The caudal fastigial nucleus and the steering of saccades toward a moving visual target

Different extrapolation of moving object locations in perception, smooth pursuit, and saccades

The ability to accurately perceive and track moving objects is crucial for many everyday activities. In this study, we use a "double-drift stimulus" to explore the processing of visual motion signals that underlie perception, pursuit, and saccade responses to a moving object. Participants were presented with peripheral moving apertures filled with noise that either drifted orthogonally to the aperture's direction or had no net motion. Participants were asked to saccade to and track these targets with their gaze as soon as they appeared and then to report their direction. In the trials with internal motion, the target disappeared at saccade onset so that the first 100 ms of the postsaccadic pursuit response was driven uniquely by peripheral information gathered before saccade onset. This provided independent measures of perceptual, pursuit, and saccadic responses to the double-drift stimulus on a trial-by-trial basis. Our analysis revealed systematic differences between saccadic responses, on one hand, and perceptual and pursuit responses, on the other. These differences are unlikely to be caused by differences in the processing of motion signals because both saccades and pursuits seem to rely on shared target position and velocity information. We conclude that our results are instead due to a difference in how the processing mechanisms underlying perception, pursuit, and saccades combine motor signals with target position. These findings advance our understanding of the mechanisms underlying dissociation in visual processing between perception and eye movements.

Bilateral lesion of the cerebellar fastigial nucleus: Effects on smooth pursuit acceleration and non-reflexive visually-guided saccades

Background

“Central dizziness” due to acute bilateral midline cerebellar disease sparing the posterior vermis has specific oculomotor signs. The oculomotor region of the cerebellar fastigial nucleus (FOR) crucially controls the accuracy of horizontal visually-guided saccades and smooth pursuit eye movements. Bilateral FOR lesions elicit bilateral saccade hypermetria with preserved pursuit. It is unknown whether the initial acceleration of smooth pursuit is impaired in patients with bilateral FOR lesions.

Objective

We studied the effect of a cerebellar lesion affecting the deep cerebellar nuclei on the initial horizontal pursuit acceleration and investigated whether saccade dysmetria also affects other types of volitional saccades, i.e., memory-guided saccades and anti-saccades, which are not performed in immediate response to the visual target.

Methods

We recorded eye movements during a sinusoidal and step-ramp target motion paradigm as well as visually-guided saccades, memory-guided saccades, and anti-saccades in one patient with a circumscribed cerebellar hemorrhage and 18 healthy control subjects using a video-based eye tracker.

Results

The lesion comprised the FOR bilaterally but spared the posterior vermis. The initial pursuit acceleration was low but not significantly different from the healthy control subjects and sinusoidal pursuit was normal. Bilateral saccade hypermetria was not only seen with visually-guided saccades but also with anti-saccades and memory-guided saccades. The final eye position remained accurate.

Conclusion

We provide new insights into the contribution of the bilateral deep cerebellar nuclei on the initial acceleration of human smooth pursuit in midline cerebellar lesions. In line with experimental bilateral FOR lesion data in non-human primates, the initial pursuit acceleration in our patient was not significantly reduced, in contrast to the effects of unilateral experimental FOR lesions. Working memory and neural representation of target locations seem to remain unimpaired. Our data argue against an impaired common command feeding the circuits controlling saccadic and pursuit eye movements and support the hypothesis of independent influences on the neural processes generating both types of eye movements in the deep cerebellar nuclei.

Posterior parietal cortex predicts upcoming movement in dynamic sensorimotor control

SignificanceMost studies in sensorimotor neurophysiology have utilized reactive movements to stationary goals pre-defined by sensory cues, but this approach is fundamentally incapable of determining whether the observed neural activity reflects current sensory stimuli or predicts future movements. In the present study, we recorded single-neuron activity from behaving monkeys engaged in a dynamic, flexible, stimulus-response contingency task that enabled us to distinguish activity co-varying with sensory inflow from that co-varying with motor outflow in the posterior parietal cortex.

A change in perspective: The interaction of saccadic and pursuit eye movements in oculomotor control and perception

Due to the close relationship between oculomotor behavior and visual processing, eye movements have been studied in many different areas of research over the last few decades. While these studies have brought interesting insights, specialization within each research area comes at the potential cost of a narrow and isolated view of the oculomotor system. In this review, we want to expand this perspective by looking at the interactions between the two most important types of voluntary eye movements: saccades and pursuit. Recent evidence indicates multiple interactions and shared signals at the behavioral and neurophysiological level for oculomotor control and for visual perception during pursuit and saccades. Oculomotor control seems to be based on shared position- and velocity-related information, which leads to multiple behavioral interactions and synergies. The distinction between position- and velocity-related information seems to be also present at the neurophysiological level. In addition, visual perception seems to be based on shared efferent signals about upcoming eye positions and velocities, which are to some degree independent of the actual oculomotor response. This review suggests an interactive perspective on the oculomotor system, based mainly on different types of sensory input, and less so on separate subsystems for saccadic or pursuit eye movements.

Bilateral control of interceptive saccades: evidence from the ipsipulsion of vertical saccades after caudal fastigial inactivation

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of saccades toward a visual target. On the basis of the organization of their efferences to the premotor burst neurons and the bilateral control of saccades, the hypothesis was proposed that the same unbalanced activity accounts for the dysmetria of all saccades during cFN unilateral inactivation, regardless of whether the saccade is horizontal, oblique, or vertical. We further tested this hypothesis by studying, in two head-restrained macaques, the effects of unilaterally inactivating the caudal fastigial nucleus on saccades toward a target moving vertically with a constant, increasing or decreasing speed. After local muscimol injection, vertical saccades were deviated horizontally toward the injected side with a magnitude that increased with saccade size. The ipsipulsion indeed depended on the tested target speed but not its instantaneous value because it did not increase (decrease) when the target accelerated (decelerated). By subtracting the effect on contralesional horizontal saccades from the effect on ipsilesional ones, we found that the net bilateral effect on horizontal saccades was strongly correlated with the effect on vertical saccades. We explain how this correlation corroborates the bilateral hypothesis and provide arguments against the suggestion that the instantaneous saccade velocity would somehow be "encoded" by the discharge of Purkinje cells in the oculomotor vermis.NEW & NOTEWORTHY Besides causing dysmetric horizontal saccades, unilateral inactivation of caudal fastigial nucleus causes an ipsipulsion of vertical saccades. This study is the first to quantitatively describe this ipsipulsion during saccades toward a moving target. By subtracting the effects on contralesional (hypometric) and ipsilesional (hypermetric) horizontal saccades, we find that this net bilateral effect is strongly correlated with the ipsipulsion of vertical saccades, corroborating the suggestion that a common disorder affects all saccades.

Population coding in the cerebellum: a machine learning perspective

The cere resembles a feedforward, three-layer network of neurons in which the "hidden layer" consists of Purkinje cells (P-cells) and the output layer consists of deep cerebellar nucleus (DCN) neurons. In this analogy, the output of each DCN neuron is a prediction that is compared with the actual observation, resulting in an error signal that originates in the inferior olive. Efficient learning requires that the error signal reach the DCN neurons, as well as the P-cells that project onto them. However, this basic rule of learning is violated in the cerebellum: the olivary projections to the DCN are weak, particularly in adulthood. Instead, an extraordinarily strong signal is sent from the olive to the P-cells, producing complex spikes. Curiously, P-cells are grouped into small populations that converge onto single DCN neurons. Why are the P-cells organized in this way, and what is the membership criterion of each population? Here, I apply elementary mathematics from machine learning and consider the fact that P-cells that form a population exhibit a special property: they can synchronize their complex spikes, which in turn suppress activity of DCN neuron they project to. Thus complex spikes cannot only act as a teaching signal for a P-cell, but through complex spike synchrony, a P-cell population may act as a surrogate teacher for the DCN neuron that produced the erroneous output. It appears that grouping of P-cells into small populations that share a preference for error satisfies a critical requirement of efficient learning: providing error information to the output layer neuron (DCN) that was responsible for the error, as well as the hidden layer neurons (P-cells) that contributed to it. This population coding may account for several remarkable features of behavior during learning, including multiple timescales, protection from erasure, and spontaneous recovery of memory.

Spatial and temporal adaptation of predictive saccades based on motion inference

Moving objects are often occluded behind larger, stationary objects, but we can easily predict when and where they reappear. Here, we show that the prediction of object reappearance is subject to adaptive learning. When monkeys generated predictive saccades to the location of target reappearance, systematic changes in the location or timing of target reappearance independently altered the endpoint or latency of the saccades. Furthermore, spatial adaptation of predictive saccades did not alter visually triggered reactive saccades, whereas adaptation of reactive saccades altered the metrics of predictive saccades. Our results suggest that the extrapolation of motion trajectory may be subject to spatial and temporal recalibration mechanisms located upstream from the site of reactive saccade adaptation. Repetitive exposure of visual error for saccades induces qualitatively different adaptation, which might be attributable to different regions in the cerebellum that regulate learning of trajectory prediction and saccades.

Dynamic combination of position and motion information when tracking moving targets

To accurately foveate a moving target, the oculomotor system needs to estimate the position of the target at the saccade end, based on information about its position and ongoing movement, while accounting for neuronal delays and execution time of the saccade. We investigated human interceptive saccades and pursuit responses to moving targets defined by high and low luminance contrast or by chromatic contrast only (isoluminance). We used step-ramps with perpendicular directions between vertical target steps of 10 deg/s and horizontal ramps of 2.5 to 20 deg/s to separate errors with respect to the position step of the target in the vertical dimension, and errors related to target motion in the horizontal dimension. Interceptive saccades to targets of high and low luminance contrast landed close to the actual target positions, suggesting relatively accurate estimates of the amount of target displacement. Interceptive saccades to isoluminant targets were less accurate. They landed at positions the target had on average 100 ms before saccade onset. One account of this finding is that the integration of target motion is compromised for isoluminant targets moving in the periphery. In this case, the oculomotor system can use an accurate, but delayed position component, but cannot account for target movement. This deficit was also present for the postsaccadic pursuit speed. For the two luminance conditions, pursuit direction and speed were adjusted depending on the saccadic landing position. The rapid postsaccadic pursuit adjustments suggest shared position- and motion-related signals of target and eye for saccade and pursuit control.

Kinematics and the neurophysiological study of visually-guided eye movements

How do we relate observations and measurements made at the behavioral and neuronal levels? Notions of kinematics have been used to "decode" the firing rate of neurons and to explain the neurophysiology underlying the generation of visually-guided eye movements. The appropriateness of their fitting to events occurring within a medium (the brain) radically different from the physical world is questioned in this chapter. Instead of embedding the eye kinematics in the firing rate of central neurons, we propose that the saccadic and pursuit eye movements in fact reflect the dynamics of transitions of brain activity, from unbalanced states to equilibrium (symmetry) between opposing directional tendencies carried by the recruited visuomotor channels, with distinct transitions characterizing each movement category. While the eyeballs conform to the physical laws of motion, the neural processes leading to their movements follow principles dictated by the intrinsic properties of the brain network and of its diverse neurons.

Neurophysiology of visually guided eye movements: critical review and alternative viewpoint

In this article, we perform a critical examination of assumptions that led to the assimilation of measurements of the movement of a rigid body in the physical world to parameters encoded within brain activity. In many neurophysiological studies of goal-directed eye movements, equivalence has indeed been made between the kinematics of the eyes or of a targeted object and the associated neuronal processes. Such a way of proceeding brings up the reduction encountered in projective geometry when a multidimensional object is being projected onto a one-dimensional segment. The measurement of a movement indeed consists of generation of a series of numerical values from which magnitudes such as amplitude, duration, and their ratio (speed) are calculated. By contrast, movement generation consists of activation of multiple parallel channels in the brain. Yet, for many years, kinematic parameters were supposed to be encoded in brain activity, even though the neuronal image of most physical events is distributed both spatially and temporally. After explaining why the "neuronalization" of such parameters is questionable for elucidating the neural processes underlying the execution of saccadic and pursuit eye movements, we propose an alternative to the framework that has dominated the last five decades. A viewpoint is presented in which these processes follow principles that are defined by intrinsic properties of the brain (population coding, multiplicity of transmission delays, synchrony of firing, connectivity). We propose reconsideration of the time course of saccadic and pursuit eye movements as the restoration of equilibria between neural populations that exert opposing motor tendencies.

Pursuit disorder and saccade dysmetria after caudal fastigial inactivation in the monkey

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of saccadic and pursuit eye movements. We investigated the consequences of unilateral inactivation on the pursuit eye movement made immediately after an interceptive saccade toward a centrifugal target. We describe here the effects when the target moved along the horizontal meridian with a 10 or 20°/s speed. After muscimol injection, the monkeys were unable to track the present location of the moving target. During contralesional tracking, the velocity of postsaccadic pursuit was reduced. This slowing was associated with a hypometria of interceptive saccades such that gaze direction always lagged behind the moving target. No correlation was found between the sizes of saccade undershoot and the decreases in pursuit speed. During ipsilesional tracking, the effects on postsaccadic pursuit were variable across the injection sessions, whereas the interceptive saccades were consistently hypermetric. Here also, the ipsilesional pursuit disorder was not correlated with the saccade hypermetria either. The lack of correlation between the sizes of saccade dysmetria and changes of postsaccadic pursuit speed suggests that cFN activity exerts independent influences on the neural processes generating the saccadic and slow eye movements. It also suggests that the cFN is one locus where the synergy between the two motor categories develops in the context of tracking a moving visual target. We explain how the different fastigial output channels can account for these oculomotor tracking disorders. NEW & NOTEWORTHY Inactivation of the caudal fastigial nucleus impairs the ability to track a moving target. The accuracy of interceptive saccades and the velocity of postsaccadic pursuit movements are both altered, but these changes are not correlated. This absence of correlation is not compatible with an impaired common command feeding the circuits producing saccadic and pursuit eye movements. However, it suggests an involvement of caudal fastigial nuclei in their synergy to accurately track a moving target.