Density gradients of trans-synaptically labeled collicular neurons after injections of rabies virus in the lateral rectus muscle of the rhesus monkey

Pathways from the superior colliculus and the nucleus of the optic tract to the posterior parietal cortex in macaque monkeys: Functional frameworks for representation updating and online movement guidance

The posterior parietal cortex (PPC) integrates multisensory and motor-related information for generating and updating body representations and movement plans. We used retrograde transneuronal transfer of rabies virus combined with a conventional tracer in macaque monkeys to identify direct and disynaptic pathways to the arm-related rostral medial intraparietal area (MIP), the ventral lateral intraparietal area (LIPv), belonging to the parietal eye field, and the pursuit-related lateral subdivision of the medial superior temporal area (MSTl). We found that these areas receive major disynaptic pathways via the thalamus from the nucleus of the optic tract (NOT) and the superior colliculus (SC), mainly ipsilaterally. NOT pathways, targeting MSTl most prominently, serve to process the sensory consequences of slow eye movements for which the NOT is the key sensorimotor interface. They potentially contribute to the directional asymmetry of the pursuit and optokinetic systems. MSTl and LIPv receive feedforward inputs from SC visual layers, which are potential correlates for fast detection of motion, perceptual saccadic suppression and visual spatial attention. MSTl is the target of efference copy pathways from saccade- and head-related compartments of SC motor layers and head-related reticulospinal neurons. They are potential sources of extraretinal signals related to eye and head movement in MSTl visual-tracking neurons. LIPv and rostral MIP receive efference copy pathways from all SC motor layers, providing online estimates of eye, head and arm movements. Our findings have important implications for understanding the role of the PPC in representation updating, internal models for online movement guidance, eye-hand coordination and optic ataxia.

The superior colliculus projection upon the macaque inferior olive

Saccade accommodation is a productive model for exploring the role of the cerebellum in behavioral plasticity. In this model, the target is moved during the saccade, gradually inducing a change in the saccade vector as the animal adapts. The climbing fiber pathway from the inferior olive provides a visual error signal generated by the superior colliculus that is believed to be crucial for cerebellar adaptation. However, the primate tecto-olivary pathway has only been explored using large injections of the central portion of the superior colliculus. To provide a more detailed picture, we have made injections of anterograde tracers into various regions of the macaque superior colliculus. As shown previously, large central injections primarily label a dense terminal field within the C subdivision at caudal end of the contralateral medial inferior olive. Several, previously unobserved, sites of sparse terminal labeling were noted: bilaterally in the dorsal cap of Kooy and ipsilaterally in the C subdivision of the medial inferior olive. Small, physiologically directed, injections into the rostral, small saccade portion of the superior colliculus produced terminal fields in the same regions of the medial inferior olive, but with decreased density. Small injections of the caudal superior colliculus, where large amplitude gaze changes are encoded, again labeled a terminal field located in the same areas. The lack of a topographic pattern within the main tecto-olivary projection suggests that either the precise vector of the visual error is not transmitted to the vermis, or that encoding of this error is via non-topographic means.

Pathways for Naturalistic Looking Behavior in Primate I: Behavioral Characteristics and Brainstem Circuits

Organisms control their visual worlds by moving their eyes, heads, and bodies. This control of "gaze" or "looking" is key to survival and intelligence, but our investigation of the underlying neural mechanisms in natural conditions is hindered by technical limitations. Recent advances have enabled measurement of both brain and behavior in freely moving animals in complex environments, expanding on historical head-fixed laboratory investigations. We juxtapose looking behavior as traditionally measured in the laboratory against looking behavior in naturalistic conditions, finding that behavior changes when animals are free to move or when stimuli have depth or sound. We specifically focus on the brainstem circuits driving gaze shifts and gaze stabilization. The overarching goal of this review is to reconcile historical understanding of the differential neural circuits for different "classes" of gaze shift with two inconvenient truths. (1) "classes" of gaze behavior are artificial. (2) The neural circuits historically identified to control each "class" of behavior do not operate in isolation during natural behavior. Instead, multiple pathways combine adaptively and non-linearly depending on individual experience. While the neural circuits for reflexive and voluntary gaze behaviors traverse somewhat independent brainstem and spinal cord circuits, both can be modulated by feedback, meaning that most gaze behaviors are learned rather than hardcoded. Despite this flexibility, there are broadly enumerable neural pathways commonly adopted among primate gaze systems. Parallel pathways which carry simultaneous evolutionary and homeostatic drives converge in superior colliculus, a layered midbrain structure which integrates and relays these volitional signals to brainstem gaze-control circuits.

Brainstem sources of input to the central mesencephalic reticular formation in the macaque

Physiological studies indicate that the central mesencephalic reticular formation (cMRF) plays a role in gaze changes, including control of disjunctive saccades. Neuroanatomical studies have demonstrated strong interconnections with the superior colliculus, along with projections to extraocular motor nuclei, the preganglionic nucleus of Edinger-Westphal, the paramedian pontine reticular formation, nucleus raphe interpositus, medullary reticular formation and cervical spinal cord, as might be expected for a structure that is intimately involved in gaze control. However, the sources of input to this midbrain structure have not been described in detail. In the present study, the brainstem cells of origin supplying the cMRF were labeled by retrograde transport of tracer (wheat germ agglutinin conjugated horseradish peroxidase) in macaque monkeys. Within the diencephalon, labeled neurons were noted in the ventromedial nucleus of the hypothalamus, pregeniculate nucleus and habenula. In the midbrain, labeled cells were found in the substantia nigra pars reticulata, medial pretectal nucleus, superior colliculus, tectal longitudinal column, periaqueductal gray, supraoculomotor area, and contralateral cMRF. In the pons they were located in the paralemniscal zone, parabrachial nucleus, locus coeruleus, nucleus prepositus hypoglossi and the paramedian pontine reticular formation. Finally, in the medulla they were observed in the medullary reticular formation. The fact that this list of input sources is very similar to those of the superior colliculus supports the view that the cMRF represents an important gaze control center.

A Novel Tectal/Pretectal Population of Premotor Lens Accommodation Neurons

Purpose

Under real-world conditions, saccades are often accompanied by changes in vergence angle and lens accommodation that compensate for changes in the distance between the current fixation point and the next target. As the superior colliculus directs saccades, we examined whether it contains premotor neurons that might control lens compensation for target distance.

Methods

Rabies virus or recombinant rabies virus was injected into the ciliary bodies of Macaca fascicularis monkeys to label circuits controlling lens accommodation via retrograde transsynaptic transport. In addition, conventional anterograde tracers were used to confirm the rabies findings with respect to projections to preganglionic Edinger–Westphal motoneurons.

Results

At time courses that rabies virus labeled lens-related premotor neurons in the supraoculomotor area and central mesencephalic reticular formation, labeled neurons were not found within the superior colliculus. They were, however, found bilaterally in the medial pretectal nucleus continuing caudally into the tectal longitudinal column, which lies on the midline, between the colliculi. A bilateral projection by this area to the preganglionic Edinger–Westphal nucleus was confirmed by anterograde tracing. Only at longer time courses were cells labeled in the superior colliculus.

Conclusions

The superior colliculus does not provide premotor input to preganglionic Edinger–Westphal nucleus motoneurons, but may provide input to lens-related premotor populations in the supraoculomotor area and central mesencephalic reticular formation. There is, however, a novel third population of lens-related premotor neurons in the tectal longitudinal column and rostrally adjacent medial pretectal nucleus. The specific function of this premotor population remains to be determined.

Superior colliculus projections to target populations in the supraoculomotor area of the macaque monkey

A projection by the superior colliculus to the supraoculomotor area (SOA) located dorsal to the oculomotor complex was first described in 1978. This projection’s targets have yet to be identified, although the initial study suggested that vertical gaze motoneuron dendrites might receive this input. Defining the tectal targets is complicated by the fact the SOA contains a number of different cell populations. In the present study, we used anterograde tracers to characterize collicular axonal arbors and retrograde tracers to label prospective SOA target populations in macaque monkeys. Close associations were not found with either superior or medial rectus motoneurons whose axons supply singly innervated muscle fibers. S-group motoneurons, which supply superior rectus multiply innervated muscle fibers, appeared to receive a very minor input, but C-group motoneurons, which supply medial rectus multiply innervated muscle fibers, received no input. A number of labeled boutons were observed in close association with SOA neurons projecting to the spinal cord, or the reticular formation in the pons and medulla. These descending output neurons are presumed to be peptidergic cells within the centrally projecting Edinger–Westphal population. It is possible the collicular input provides a signaling function for neurons in this population that serve roles in either stress responses, or in eating and drinking behavior. Finally, a number of close associations were observed between tectal terminals and levator palpebrae superioris motoneurons, suggesting the possibility that the superior colliculus provides a modest direct input for raising the eyelids during upward saccades.

Viruses in connectomics: Viral transneuronal tracers and genetically modified recombinants as neuroscience research tools

Connectomic studies have become 'viral', as viral pathogens have been turned into irreplaceable neuroscience research tools. Highly sensitive viral transneuronal tracing technologies are available, based on the use of alpha-herpesviruses and a rhabdovirus (rabies virus), which function as self-amplifying markers by replicating in recipient neurons. These viruses highly differ with regard to host range, cellular receptors, peripheral uptake, replication, transport direction and specificity. Their characteristics, that make them useful for different purposes, will be highlighted and contrasted. Only transneuronal tracing with rabies virus is entirely specific. The neuroscientist toolbox currently include wild-type alpha-herpesviruses and rabies virus strains enabling polysynaptic tracing of neuronal networks across multiple synapses, as well as genetically modified viral tracers for dual transneuronal tracing, and complementary viral tools including defective and chimeric recombinants that function as single step or monosynaptically restricted tracers, or serve for monitoring and manipulating neuronal activity and gene expression. Methodological issues that are crucial for appropriate use of these technologies will be summarized. Among wild-type and genetically engineered viral tools, rabies virus and chimeric recombinants based on rabies virus as virus backbone are the most powerful, because of the ability of rabies virus to propagate exclusively among connected neurons unidirectionally (retrogradely), without affecting neuronal function. Understanding in depth viral properties is essential for neuroscientists who intend to exploit alpha-herpesviruses, rhabdoviruses or derived recombinants as research tools. Key knowledge will be summarized regarding their cellular receptors, intracellular trafficking and strategies to contrast host defense that explain their different pathophysiology and properties as research tools.

The Foveal Visual Representation of the Primate Superior Colliculus

A defining feature of the primate visual system is its foveated nature. Processing of foveal retinal input is important not only for high-quality visual scene analysis but also for ensuring precise, albeit tiny, gaze shifts during high-acuity visual tasks. The representations of foveal retinal input in the primate lateral geniculate nucleus and early visual cortices have been characterized. However, how such representations translate into precise eye movements remains unclear. Here, we document functional and structural properties of the foveal visual representation of the midbrain superior colliculus. We show that the superior colliculus, classically associated with extra-foveal spatial representations needed for gaze shifts, is highly sensitive to visual input impinging on the fovea. The superior colliculus also represents such input in an orderly and very specific manner, and it magnifies the representation of foveal images in neural tissue as much as the primary visual cortex does. The primate superior colliculus contains a high-fidelity visual representation, with large foveal magnification, perfectly suited for active visuomotor control and perception.

Integrating Brain and Biomechanical Models-A New Paradigm for Understanding Neuro-muscular Control

To date, realistic models of how the central nervous system governs behavior have been restricted in scope to the brain, brainstem or spinal column, as if these existed as disembodied organs. Further, the model is often exercised in relation to an in vivo physiological experiment with input comprising an impulse, a periodic signal or constant activation, and output as a pattern of neural activity in one or more neural populations. Any link to behavior is inferred only indirectly via these activity patterns. We argue that to discover the principles of operation of neural systems, it is necessary to express their behavior in terms of physical movements of a realistic motor system, and to supply inputs that mimic sensory experience. To do this with confidence, we must connect our brain models to neuro-muscular models and provide relevant visual and proprioceptive feedback signals, thereby closing the loop of the simulation. This paper describes an effort to develop just such an integrated brain and biomechanical system using a number of pre-existing models. It describes a model of the saccadic oculomotor system incorporating a neuromuscular model of the eye and its six extraocular muscles. The position of the eye determines how illumination of a retinotopic input population projects information about the location of a saccade target into the system. A pre-existing saccadic burst generator model was incorporated into the system, which generated motoneuron activity patterns suitable for driving the biomechanical eye. The model was demonstrated to make accurate saccades to a target luminance under a set of environmental constraints. Challenges encountered in the development of this model showed the importance of this integrated modeling approach. Thus, we exposed shortcomings in individual model components which were only apparent when these were supplied with the more plausible inputs available in a closed loop design. Consequently we were able to suggest missing functionality which the system would require to reproduce more realistic behavior. The construction of such closed-loop animal models constitutes a new paradigm of computational neurobehavior and promises a more thoroughgoing approach to our understanding of the brain's function as a controller for movement and behavior.

Modeling eye-head gaze shifts in multiple contexts without motor planning

During gaze shifts, the eyes and head collaborate to rapidly capture a target (saccade) and fixate it. Accordingly, models of gaze shift control should embed both saccadic and fixation modes and a mechanism for switching between them. We demonstrate a model in which the eye and head platforms are driven by a shared gaze error signal. To limit the number of free parameters, we implement a model reduction approach in which steady-state cerebellar effects at each of their projection sites are lumped with the parameter of that site. The model topology is consistent with anatomy and neurophysiology, and can replicate eye-head responses observed in multiple experimental contexts: 1) observed gaze characteristics across species and subjects can emerge from this structure with minor parametric changes; 2) gaze can move to a goal while in the fixation mode; 3) ocular compensation for head perturbations during saccades could rely on vestibular-only cells in the vestibular nuclei with postulated projections to burst neurons; 4) two nonlinearities suffice, i.e., the experimentally-determined mapping of tectoreticular cells onto brain stem targets and the increased recruitment of the head for larger target eccentricities; 5) the effects of initial conditions on eye/head trajectories are due to neural circuit dynamics, not planning; and 6) "compensatory" ocular slow phases exist even after semicircular canal plugging, because of interconnections linking eye-head circuits. Our model structure also simulates classical vestibulo-ocular reflex and pursuit nystagmus, and provides novel neural circuit and behavioral predictions, notably that both eye-head coordination and segmental limb coordination are possible without trajectory planning.

Characterization of last-order premotor interneurons by transneuronal tracing with rabies virus in the neonatal mouse spinal cord

We characterized the interneurons involved in the control of ankle extensor (triceps surae [TS] muscles) motoneurons (MNs) in the lumbar enlargement of mouse neonates by retrograde transneuronal tracing using rabies virus (RV). Examination of the kinetics of retrograde transneuronal transfer at sequential intervals post inoculation enabled us to determine the time window during which only the first-order interneurons, i.e., interneurons likely monosynaptically connected to MNs (last-order interneurons [loINs]) were RV-infected. The infection of the network resulted exclusively from a retrograde transport of RV along the motor pathway. About 80% of the loINs were observed ipsilaterally to the injection. They were distributed all along the lumbar enlargement, but the majority was observed in L4 and L5 segments where TS MNs were localized. Most loINs were distributed in laminae V-VII, whereas the most superficial laminae were devoid of RV infection. Contralaterally, commissural loINs were found essentially in lamina VIII of all lumbar segments. Groups of loINs were characterized by their chemical phenotypes using dual immunolabeling. Glycinergic neurons connected to TS MNs represented 50% of loINs ipsilaterally and 10% contralaterally. As expected, the ipsilateral glycinergic loINs included Renshaw cells, the most ventral neurons expressing calbindin. We also demonstrated a direct connection between a group of cholinergic interneurons observed ipsilaterally in L3 and the rostral part of L4, and TS MNs. To conclude, transneuronal tracing with RV, combined with an immunohistochemical detection of neuronal determinants, allows a very specific mapping of motor networks involved in the control of single muscles.

Optimal control of saccades by spatial-temporal activity patterns in the monkey superior colliculus

A major challenge in computational neurobiology is to understand how populations of noisy, broadly-tuned neurons produce accurate goal-directed actions such as saccades. Saccades are high-velocity eye movements that have stereotyped, nonlinear kinematics; their duration increases with amplitude, while peak eye-velocity saturates for large saccades. Recent theories suggest that these characteristics reflect a deliberate strategy that optimizes a speed-accuracy tradeoff in the presence of signal-dependent noise in the neural control signals. Here we argue that the midbrain superior colliculus (SC), a key sensorimotor interface that contains a topographically-organized map of saccade vectors, is in an ideal position to implement such an optimization principle. Most models attribute the nonlinear saccade kinematics to saturation in the brainstem pulse generator downstream from the SC. However, there is little data to support this assumption. We now present new neurophysiological evidence for an alternative scheme, which proposes that these properties reside in the spatial-temporal dynamics of SC activity. As predicted by this scheme, we found a remarkably systematic organization in the burst properties of saccade-related neurons along the rostral-to-caudal (i.e., amplitude-coding) dimension of the SC motor map: peak firing-rates systematically decrease for cells encoding larger saccades, while burst durations and skewness increase, suggesting that this spatial gradient underlies the increase in duration and skewness of the eye velocity profiles with amplitude. We also show that all neurons in the recruited population synchronize their burst profiles, indicating that the burst-timing of each cell is determined by the planned saccade vector in which it participates, rather than by its anatomical location. Together with the observation that saccade-related SC cells indeed show signal-dependent noise, this precisely tuned organization of SC burst activity strongly supports the notion of an optimal motor-control principle embedded in the SC motor map as it fully accounts for the straight trajectories and kinematic nonlinearity of saccades.

Rabies virus as a transneuronal tracer of neuronal connections

Powerful transneuronal tracing technologies exploit the ability of some neurotropic viruses to travel across neuronal pathways and to function as self-amplifying markers. Rabies virus is the only viral tracer that is entirely specific, as it propagates exclusively between connected neurons by strictly unidirectional (retrograde) transneuronal transfer, allowing for the stepwise identification of neuronal connections of progressively higher order. Transneuronal tracing studies in primates and rodent models prior to the development of clinical disease have provided valuable information on rabies pathogenesis. We have shown that rabies virus propagation occurs at chemical synapses but not via gap junctions or cell-to-cell spread. Infected neurons remain viable, as they can express their neurotransmitters and cotransport other tracers. Axonal transport occurs at high speed, and all populations of the same synaptic order are infected simultaneously regardless of their neurotransmitters, synaptic strength, and distance, showing that rabies virus receptors are ubiquitously distributed within the CNS. Conversely, in the peripheral nervous system, rabies virus receptors are present only on motor endplates and motor axons, since uptake and transneuronal transmission to the CNS occur exclusively via the motor route, while sensory and autonomic endings are not infected. Infection of sensory and autonomic ganglia requires longer incubation times, as it reflects centrifugal propagation from the CNS to the periphery, via polysynaptic connections from sensory and autonomic neurons to the initially infected motoneurons. Virus is recovered from end organs only after the development of rabies because anterograde spread to end organs is likely mediated by passive diffusion, rather than active transport mechanisms.

Neural network simulations of the primate oculomotor system. V. Eye-head gaze shifts

We examined the performance of a dynamic neural network that replicates much of the psychophysics and neurophysiology of eye-head gaze shifts without relying on gaze feedback control. For example, our model generates gaze shifts with ocular components that do not exceed 35 degrees in amplitude, whatever the size of the gaze shifts (up to 75 degrees in our simulations), without relying on a saturating nonlinearity to accomplish this. It reproduces the natural patterns of eye-head coordination in that head contributions increase and ocular contributions decrease together with the size of gaze shifts and this without compromising the accuracy of gaze realignment. It also accounts for the dependence of the relative contributions of the eyes and the head on the initial positions of the eyes, as well as for the position sensitivity of saccades evoked by electrical stimulation of the superior colliculus. Finally, it shows why units of the saccadic system could appear to carry gaze-related signals even if they do not operate within a gaze control loop and do not receive head-related information.

Geometry of the superior colliculus mapping and efficient oculomotor computation

Numerous brain regions encode variables using spatial distribution of activity in neuronal maps. Their specific geometry is usually explained by sensory considerations only. We provide here, for the first time, a theory involving the motor function of the superior colliculus to explain the geometry of its maps. We use six hypotheses in accordance with neurobiology to show that linear and logarithmic mappings are the only ones compatible with the generation of saccadic motor command. This mathematical proof gives a global coherence to the neurobiological studies on which it is based. Moreover, a new solution to the problem of saccades involving both colliculi is proposed. Comparative simulations show that it is more precise than the classical one.

Comparison of Saccade-Associated Neuronal Activity in the Primate Central Mesencephalic and Paramedian Pontine Reticular Formations

The oculomotor system must convert signals representing the target of an intended eye movement into appropriate input to drive the individual extraocular muscles. Neural models propose that this transformation may involve either a decomposition of the intended eye displacement signal into horizontal and vertical components or an implicit process whereby component signals do not predominate until the level of the motor neurons. Thus decomposition models predict that premotor neurons should primarily encode component signals while implicit models predict encoding of off-cardinal optimal directions by premotor neurons. The central mesencephalic reticular formation (cMRF) and paramedian pontine reticular formation (PPRF) are two brain stem regions that likely participate in the development of motor activity since both structures are anatomically connected to nuclei that encode movement goal (superior colliculus) and generate horizontal eye movements (abducens nucleus). We compared cMRF and PPRF neurons and found they had similar relationships to saccade dynamics, latencies, and movement fields. Typically, the direction preference of these premotor neurons was horizontal, suggesting they were related to saccade components. To confirm this supposition, we studied the neurons during a series of oblique saccades that caused “component stretching” and thus allowed the vectorial (overall) saccade velocity to be dissociated from horizontal component velocity. The majority of cMRF and PPRF neurons encoded component velocity across all saccades, supporting decomposition models that suggest horizontal and vertical signals are generated before the level of the motoneurons. However, we also found novel vectorial eye velocity encoding neurons that may have important implications for saccade control.

Horizontal eye movement networks in primates as revealed by retrograde transneuronal transfer of rabies virus: differences in monosynaptic input to "slow" and "fast" abducens motoneurons

The sources of monosynaptic input to "fast" and "slow" abducens motoneurons (MNs) were revealed in primates by retrograde transneuronal tracing with rabies virus after injection either into the distal or central portions of the lateral rectus (LR) muscle, containing, respectively, "en grappe" endplates innervating slow muscle fibers or "en plaque" motor endplates innervating fast fibers. Rabies uptake involved exclusively motor endplates within the injected portion of the muscle. At 2.5 days after injections, remarkable differences of innervation of slow and fast MNs were demonstrated. Premotor connectivity of slow MNs, revealed here for the first time, involves mainly the supraoculomotor area, central mesencephalic reticular formation, and portions of medial vestibular and prepositus hypoglossi nuclei carrying eye position and smooth pursuit signals. Results suggest that slow MNs are involved exclusively in slow eye movements (vergence and possibly smooth pursuit), muscle length stabilization and gaze holding (fixation), and rule out their participation in fast eye movements (saccades, vestibulo-ocular reflex). By contrast, all known monosynaptic pathways to LR MNs innervate fast MNs, showing their participation in the entire horizontal eye movements repertoire. Hitherto unknown monosynaptic connections were also revealed, such as those derived from the central mesencephalic reticular formation and vertical eye movements pathways (Y group, interstitial nucleus of Cajal, rostral interstitial nucleus of the medial longitudinal fasciculus). The different connectivity of fast and slow MNs parallel differences in properties of muscle fibers that they innervate, suggesting that muscle fibers properties, rather than being self-determined, are the result of differences of their premotor innervation.

Kinematics and eye-head coordination of gaze shifts evoked from different sites in the superior colliculus of the cat

Shifting gaze requires precise coordination of eye and head movements. It is clear that the superior colliculus (SC) is involved with saccadic gaze shifts. Here we investigate its role in controlling both eye and head movements during gaze shifts. Gaze shifts of the same amplitude can be evoked from different SC sites by controlled electrical microstimulation. To describe how the SC coordinates the eye and the head, we compare the characteristics of these amplitude-matched gaze shifts evoked from different SC sites. We show that matched amplitude gaze shifts elicited from progressively more caudal sites are progressively slower and associated with a greater head contribution. Stimulation at more caudal SC sites decreased the peak velocity of the eye but not of the head, suggesting that the lower peak gaze velocity for the caudal sites is due to the increased contribution of the slower-moving head. Eye-head coordination across the SC motor map is also indicated by the relative latencies of the eye and head movements. For some amplitudes of gaze shift, rostral stimulation evoked eye movement before head movement, whereas this reversed with caudal stimulation, which caused the head to move before the eyes. These results show that gaze shifts of similar amplitude evoked from different SC sites are produced with different kinematics and coordination of eye and head movements. In other words, gaze shifts evoked from different SC sites follow different amplitude-velocity curves, with different eye-head contributions. These findings shed light on mechanisms used by the central nervous system to translate a high-level motor representation (a desired gaze displacement on the SC map) into motor commands appropriate for the involved body segments (the eye and the head).

Discrimination between active and passive head movements by macaque ventral and medial intraparietal cortex neurons

An important prerequisite for effective motor action is the discrimination between active and passive body movements. Passive movements often require immediate reflexes, whereas active movements may demand suppression of the latter. The vestibular system maintains correct body and head posture in space through reflexes. Since vestibular inputs have been reported to be largely suppressed in the vestibular nuclei during active head movements, we investigated whether head movement-related signals in the primate parietal cortex, a brain region involved in self-motion perception, could support both reflex functions and self-movement behaviour. We employed a paradigm that made available direct comparison of neuronal discharge under active and passive movement conditions. In this study, we demonstrate that a population of intraparietal (VIP (ventral) and MIP (medial)) cortex neurons change their preferred directions during horizontal head rotations depending on whether animals have performed active movements, or if they were moved passively. In other neurons no such change occurred. A combination of these signals would provide differential information about the active or passive nature of an ongoing movement. Moreover, some neurons' responses clearly anticipated the upcoming active head movement, providing a possible basis for vestibular-related reflex suppression. Intraparietal vestibular neurons thus distinguish between active and passive head movements, and their responses differ substantially from those reported in brainstem vestibular neurons, regarding strength, timing, and direction selectivity. We suggest that the contextual firing characteristics of these neurons have far-reaching implications for the suppression of reflex movements during active movement, and for the representation of space during self-movement.

C fragment of tetanus toxin hybrid proteins evaluated for muscle-specific transsynaptic mapping of spinal motor circuitry in the newborn mouse

We investigated whether the non-toxic C fragment of tetanus toxin (TTC) fused to either beta-galactosidase or green fluorescent protein could be utilized to transsynaptically trace muscle-specific spinal circuitry in the neonatal mouse after i.m. injection into a single hindlimb muscle. We found that even with careful low volume injection (0.2-1.0 microl) into a single muscle (medial gastrocnemius), the TTC hybrid proteins spread rapidly to many other hindlimb muscles and to trunk musculature such that retrograde labeling of motoneurons could not be constrained to a single motoneuron pool. Retrogradely labeled motoneurons in the lower lumbar segments harboring the medial gastrocnemius motoneuron pool were first observed two hours after the medial gastrocnemius injection. Within the next 10 h, additional lumbar and lower thoracic motoneurons became labeled, and punctate labeling in the neuropil surrounding the motoneurons appeared. Many of the TTC hybrid protein-labeled puncta in the neuropil co-localized synaptotagmin, indicating that they represent presynaptic axon terminals onto motoneurons. Although this is consistent with retrograde transsynaptic passage, we found no evidence that the TTC hybrid proteins were transported further along premotor axons to label interneuron somata. The i.m. TTC injection procedure described here therefore provides an important tool for the study of presynaptic terminals onto motoneurons. However, additional technical modifications will be required to utilize TTC tracers for transsynaptic mapping of muscle-specific spinal motor circuitry in the neonatal mouse. We provide here a set of criteria for assessing the i.m. delivery of TTC tracers as a basis for future improvements in this technique.

Neurones associated with saccade metrics in the monkey central mesencephalic reticular formation

Neurones in the central mesencephalic reticular formation (cMRF) begin to discharge prior to saccades. These long lead burst neurones interact with major oculomotor centres including the superior colliculus (SC) and the paramedian pontine reticular formation (PPRF). Three different functions have been proposed for neurones in the cMRF: (1) to carry eye velocity signals that provide efference copy information to the SC (feedback), (2) to provide duration signals from the omnipause neurones to the SC (feedback), or (3) to participate in the transformation from the spatial encoding of a target selection signal in the SC into the temporal pattern of discharge used to drive the excitatory burst neurones in the pons (feed-forward). According to each respective proposal, specific predictions about cMRF neuronal discharge have been formulated. Individual neurones should: (1) encode instantaneous eye velocity, (2) burst specifically in relation to saccade duration but not to other saccade metrics, or (3) have a spectrum of weak to strong correlations to saccade dynamics. To determine if cMRF neurones could subserve these multiple oculomotor roles, we examined neuronal activity in relation to a variety of saccade metrics including amplitude, velocity and duration. We found separate groups of cMRF neurones that have the characteristics predicted by each of the proposed models. We also identified a number of subgroups for which no specific model prediction had previously been established. We found that we could accurately predict the neuronal firing pattern during one type of saccade behaviour (visually guided) using the activity during an alternative behaviour with different saccade metrics (memory guided saccades). We suggest that this evidence of a close relationship of cMRF neuronal discharge to individual saccade metrics supports the hypothesis that the cMRF participates in multiple saccade control pathways carrying saccade amplitude, velocity and duration information within the brainstem.

Temporal characteristics of neurons in the central mesencephalic reticular formation of head unrestrained monkeys

The accompanying paper demonstrated two distinct types of central mesencephalic reticular formation (cMRF) neuron that discharged before or after the gaze movement: pre-saccadic or post-saccadic. The movement fields of pre-saccadic neurons were most closely associated with gaze displacement. The movement fields of post-saccadic neurons were most closely associated with head displacement. Here we examine the relationships of the discharge patterns of these cMRF neurons with the temporal aspects of gaze or head movement. For pre-saccadic cMRF neurons with monotonically open movement fields, we demonstrate that burst duration correlated closely with gaze duration. In addition, the peak discharge of the majority of pre-saccadic neurons was closely correlated with peak gaze velocity. In contrast, discharge parameters of post-saccadic neurons were best correlated with the time of peak head velocity. However, the duration and peak discharge of post-saccadic discharge was only weakly related to the duration and peak velocity of head movement. As a result, for the majority of post-saccadic neurons the discharge waveform poorly correlated with the dynamics of head movement. We suggest that the discharge characteristics of pre-saccadic cMRF neurons with monotonically open movement fields are similar to that of direction long-lead burst neurons found previously in the paramedian portion of the pontine reticular formation (PPRF; Hepp and Henn 1983). In light of their anatomic connections with the PPRF, these pre-saccadic neurons could form a parallel pathway that participates in the transformation from the spatial coding of gaze in the superior colliculus (SC) to the temporal coding displayed by excitatory burst neurons of the PPRF. In contrast, closed and non-monotonically open movement field pre-saccadic neurons could play a critical role in feedback to the SC. The current data do not support a role for post-saccadic cMRF neurons in the direct control of head movements, but suggest that they may serve a feedback or reafference function, providing a signal of current head amplitude to upstream regions involved in head control.

Spatial characteristics of neurons in the central mesencephalic reticular formation (cMRF) of head-unrestrained monkeys

Prior studies of the central portion of the mesencephalic reticular formation (cMRF) have shown that in head-restrained monkeys, neurons discharge prior to saccades. Here, we provide a systematic analysis of the patterns of activity in cMRF neurons during head unrestrained gaze shifts. Two types of cMRF neurons were found: presaccadic neurons began to discharge before the onset of gaze movements, while postsaccadic neurons began to discharge after gaze shift onset and typically after the end of the gaze shift. Presaccadic neuronal responses were well correlated with gaze movements, while the discharge of postsaccadic neurons was more closely associated with head movements. The activity of presaccadic neurons was organized into gaze movement fields, while the activity of postsaccadic neurons was better organized into movement fields associated with head displacement. We found that cMRF neurons displayed both open and closed movement field responses. Neurons with closed movement fields discharged before a specific set of gaze (presaccadic) or head (postsaccadic) movement amplitudes and directions and had a clear distal boundary. Neurons with open movement fields discharged for gaze or head movements of a specific direction and also for movement amplitudes up to the limit of measurement (70 degrees). A subset of open movement field neurons displayed an increased discharge with increased gaze shift amplitudes, similar to pontine burst neurons, and were called monotonically increasing open movement field neurons. In contrast, neurons with non-monotonically open movement fields demonstrated activity for all gaze shift amplitudes, but their activity reached a plateau or declined gradually for gaze shifts beyond specific amplitudes. We suggest that presaccadic neurons with open movement fields participate in a descending pathway providing gaze signals to medium-lead burst neurons in the paramedian pontine reticular formation, while presaccadic closed movement field neurons may participate in feedback to the superior colliculus. The previously unrecognized group of postsaccadic cMRF neurons may provide signals of head position or velocity to the thalamus, cerebellum, or spinal cord.

Oculomotor areas of the primate frontal lobes: a transneuronal transfer of rabies virus and [14C]-2-deoxyglucose functional imaging study

We used the [14C]-2-deoxyglucose method to study the location and extent of primate frontal lobe areas activated for saccades and fixation and the retrograde transneuronal transfer of rabies virus to determine whether these regions are oligosynaptically connected with extraocular motoneurons. Fixation-related increases of local cerebral glucose utilization (LCGU) values were found around the fundus of the inferior limb of the arcuate sulcus (AS) just ventral to its genu, in the dorsomedial frontal cortex (DMFC), cingulate cortex, and orbitofrontal cortex. Significant increases of LCGU values were found in and around both banks of the AS, DMFC, and caudal principal, cingulate, and orbitofrontal cortices of monkeys executing visually guided saccades. All of these areas are oligosynaptically connected to extraocular motoneurons, as shown by the presence of retrogradely transneuronally labeled cells after injection of rabies virus in the lateral rectus muscle. Our data demonstrate that the arcuate oculomotor cortex occupies a region considerably larger than the classic, electrical stimulation-defined, frontal eye field. Besides a large part of the anterior bank of the AS, it includes the caudal prearcuate convexity and part of the premotor cortex in the posterior bank of the AS. They also demonstrate that the oculomotor DMFC occupies a small area straddling the ridge of the brain medial to the superior ramus of the AS. Our results support the notion that a network of several interconnected frontal lobe regions is activated during rapid, visually guided eye movements and that their output is conveyed in parallel to subcortical structures projecting to extraocular motoneurons.

Vestibular response kinematics in posterior parietal cortex neurons of macaque monkeys

Perception of extrapersonal space is a fundamental requirement for accurate interaction with the environment and moving in it. Parietal cortical areas are thought to play an important role in this function. A significant sensory input to this area arrives from the vestibular system. We quantified neuronal responses in the ventral intraparietal area and the medial intraparietal area of awake head-fixed macaque monkeys during classical vestibular sinusoidal stimulation protocols and with a newly developed random vestibular testing paradigm. The goal was to study more specifically the signal content of parietal vestibular neurons with respect to head movement kinematics. Traditional sinusoidal stimulation analysis revealed that about one-third of the neurons responded in phase with either head position or head acceleration, besides classical head velocity tuning. Random vestibular stimulation revealed more complex signal profiles in the majority of neurons, although quantification of the kinematic variables that drove the neurons most effectively led to similar results to phase shift analysis. Thus, a majority of cells was principally driven by head velocity, and a minority by either acceleration or position. Nevertheless, random stimulation also revealed the simultaneous presence of all three kinematic response parameters (i.e. velocity, position and acceleration) in a majority of neurons. A minority of cells coded only two kinematic variables, i.e. head velocity coupled with either acceleration or position. Neurons coding only one kinematic variable were not found. We hereby demonstrate for the first time that central vestibular neurons carry several head movement kinematic variables simultaneously.

Connectivity of the goldfish optic tectum with the mesencephalic and rhombencephalic reticular formation

The optic tectum of goldfish, as in other vertebrates, plays a major role in the generation of orienting movements, including eye saccades. To perform these movements, the optic tectum sends a motor command through the mesencephalic and rhombencephalic reticular formation, to the extraocular motoneurons. Furthermore, the tectal command is adjusted by a feedback signal arising from the reticular targets. Since the features of the motor command change with respect to the tectal site, the present work was devoted to determining, quantitatively, the particular reciprocal connectivity between the reticular regions and tectal sites having different motor properties. With this aim, the bidirectional tracer, biotin dextran amine, was injected into anteromedial tectal sites, where eye movements with small horizontal and large vertical components were evoked, or into posteromedial tectal sites, where eye movements with large horizontal and small vertical components were evoked. Labeled boutons and somas were then located and counted in the reticular formation. Both were more numerous in the mesencephalon than in the rhombencephalon, and ipsilaterally than contralaterally, with respect to the injection site. Furthermore, the somas showed a tendency to be located in the area containing the most dense labeling of synaptic endings. In addition, labeled boutons were often observed in close association with retrogradely stained neurons, suggesting the presence of a tectoreticular feedback circuit. Following the injection in the anteromedial tectum, most of the boutons and labeled neurons were found in the reticular formation rostral to the oculomotor nucleus. Conversely, following the injection in the posteromedial tectum, most of the boutons and neurons were also located in the caudal mesencephalic reticular formation. Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus. They were more numerous following the injection in the posteromedial tectum. These results demonstrate characteristic patterns of reciprocal connectivity between physiologically different tectal sites and the mesencephalic and rhombencephalic reticular formation. These patterns are discussed in the framework of the neural substratum that underlies the codification of orienting movements in goldfish.