1), were elicited from areas 1, 2, 7b, and M1. Other movements evoked from these regions were similar to ethologically relevant movements that have been described in other primates. These include combined forelimb and mouth movements and full hand grasps. However, many other movements were much more complex and could not be categorized into any of the previously described ethological categories. Movements involving specific digits, which mimic precision grips, are unique to macaques and have not been described in New World or prosimian primates. We propose that these multiple and expanded motor representations of the digits co-evolved with the emergence of the opposable thumb and alterations in grip type in some anthropoid lineages.

Cortical Reorganization of Sensorimotor Systems and the Role of Intracortical Circuits After Spinal Cord Injury

The plasticity of sensorimotor systems in mammals underlies the capacity for motor learning as well as the ability to relearn following injury. Spinal cord injury, which both deprives afferent input and interrupts efferent output, results in a disruption of cortical somatotopy. While changes in corticospinal axons proximal to the lesion are proposed to support the reorganization of cortical motor maps after spinal cord injury, intracortical horizontal connections are also likely to be critical substrates for rehabilitation-mediated recovery. Intrinsic connections have been shown to dictate the reorganization of cortical maps that occurs in response to skilled motor learning as well as after peripheral injury. Cortical networks incorporate changes in motor and sensory circuits at subcortical or spinal levels to induce map remodeling in the neocortex. This review focuses on the reorganization of cortical networks observed after injury and posits a role of intracortical circuits in recovery.

Evolution, immunity and the emergence of brain superautoantigens

While some autoimmune disorders remain extremely rare, others largely predominate the epidemiology of human autoimmunity. Notably, these include psoriasis, diabetes, vitiligo, thyroiditis, rheumatoid arthritis and multiple sclerosis. Thus, despite the quasi-infinite number of "self" antigens that could theoretically trigger autoimmune responses, only a limited set of antigens, referred here as superautoantigens, induce pathogenic adaptive responses. Several lines of evidence reviewed in this paper indicate that, irrespective of the targeted organ (e.g. thyroid, pancreas, joints, brain or skin), a significant proportion of superautoantigens are highly expressed in the synaptic compartment of the central nervous system (CNS). Such an observation applies notably for GAD65, AchR, ribonucleoproteins, heat shock proteins, collagen IV, laminin, tyrosine hydroxylase and the acetylcholinesterase domain of thyroglobulin. It is also argued that cognitive alterations have been described in a number of autoimmune disorders, including psoriasis, rheumatoid arthritis, lupus, Crohn's disease and autoimmune thyroiditis. Finally, the present paper points out that a great majority of the "incidental" autoimmune conditions notably triggered by neoplasms, vaccinations or microbial infections are targeting the synaptic or myelin compartments. On this basis, the concept of an immunological homunculus, proposed by Irun Cohen more than 25 years ago, is extended here in a model where physiological autoimmunity against brain superautoantigens confers both: i) a crucial evolutionary-determined advantage via cognition-promoting autoimmunity; and ii) a major evolutionary-determined vulnerability, leading to the emergence of autoimmune disorders in Homo sapiens. Moreover, in this theoretical framework, the so called co-development/co-evolution model, both the development (at the scale of an individual) and evolution (at the scale of species) of the antibody and T-cell repertoires are coupled to those of the neural repertoires (i.e. the distinct neuronal populations and synaptic circuits supporting cognitive and sensorimotor functions). Clinical implications and future experimental insights are also presented and discussed.

The origins of thalamic inputs to grasp zones in frontal cortex of macaque monkeys

The hand representation in primary motor cortex (M1) is instrumental to manual dexterity in primates. In Old World monkeys, rostral and caudal aspects of the hand representation are located in the precentral gyrus and the anterior bank of the central sulcus, respectively. We previously reported the organization of the cortico-cortical connections of the grasp zone in rostral M1. Here we describe the organization of thalamocortical connections that were labeled from the same tracer injections. Thalamocortical connections of a grasp zone in ventral premotor cortex (PMv) and the M1 orofacial representation are included for direct comparison. The M1 grasp zone was primarily connected with ventral lateral divisions of motor thalamus. The largest proportion of inputs originated in the posterior division (VLp) followed by the medial and the anterior divisions. Thalamic inputs to the M1 grasp zone originated in more lateral aspects of VLp as compared to the origins of thalamic inputs to the M1 orofacial representation. Inputs to M1 from thalamic divisions connected with cerebellum constituted three fold the density of inputs from divisions connected with basal ganglia, whereas the ratio of inputs was more balanced for the grasp zone in PMv. Privileged access of the cerebellothalamic pathway to the grasp zone in rostral M1 is consistent with the connection patterns previously reported for the precentral gyrus. Thus, cerebellar nuclei are likely more involved than basal ganglia nuclei with the contributions of rostral M1 to manual dexterity.

Patterns of myeloarchitecture in lower limb amputees: an MRI study

Functional studies of cortical plasticity in humans suggest that the motor cortex reorganizes when the descending motor output pathway is disrupted as a result of limb amputation. The question thus arises if the underlying anatomical organization of the motor cortex is also altered in limb amputation. Owing to challenges involved in imaging the thin cerebral cortex in vivo, there is limited data available on the anatomical or morphological plasticity of the motor cortex in amputation. In this paper, we study the morphology of the primary motor cortex in four lower limb amputees with 37 or more years of amputation and four age and gender-matched controls using 0.7 mm isotropic, T₁-weighted MRI optimized to produce enhanced intracortical contrast based on myelin content. We segment the cortex into myelinated and unmyelinated gray matter. We determine the myelinated thickness which is the thickness of the well-myelinated tissue in the deeper layers of the cortex. We compare the bilateral differences in the myelinated thickness between amputees and controls. We also compare bilateral differences in cortical thickness between the two groups. Our measurements show no statistically significant difference between the amputees and controls in the myelinated thickness and in cortical thickness, in the region of the primary motor cortex representing the lower leg.

Cortical reorganization after spinal cord injury: always for good?

Plasticity constitutes the basis of behavioral changes as a result of experience. It refers to neural network shaping and re-shaping at the global level and to synaptic contacts remodeling at the local level, either during learning or memory encoding, or as a result of acute or chronic pathological conditions. 'Plastic' brain reorganization after central nervous system lesions has a pivotal role in the recovery and rehabilitation of sensory and motor dysfunction, but can also be "maladaptive". Moreover, it is clear that brain reorganization is not a "static" phenomenon but rather a very dynamic process. Spinal cord injury immediately initiates a change in brain state and starts cortical reorganization. In the long term, the impact of injury - with or without accompanying therapy - on the brain is a complex balance between supraspinal reorganization and spinal recovery. The degree of cortical reorganization after spinal cord injury is highly variable, and can range from no reorganization (i.e. "silencing") to massive cortical remapping. This variability critically depends on the species, the age of the animal when the injury occurs, the time after the injury has occurred, and the behavioral activity and possible therapy regimes after the injury. We will briefly discuss these dependencies, trying to highlight their translational value. Overall, it is not only necessary to better understand how the brain can reorganize after injury with or without therapy, it is also necessary to clarify when and why brain reorganization can be either "good" or "bad" in terms of its clinical consequences. This information is critical in order to develop and optimize cost-effective therapies to maximize functional recovery while minimizing maladaptive states after spinal cord injury.

The reactivation of somatosensory cortex and behavioral recovery after sensory loss in mature primates

In our experiments, we removed a major source of activation of somatosensory cortex in mature monkeys by unilaterally sectioning the sensory afferents in the dorsal columns of the spinal cord at a high cervical level. At this level, the ascending branches of tactile afferents from the hand are cut, while other branches of these afferents remain intact to terminate on neurons in the dorsal horn of the spinal cord. Immediately after such a lesion, the monkeys seem relatively unimpaired in locomotion and often use the forelimb, but further inspection reveals that they prefer to use the unaffected hand in reaching for food. In addition, systematic testing indicates that they make more errors in retrieving pieces of food, and start using visual inspection of the rotated hand to confirm the success of the grasping of the food. Such difficulties are not surprising as a complete dorsal column lesion totally deactivates the contralateral hand representation in primary somatosensory cortex (area 3b). However, hand use rapidly improves over the first post-lesion weeks, and much of the hand representational territory in contralateral area 3b is reactivated by inputs from the hand in roughly a normal somatotopic pattern. Quantitative measures of single neuron response properties reveal that reactivated neurons respond to tactile stimulation on the hand with high firing rates and only slightly longer latencies. We conclude that preserved dorsal column afferents after nearly complete lesions contribute to the reactivation of cortex and the recovery of the behavior, but second-order sensory pathways in the spinal cord may also play an important role. Our microelectrode recordings indicate that these preserved first-order, and second-order pathways are initially weak and largely ineffective in activating cortex, but they are potentiated during the recovery process. Therapies that would promote this potentiation could usefully enhance recovery after spinal cord injury.

Median nerve stimulation modulates extracellular signals in the primary motor area of a macaque monkey

Aiming to better define the functional influence of somatosensory stimuli on the primary motor cortex (M1) of primates, we investigated changes in extracellular neural activity induced by repetitive median nerve stimulation (MNS). We described neural adaptation and signal integration in both the multiunit activity (MUA) and the local field potential (LFP). To identify integration of initial M1 activity in the MNS response, we tested the correlation between peak amplitude responses and band energy preceding the peaks. Most of the sites studied in the M1 resulted responsive to MNS. MUA response peak amplitude decreased significantly in time in all sites during repetitive MNS, LFP response peak amplitude instead resulted more variable. Similarly, correlation analysis with the initial activity revealed a significant influence when tested using MUA peak amplitude modulation and a less significant correlation when tested using LFP peak amplitude. Our findings improve current knowledge on mechanisms underlying early M1 changes consequent to afferent somatosensory stimuli.

Impairment and recovery of hand use after unilateral section of the dorsal columns of the spinal cord in squirrel monkeys

Damage to the ascending forelimb afferents in the dorsal columns (DCs) of the cervical spinal cord in monkeys impairs forelimb use, particularly hand dexterity. Although considerable recovery has been reported, interpretation of the results is complicated by the reproducibility of the lesion and behavioral assessment. Here, we examined the effects of a unilateral DC lesion at the C4-C6 spinal cord level in four adult squirrel monkeys. Behavioral performance was assessed on a reach-to-grasp task over 5-13 weeks after lesion. Retrograde tracers were injected into the skin of the fingertips to determine the distribution of axon terminals in the cuneate nucleus and estimate the effectiveness of lesion at the conclusion of each case. The size and level of DC lesion was reflected in the proportion of spared afferents, which ranged from 1 to 25% across monkeys. The experiments produced two major findings. First, the extent of deafferentation in the DC is directly related to the degree of reaching and grasping impairments, and to the reactivation profile and somatotopic reorganization in contralateral primary somatosensory cortex. Second, considerable behavioral recovery and cortical reorganization occurred even in the monkey with only 1% of axons spared in the DC. Our findings suggest that cutaneous inputs from the hand and forelimb are critical to the integrity of functions such as reaching and grasping. In addition, axon branches from peripheral afferents that terminate on neurons in the dorsal horn of the spinal cord are likely central to the functional recovery.

Cell and neuron densities in the primary motor cortex of primates

Cell and neuron densities vary across the cortical sheet in a predictable manner across different primate species (Collins et al., 2010b). Primary motor cortex, M1, is characterized by lower neuron densities relative to other cortical areas. M1 contains a motor representation map of contralateral body parts from tail to tongue in a mediolateral sequence. Different functional movement representations within M1 likely require specialized microcircuitry for control of different body parts, and these differences in circuitry may be reflected by variation in cell and neuron densities. Here we determined cell and neuron densities for multiple sub-regions of M1 in six primate species, using the semi-automated flow fractionator method. The results verify previous reports of lower overall neuron densities in M1 compared to other parts of cortex in the six primate species examined. The most lateral regions of M1 that correspond to face and hand movement representations, are more neuron dense relative to medial locations in M1, which suggests differences in cortical circuitry within movement zones.

Cortical networks for ethologically relevant behaviors in primates

Parietal-frontal networks in primate brains are central to mediating actions. Physiological and anatomical investigations have shown that the parietal-frontal network is consistently organized across several branches of primate evolution that include prosimian galagos, New World owl and squirrel monkeys, and Old World macaque monkeys. Electrical stimulation with 0.5-sec trains of pulses delivered via microelectrodes evoked ethologically relevant actions from both posterior parietal cortex (PPC) and frontal motor cortex (FMC). Reaching, grasping, defensive, and other complex movement patterns were evoked from domains that had a characteristic organization in both FMC and PPC. Although a PPC domain (e.g. reaching) may be connected with other PPC domains (e.g. grasping and defensive), its connections with FMC are preferential for a matching domain (reaching). Similarly, electrical stimulation of a PPC domain and concurrent optical imaging of FMC, showed activation patterns consistent with the preferential connectivity of PPC and FMC domains. The evidence for similar arrangements of interconnected functional domains in PPC and FMC of members of three major branches of the primate radiation suggests that the parietal-frontal networks emerged early in the evolution of primates. The small size of PPC in the close relatives of primates including lagomorphs, rodents, and tree shrews, suggests a limited involvement of PPC in motor behavior before archaic primates emerged. However, functional domains may have evolved in motor cortex before the emergence of archaic primates.

Reorganization of somatosensory cortical areas 3b and 1 after unilateral section of dorsal columns of the spinal cord in squirrel monkeys

An incomplete lesion of the ascending afferents from the hand in the dorsal columns of the spinal cord in monkeys is followed after weeks of recovery by a reactivation of much of the territory of the hand representations in primary somatosensory cortex (area 3b). However, the relationship between the extent of the dorsal column lesion and the amount of cortical reactivation has not been clear. Largely, this is due to the uncertainties about axon sparing after spinal cord lesions. Here, we unilaterally sectioned dorsal column afferents in the cervical spinal cord (C4-C6) in adult squirrel monkeys. After weeks of recovery, cholera toxin subunit B (CTB) was injected into the distal phalanges to label normal and surviving afferents to the cuneate nuclei representing the hands. Days later, the responsiveness of neurons in cortical areas 3b and 1 to tactile stimulation on the hand was evaluated in a microelectrode mapping session. The sizes and densities of CTB-labeled patches in the cuneate nuclei of both sides were quantified and compared. The results indicate that extensive reactivations of the hand representations in cortical areas 3b and 1 occur contralateral to the spinal cord lesion, even when <1% of labeled dorsal column terminations in the cuneate nucleus remained. These results raise the possibilities that secondary afferents from innervated neurons in the spinal cord contribute to the reactivation, and that the reactivation of area 1 is not completely dependent on inputs from area 3b.

Reorganization of the primary motor cortex of adult macaque monkeys after sensory loss resulting from partial spinal cord injuries

Long-term injuries to the dorsal columns of the spinal cord at cervical levels result in large-scale somatotopic reorganization of the somatosensory areas of the cortex and the ventroposterior nucleus of the thalamus. As a result of this reorganization, intact inputs from the face expand into the deafferented hand representations. Dorsal column injuries also result in permanent deficits in the use of digits for precision grip and a loss of fractionated movements of the digits. We determined whether the chronic loss of sensory inputs and the behavioral deficits caused by lesions of the dorsal columns in adult macaque monkeys affect organization of the motor cortex. The results show that, in the primary motor cortex, intracortical microstimulation evokes extension-flexion movements of the thumb at significantly fewer sites compared with the normal monkeys. There is a corresponding increase in the adduction-abduction movements. Furthermore, there is a significant increase in the thresholds of the currents required to evoke movements of the digits. Thus, long-term sensory loss in adult monkeys does not change the overall topography of the movement representation in the motor cortex but results in changes in the details of movement representations.