Electrical stimulation of the frontal eye fields in the head-free macaque evokes kinematically normal 3D gaze shifts

Disrupted voxel-mirrored homotopic connectivity in congenital nystagmus using resting-state fMRI

OBJECTIVES

Idiopathic congenital nystagmus (CN) is a rare eye disease that can cause early blindness (EB). CN deficits are observed most frequently with oculomotor dysfunction; however, it is still unclear what neuromechanics underly CN with EB. Based on that visual experience requires the functional integration of both hemispheres, we hypothesized that CN adolescents with EB might exhibit impaired interhemispheric synchrony. Our study aimed to investigate the interhemispheric functional connectivity alterations using voxel-mirrored homotopic connectivity (VMHC) and their relationships with clinical features in CN patients.

MATERIALS AND METHODS

This study included 21 patients with CN and EB, and 21 sighted controls (SC), who were matched for sex, age and educational level. The 3.0 T MRI scan and ocular examination were performed. The VMHC differences were examined between the two groups, and the relationships between mean VMHC values in altered brain regions and clinical variables in the CN group were evaluated by Pearson correlation analysis.

RESULTS

Compared with the SC group, the CN had increased VMHC values in the bilateral cerebellum posterior and anterior lobes/cerebellar tonsil/declive/pyramis/culmen/pons, middle frontal gyri (BA 10) and frontal eye field/superior frontal gyri (BA 6 and BA 8). No particular areas of the brain had lower VMHC values. Furthermore, no correlation with the duration of disease or blindness could be demonstrated in CN.

CONCLUSION

Our results suggest the existence of interhemispheric connectivity changes and provide further evidence for the neurological basis of CN with EB.

Ventral premotor cortex encodes task relevant features during eye and head movements

Visual exploration of the environment is achieved through gaze shifts or coordinated movements of the eyes and the head. The kinematics and contributions of each component can be decoupled to fit the context of the required behavior, such as redirecting the visual axis without moving the head or rotating the head without changing the line of sight. A neural controller of these effectors, therefore, must show code relating to multiple muscle groups, and it must also differentiate its code based on context. In this study we tested whether the ventral premotor cortex (PMv) in monkey exhibits a population code relating to various features of eye and head movements. We constructed three different behavioral tasks or contexts, each with four variables to explore whether PMv modulates its activity in accordance with these factors. We found that task related population code in PMv differentiates between all task related features and conclude that PMv carries information about task relevant features during eye and head movements. Furthermore, this code represents both lower-level (effector and movement direction) and higher-level (context) information.

Visual Attention in the Prefrontal Cortex

Voluntary attention selects behaviorally relevant signals for further processing while filtering out distracter signals. Neural correlates of voluntary visual attention have been reported across multiple areas of the primate visual processing streams, with the earliest and strongest effects isolated in the prefrontal cortex. In this article, I review evidence supporting the hypothesis that signals guiding the allocation of voluntary attention emerge in areas of the prefrontal cortex and reach upstream areas to modulate the processing of incoming visual information according to its behavioral relevance. Areas located anterior and dorsal to the arcuate sulcus and the frontal eye fields produce signals that guide the allocation of spatial attention. Areas located anterior and ventral to the arcuate sulcus produce signals for feature-based attention. Prefrontal microcircuits are particularly suited to supporting voluntary attention because of their ability to generate attentional template signals and implement signal gating and their extensive connectivity with the rest of the brain. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Spatiotemporal transformations for gaze control

Sensorimotor transformations require spatiotemporal coordination of signals, that is, through both time and space. For example, the gaze control system employs signals that are time-locked to various sensorimotor events, but the spatial content of these signals is difficult to assess during ordinary gaze shifts. In this review, we describe the various models and methods that have been devised to test this question, and their limitations. We then describe a new method that can (a) simultaneously test between all of these models during natural, head-unrestrained conditions, and (b) track the evolving spatial continuum from target (T) to future gaze coding (G, including errors) through time. We then summarize some applications of this technique, comparing spatiotemporal coding in the primate frontal eye field (FEF) and superior colliculus (SC). The results confirm that these areas preferentially encode eye-centered, effector-independent parameters, and show-for the first time in ordinary gaze shifts-a spatial transformation between visual and motor responses from T to G coding. We introduce a new set of spatial models (T-G continuum) that revealed task-dependent timing of this transformation: progressive during a memory delay between vision and action, and almost immediate without such a delay. We synthesize the results from our studies and supplement it with previous knowledge of anatomy and physiology to propose a conceptual model where cumulative transformation noise is realized as inaccuracies in gaze behavior. We conclude that the spatiotemporal transformation for gaze is both local (observed within and across neurons in a given area) and distributed (with common signals shared across remote but interconnected structures).

Eye-head-hand coordination during visually guided reaches in head-unrestrained macaques

Nonhuman primates have been used extensively to study eye-head coordination and eye-hand coordination, but the combination-eye-head-hand coordination-has not been studied. Our goal was to determine whether reaching influences eye-head coordination (and vice versa) in rhesus macaques. Eye, head, and hand motion were recorded in two animals with search coil and touch screen technology, respectively. Animals were seated in a customized "chair" that allowed unencumbered head motion and reaching in depth. In the reach condition, animals were trained to touch a central LED at waist level while maintaining central gaze and were then rewarded if they touched a target appearing at 1 of 15 locations in a 40° × 20° (visual angle) array. In other variants, initial hand or gaze position was varied in the horizontal plane. In similar control tasks, animals were rewarded for gaze accuracy in the absence of reach. In the Reach task, animals made eye-head gaze shifts toward the target followed by reaches that were accompanied by prolonged head motion toward the target. This resulted in significantly higher head velocities and amplitudes (and lower eye-in-head ranges) compared with the gaze control condition. Gaze shifts had shorter latencies and higher velocities and were more precise, despite the lack of gaze reward. Initial hand position did not influence gaze, but initial gaze position influenced reach latency. These results suggest that eye-head coordination is optimized for visually guided reach, first by quickly and accurately placing gaze at the target to guide reach transport and then by centering the eyes in the head, likely to improve depth vision as the hand approaches the target.NEW & NOTEWORTHY Eye-head and eye-hand coordination have been studied in nonhuman primates but not the combination of all three effectors. Here we examined the timing and kinematics of eye-head-hand coordination in rhesus macaques during a simple reach-to-touch task. Our most novel finding was that (compared with hand-restrained gaze shifts) reaching produced prolonged, increased head rotation toward the target, tending to center the binocular field of view on the target/hand.

Motor action of the frontal eye field on the eyes and neck in the monkey

Focal stimulation in the frontal eye field (FEF) evoked eye movements that were often accompanied by neck movements. Experiments were performed with concurrent recording of both movements in trained monkeys. We recorded neck forces under a head-restrained condition with a force-measuring system. With the system, we measured forces along the x-, y-, and z-axes and torque about the z-axis. Torque about the z-axis that represented yaw rotation of the head was significantly affected by stimulation. We found that stimulation generated two types of motor actions of the eyes and neck. In the first type, contraversive neck forces were evoked by stimulation of the medial part of the FEF, where contraversive saccadic eye movements with large amplitudes were evoked. When the stimulus intensity was increased, saccades were evoked in an all-or-none manner, whereas the amplitude of neck forces increased gradually. In the second type, contraversive neck forces were evoked by stimulation of the medial and caudal part of the FEF, where ipsiversive slow eye movements were evoked. The depth profiles of amplitudes of neck forces were almost parallel to those of eye movements in individual stimulation tracks. The present results suggest that the FEF is involved in the control of motor actions of the neck as well as the eyes. The FEF area associated with contraversive saccades and contraversive neck movements may contribute to a gaze shift process, whereas that associated with ipsiversive slow eye movements and contraversive neck movements may contribute to a visual stabilization process. NEW & NOTEWORTHY Focal stimulation in the frontal eye field (FEF) evoked eye and neck movements. We recorded neck forces under a head-restrained condition with a force-measuring system. Taking advantage of this approach, we could analyze slow eye movements that were dissociated from the vestibuloocular reflex. We found ipsiversive slow eye movements in combination with contraversive neck forces, suggesting that the FEF may be a source of a corollary discharge signal for compensatory eye movements during voluntary neck movements.

Transition from Target to Gaze Coding in Primate Frontal Eye Field during Memory Delay and Memory-Motor Transformation

The frontal eye fields (FEFs) participate in both working memory and sensorimotor transformations for saccades, but their role in integrating these functions through time remains unclear. Here, we tracked FEF spatial codes through time using a novel analytic method applied to the classic memory-delay saccade task. Three-dimensional recordings of head-unrestrained gaze shifts were made in two monkeys trained to make gaze shifts toward briefly flashed targets after a variable delay (450-1500 ms). A preliminary analysis of visual and motor response fields in 74 FEF neurons eliminated most potential models for spatial coding at the neuron population level, as in our previous study (Sajad et al., 2015). We then focused on the spatiotemporal transition from an eye-centered target code (T; preferred in the visual response) to an eye-centered intended gaze position code (G; preferred in the movement response) during the memory delay interval. We treated neural population codes as a continuous spatiotemporal variable by dividing the space spanning T and G into intermediate T–G models and dividing the task into discrete steps through time. We found that FEF delay activity, especially in visuomovement cells, progressively transitions from T through intermediate T–G codes that approach, but do not reach, G. This was followed by a final discrete transition from these intermediate T–G delay codes to a “pure” G code in movement cells without delay activity. These results demonstrate that FEF activity undergoes a series of sensory–memory–motor transformations, including a dynamically evolving spatial memory signal and an imperfect memory-to-motor transformation.

Eye position information is used to compensate the consequences of ocular torsion on V1 receptive fields

It is commonly held that the receptive fields (RFs) of neurons in primary visual cortex (V1) are fixed relative to the retina. Hence, V1 should be unable to distinguish between retinal image shifts due to object motion and image shifts resulting from ego motion. Here we show that, in contrast to this belief, a particular class of neurons in V1 of non-human primates have RFs that are actually head centred, despite intervening eye movements. They use eye position information to shift their RFs location and to change their orientation tuning on the retina so as to fully compensate for the retinal consequences of a particular type of reflexive eye movements, ocular counter-roll, an eye rotation around the line of sight partially counterpoising head tilt. In other words, V1 uses eye position information to resolve the ambiguity if retinal image tilt is the result of the tilting of an object or of the ocular counter-roll.

Visual attention: Linking prefrontal sources to neuronal and behavioral correlates

Attention is a means of flexibly selecting and enhancing a subset of sensory input based on the current behavioral goals. Numerous signatures of attention have been identified throughout the brain, and now experimenters are seeking to determine which of these signatures are causally related to the behavioral benefits of attention, and the source of these modulations within the brain. Here, we review the neural signatures of attention throughout the brain, their theoretical benefits for visual processing, and their experimental correlations with behavioral performance. We discuss the importance of measuring cue benefits as a way to distinguish between impairments on an attention task, which may instead be visual or motor impairments, and true attentional deficits. We examine evidence for various areas proposed as sources of attentional modulation within the brain, with a focus on the prefrontal cortex. Lastly, we look at studies that aim to link sources of attention to its neuronal signatures elsewhere in the brain.

Evidence for a functional subdivision of Premotor Ear-Eye Field (Area 8B)

The Supplementary Eye Field (SEF) and the Frontal Eye Field (FEF) have been described as participating in gaze shift control. Recent evidence suggests, however, that other areas of the dorsomedial prefrontal cortex also influence gaze shift. Herein, we have investigated electrically evoked ear- and eye movements from the Premotor Ear-Eye Field, or PEEF (area 8B) of macaque monkeys. We stimulated PEEF during spontaneous condition (outside the task performance) and during the execution of a visual fixation task (VFT). In the first case, we functionally identified two regions within the PEEF: a core and a belt. In the core region, stimulation elicited forward ear movements; regarding the evoked eye movements, in some penetrations, stimulation elicited contraversive fixed-vectors with a mean amplitude of 5.14°; while in other penetrations, we observed prevalently contralateral goal-directed eye movements having end-points that fell within 15° in respect to the primary eye position. On the contrary, in the belt region, stimulation elicited backward ear movements; regarding the eye movements, in some penetrations stimulation elicited prevalently contralateral goal-directed eye movements having end-points that fell within 15° in respect to the primary eye position, while in the lateral edge of the investigated region, stimulation elicited contralateral goal-directed eye movements having end-points that fell beyond 15° in respect to the primary eye position. Stimulation during VFT either did not elicit eye movements or evoked saccades of only a few degrees. Finally, even though no head rotation movements were observed during the stimulation period, we viewed a relationship between the duration of stimulation and the neck forces exerted by the monkey's head. We propose an updated vision of the PEEF composed of two functional regions, core and belt, which may be involved in integrating auditory and visual information important to the programming of gaze orienting movements.

Visual-Motor Transformations Within Frontal Eye Fields During Head-Unrestrained Gaze Shifts in the Monkey

A fundamental question in sensorimotor control concerns the transformation of spatial signals from the retina into eye and head motor commands required for accurate gaze shifts. Here, we investigated these transformations by identifying the spatial codes embedded in visually evoked and movement-related responses in the frontal eye fields (FEFs) during head-unrestrained gaze shifts. Monkeys made delayed gaze shifts to the remembered location of briefly presented visual stimuli, with delay serving to dissociate visual and movement responses. A statistical analysis of nonparametric model fits to response field data from 57 neurons (38 with visual and 49 with movement activities) eliminated most effector-specific, head-fixed, and space-fixed models, but confirmed the dominance of eye-centered codes observed in head-restrained studies. More importantly, the visual response encoded target location, whereas the movement response mainly encoded the final position of the imminent gaze shift (including gaze errors). This spatiotemporal distinction between target and gaze coding was present not only at the population level, but even at the single-cell level. We propose that an imperfect visual–motor transformation occurs during the brief memory interval between perception and action, and further transformations from the FEF's eye-centered gaze motor code to effector-specific codes in motor frames occur downstream in the subcortical areas.

Vestibulocollic reflexes in the absence of head postural control

Percutaneous electrical vestibular stimulation evokes reflexive responses in appendicular muscles that are suppressed during tasks in which the muscles are not contributing to balance control. In neck muscles, which stabilize the head on the torso and in space, it is unclear whether similar postural task dependence shapes vestibular reflexes. We investigated whether vestibulocollic reflexes are modulated during tasks in which vestibular information is not directly relevant to maintaining the head balanced on the torso. We hypothesized that vestibulocollic reflexes would be 1) evoked when neck muscles are not involved in balancing the head on the torso and 2) invariant across synergistic neck muscle contraction tasks. Muscle activity was recorded bilaterally in sternocleidomastoid and splenius capitis muscles during head-free and head-fixed conditions while subjects were exposed to stochastic electrical vestibular stimulation (± 5 mA, 0-75 Hz). Significant vestibular reflex responses (P < 0.05) were observed during head-free and head-fixed trials. Response magnitude and timing were similar between head-free and head-fixed trials for sternocleidomastoid, but splenius capitis magnitudes decreased with the head fixed by ∼ 25% (P < 0.05). Nevertheless, this indicates that vestibulocollic responses are evoked independent of the requirement to maintain postural control of the head on the torso. Response magnitude and timing were similar across focal muscle contractions (i.e., axial rotation/flexion/extension) provided the muscle was active. In contrast, when subjects cocontracted neck muscles, vestibular-evoked responses decreased in sternocleidomastoid by ∼ 30-45% (P < 0.05) compared with focal muscle contractions but remained unchanged in splenius capitis. These results indicate robust vestibulocollic reflex coupling, which we suggest functions through its closed-loop influence on head posture to ensure cervical spine stabilization.

Effects of unilateral deactivations of dorsolateral prefrontal cortex and anterior cingulate cortex on saccadic eye movements

The dorsolateral prefrontal cortex (dlPFC) and anterior cingulate cortex (ACC) have both been implicated in the cognitive control of saccadic eye movements by single neuron recording studies in nonhuman primates and functional imaging studies in humans, but their relative roles remain unclear. Here, we reversibly deactivated either dlPFC or ACC subregions in macaque monkeys while the animals performed randomly interleaved pro- and antisaccades. In addition, we explored the whole-brain functional connectivity of these two regions by applying a seed-based resting-state functional MRI analysis in a separate cohort of monkeys. We found that unilateral dlPFC deactivation had stronger behavioral effects on saccades than unilateral ACC deactivation, and that the dlPFC displayed stronger functional connectivity with frontoparietal areas than the ACC. We suggest that the dlPFC plays a more prominent role in the preparation of pro- and antisaccades than the ACC.

Frames of reference for eye-head gaze shifts evoked during frontal eye field stimulation

The frontal eye field (FEF), in the prefrontal cortex, participates in the transformation of visual signals into saccade motor commands and in eye-head gaze control. The FEF is thought to show eye-fixed visual codes in head-restrained monkeys, but it is not known how it transforms these inputs into spatial codes for head-unrestrained gaze commands. Here, we tested if the FEF influences desired gaze commands within a simple eye-fixed frame, like the superior colliculus (SC), or in more complex egocentric frames like the supplementary eye fields (SEFs). We electrically stimulated 95 FEF sites in two head-unrestrained monkeys to evoke 3D eye-head gaze shifts and then mathematically rotated these trajectories into various reference frames. In theory, each stimulation site should specify a specific spatial goal when the evoked gaze shifts are plotted in the appropriate frame. We found that these motor output frames varied site by site, mainly within the eye-to-head frame continuum. Thus, consistent with the intermediate placement of the FEF within the high-level circuits for gaze control, its stimulation-evoked output showed an intermediate trend between the multiple reference frame codes observed in SEF-evoked gaze shifts and the simpler eye-fixed reference frame observed in SC-evoked movements. These results suggest that, although the SC, FEF and SEF carry eye-fixed information at the level of their unit response fields, this information is transformed differently in their output projections to the eye and head controllers.

Contribution of the frontal eye field to gaze shifts in the head-unrestrained rhesus monkey: neuronal activity

The frontal eye field (FEF) has a strong influence on saccadic eye movements with the head restrained. With the head unrestrained, eye saccades combine with head movements to produce large gaze shifts, and microstimulation of the FEF evokes both eye and head movements. To test whether the dorsomedial FEF provides commands for the entire gaze shift or its separate eye and head components, we recorded extracellular single-unit activity in monkeys trained to make large head-unrestrained gaze shifts. We recorded 80 units active during gaze shifts, and closely examined 26 of these that discharged a burst of action potentials that preceded horizontal gaze movements. These units were movement or visuomovement related and most exhibited open movement fields with respect to amplitude. To reveal the relations of burst parameters to gaze, eye, and/or head movement metrics, we used behavioral dissociations of gaze, eye, and head movements and linear regression analyses. The burst number of spikes (NOS) was strongly correlated with movement amplitude and burst temporal parameters were strongly correlated with movement temporal metrics for eight gaze-related burst neurons and five saccade-related burst neurons. For the remaining 13 neurons, the NOS was strongly correlated with the head movement amplitude, but burst temporal parameters were most strongly correlated with eye movement temporal metrics (head-eye-related burst neurons, HEBNs). These results suggest that FEF units do not encode a command for the unified gaze shift only; instead, different units may carry signals related to the overall gaze shift or its eye and/or head components. Moreover, the HEBNs exhibit bursts whose magnitude and timing may encode a head displacement signal and a signal that influences the timing of the eye saccade, thereby serving as a mechanism for coordinating the eye and head movements of a gaze shift.

Neural mechanisms for predictive head movement strategies during sequential gaze shifts

Humans adopt very different head movement strategies for different gaze behaviors, for example, when playing sports versus watching sports on television. Such strategy switching appears to depend on both context and expectation of future gaze positions. Here, we explored the neural mechanisms for such behaviors by training three monkeys to make head-unrestrained gaze shifts toward eccentric radial targets. A randomized color cue provided predictive information about whether that target would be followed by either a return gaze shift to center or another, more eccentric gaze shift, but otherwise animals were allowed to develop their own eye-head coordination strategy. In the first two animals we then stimulated the frontal eye fields (FEF) in conjunction with the color cue, and in the third animal we recorded from neurons in the superior colliculus (SC). Our results show that 1) monkeys can optimize eye-head coordination strategies from trial to trial, based on learned associations between color cues and future gaze sequences, 2) these cue-dependent coordination strategies were preserved in gaze saccades evoked during electrical stimulation of the FEF, and 3) two types of SC responses (the saccade burst and a more prolonged response related to head movement) modulated with these cue-dependent strategies, although only one (the saccade burst) varied in a predictive fashion. These data show that from one moment to the next, the brain can use contextual sensory cues to set up internal "coordination states" that convert fixed cortical gaze commands into the brain stem signals required for predictive head motion.

Recruitment of a contralateral head turning synergy by stimulation of monkey supplementary eye fields

The supplementary eye fields (SEF) are thought to enable higher-level aspects of oculomotor control. The goal of the present experiment was to learn more about the SEF's role in orienting, specifically by examining neck muscle recruitment evoked by stimulation of the SEF. Neck muscle activity was recorded from multiple muscles in two monkeys during SEF stimulation (100 μA, 150–300 ms, 300 Hz, with the head restrained or unrestrained) delivered 200 ms into a gap period, before a visually guided saccade initiated from a central position (doing so avoids confounds between initial position and prestimulation neck muscle activity). SEF stimulation occasionally evoked overt gaze shifts and/or head movements but almost always evoked a response that invariably consisted of a contralateral head turning synergy by increasing activity on contralateral turning muscles and decreasing activity on ipsilateral turning muscles (when background activity was present). Neck muscle responses began well in advance of evoked gaze shifts (∼30 ms after stimulation onset, leading gaze shifts by ∼40–70 ms on average), started earlier and attained a larger magnitude when accompanied by progressively larger gaze shifts, and persisted on trials without overt gaze shifts. The patterns of evoked neck muscle responses resembled those evoked by frontal eye field (FEF) stimulation, except that response latencies from the SEF were ∼10 ms longer. This basic description of the cephalomotor command evoked by SEF stimulation suggests that this structure, while further removed from the motor periphery than the FEF, accesses premotor orienting circuits in the brain stem and spinal cord in a similar manner.

Three-dimensional transformations for goal-directed action

Much of the central nervous system is involved in visuomotor transformations for goal-directed gaze and reach movements. These transformations are often described in terms of stimulus location, gaze fixation, and reach endpoints, as viewed through the lens of translational geometry. Here, we argue that the intrinsic (primarily rotational) 3-D geometry of the eye-head-reach systems determines the spatial relationship between extrinsic goals and effector commands, and therefore the required transformations. This approach provides a common theoretical framework for understanding both gaze and reach control. Combined with an assessment of the behavioral, neurophysiological, imaging, and neuropsychological literature, this framework leads us to conclude that (a) the internal representation and updating of visual goals are dominated by gaze-centered mechanisms, but (b) these representations must then be transformed as a function of eye and head orientation signals into effector-specific 3-D movement commands.

A cortical substrate for memory-guided orienting in the rat

Anatomical, stimulation, and lesion data have suggested a homology between the rat frontal orienting fields (FOF) (centered at +2 AP, ±1.3 ML mm from Bregma) and primate frontal cortices such as the frontal or supplementary eye fields. We investigated the functional role of the FOF using rats trained to perform a memory-guided orienting task, in which there was a delay period between the end of a sensory stimulus instructing orienting direction and the time of the allowed motor response. Unilateral inactivation of the FOF resulted in impaired contralateral responses. Extracellular recordings of single units revealed that 37% of FOF neurons had delay period firing rates that predicted the direction of the rats' later orienting motion. Our data provide the first electrophysiological and pharmacological evidence supporting the existence in the rat, as in the primate, of a frontal cortical area involved in the preparation and/or planning of orienting responses.