Neural activity in the macaque putamen associated with saccades and behavioral outcome

Good vision without peripheries: behavioral and fMRI evidence

In healthy vision, bright slow-motion stimuli are processed primarily by the regions of the visual system that receive input from the central part of the scene, whereas processing of dark fast-motion stimuli is more dependent on peripheral visual input. We tested 31 retinitis pigmentosa (RP) patients with long-term loss of peripheral photoreceptors and healthy controls with temporarily limited peripheral vision. We measured motion-based acuity using random-dot kinematograms, establishing individual thresholds for differentiating a circle from an ellipse. Participants subsequently performed a functional magnetic resonance imaging (fMRI) task set at a constant level of difficulty. The results showed that limiting vision did not affect motion-acuity thresholds in control participants but did cause different brain activations than those in RP patients, indicating prompt implementation of the strategy that would be perceptually successful. Compared with controls with both full and limited vision, impaired motion acuity in RP patients led to decreased brain activation, particularly in the primary peripheral visual areas V1-3. Importantly, compared with controls in full vision, matched decreased activation in MT+/V5, salience-processing cortices and the superior temporal cortex were detected in RP patients and in controls with limited peripheral vision, revealing brain networks that compensate for the loss of peripheral vision.

Assessment of Oculomotor Functions as a Biomarker in Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI), or concussion, is a major public health problem, and ambiguity still exists regarding its diagnosis. While functional magnetic resonance imaging (fMRI) has been identified as a helpful screening tool for concussion, its limited accessibility in clinical or field settings necessitates a more efficient alternative. Oculomotor function deficit is an often-reported pathology in mTBI. Due to the neuroanatomical overlap between eye-movement circuitry and mTBI pathophysiology, visual deficits are expected. In this study, we investigate the possibility of using an oculomotor assessment tool for finding biomarkers in concussion. We used fMRI with tasks evaluating oculomotor functions: smooth pursuit (SP), saccades, anti-saccades, and optokinetic nystagmus (OKN). Before the scanning, the testing with a system of virtual reality goggles with integrated eye- and head-tracking was used where subjects performed the same tasks as those used in fMRI. Twenty-nine concussed symptomatic adults (CSA) within 1-month postconcussion and 29 age- and sex-matched healthy controls (HCS) were tested to examine blood oxygen level-dependent (BOLD) fMRI alterations associated with performances in oculomotor function after mTBI and evaluate the efficacy of the oculomotor assessment in detecting oculomotor and gaze deficits following mTBI. Comparing CSA with HCS, significant differences were observed in anti-saccades and OKN performance. CSA group exhibited elevated %BOLD signal change on each task compared with HCS: in the superior frontal gyrus during the smooth pursuit, inferior frontal gyrus during the saccades, putamen and dorsolateral prefrontal cortex (DLPFC) during the anti-saccades, and lingual gyrus and IFG during the OKN. Key findings include the following: (1) oculomotor deficits in concussed subjects compared with controls, (2) abnormal activation patterns in areas related to the regulation and control of oculomotor movements, suggesting concussion-induced disruptions, and (3) the potential of oculomotor assessment as a promising approach for mTBI biomarkers, with anti-saccades and OKN identified as the most sensitive tasks.

Cortico-Subcortical Functional Connectivity Profiles of Resting-State Networks in Marmosets and Humans

Understanding the similarity of cortico-subcortical networks topologies between humans and nonhuman primate species is critical to study the origin of network alternations underlying human neurologic and neuropsychiatric diseases. The New World common marmoset (Callithrix jacchus) has become popular as a nonhuman primate model for human brain function. Most marmoset connectomic research, however, has exclusively focused on cortical areas, with connectivity to subcortical networks less extensively explored. Here, we aimed to first isolate patterns of subcortical connectivity with cortical resting-state networks in awake marmosets using resting-state fMRI, then to compare these networks with those in humans using connectivity fingerprinting. In this study, we used 5 marmosets (4 males, 1 female). While we could match several marmoset and human resting-state networks based on their functional fingerprints, we also found a few striking differences, for example, strong functional connectivity of the default mode network with the superior colliculus in marmosets that was much weaker in humans. Together, these findings demonstrate that many of the core cortico-subcortical networks in humans are also present in marmosets, but that small, potentially functionally relevant differences exist.SIGNIFICANCE STATEMENT The common marmoset is becoming increasingly popular as an additional preclinical nonhuman primate model for human brain function. Here we compared the functional organization of cortico-subcortical networks in marmosets and humans using ultra-high field fMRI. We isolated the patterns of subcortical connectivity with cortical resting-state networks (RSNs) in awake marmosets using resting-state fMRI and then compared these networks with those in humans using connectivity fingerprinting. While we could match several marmoset and human RSNs based on their functional fingerprints, we also found several striking differences. Together, these findings demonstrate that many of the core cortico-subcortical RSNs in humans are also present in marmosets, but that small, potentially functionally relevant differences exist.

The Caudal Part of Putamen Represents the Historical Object Value Information

The basal ganglia, especially the circuits originating from the putamen, are essential for controlling normal body movements. Notably, the putamen receives inputs not only from motor cortical areas but also from multiple sensory cortices. However, how these sensory signals are processed in the putamen remains unclear. We recorded the activity of tentative medium spiny neurons in the caudal part of the putamen when the monkey viewed many fractal objects. We found many neurons that responded to these objects, mostly in the ventral region. We called this region "putamen tail" (PUTt), as it is dorsally adjacent to "caudate tail" (CDt). Although PUTt and CDt are mostly separated by a thin layer of white matter, their neurons shared several features. Almost all of them had receptive fields in the contralateral hemifield. Moreover, their responses were object selective (i.e., variable across objects). The object selectivity was higher in the ventral region (i.e., CDt > PUTt). Some neurons above PUTt, which we called the caudal-dorsal putamen (cdPUT), also responded to objects, but less selectively than PUTt. Next, we examined whether these visual neurons changed their responses based on the reward outcome. We found that many neurons encoded the values of many objects based on long-term memory, but not based on short-term memory. Such stable value responses were stronger in PUTt and CDt than in cdPUT. These results suggest that PUTt, together with CDt, controls saccade/attention among objects with different historical values, and may control other motor actions as well.SIGNIFICANCE STATEMENT Although the putamen receives inputs not only from motor cortical areas but also from sensory cortical areas, how these sensory signals are processed remains unclear. Here we found that neurons in the caudal-ventral part of the putamen (putamen tail) process visual information including spatial and object features. These neurons discriminate many objects, first by their visual features and later by their reward values as well. Importantly, the value discrimination was based on long-term memory, but not on short-term memory. These results suggest that the putamen tail controls saccade/attention among objects with different historical values and might control other motor actions as well.

The Relationship between Saccades and Locomotion

Human locomotion involves a complex interplay among multiple brain regions and depends on constant feedback from the visual system. We summarize here the current understanding of the relationship among fixations, saccades, and gait as observed in studies sampling eye movements during locomotion, through a review of the literature and a synthesis of the relevant knowledge on the topic. A significant overlap in locomotor and saccadic neural circuitry exists that may support this relationship. Several animal studies have identified potential integration nodes between these overlapping circuitries. Behavioral studies that explored the relationship of saccadic and gait-related impairments in normal conditions and in various disease states are also discussed. Eye movements and locomotion share many underlying neural circuits, and further studies can leverage this interplay for diagnostic and therapeutic purposes.

Theta and beta synchrony coordinate frontal eye fields and anterior cingulate cortex during sensorimotor mapping

The frontal eye fields (FEFs) and the anterior cingulate cortex (ACC) are commonly coactivated for cognitive saccade tasks, but whether this joined activation indexes coordinated activity underlying successful guidance of sensorimotor mapping is unknown. Here we test whether ACC and FEF circuits coordinate through phase synchronization of local field potential and neural spiking activity in macaque monkeys performing memory-guided and pro- and anti-saccades. We find that FEF and ACC showed prominent synchronization at a 3–9 Hz theta and a 12–30 Hz beta frequency band during the delay and preparation periods with a strong Granger-causal influence from ACC to FEF. The strength of theta- and beta-band coherence between ACC and FEF but not variations in power predict correct task performance. Taken together, the results support a role of ACC in cognitive control of frontoparietal networks and suggest that narrow-band theta and to some extent beta rhythmic activity indexes the coordination of relevant information during periods of enhanced control demands.

Frontal eye fields (FEF) and anterior cingulate cortex (ACC) are coactivated during cognitive tasks, but the precise format of their interaction is not known. Here the authors show that phase coupling between ACC -FEF in theta and beta frequency bands better predicts behavioural performance.

Conflict Resolution as Near-Threshold Decision-Making: A Spiking Neural Circuit Model with Two-Stage Competition for Antisaccadic Task

Automatic responses enable us to react quickly and effortlessly, but they often need to be inhibited so that an alternative, voluntary action can take place. To investigate the brain mechanism of controlled behavior, we investigated a biologically-based network model of spiking neurons for inhibitory control. In contrast to a simple race between pro- versus anti-response, our model incorporates a sensorimotor remapping module, and an action-selection module endowed with a “Stop” process through tonic inhibition. Both are under the modulation of rule-dependent control. We tested the model by applying it to the well known antisaccade task in which one must suppress the urge to look toward a visual target that suddenly appears, and shift the gaze diametrically away from the target instead. We found that the two-stage competition is crucial for reproducing the complex behavior and neuronal activity observed in the antisaccade task across multiple brain regions. Notably, our model demonstrates two types of errors: fast and slow. Fast errors result from failing to inhibit the quick automatic responses and therefore exhibit very short response times. Slow errors, in contrast, are due to incorrect decisions in the remapping process and exhibit long response times comparable to those of correct antisaccade responses. The model thus reveals a circuit mechanism for the empirically observed slow errors and broad distributions of erroneous response times in antisaccade. Our work suggests that selecting between competing automatic and voluntary actions in behavioral control can be understood in terms of near-threshold decision-making, sharing a common recurrent (attractor) neural circuit mechanism with discrimination in perception.

We propose a novel neural circuit mechanism and construct a spiking neural network model for resolving conflict between an automatic response and a volitional one. In this mechanism the two types of responses compete against each other under the modulation of top-down control via multiple neural pathways. The model is able to reproduce a wide range of neuronal and behavioral features observed in various studies and provides insights into not just how subjects make correct responses and fast errors, but also why they make slow errors, a type of error often overlooked by previous modeling studies. The model suggests critical roles of tonic (non-racing) top-down inhibition and near-threshold decision-making in neural competition.

Changes in Effective Connectivity of the Superior Parietal Lobe during Inhibition and Redirection of Eye Movements

Executive control is the ability to flexibly control behavior and is frequently studied with saccadic eye movements. Contrary to frontal oculomotor areas, the role of the superior parietal lobe (SPL) in the executive control of saccades remains unknown. To explore the role of SPL networks in saccade control, we performed a saccadic search-step task while acquiring functional magnetic resonance imaging data for 41 participants. Psychophysiological interaction analyses assessed task-related differences in the effective connectivity of SPL with other brain regions during the inhibition and redirection of saccades. Results indicate an increased coupling of SPL with frontal, posterior, and striatal oculomotor areas for redirected saccades versus visually guided saccades. Saccade inhibition versus unsuccessful inhibition revealed an increased coupling of SPL with dorsolateral prefrontal cortex and anterior cingulate cortex. We discuss how these findings relate to ongoing debates about the implementation of executive control and conclude that early attentional control and rapid updating of saccade goals are important signals for executive control.

The Striatum and Subthalamic Nucleus as Independent and Collaborative Structures in Motor Control

The striatum and the subthalamic nucleus (STN) are two separate input structures into the basal ganglia (BG). Accordingly, research to date has primarily focused on the distinct roles of these structures in motor control and cognition, often through investigation of Parkinson’s disease (PD). Both structures are divided into sensorimotor, associative, and limbic subdivisions based on cortical connectivity. The more recent discovery of the STN as an input structure into the BG drives comparison of these two structures and their respective roles in cognition and motor control. This review compares the role of the striatum and STN in motor response inhibition and execution, competing motor programs, feedback based learning, and response planning. Through comparison, it is found that the striatum and STN have highly independent roles in motor control but also collaborate in order to execute desired actions. There is also the possibility that inhibition or activation of one of these structures indirectly contributes to the function of other connected anatomical structures. Both structures contribute to selective motor response inhibition, which forms the basis of many tasks, but the STN additionally contributes to global inhibition through the hyperdirect pathway. Research is warranted on the functional connectivity of the network for inhibition involving the rIFG, preSMA, striatum, and STN.

Saccade abnormalities associated with focal cerebral lesions - How cortical and basal ganglia commands shape saccades in humans

OBJECTIVE

To study saccade abnormalities associated with focal cerebral lesions, including the cerebral cortex and basal ganglia (BG).

METHODS

We studied the latency and amplitude of reflexive and voluntary saccades in 37 patients with focal lesions of the frontal and parietal cortices and BG (caudate and putamen), and 51 age-matched controls, along with the ability to inhibit unwanted reflexive saccades.

RESULTS

Latencies of reflexive saccades were prolonged in patients with parietal lesions involving the parietal eye field (PEF), whereas their amplitude was decreased with parietal or putaminal lesions. In contrast, latency of voluntary saccades was prolonged and their success rate reduced with frontal lesions including the frontal eye field (FEF) or its outflow tract as well as the dorsolateral/medial prefrontal cortex, and caudate lesions, whereas their amplitude was decreased with parietal lesions. Inhibitory control of reflexive saccades was impaired with frontal, caudate and, less prominently, parietal lesions.

CONCLUSIONS

PEF is important in triggering reflexive saccades, also determining their amplitude. Whereas FEF and the caudate emit commands for initiating voluntary saccades, their amplitude is mainly determined by PEF. Commands not only from FEF and dorsolateral/medial prefrontal cortex but also from the caudate and PEF serve to inhibit unnecessary reflexive saccades.

SIGNIFICANCE

The findings suggested how cortical and BG commands shape reflexive and voluntary saccades in humans.

Parallel basal ganglia circuits for voluntary and automatic behaviour to reach rewards

The basal ganglia control body movements, value processing and decision-making. Many studies have shown that the inputs and outputs of each basal ganglia structure are topographically organized, which suggests that the basal ganglia consist of separate circuits that serve distinct functions. A notable example is the circuits that originate from the rostral (head) and caudal (tail) regions of the caudate nucleus, both of which target the superior colliculus. These two caudate regions encode the reward values of visual objects differently: flexible (short-term) values by the caudate head and stable (long-term) values by the caudate tail. These value signals in the caudate guide the orienting of gaze differently: voluntary saccades by the caudate head circuit and automatic saccades by the caudate tail circuit. Moreover, separate groups of dopamine neurons innervate the caudate head and tail and may selectively guide the flexible and stable learning/memory in the caudate regions. Studies focusing on manual handling of objects also suggest that rostrocaudally separated circuits in the basal ganglia control the action differently. These results suggest that the basal ganglia contain parallel circuits for two steps of goal-directed behaviour: finding valuable objects and manipulating the valuable objects. These parallel circuits may underlie voluntary behaviour and automatic skills, enabling animals (including humans) to adapt to both volatile and stable environments. This understanding of the functions and mechanisms of the basal ganglia parallel circuits may inform the differential diagnosis and treatment of basal ganglia disorders.

Comparative diffusion tractography of corticostriatal motor pathways reveals differences between humans and macaques

The primate corticobasal ganglia circuits are understood to be segregated into parallel anatomically and functionally distinct loops. Anatomical and physiological studies in macaque monkeys are summarized as showing that an oculomotor loop begins with projections from the frontal eye fields (FEF) to the caudate nucleus, and a motor loop begins with projections from the primary motor cortex (M1) to the putamen. However, recent functional and structural neuroimaging studies of the human corticostriatal system report evidence inconsistent with this organization. To obtain conclusive evidence, we directly compared the pattern of connectivity between cortical motor areas and the striatum in humans and macaques in vivo using probabilistic diffusion tractography. In macaques we found that FEF is connected with the head of the caudate and anterior putamen, and M1 is connected with more posterior sections of the caudate and putamen, corroborating neuroanatomical tract tracing findings. However, in humans FEF and M1 are connected to largely overlapping portions of posterior putamen and only a small portion of the caudate. These results demonstrate that the corticobasal connectivity for the oculomotor and primary motor loop is not entirely segregated for primates at a macroscopic level and that the description of the anatomical connectivity of corticostriatal motor systems in humans does not parallel that of macaques, perhaps because of an expansion of prefrontal projections to striatum in humans.

N-methyl-d-aspartate receptor antagonist ketamine impairs action-monitoring activity in the prefrontal cortex

Failures in monitoring of self-generated actions are thought to underlie the positive symptoms in schizophrenia. It has been hypothesized that these deficits may be caused by a dysfunction of N-methyl-d-aspartate receptors (NMDARs). Here we recorded the activity of prefrontal neurons in monkeys performing an antisaccade task, while we administered a subanesthetic dose of the noncompetitive NMDAR antagonist ketamine. Many neurons discriminated between correct antisaccades and response errors in their postresponse activity. Ketamine increased the activity for the neurons' nonpreferred response, thereby decreasing the neurons' performance selectivity. Ketamine also affected the monkeys' behavior after an error, consistent with a deficit in error detection. The results show that NMDARs play an important role in action monitoring in primates. The decrease in performance selectivity of prefrontal neurons after ketamine can help to explain the deficits in action monitoring found in humans after ketamine administration and provides support for the hypothesis that an NMDAR dysfunction underlies self-monitoring deficits and psychotic symptoms in schizophrenia.

Functional connectivity patterns of medial and lateral macaque frontal eye fields reveal distinct visuomotor networks

It has been previously shown that small- and large-amplitude saccades have different functions during vision in natural environments. Large saccades are associated with reaching movements toward objects, whereas small saccades facilitate the identification of more detailed object features necessary for successful grasping and manual manipulation. To determine whether these represent dichotomous processing streams, we used resting-state functional MRI to examine the functional connectivity patterns of the medial and lateral frontal eye field (FEF) regions that encode large- and small-amplitude saccades, respectively. We found that the spontaneous blood oxygen level-dependent signals of the medial FEF were functionally correlated with areas known to be involved in reaching movements and executive control processes, whereas lateral FEF was functionally correlated with cortical areas involved in object processing and in grasping, fixation, and manipulation of objects. The results provide strong evidence for two distinct visuomotor network systems in the primate brain that likely reflect the alternating phases of vision for action in natural environments.