Comparison of oculomotor neuronal activity in paralaminar and mediodorsal thalamus in the rhesus monkey

The Arousal-Related "Central Thalamus" Stimulation Site Simultaneously Innervates Multiple High-Level Frontal and Parietal Areas

In human and nonhuman primates, deep brain stimulation applied at or near the internal medullary lamina of the thalamus [a region referred to as "central thalamus," (CT)], but not at nearby thalamic sites, elicits major changes in the level of consciousness, even in some minimally conscious brain-damaged patients. The mechanisms behind these effects remain mysterious, as the connections of CT had not been specifically mapped in primates. In marmoset monkeys (Callithrix jacchus) of both sexes, we labeled the axons originating from each of the various CT neuronal populations and analyzed their arborization patterns in the cerebral cortex and striatum. We report that, together, these CT populations innervate an array of high-level frontal, posterior parietal, and cingulate cortical areas. Some populations simultaneously target the frontal, parietal, and cingulate cortices, while others predominantly target the dorsal striatum. Our data indicate that CT stimulation can simultaneously engage a heterogeneous set of projection systems that, together, target the key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.SIGNIFICANCE STATEMENT In human and nonhuman primates, deep brain stimulation at a specific site near the internal medullary lamina of the thalamus ["central thalamus," (CT)] had been shown to restore arousal and awareness in anesthetized animals, as well as in some brain-damaged patients. The mechanisms behind these effects remain mysterious, as CT connections remain poorly defined in primates. In marmoset monkeys, we mapped with sensitive axon-labeling methods the pathways originated from CT. Our data indicate that stimulation applied in CT can simultaneously engage a heterogeneous set of projection systems that, together, target several key nodes of the attention, executive control, and working-memory networks of the brain. Increased functional connectivity in these networks has been previously described as a signature of consciousness.

Central Thalamic-Medial Prefrontal Control of Adaptive Responding in the Rat: Many Players in the Chamber

The medial prefrontal cortex (mPFC) has robust afferent and efferent connections with multiple nuclei clustered in the central thalamus. These nuclei are elements in large-scale networks linking mPFC with the hippocampus, basal ganglia, amygdala, other cortical areas, and visceral and arousal systems in the brainstem that give rise to adaptive goal-directed behavior. Lesions of the mediodorsal nucleus (MD), the main source of thalamic input to middle layers of PFC, have limited effects on delayed conditional discriminations, like DMTP and DNMTP, that depend on mPFC. Recent evidence suggests that MD sustains and amplifies neuronal responses in mPFC that represent salient task-related information and is important for detecting and encoding contingencies between actions and their consequences. Lesions of rostral intralaminar (rIL) and ventromedial (VM) nuclei produce delay-independent impairments of egocentric DMTP and DNMTP that resemble effects of mPFC lesions on response speed and accuracy: results consistent with projections of rIL to striatum and VM to motor cortices. The ventral midline and anterior thalamic nuclei affect allocentric spatial cognition and memory consistent with their connections to mPFC and hippocampus. The dorsal midline nuclei spare DMTP and DNMTP. They have been implicated in behavioral-state control and response to salient stimuli in associative learning. mPFC functions are served during DNMTP by discrete populations of neurons with responses related to motor preparation, movements, lever press responses, reinforcement anticipation, reinforcement delivery, and memory delay. Population analyses show that different responses are timed so that they effectively tile the temporal interval from when DNMTP trials are initiated until the end. Event-related responses of MD neurons during DNMTP are predominantly related to movement and reinforcement, information important for DNMTP choice. These responses closely mirror the activity of mPFC neurons with similar responses. Pharmacological inactivation of MD and adjacent rIL affects the expression of diverse action- and outcome-related responses of mPFC neurons. Lesions of MD before training are associated with a shift away from movement-related responses in mPFC important for DNMTP choice. These results suggest that MD has short-term effects on the expression of event-related activity in mPFC and long-term effects that tune mPFC neurons to respond to task-specific information.

Effects of Optogenetic Suppression of Cortical Input on Primate Thalamic Neuronal Activity during Goal-Directed Behavior

The motor thalamus relays signals from subcortical structures to the motor cortical areas. Previous studies in songbirds and rodents suggest that cortical feedback inputs crucially contribute to the generation of movement-related activity in the motor thalamus. In primates, however, it remains uncertain whether the corticothalamic projections may play a role in shaping neuronal activity in the motor thalamus. Here, using an optogenetic inactivation technique with the viral vector system expressing halorhodopsin, we investigated the role of cortical input in modulating thalamic neuronal activity during goal-directed behavior. In particular, we assessed whether the suppression of signals originating from the supplementary eye field at the corticothalamic terminals could change the task-related neuronal modulation in the oculomotor thalamus in monkeys performing a self-initiated saccade task. We found that many thalamic neurons exhibited changes in their firing rates depending on saccade direction or task event, indicating that optical stimulation exerted task-specific effects on neuronal activity beyond the global changes in baseline activity. These results suggest that the corticothalamic projections might be actively involved in the signal processing necessary for goal-directed behavior. However, we also found that some thalamic neurons exhibited overall, non-task-specific changes in the firing rate during optical stimulation, even in control animals without vector injections. The stimulation effects in these animals started with longer latency, implying a possible thermal effect on neuronal activity. Thus, our results not only reveal the importance of direct cortical input in neuronal activity in the primate motor thalamus, but also provide useful information for future optogenetic studies.

Differential Roles of Mediodorsal Nucleus of the Thalamus and Prefrontal Cortex in Decision-Making and State Representation in a Cognitive Control Task Measuring Deficits in Schizophrenia

The mediodorsal nucleus of the thalamus (MD) is reciprocally connected with the prefrontal cortex (PFC), and although the MD has been implicated in a range of PFC-dependent cognitive functions (Watanabe and Funahashi, 2012; Mitchell and Chakraborty, 2013; Parnaudeau et al., 2018), little is known about how MD neurons in the primate participate specifically in cognitive control, a capability that reflects the ability to use contextual information (such as a rule) to modify responses to environmental stimuli. To learn how the MD-PFC thalamocortical network is engaged to mediate forms of cognitive control that are selectively disrupted in schizophrenia, we trained male monkeys to perform a variant of the AX continuous performance task, which reliably measures cognitive control deficits in patients (Henderson et al., 2012) and used linear multielectrode arrays to record neural activity in the MD and PFC simultaneously. We found that the two structures made clearly different contributions to distributed processing for cognitive control: MD neurons were specialized for decision-making and response selection, whereas prefrontal neurons were specialized to preferentially encode the environmental state on which the decision was based. In addition, we observed that functional coupling between MD and PFC was strongest when the decision as to which of the two responses in the task to execute was being made. These findings delineate unique contributions of MD and PFC to distributed processing for cognitive control and characterized neural dynamics in this network associated with normative cognitive control performance.SIGNIFICANCE STATEMENT Cognitive control is fundamental to healthy human executive functioning (Miller and Cohen, 2001) and deficits in patients with schizophrenia relate to decreased functional activation of the MD thalamus and the prefrontal cortex (Minzenberg et al., 2009), which are reciprocally linked (Goldman-Rakic and Porrino, 1985; Xiao et al., 2009). We carry out simultaneous neural recordings in the MD and PFC while monkeys perform a cognitive control task translated from patients with schizophrenia to relate thalamocortical dynamics to cognitive control performance. Our data suggest that state representation and decision-making computations for cognitive control are preferentially performed by PFC and MD, respectively. This suggests experiments to parse decision-making and state representation deficits in patients while providing novel computational targets for future therapies.

Central thalamic inactivation impairs the expression of action- and outcome-related responses of medial prefrontal cortex neurons in the rat

The mediodorsal (MD) and adjacent intralaminar (IL) and midline nuclei provide the main thalamic input to the medial prefrontal cortex (mPFC) and are critical for associative learning and decision-making. MD neurons exhibit activity related to actions and outcomes that mirror responses of mPFC neurons in rats during dynamic delayed non-match to position (dDNMTP), a variation of DNMTP where start location is varied randomly within an open octagonal arena to avoid confounding behavioral events with spatial location. To test whether the thalamus affects the expression of these responses in mPFC, we inhibited the central thalamus unilaterally by microinjecting muscimol at doses and sites found to affect decision-making when applied bilaterally. Unilateral inactivation reduced normalized task-related responses in the ipsilateral mPFC without disrupting behavior needed to characterize event-related neuronal activity. Our results extend earlier findings that focused on delay-related activity by showing that central thalamic inactivation interferes with responses related to actions and outcomes that occur outside the period of memory delay. These findings are consistent with the broad effects of central thalamic lesions on behavioral measures of reinforcement-guided responding. Most (7/8) of the prefrontal response types affected by thalamic inactivation have also been observed in MD during dDNMTP. These results support the hypothesis that MD and IL act as transthalamic gates: monitoring prefrontal activity through corticothalamic inputs; integrating this information with signals from motivational and sensorimotor systems that converge in thalamus; and acting through thalamocortical projections to enhance expression of neuronal responses in the PFC that support adaptive goal-directed behavior.

Macaque parvocellular mediodorsal thalamus: dissociable contributions to learning and adaptive decision-making

Distributed brain networks govern adaptive decision‐making, new learning and rapid updating of information. However, the functional contribution of the rhesus macaque monkey parvocellular nucleus of the mediodorsal thalamus (MDpc) in these key higher cognitive processes remains unknown. This study investigated the impact of MDpc damage in cognition. Preoperatively, animals were trained on an object‐in‐place scene discrimination task that assesses rapid learning of novel information within each session. Bilateral neurotoxic (NMDA and ibotenic acid) MDpc lesions did not impair new learning unless the monkey had also sustained damage to the magnocellular division of the MD (MDmc). Contralateral unilateral MDpc and MDmc damage also impaired new learning, while selective unilateral MDmc damage produced new learning deficits that eventually resolved with repeated testing. In contrast, during food reward (satiety) devaluation, monkeys with either bilateral MDpc damage or combined MDpc and MDmc damage showed attenuated food reward preferences compared to unoperated control monkeys; the selective unilateral MDmc damage left performance intact. Our preliminary results demonstrate selective dissociable roles for the two adjacent nuclei of the primate MD, namely, MDpc, as part of a frontal cortical network, and the MDmc, as part of a frontal‐temporal cortical network, in learning, memory and the cognitive control of behavioural choices after changes in reward value. Moreover, the functional cognitive deficits produced after differing MD damage show that the different subdivisions of the MD thalamus support distributed neural networks to rapidly and fluidly incorporate task‐relevant information, in order to optimise the animals’ ability to receive rewards.

Frontal eye field in prosimian galagos: Intracortical microstimulation and tracing studies

The frontal eye field (FEF) in prosimian primates was identified as a small cortical region, above and anterior to the anterior frontal sulcus, from which saccadic eye movements were evoked with electrical stimulation. Tracer injections revealed FEF connections with cortical and subcortical structures participating in higher order visual processing. Ipsilateral cortical connections were the densest with adjoining parts of the dorsal premotor and prefrontal cortex (PFC). Label in a region corresponding to supplementary eye field (SEF) of other primates, suggests the existence of SEF in galagos. Other connections were with ventral premotor cortex (PMV), the caudal half of posterior parietal cortex, cingulate cortex, visual areas within the superior temporal sulcus, and inferotemporal cortex. Callosal connections were mostly with the region of the FEF of another hemisphere, SEF, PFC, and PMV. Most subcortical connections were ipsilateral, but some were bilateral. Dense bilateral connections were to caudate nuclei. Densest reciprocal ipsilateral connections were with the paralamellar portion of mediodorsal nucleus, intralaminar nuclei and magnocellular portion of ventral anterior nucleus. Other FEF connections were with the claustrum, reticular nucleus, zona incerta, lateral posterior and medial pulvinar nuclei, nucleus limitans, pretectal area, nucleus of Darkschewitsch, mesencephalic and pontine reticular formation and pontine nuclei. Surprisingly, the superior colliculus (SC) contained only sparse anterograde label. Although most FEF connections in galagos are similar to those in monkeys, the FEF-SC connections appear to be much less. This suggests that a major contribution of the FEF to visuomotor functions of SC emerged with the evolution of anthropoid primates.

Cholinergic Oculomotor Nucleus Activity Is Induced by REM Sleep Deprivation Negatively Impacting on Cognition

Several efforts have been made to understand the involvement of rapid eye movement (REM) sleep for cognitive processes. Consolidation or retention of recognition memories is severely disrupted by REM sleep deprivation (REMSD). In this regard, pedunculopontine tegmental nucleus (PPT) and other brainstem nuclei, such as pontine nucleus (Pn) and oculomotor nucleus (OCM), appear to be candidates to take part in this REM sleep circuitry with potential involvement in cognition. Therefore, the objective of this study was to investigate a possible association between the performance of Wistar rats in a declarative memory and PPT, Pn, and OCM activities after different periods of REMSD. We examined c-Fos and choline acetyltransferase (ChaT) expressions as indicators of neuronal activity as well as a familiarity-based memory test. The animals were distributed in groups: control, REMSD, and sleep rebound (REB). At the end of the different REMSD (24, 48, 72, and 96 h) and REB (24 h) time points, the rats were immediately tested in the object recognition test and then the brains were collected. Results indicated that OCM neurons presented an increased activity, due to ChaT-labeling associated with REMSD that negatively correlated (r = -0.32) with the cognitive performance. This suggests the existence of a cholinergic compensatory mechanism within the OCM during REMSD. We also showed that 24 h of REMSD impacted similarly in memory, compared to longer periods of REMSD. These data extend the notion that REM sleep is influenced by areas other than PPT, i.e., Pn and OCM, which could be key players in both sleep processes and cognition.

Task dependence of decision- and choice-related activity in monkey oculomotor thalamus

Oculomotor signals circulate within putative recurrent feedback loops that include the frontal eye field (FEF) and the oculomotor thalamus (OcTh). To examine how OcTh contributes to visuomotor control, and perceptually informed saccadic choices in particular, neural correlates of perceptual judgment and motor selection in OcTh were evaluated and compared with those previously reported for FEF in the same subjects. Monkeys performed three tasks: a choice task in which perceptual decisions are urgent, a choice task in which identical decisions are made without time pressure, and a single-target, delayed saccade task. The OcTh yielded far fewer task-responsive neurons than the FEF, but across responsive pools, similar neuron types were found, ranging from purely visual to purely saccade related. Across such types, the impact of the perceptual information relevant to saccadic choices was qualitatively the same in FEF and OcTh. However, distinct from that in FEF, activity in OcTh was strongly task dependent, typically being most vigorous in the urgent task, less so in the easier choice task, and least in the single-target task. This was true for responsive and nonresponsive cells alike. Neurons with exclusively motor-related activity showed strong task dependence, fired less, and differed most patently from their FEF counterparts, whereas those that combined visual and motor activity fired most similarly to their FEF counterparts. The results suggest that OcTh activity is more distantly related to saccade production per se, because its degree of commitment to a motor choice varies markedly as a function of ongoing cognitive or behavioral demands.

What does the mediodorsal thalamus do?

Dense amnesia can result from damage to the medial diencephalon in humans and in animals. In humans this damage is diffuse and can include the mediodorsal nuclei of the thalamus. In animal models, lesion studies have confirmed the mediodorsal thalamus (MD) has a role in memory and other cognitive tasks, although the extent of deficits is mixed. Anatomical tracing studies confirm at least three different subgroupings of the MD: medial, central, and lateral, each differentially interconnected to the prefrontal cortex (PFC). Moreover, these subgroupings of the MD also receive differing inputs from other brain structures, including the basal ganglia thus the MD subgroupings form key nodes in interconnected frontal-striatal-thalamic neural circuits, integrating critical information within the PFC. We will provide a review of data collected from non-human primates and rodents after selective brain injury to the whole of the MD as well as these subgroupings to highlight the extent of deficits in various cognitive tasks. This research highlights the neural basis of memory and cognitive deficits associated with the subgroupings of the MD and their interconnected neural networks. The evidence shows that the MD plays a critical role in many varied cognitive processes. In addition, the MD is actively processing information and integrating it across these neural circuits for successful cognition. Having established that the MD is critical for memory and cognition, further research is required to understand how the MD specifically influences these cognitive processing carried out by the brain.

Contribution of the central thalamus to the generation of volitional saccades

Lesions in the motor thalamus can cause deficits in somatic movements. However, the involvement of the thalamus in the generation of eye movements has only recently been elucidated. In this article, we review recent advances into the role of the thalamus in eye movements. Anatomically, the anterior group of the intralaminar nuclei and paralaminar portion of the ventrolateral, ventroanterior and mediodorsal nuclei of the thalamus send massive projections to the frontal eye field and supplementary eye field. In addition, these parts of the thalamus, collectively known as the 'oculomotor thalamus', receive inputs from the cerebellum, the basal ganglia and virtually all stages of the saccade-generating pathways in the brainstem. In their pioneering work in the 1980s, Schlag and Schlag-Rey found a variety of eye movement-related neurons in the oculomotor thalamus, and proposed that this region might constitute a 'central controller' playing a role in monitoring eye movements and generating self-paced saccades. This hypothesis has been evaluated by recent experiments in non-human primates and by clinical observations of subjects with thalamic lesions. In addition, several recent studies have also addressed the involvement of the oculomotor thalamus in the generation of anti-saccades and the selection of targets for saccades. These studies have revealed the impact of subcortical signals on the higher-order cortical processing underlying saccades, and suggest the possibility of future studies using the oculomotor system as a model to explore the neural mechanisms of global cortico-subcortical loops and the neural basis of a local network between the thalamus and cortex.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

Spatially distributed encoding of covert attentional shifts in human thalamus

Spatial attention modulates signal processing within visual nuclei of the thalamus—but do other nuclei govern the locus of attention in top-down mode? We examined functional MRI (fMRI) data from three subjects performing a task requiring covert attention to 1 of 16 positions in a circular array. Target position was cued after stimulus offset, requiring subjects to perform target detection from iconic visual memory. We found positionally specific responses at multiple thalamic sites, with individual voxels activating at more than one direction of attentional shift. Voxel clusters at anatomically equivalent sites across subjects revealed a broad range of directional tuning at each site, with little sign of contralateral bias. By reference to a thalamic atlas, we identified the nuclear correspondence of the four most reliably activated sites across subjects: mediodorsal/central-intralaminar (oculomotor thalamus), caudal intralaminar/parafascicular, suprageniculate/limitans, and medial pulvinar/lateral posterior. Hence, the cortical network generating a top-down control signal for relocating attention acts in concert with a spatially selective thalamic apparatus—the set of active nuclei mirroring the thalamic territory of cortical “eye-field” areas, thus supporting theories which propose the visuomotor origins of covert attentional selection.

Substantia Nigra Output to Prefrontal Cortex Via Thalamus in Monkeys. II. Activity of Thalamic Relay Neurons in Delayed Conditional Go/No-Go Discrimination Task

The present report investigated the involvement of primate nigro-thalamo-cortical projections in discrimination of visual signals with behavioral meaning. We tested the extracellular unit activity of mediodorsal (MD) and ventral anterior (VA) thalamic neurons monosynaptically receiving inhibitory input from the substantia nigra pars reticulata (SNr) and projecting to the frontal cortex in Japanese monkeys performing a delayed conditional go/no-go discrimination task. In the task two colored stimuli (S1, S2) intervened by delay period required the monkeys lifting a lever (go) or not (no-go); the same and different colored pairs of S1 and S2 meant go and no-go signals, respectively. Prominent task-relevant responses were sustained activity with color preference to S1 during delay period and S2-related activity with different firing rates between go and no-go trials. In particular, a high proportion of such go/no-go differential S2-related activity was found in thalamic relay neurons, receiving input from the caudolateral SNr and projecting to the prefrontal area (PSv) ventral to the principal sulcus, in the rostrolateral MD. The findings suggest that the caudolateral SNr–rostrolateral MD–PSv pathways may be possible conduits of signals coding the behavioral meaning of the visual stimuli and thus may be responsible for generating similar neuronal activity in the PSv.

Memory enhancement with event-related stimulation of the rostral intralaminar thalamic nuclei

The rostral intralaminar thalamic nuclei (ILn) are organized to activate pathways originating in medial prefrontal cortex (mPF) that mediate memory-guided responding during alert, wakeful states. Previous studies have shown that rostral ILn or mPF lesions produce deficits in delayed matching to position (DMTP). Here, we manipulated rostral ILn activity in rats by microinjecting drugs or applying electrical current and examined effects on DMTP. Inhibiting activity with the GABA(A) agonist muscimol impaired DMTP. Decreasing GABA(A) tone with FG-7142 (N-methyl-beta-carboline-3-carboxamide) improved DMTP at low but not high doses. Orexin A, which depolarizes thalamocortical neurons locally within the ILn, improved DMTP, whereas the cholinergic agonist carbachol impaired performance at the highest dose tested. These drug effects were unaffected by partial mPF lesions in a subset of animals. Microinjection results are consistent with an inverted-U relationship between thalamic activity and DMTP. This relationship was confirmed by event-related electrical stimulation, which produced improvement at low stimulation currents and impairment at higher currents. Electrical stimulation affected DMTP when applied at the start of the memory delay or choice response, but not earlier when trials began or the sample lever was presented. Our results are consistent with evidence that the rostral ILn play a role in retrieval, carrying response-related information across brief memory delays and facilitating memory-guided responding. They also provide evidence that treatments stimulating rostral ILn activity may be an effective means to enhance working memory and related cognitive processes and thus to treat disorders that affect these functions.

Cholinergic modulation of visuospatial responding in central thalamus

Central thalamus has extensive connections with basal ganglia and frontal cortex that are thought to play a critical role in sensory-guided goal-directed behavior. Central thalamic activity is influenced by cholinergic projections from mesopontine nuclei. To elucidate this function we trained rats to respond to lights in a reaction time (RT) task and compared effects of muscarinic (2.4, 7.3, 22 nmol scopolamine) and nicotinic (5.4, 16, 49, 98 nmol mecamylamine) antagonists with the GABA(A) agonist muscimol (0.1, 0.3, 1.0 nmol) in central thalamus. We compared this with subcutaneous (systemic) effects of mecamylamine (3.2, 9.7, 29 micromol/kg) and scopolamine (0.03, 0.09, 0.26 micromol/kg). Subcutaneous scopolamine increased omissions (failure to respond within a 3-s response window) at the highest dose tested. Subcutaneous mecamylamine increased omissions at the highest dose tested while impairing RT and per cent correct at lower doses. Intrathalamic injections of muscimol and mecamylamine decreased per cent correct at doses that did not affect omissions or RT. Intrathalamic scopolamine increased omissions and RT at doses that had little effect on per cent correct. Anatomical controls indicated that the effects of mecamylamine were localized in central thalamus and those of scopolamine were not. Drug effects did not interact with attention-demanding manipulations of stimulus duration, proximity of stimulus and response locations, or stimulus array size. These results are consistent with the hypothesis that central thalamus mediates decisional processes linking sensory stimuli with actions, downstream from systems that detect sensory signals. They also provide evidence that this function is specifically influenced by nicotinic cholinergic receptors.

Cognitive signals in the primate motor thalamus predict saccade timing

We often generate movements without any external event that immediately triggers them. How the brain decides the timing of self-initiated movements remains unclear. Previous studies suggest that the basal ganglia-thalamocortical pathways play this role, but the subcortical signals that determine movement timing have not been identified. The present study reports that a subset of thalamic neurons predicts the timing of self-initiated saccadic eye movements. When monkeys made a saccade in response to the fixation point (FP) offset in the traditional memory saccade task, neurons in the ventrolateral and the ventroanterior nuclei of the thalamus exhibited a gradual buildup of activity that peaked around the most probable time of the FP offset; however, neither the timing nor the magnitude of neuronal activity correlated with saccade latencies, suggesting that the brain is unlikely to have used this information to decide the times of saccades in the traditional memory saccade task. In contrast, when monkeys were required to make a self-timed saccade within a fixed time interval after an external cue, the same neurons again exhibited a strong buildup of activity that preceded saccades by several hundred milliseconds, showing a close correlation between the times of neuronal activity and the times of self-initiated saccades. The results suggest that neurons in the motor thalamus carry subjective time information, which is used by cortical networks to determine the timing of self-initiated saccades.

Spatiotemporal Properties of Eye Position Signals in the Primate Central Thalamus

Although both sensory and motor signals in multiple cortical areas are modulated by eye position, the origin of eye position signals for cortical neurons remains uncertain. One likely source is the central thalamus, which contains neurons sensitive to eye position. Because the central thalamus receives inputs from the brainstem, these neurons may transmit eye position signals arising from the neural integrator or from proprioceptive feedback. However, because the central thalamus also receives inputs from many cortical areas, eye position signals in the central thalamus could come from the cerebral cortex. To clarify these possibilities, spatial and temporal properties of eye position signals in the central thalamus were examined in trained monkeys. Data showed that eye position signals were decomposed into horizontal and vertical components, suggesting that the central thalamus lies within pathways that transmit brainstem eye position signals to the cortex. Further quantitative analyses suggested that 2 distinct groups of thalamic neurons mediate eye position signals from different subcortical origins, and that the signals are modified dynamically through ascending pathways. Eye position signals through the central thalamus may play essential roles in spatial transformation performed by cortical networks.

Dopaminergic control of working memory and its relevance to schizophrenia: A circuit dynamics perspective

This article argues how dopamine controls working memory and how the dysregulation of the dopaminergic system is related to schizophrenia. In the dorsolateral prefrontal cortex, which is the principal part of the working memory system, recurrent excitation is subtly balanced with intracortical inhibition. A potent controller of the dorsolateral prefrontal cortical circuit is the mesocortical dopaminergic system. To understand the characteristics of the dopaminergic control of working memory, the stability of the circuit dynamics under the influence of dopamine has been studied. Recent computational studies suggest that the hyperdopaminergic state is usually stable but the hypodopaminergic state tends to be unstable. The stability also depends on the efficacy of the glutamatergic transmission in the corticomesencephalic projections to dopamine neurons. When this cortical feedback is hypoglutamatergic, the circuit of the dorsolateral prefrontal cortex tends to be unstable, such that a slight increase in dopamine releasability causes a catastrophic jump of the dorsolateral prefrontal cortex activity from a low to a high level. This may account for the seemingly paradoxical overactivation of the dorsolateral prefrontal cortex observed in schizophrenic patients. Given that dopamine transmission is abnormal in the brains of patients with schizophrenia and working memory deficit is a core dysfunction in schizophrenia, the concept of circuit stability would be useful not only for understanding the mechanisms of working memory processing but for developing therapeutic strategies to enhance cognitive functions in schizophrenia.

Cortico-cortical networks and cortico-subcortical loops for the higher control of eye movements

There are multiple distinct regions, or eye fields, in the cerebral cortex that contribute directly to the initiation and control of voluntary eye movements. We concentrate on six of these: the frontal eye field, parietal eye field, supplementary eye field, middle superior temporal area, prefrontal eye field, and area 7 m (precuneus in humans). In each of these regions: (1) there is neural activity closely related to eye movements; (2) electrical microstimulation produces or modifies eye movements; (3) surgical lesions or chemical inactivation impairs eye movements; (4) there are direct neural projections to major structures in the brainstem oculomotor system; and (5) increased activity is observed during eye movement tasks in functional magnetic resonance imaging or positron emission tomography experiments in humans. Each of these eye fields is reciprocally connected with the other eye fields, and each receives visual information directly from visual association cortex. Each eye field has distinct subregions that are concerned with either saccadic or pursuit eye movements. The saccadic subregions are preferentially interconnected with other saccade subregions and the pursuit subregions are preferentially interconnected with other pursuit subregions. Current evidence strongly supports the proposal that there are parallel cortico-cortical networks that control purposeful saccadic and pursuit eye movements, and that the activity in those networks is modulated by feedback information, via the thalamus, from the superior colliculus, basal ganglia, and cerebellum.

Inactivation of the central thalamus delays self-timed saccades

The central thalamus transmits corollary discharge signals for eye movement control, but its role in eye movement generation remains uncertain. Inactivation of the paralaminar part of the ventrolateral thalamus delayed the initiation of contraversive saccades, particularly during a new memory-guided saccade task that required self-triggering of the movement. The results suggest that signals through the thalamus regulate the timing of self-initiated saccades.

Involvement of the central thalamus in the control of smooth pursuit eye movements

During maintenance of smooth pursuit eye movements, the brain must keep track of pursuit velocity to reconstruct target velocity from motion of retinal images. Although a recent study showed that corollary discharge signals through the thalamus to the cortex are used for internal monitoring of saccades, it remains unknown whether signals in the thalamus also contribute to monitoring and on-line regulation of smooth pursuit. The present study sought possible roles of the thalamocortical pathways in pursuit by recording activities of single thalamic neurons and by analyzing the effects of local inactivation. Data showed that many neurons in the ventrolateral thalamus exhibited directional modulation during pursuit. Most neurons discharged before or during initiation of pursuit, and the firing rate was proportional to the speed of target motion in a preferred direction. When the tracking target was extinguished briefly during maintenance of pursuit, these neurons continued firing, indicating that they carried extra-retinal, eye movement signals. The majority of neurons showed no change in activity around the time of small catch-up saccades during pursuit but responded transiently to large (16 degrees) memory-guided saccades in the preferred pursuit direction. Local inactivation of the recording sites did not alter pursuit latency but reduced eye velocity modestly during initiation and maintenance of ipsiversive pursuit. The results suggest that the central thalamus lies within pathways that regulate and monitor smooth pursuit eye movements.

Distribution of Activity Across the Monkey Cerebral Cortical Surface, Thalamus and Midbrain during Rapid, Visually Guided Saccades

To examine the distribution of visual and oculomotor activity across the macaque brain, we performed functional magnetic resonance imaging (fMRI) on awake, behaving monkeys trained to perform visually guided saccades. Two subjects alternated between periods of making saccades and central fixations while blood oxygen level dependent (BOLD) images were collected [3 T, (1.5 mm)3 spatial resolution]. BOLD activations from each of four cerebral hemispheres were projected onto the subjects' cortical surfaces and aligned to a surface-based atlas for comparison across hemispheres and subjects. This surface-based analysis revealed patterns of visuo-oculomotor activity across much of the cerebral cortex, including activations in the posterior parietal cortex, superior temporal cortex and frontal lobe. For each cortical domain, we show the anatomical position and extent of visuo-oculomotor activity, including evidence that the dorsolateral frontal activation, which includes the frontal eye field (on the anterior bank of the arcuate sulcus), extends anteriorly into posterior principal sulcus (area 46) and posteriorly into part of dorsal premotor cortex (area 6). Our results also suggest that subcortical BOLD activity in the pulvinar thalamus may be lateralized during voluntary eye movements. These findings provide new neuroanatomical information as to the complex neural substrates that underlie even simple goal-directed behaviors.