State-dependent modulation of activity in distinct layer 6 corticothalamic neurons in barrel cortex of awake mice.

Layer 6 corticothalamic (L6 CT) neurons are in a strategic position to control sensory input to the neocortex, yet we understand very little about their functions. Apart from studying their anatomical, physiological and synaptic properties, most recent efforts have focused on the activity-dependent influences CT cells can exert on thalamic and cortical neurons through causal optogenetic manipulations. However, few studies have attempted to study them during behavior. To address this gap, we performed juxtacellular recordings from optogenetically identified CT neurons in whisker-related primary somatosensory cortex (wS1) of awake, head-fixed mice (either sex) free to rest quietly or self-initiate bouts of whisking and locomotion. We found a rich diversity of response profiles exhibited by CT cells. Their spiking patterns were either modulated by whisking-related behavior (∼28%) or not (∼72%). Whisking-responsive neurons exhibited either increases, activated-type, or decreases in firing rates, suppressed-type, that aligned with whisking onset better than locomotion. We also encountered responsive neurons with preceding modulations in firing rate before whisking onset. Overall, whisking better explained these changes in rates than overall changes in arousal. Whisking-unresponsive CT cells were generally quiet, with many having low spontaneous firing rates, sparse-type, and others being completely silent. Remarkably, the sparse firing CT population preferentially spiked at the state transition point when pupil diameter constricted and the mouse entered quiet wakefulness. Thus, our results demonstrate that L6 CT cells in wS1 show diverse spiking patterns, perhaps subserving distinct functional roles related to precisely timed responses during complex behaviors and transitions between discrete waking states.SIGNIFICANCE STATEMENTLayer 6 corticothalamic neurons provide a massive input to the sensory thalamus and local connectivity within cortex, but their role in thalamocortical processing remains unclear due to difficulty accessing and isolating their activity. Although several recent optogenetic studies reveal that the net influence of corticothalamic actions, suppression versus enhancement, depends critically on the rate these neurons fire, the factors that influence their spiking are poorly understood, particularly during wakefulness. Using the well-established Ntsr1-Cre line to target this elusive population in the whisker somatosensory cortex of awake mice, we found that corticothalamic neurons show diverse state-related responses and modulations in firing rate. These results suggest separate corticothalamic populations can differentially influence thalamocortical excitability during rapid state transitions in awake, behaving animals.

Frontal eye field inactivation alters the readout of superior colliculus activity for saccade generation in a task-dependent manner.

Saccades require a spatiotemporal transformation of activity between the intermediate layers of the superior colliculus (iSC) and downstream brainstem burst generator. The dynamic linear ensemble-coding model (Goossens and Van Opstal 2006) proposes that each iSC spike contributes a fixed mini-vector to saccade displacement. Although biologically-plausible, this model assumes cortical areas like the frontal eye fields (FEF) simply provide the saccadic goal to be executed by the iSC and brainstem burst generator. However, the FEF and iSC operate in unison during saccades, and a pathway from the FEF to the brainstem burst generator that bypasses the iSC exists. Here, we investigate the impact of large yet reversible inactivation of the FEF on iSC activity in the context of the model across four saccade tasks. We exploit the overlap of saccade vectors generated when the FEF is inactivated or not, comparing the number of iSC spikes for metrically-matched saccades. We found that the iSC emits fewer spikes for metrically-matched saccades during FEF inactivation. The decrease in spike count is task-dependent, with a greater decrease accompanying more cognitively-demanding saccades. Our results show that FEF integrity influences the readout of iSC activity in a task-dependent manner. We propose that the dynamic linear ensemble-coding model be modified so that FEF inactivation increases the gain of a readout parameter, effectively increasing the influence of a single iSC spike. We speculate that this modification could be instantiated by FEF and iSC pathways to the cerebellum that could modulate the excitability of the brainstem burst generator.

Impairment but not abolishment of express saccades after unilateral or bilateral inactivation of the frontal eye fields.

Express saccades are a manifestation of a visual grasp reflex triggered when visual information arrives in the intermediate layers of the superior colliculus (SCi), which in turn orchestrates the lower level brainstem saccade generator to evoke a saccade with a very short latency (~100 ms or less). A prominent theory regarding express saccades generation is that they are facilitated by preparatory signals, presumably from cortical areas, which prime the SCi before the arrival of visual information. Here, we test this theory by reversibly inactivating a key cortical input to the SCi, the frontal eye fields (FEF), while monkeys perform an oculomotor task that promotes express saccades. Across three tasks with a different combination of potential target locations and unilateral or bilateral FEF inactivation, we found a spared ability for monkeys to generate express saccades, despite decreases in express saccade frequency during FEF inactivation. This result is consistent with the FEF having a facilitatory but not critical role in express saccade generation, likely because other cortical areas compensate for the loss of preparatory input to the SCi. However, we also found decreases in the accuracy and peak velocity of express saccades generated during FEF inactivation, which argues for an influence of the FEF on the saccadic burst generator even during express saccades. Overall, our results shed further light on the role of the FEF in the shortest-latency visually-guided eye movements.NEW & NOTEWORTHY Express saccades are the shortest-latency saccade. The frontal eye fields (FEF) are thought to promote express saccades by presetting the superior colliculus. Here, by reversibly inactivating the FEF either unilaterally or bilaterally via cortical cooling, we support this by showing that the FEF plays a facilitative but not critical role in express saccade generation. We also found that FEF inactivation lowered express saccade peak velocity, emphasizing a contribution of the FEF to express saccade kinematics.

A Causal Role for the Cortical Frontal Eye Fields in Microsaccade Deployment.

Microsaccades aid vision by helping to strategically sample visual scenes. Despite the importance of these small eye movements, no cortical area has ever been implicated in their generation. Here, we used unilateral and bilateral reversible inactivation of the frontal eye fields (FEF) to identify a cortical drive for microsaccades. Unexpectedly, FEF inactivation altered microsaccade metrics and kinematics. Such inactivation also impaired microsaccade deployment following peripheral cue onset, regardless of cue side or inactivation configuration. Our results demonstrate that the FEF provides critical top-down drive for microsaccade generation, particularly during the recovery of microsaccades after disruption by sensory transients. Our results constitute the first direct evidence, to our knowledge, for the contribution of any cortical area to microsaccade generation, and they provide a possible substrate for how cognitive processes can influence the strategic deployment of microsaccades.

Frontal Eye Field Inactivation Diminishes Superior Colliculus Activity, But Delayed Saccadic Accumulation Governs Reaction Time Increases.

Stochastic accumulator models provide a comprehensive framework for how neural activity could produce behavior. Neural activity within the frontal eye fields (FEFs) and intermediate layers of the superior colliculus (iSC) support such models for saccade initiation by relating variations in saccade reaction time (SRT) to variations in such parameters as baseline, rate of accumulation of activity, and threshold. Here, by recording iSC activity during reversible cryogenic inactivation of the FEF in four male nonhuman primates, we causally tested which parameter(s) best explains concomitant increases in SRT. While FEF inactivation decreased all aspects of ipsilesional iSC activity, decreases in accumulation rate and threshold poorly predicted accompanying increases in SRT. Instead, SRT increases best correlated with delays in the onset of saccade-related accumulation. We conclude that FEF signals govern the onset of saccade-related accumulation within the iSC, and that the onset of accumulation is a relevant parameter for stochastic accumulation models of saccade initiation.SIGNIFICANCE STATEMENT The superior colliculus (SC) and frontal eye fields (FEFs) are two of the best-studied areas in the primate brain. Surprisingly, little is known about what happens in the SC when the FEF is temporarily inactivated. Here, we show that temporary FEF inactivation decreases all aspects of functionally related activity in the SC. This combination of techniques also enabled us to relate changes in SC activity to concomitant increases in saccadic reaction time (SRT). Although stochastic accumulator models relate SRT increases to reduced rates of accumulation or increases in threshold, such changes were not observed in the SC. Instead, FEF inactivation delayed the onset of saccade-related accumulation, emphasizing the importance of this parameter in biologically plausible models of saccade initiation.

Encoding of smooth-pursuit eye movement initiation by a population of vermal Purkinje cells.

Lesion studies suggest that the oculomotor vermis (OMV) is critical for the initiation of smooth-pursuit eye movements (SPEMs); yet, its specific role has remained elusive. In this study, we tested the hypothesis that vermal Purkinje cells (PCs) may be needed to fine-tune the kinematic description of SPEM initiation. Recording from identified PCs from the monkey OMV, we observed that SPEM-related PCs were characterized by a formidable diversity of response profiles with typically only modest reflection of eye movement kinematics. In contrast, the PC population discharge could be perfectly predicted based on a linear combination of eye acceleration, velocity, and position. This finding is in full accord with a role of the OMV in shaping eye movement kinematics. It, moreover, supports the notion that this shaping action is based on a population code, whose anatomic basis is the convergence of PCs on target neurons in the cerebellar nuclei.

Characteristics of responses of Golgi cells and mossy fibers to eye saccades and saccadic adaptation recorded from the posterior vermis of the cerebellum.

The anatomical organization of the granular layer of the cerebellum suggests an important function for Golgi cells (GC) in the pathway conveying mossy fiber (MF) afferents to Purkinje cells. Based on such anatomic observations, early proposals have attributed a role in "gain control" for GCs, a function disputed by recent investigations, which assert that GCs instead contribute to oscillatory mechanisms. However, conclusive physiological evidence based on studies of cerebellum-dependent behavior supporting/dismissing the gain control proposition has been lacking as of yet. We addressed the possible function of this interneuron by recording the activity of a large number of both MFs and GCs during saccadic eye movements from the same cortical area of the monkey cerebellum, namely the oculomotor vermis (OMV). Our cellular identification conformed to previously established criteria, mainly to juxtacellular labeling studies correlating physiological parameters with cell morphology. Response patterns of both MFs and GCs were highly heterogeneous. MF discharges correlated linearly with eye saccade metrics and timing, showing directional preference and precise direction tuning. In contrast, GC discharges did not correlate strongly with the metrics or direction of movement. Their discharge properties were also unaffected by motor learning during saccadic adaptation. The OMV therefore receives a barrage of information about eye movements from different oculomotor areas over the MF pathway, which is not reflected in GCs. The unspecificity of GCs has important implications for the intricacies of neuronal processing in the granular layer, clearly discrediting their involvement in gain control and instead suggesting a more secluded role for these interneurons.

Specific vermal complex spike responses build up during the course of smooth-pursuit adaptation, paralleling the decrease of performance error.

Contemporary theories of the cerebellum hold that the complex spike (CS) fired by cerebellar Purkinje cells (PCs) reports the error signal essential for motor adaptation, i.e., the CS serves as a teacher reducing the performance error. This hypothesis suggests a monotonic relationship between CS modulation and performance error: the modulation of CS responses should be maximal at adaptation onset and turn back to its pre-adaptation state when the error is nulled. An alternative viewpoint based on studies of saccades suggests that the modulation of the CS discharge builds up as performance error decreases, and maximum and stable CS modulation is found after adaptation has been completed (Catz et al. 2005). We wanted to know whether this pattern can be generalized to other forms of motor adaptation. We resorted to smooth-pursuit adaptation (SPA) as an example of cerebellar-dependent adaptation. SPA is induced by increasing or decreasing target velocity during pursuit initiation that leads to a gradual increase or decrease in eye velocity. We trained 2 rhesus monkeys and recorded CS from PC in vermal lobuli VI and VII during SPA. We find that SPA is accompanied by a pattern of CS firing, which at the onset of adaptation, i.e., when the error is large, is not modulated significantly. On the other hand, when initial eye velocity is stably increased or decreased by adaptation, the probability of CS occurrence during pursuit initiation decreases or increases, respectively. Overall, our results deviate from the predictions made by the classical error-coding concept.

Demonstration of an eye-movement-induced visual motion illusion (Filehne illusion) in Rhesus monkeys.

During pursuit eye movements, the world around us remains perceptually stable despite the retinal-image slip induced by the eye movement. It is commonly held that this perceptual invariance is achieved by subtracting an internal reference signal, reflecting the eye movement, from the retinal motion signal. However, if the reference signal is too small or too large, a false eye-movement-induced motion of the external world, the Filehne illusion (FI), will be perceived. A reference signal of inadequate size can be simulated experimentally by asking human subjects to pursue a target across backgrounds with externally added motion that are perceived as moving. In the present study we asked if non-human primates respond to such manipulation in a way comparable to humans. Using psychophysical methods, we demonstrate that Rhesus monkeys do indeed experience a percept of pursuit-induced background motion. In this study we show that an FI can be predictably induced in Rhesus monkeys. The monkey FI shows dependencies on the size and direction of background movement, which is very similar to the ones characterizing the human FI. This congruence suggests that the perception of self-induced visual motion is based on similar inferential mechanisms in non-human and human primates.

Frontal Eye Field Inactivation Reduces Saccade Preparation in the Superior Colliculus but Does Not Alter How Preparatory Activity Relates to Saccades of a Given Latency.

A neural correlate for saccadic reaction times (SRTs) in the gap saccade task is the level of low-frequency activity in the intermediate layers of the superior colliculus (iSC) just before visual target onset: greater levels of such preparatory iSC low-frequency activity precede shorter SRTs. The frontal eye fields (FEFs) are one likely source of iSC preparatory activity, since FEF preparatory activity is also inversely related to SRT. To better understand the FEF's role in saccade preparation, and the way in which such preparation relates to SRT, in two male rhesus monkeys, we compared iSC preparatory activity across unilateral reversible cryogenic inactivation of the FEF. FEF inactivation increased contralesional SRTs, and lowered ipsilesional iSC preparatory activity. FEF inactivation also reduced rostral iSC activity during the gap period. Importantly, the distributions of SRTs generated with or without FEF inactivation overlapped, enabling us to conduct a novel population-level analyses examining iSC preparatory activity just before generation of SRT-matched saccades. When matched for SRTs, we observed no change during FEF inactivation in the relationship between iSC preparatory activity and SRT-matched saccades across a range of SRTs, even for the occasional express saccade. Thus, while our results emphasize that the FEF has an overall excitatory influence on preparatory activity in the iSC, the communication between the iSC and downstream oculomotor brainstem is unaltered for SRT-matched saccades.

A State Space Model for Spatial Updating of Remembered Visual Targets during Eye Movements.

In the oculomotor system, spatial updating is the ability to aim a saccade toward a remembered visual target position despite intervening eye movements. Although this has been the subject of extensive experimental investigation, there is still no unifying theoretical framework to explain the neural mechanism for this phenomenon, and how it influences visual signals in the brain. Here, we propose a unified state-space model (SSM) to account for the dynamics of spatial updating during two types of eye movement; saccades and smooth pursuit. Our proposed model is a non-linear SSM and implemented through a recurrent radial-basis-function neural network in a dual Extended Kalman filter (EKF) structure. The model parameters and internal states (remembered target position) are estimated sequentially using the EKF method. The proposed model replicates two fundamental experimental observations: continuous gaze-centered updating of visual memory-related activity during smooth pursuit, and predictive remapping of visual memory activity before and during saccades. Moreover, our model makes the new prediction that, when uncertainty of input signals is incorporated in the model, neural population activity and receptive fields expand just before and during saccades. These results suggest that visual remapping and motor updating are part of a common visuomotor mechanism, and that subjective perceptual constancy arises in part from training the visual system on motor tasks.

Superior Colliculus Responses to Attended, Unattended, and Remembered Saccade Targets during Smooth Pursuit Eye Movements.

In realistic environments, keeping track of multiple visual targets during eye movements likely involves an interaction between vision, top-down spatial attention, memory, and self-motion information. Recently we found that the superior colliculus (SC) visual memory response is attention-sensitive and continuously updated relative to gaze direction. In that study, animals were trained to remember the location of a saccade target across an intervening smooth pursuit (SP) eye movement (Dash et al., 2015). Here, we modified this paradigm to directly compare the properties of visual and memory updating responses to attended and unattended targets. Our analysis shows that during SP, active SC visual vs. memory updating responses share similar gaze-centered spatio-temporal profiles (suggesting a common mechanism), but updating was weaker by ~25%, delayed by ~55 ms, and far more dependent on attention. Further, during SP the sum of passive visual responses (to distracter stimuli) and memory updating responses (to saccade targets) closely resembled the responses for active attentional tracking of visible saccade targets. These results suggest that SP updating signals provide a damped, delayed estimate of attended location that contributes to the gaze-centered tracking of both remembered and visible saccade targets.

Continuous updating of visuospatial memory in superior colliculus during slow eye movements.

BACKGROUND: Primates can remember and spatially update the visual direction of previously viewed objects during various types of self-motion. It is known that the brain "remaps" visual memory traces relative to gaze just before and after, but not during, discrete gaze shifts called saccades. However, it is not known how visual memory is updated during slow, continuous motion of the eyes. RESULTS: Here, we recorded the midbrain superior colliculus (SC) of two rhesus monkeys that were trained to spatially update the location of a saccade target across an intervening smooth pursuit (SP) eye movement. Saccade target location was varied across trials so that it passed through the neuron's receptive field at different points of the SP trajectory. Nearly all (99% of) visual responsive neurons, but no motor neurons, showed a transient memory response that continuously updated the saccade goal during SP. These responses were gaze centered (i.e., shifting across the SC's retinotopic map in opposition to gaze). Furthermore, this response was strongly enhanced by attention and/or saccade target selection. CONCLUSIONS: This is the first demonstration of continuous updating of visual memory responses during eye motion. We expect that this would generalize to other visuomotor structures when gaze shifts in a continuous, unpredictable fashion.

Cerebellum-dependent motor learning: lessons from adaptation of eye movements in primates.

In order to ameliorate the consequences of ego motion for vision, human and nonhuman observers generate reflexive, compensatory eye movements based on visual as well as vestibular information, helping to stabilize the images of visual scenes on the retina despite ego motion. And in order to fully exploit the advantages of foveal vision, they make saccades to shift the image of an object onto the fovea and smooth pursuit eye movements to stabilize it there despite continuing object movement relative to the observer. With the exception of slow visually driven eye movements, which can be understood as manifestations of relatively straightforward feedback systems, most eye movements require a direct conversion of sensory input into appropriate motor responses in the absence of immediate sensory feedback. Hence, in order to generate appropriate oculomotor responses, the parameters linking input and output must be chosen suitably. Moreover, as the parameters may change from one manifestation of a movement to the next, for instance because of oculomotor fatigue, the choices should also be quickly modifiable. This chapter will present evidence showing that this fast parametric optimization, understood as a functionally distinct example of motor learning, is an accomplishment of specific parts of the cerebellum devoted to the control of eye movements. It will also discuss recent electrophysiological results suggesting how this specific form of motor learning may emerge from information processing in cerebellar circuits.

Smooth pursuit adaptation (SPA) exhibits features useful to compensate changes in the properties of the smooth pursuit eye movement system due to usage.

Smooth-pursuit adaptation (SPA) refers to the fact that pursuit gain in the early, still open-loop response phase of the pursuit eye movement can be adjusted based on experience. For instance, if the target moves initially at a constant velocity for ~100-200 ms and then steps to a higher velocity, subjects learn to up-regulate the pursuit gain associated with the initial target velocity (gain-increase SPA) in order to reduce the retinal error resulting from the velocity step. Correspondingly, a step to a lower target velocity leads to a decrease in gain (gain-decrease SPA). In this study we demonstrate that the increase in peak eye velocity during gain-increase SPA is a consequence of expanding the duration of the eye acceleration profile while the decrease in peak velocity during gain-decrease SPA results from reduced peak eye acceleration but unaltered duration. Furthermore, we show that carrying out stereotypical smooth pursuit eye movements elicited by constant velocity target ramps for several hundred trials (=test of pursuit resilience) leads to a clear drop in initial peak acceleration, a reflection of oculomotor and/or cognitive fatigue. However, this drop in acceleration gets compensated by an increase in the duration of the acceleration profile, thereby keeping initial pursuit gain constant. The compensatory expansion of the acceleration profile in the pursuit resilience experiment is reminiscent of the one leading to gain-increase SPA, suggesting that both processes tap one and the same neuronal mechanism warranting a precise acceleration-duration trade-off. Finally, we show that the ability to adjust acceleration duration during pursuit resilience depends on the integrity of the oculomotor vermis (OMV) as indicated by the complete loss of the duration adjustment following a surgical lesion of the OMV in one rhesus monkey we could study.

A vermal Purkinje cell simple spike population response encodes the changes in eye movement kinematics due to smooth pursuit adaptation.

Smooth pursuit adaptation (SPA) is an example of cerebellum-dependent motor learning that depends on the integrity of the oculomotor vermis (OMV). In an attempt to unveil the neuronal basis of the role of the OMV in SPA, we recorded Purkinje cell simple spikes (PC SS) of trained monkeys. Individual PC SS exhibited specific changes of their discharge patterns during the course of SPA. However, these individual changes did not provide a reliable explanation of the behavioral changes. On the other hand, the population response of PC SS perfectly reflected the changes resulting from adaptation. Population vector was calculated using all cells recorded independent of their location. A population code conveying the behavioral changes is in full accordance with the anatomical convergence of PC axons on target neurons in the cerebellar nuclei. Its computational advantage is the ease with which it can be adjusted to the needs of the behavior by changing the contribution of individual PC SS based on error feedback.