Saccades normally place the eye on target with one smooth movement. In late-onset Tay—Sachs (LOTS), intrasaccadic transient decelerations occur that may result from (1) premature omnipause neuron (OPN) re-activation due to malfunction of the latch circuit that inhibits OPNs for the duration of the saccade or (2) premature inhibitory burst neuron (IBN) activation due to fastigial nucleus (FN) dysregulation by the dorsal cerebellar vermis. Neuroanatomic analysis of a LOTS brain was performed. Purkinje cells were absent and gliosis of the granular cell layer was present in the dorsal cerebellar vermis. Deep cerebellar nuclei contained large inclusions. IBNs were present with small inclusions. The sample did not contain the complete OPN region; however, neurons in the OPN region contained massive inclusions. Pathologic findings suggest that premature OPN re-activation and/or inappropriate firing of IBNs may be responsible for interrupted saccades in LOTS. Cerebellar clinical dysfunction, lack of saccadic slowing, and significant loss of cerebellar cells suggest that the second cause is more likely.
fastigial nucleus; omnipause neurons; burst neurons; latch circuit; brainstem
Saccadic eye movements rapidly orient the line of sight towards the object of interest. Pre-motor burst neurons (BNs) controlling saccades receive excitation from superior colliculus and cerebellum, but inhibition by omnipause neurons (OPNs) prevents saccades. When the OPNs pause, BNs begin to fire. It has been presumed that part of the BN burst comes from post-inhibitory rebound (PIR). We hypothesized that in the absence of prior inhibition from OPNs there would be no PIR, and thus the increase in initial firing rate of BNs would be reduced. Consequently, saccade acceleration would be reduced. We measured eye movements and showed that sustained eye closure, which inhibits the activity of OPNs and thus hypothetically should weaken PIR, reduced the peak velocity, acceleration, and deceleration of saccades in healthy human subjects. Saccades under closed eyelids also had irregular trajectories; the frequency of the oscillations underlying this irregularity was similar to that of high-frequency ocular flutter (back-to-back saccades) often seen in normal subjects during attempted fixation at straight ahead while eyes are closed. Saccades and quick phases of nystagmus are generated by the same pre-motor neurons, and we found that the quick-phase velocity of nystagmus was also reduced by lid closure. These changes were not due to a mechanical hindrance to the eyes, because lid closure did not affect the peak velocities or accelerations of the eyes in the “slow-phase” response to rapid head movements of comparable speeds to those of saccades. These results indicate a role for OPNs in generating the abrupt onset and high velocities of saccades. We hypothesize that the mechanism involved is PIR in pre-motor burst neurons.
Omnipause neurons; Burst neurons; Oscillations; Ballistic movement; Post-inhibitory rebound
The superior colliculus (SC) provides signals for the generation of
saccades via a direct pathway to the brain stem burst generator (BG). In
addition, it sends saccade-related activity to the BG indirectly through the
cerebellum via a relay in the nucleus reticularis tegmenti pontis (NRTP).
Lesions of the oculomotor vermis, lobules VIc and VII, and inactivation of the
caudal fastigial nucleus, the cerebellar output nucleus to which it projects,
produce saccade dysmetria but have little effect on saccade peak velocity and
duration. We expected similar deficits from inactivation of the NRTP. Instead,
injections as small as 80 nl into the NRTP first slowed ipsiversive saccades and
then gradually reduced their amplitudes. Postinjection saccades had slower peak
velocities and longer durations than preinjection saccades with similar
amplitudes. Contraversive saccades retained their normal kinematics. When the
gains of ipsiversive saccades to 10° target steps had fallen to
their lowest values (0.28 ± 0.19; mean ± SD;
n = 10 experiments), the gains of contraversive
saccades to 10° target steps had decreased very little (0.82
± 0.11). Eventually, ipsiversive saccades did not exceed
5°, even to 20° target steps. Moreover, these small
remaining saccades apparently were made with considerable difficulty because
their latencies increased substantially. When ipsiversive saccade gain was at
its lowest, the gain and kinematics of vertical saccades to 10°
target steps exhibited inconsistent changes. We argue that our injections did
not compromise the direct SC pathway. Therefore these data suggest that the
cerebellar saccade pathway does not simply modulate BG activity but is required
for horizontal saccades to occur at all.
Current knowledge of saccade-blink interactions suggests that blinks have paradoxical effects on saccade generation. Blinks suppress saccade generation by attenuating the oculomotor drive command in structures like the superior colliculus (SC), but they also disinhibit the saccadic system by removing the potent inhibition of pontine omnipause neurons (OPNs). To better characterize these effects, we evoked the trigeminal blink reflex by delivering an air puff to one eye as saccades were evoked by sub-optimal stimulation of the SC. For every stimulation site, the peak and average velocities of stimulation with blink movements (SwBMs) were lower than stimulation-only saccades (SoMs), supporting the notion that the oculomotor drive is weakened in the presence of a blink. In contrast, the duration of the SwBMs was longer, consistent with the hypothesis that the blink-induced inhibition of the OPNs could prolong the window of time available for oculomotor commands to drive an eye movement. The amplitude of the SwBM could also be larger than the SoM amplitude obtained from the same site, particularly for cases in which blink-associated eye movements exhibited the slowest kinematics. The results are interpreted in terms of neural signatures of saccade-blink interactions.
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.
As the fovea is the only spot on the retina with high spatial resolution, primates need to move their eyes to peripheral targets for detailed inspection. Saccades are the fastest movements of the body, and theoretical studies suggest that their trajectories are optimized to bring the fovea as fast and accurately as possible on target. Speed-accuracy optimization principles explain the stereotyped nonlinear ‘main-sequence’ relationship between saccade amplitude, duration, and peak velocity. Earlier models attributed these kinematic properties to nonlinear neural circuitry in the brainstem but this creates problems for oblique saccades. Here, we demonstrate how the brainstem can be linear, and how instead the midbrain superior colliculus (SC) could optimize saccadic speed-accuracy tradeoff. Each saccade involves the recruitment of a large population of SC neurons. We show that peak firing-rate and burst shape of the recruited cells systematically vary with their location in the SC, and that burst shapes nicely match the eye-velocity profiles. This organization of burst properties fully explains the main-sequence. Moreover, all cells synchronize their bursts, thus maximizing the total instantaneous input to the brainstem, and ensuring that oblique saccades have straight trajectories. We thus discovered a sophisticated neural mechanism underlying optimal motor control in the brain.
The anatomy and neurophysiology of the saccadic eye movement system have been well studied, but the roles of certain key neurons in this system are not fully appreciated. Important clues about the functional interactions in the saccadic system can be gleaned from the histochemistry of different saccadic neurons. The most prominent inhibitory neurons in the circuit are the omnidirectional pause neurons (OPN), which inhibit the premotor burst neurons that drive the eye. Most inhibitory neurons in the brain transmit γ-aminobutyric acid (GABA), but OPN transmit glycine (Gly). It is interesting to ask whether the saccadic system would work any differently if OPN were GABA-ergic. Gly and GABA receptors both provide a channel for a hyperpolarizing Cl- current that inhibits its target neuron. Depolarizing currents that excite the neurons come through several channels, including the NMDA receptor (NMDAR). The NMDAR is unique among receptors in that it has active sites for two different neurotransmitters, glutamate (Glu) and Gly. Gly is a co-agonist that acts to amplify the current produced by Glu. We have proposed a model of the saccadic brain stem circuitry that exploits this dual role of Gly to produce both inhibition of the saccadic circuit during fixation, and to increase its responsiveness, or gain, during movements. This suggests that OPNs act more as a regulator of the saccadic circuit’s gain, rather than as a gate for allowing saccades. We propose a new hypothesis: the OPNs play a general role as a modulator of arousal in orienting subsystems, such as saccades, pursuit, head movements, etc.
glycine; burst neurons; brainstem; saccades
Individuals with autism spectrum disorder (ASD) show atypical scan paths during social interaction and when viewing faces, and recent evidence suggests that they also show abnormal saccadic eye movement dynamics and accuracy when viewing less complex and non-social stimuli. Eye movements are a uniquely promising target for studies of ASD as their spatial and temporal characteristics can be measured precisely and the brain circuits supporting them are well-defined. Control of saccade metrics is supported by discrete circuits within the cerebellum and brainstem - two brain regions implicated in magnetic resonance (MR) morphometry and histopathological studies of ASD. The functional integrity of these distinct brain systems can be examined by evaluating different parameters of visually-guided saccades.
A total of 65 participants with ASD and 43 healthy controls, matched on age (between 6 and 44-years-old), gender and nonverbal IQ made saccades to peripheral targets. To examine the influence of attentional processes, blocked gap and overlap trials were presented. We examined saccade latency, accuracy and dynamics, as well as the trial-to-trial variability of participants’ performance.
Saccades of individuals with ASD were characterized by reduced accuracy, elevated variability in accuracy across trials, and reduced peak velocity and prolonged duration. In addition, their saccades took longer to accelerate to peak velocity, with no alteration in the duration of saccade deceleration. Gap/overlap effects on saccade latencies were similar across groups, suggesting that visual orienting and attention systems are relatively spared in ASD. Age-related changes did not differ across groups.
Deficits precisely and consistently directing eye movements suggest impairment in the error-reducing function of the cerebellum in ASD. Atypical increases in the duration of movement acceleration combined with lower peak saccade velocities implicate pontine nuclei, specifically suggesting reduced excitatory activity in burst cells that drive saccades relative to inhibitory activity in omnipause cells that maintain stable fixation. Thus, our findings suggest that both cerebellar and brainstem abnormalities contribute to altered sensorimotor control in ASD.
Electronic supplementary material
The online version of this article (doi:10.1186/2040-2392-5-47) contains supplementary material, which is available to authorized users.
Autism spectrum disorder (ASD); Sensorimotor; Eye movement; Saccade; Cerebellum; Brainstem
Saccade accuracy is maintained by adaptive mechanisms that continually modify saccade amplitude to reduce dysmetria. Previous studies suggest that adaptation occurs upstream of the caudal fastigial nucleus (CFN), the output of the oculomotor cerebellar vermis but downstream from the superior colliculus (SC). The nucleus reticularis tegmenti pontis (NRTP) is a major source of afferents to both the oculomotor vermis and the CFN and in turn receives direct input from the SC. Here we examine the activity of NRTP neurons in four rhesus monkeys during behaviorally induced changes in saccade amplitude to assess whether their discharge might reveal adaptation mechanisms that mediate changes in saccade amplitude. During amplitude decrease adaptation (average, 22%), the gradual reduction of saccade amplitude was accompanied by an increase in the number of spikes in the burst of 19/34 neurons (56%) and no change for 15 neurons (44%). For the neurons that increased their discharge, the additional spikes were added at the beginning of the saccadic burst and adaptation also delayed the peak-firing rate in some neurons. Moreover, after amplitude reduction, the movement fields changed shape in all 15 open field neurons tested. Our data show that saccadic amplitude reduction affects the number of spikes in the burst of more than half of NRTP neurons tested, primarily by increasing burst duration not frequency. Therefore adaptive changes in saccade amplitude are reflected already at a major input to the oculomotor cerebellum.
Rapid shifts of the point of visual fixation between equidistant targets require equal-sized saccades of each eye. The brainstem medial longitudinal fasciculus (MLF) plays a cardinal role in ensuring that horizontal saccades between equidistant targets are tightly yoked. Lesions of the MLF—internuclear ophthalmoparesis (INO)—cause horizontal saccades to become disjunctive: adducting saccades are slow, small, or absent. However, in INO, convergence movements may remain intact. We studied horizontal gaze shifts between equidistant targets and between far and near targets aligned on the visual axis of one eye (Müller test paradigm) in five cases of INO and five control subjects. We estimated the saccadic component of each movement by measuring peak velocity and peak acceleration. We tested whether the ratio of the saccadic component of the adducting/abducting eyes stayed constant or changed for the two types of saccades. For saccades made by control subjects between equidistant targets, the group mean ratio (±SD) of adducting/abducting peak velocity was 0.96 ± 0.07 and adducting/abducting peak acceleration was 0.94 ± 0.09. Corresponding ratios for INO cases were 0.45 ± 0.10 for peak velocity and 0.27 ± 0.11 for peak acceleration, reflecting reduced saccadic pulses for adduction. For control subjects, during the Müller paradigm, the adducting/abducting ratio was 1.25 ± 0.14 for peak velocity and 1.03 ± 0.12 for peak acceleration. Corresponding ratios for INO cases were 0.82 ± 0.18 for peak velocity and 0.48 ± 0.13 for peak acceleration. When adducting/abducting ratios during Müller versus equidistant targets paradigms were compared, INO cases showed larger relative increases for both peak velocity and peak acceleration compared with control subjects. Comparison of similar-sized movements during the two test paradigms indicated that whereas INO patients could decrease peak velocity of their abducting eye during the Müller paradigm, they were unable to modulate adducting velocity in response to viewing conditions. However, the initial component of each eye’s movement was similar in both cases, possibly reflecting activation of saccadic burst neurons. These findings support the hypothesis that horizontal saccades are governed by disjunctive signals, preceded by an initial, high-acceleration conjugate transient and followed by a slower vergence component.
Eye movements; Saccades; Vergence; Medial longitudinal fasciculus; Hering’s law; Multiple sclerosis; Internuclear ophthalmoplegia
Omnipause neurons (OPNs) within the nucleus raphe interpositus (RIP) help gate the transition between fixation and saccadic eye movements by monosynaptically suppressing activity in premotor burst neurons during fixation, and releasing them during saccades. Premotor neuron activity is initiated by excitatory input from the superior colliculus (SC), but how the tectum's saccade-related activity turns off OPNs is not known. Since the central mesencephalic reticular formation (cMRF) is a major SC target, we explored whether this nucleus has the appropriate connections to support tectal gating of OPN activity. In dual-tracer experiments undertaken in macaque monkeys (Macaca fascicularis), cMRF neurons labeled retrogradely from injections into RIP had numerous anterogradely labeled terminals closely associated with them following SC injections. This suggested the presence of an SC–cMRF–RIP pathway. Furthermore, anterograde tracers injected into the cMRF of other macaques labeled axonal terminals in RIP, confirming this cMRF projection. To determine whether the cMRF projections gate OPN activity, postembedding electron microscopic immunochemistry was performed on anterogradely labeled cMRF terminals with antibody to GABA or glycine. Of the terminals analyzed, 51.4% were GABA positive, 35.5% were GABA negative, and most contacted glycinergic cells. In summary, a trans-cMRF pathway connecting the SC to the RIP is present. This pathway contains inhibitory elements that could help gate omnipause activity and allow other tectal drives to induce the bursts of firing in premotor neurons that are necessary for saccades. The non-GABAergic cMRF terminals may derive from fixation units in the cMRF.
A neural network model of biophysical neurons in the midbrain is presented to drive a muscle fiber oculomotor plant during horizontal monkey saccades. Neural circuitry, including omnipause neuron, premotor excitatory and inhibitory burst neurons, long lead burst neuron, tonic neuron, interneuron, abducens nucleus, and oculomotor nucleus, is developed to examine saccade dynamics. The time-optimal control strategy by realization of agonist and antagonist controller models is investigated. In consequence, each agonist muscle fiber is stimulated by an agonist neuron, while an antagonist muscle fiber is unstimulated by a pause and step from the antagonist neuron. It is concluded that the neural network is constrained by a minimum duration of the agonist pulse and that the most dominant factor in determining the saccade magnitude is the number of active neurons for the small saccades. For the large saccades, however, the duration of agonist burst firing significantly affects the control of saccades. The proposed saccadic circuitry establishes a complete model of saccade generation since it not only includes the neural circuits at both the premotor and motor stages of the saccade generator, but also uses a time-optimal controller to yield the desired saccade magnitude.
When normal subjects fix their eyes upon a stationary target, their gaze is not perfectly still, due to small movements that prevent visual fading. Visual loss is known to cause greater instability of gaze, but reported comparisons with normal subjects using reliable measurement techniques are few. We measured binocular gaze using the magnetic search coil technique during attempted fixation (monocular or binocular viewing) of 4 individuals with childhood-onset of monocular visual loss, 2 individuals with late-onset monocular visual loss due to age-related macular degeneration, 2 individuals with bilateral visual loss, and 20 healthy control subjects. We also measured saccades to visual or somatosensory cues. We tested the hypothesis that gaze instability following visual impairment is caused by loss of inputs that normally optimize the performance of the neural network (integrator), which ensures both monocular and conjugate gaze stability. During binocular viewing, patients with early-onset monocular loss of vision showed greater instability of vertical gaze in the eye with visual loss and, to a lesser extent, in the normal eye, compared with control subjects. These vertical eye drifts were much more disjunctive than upward saccades. In individuals with late monocular visual loss, gaze stability was more similar to control subjects. Bilateral visual loss caused eye drifts that were larger than following monocular visual loss or in control subjects. Accurate saccades could be made to somatosensory cues by an individual with acquired blindness, but voluntary saccades were absent in an individual with congenital blindness. We conclude that the neural gaze-stabilizing network, which contains neurons with both binocular and monocular discharge preferences, is under adaptive visual control. Whereas monocular visual loss causes disjunctive gaze instability, binocular blindness causes both disjunctive and conjugate gaze instability (drifts and nystagmus). Inputs that bypass this neural network, such as projections to motoneurons for upward saccades, remain conjugate.
Eye movements create an ever-changing image of the world on the retina. In
particular, frequent saccades call for a compensatory mechanism to transform the
changing visual information into a stable percept. To this end, the brain
presumably uses internal copies of motor commands. Electrophysiological
recordings of visual neurons in the primate lateral intraparietal cortex, the
frontal eye fields, and the superior colliculus suggest that the receptive
fields (RFs) of special neurons shift towards their post-saccadic positions
before the onset of a saccade. However, the perceptual consequences of these
shifts remain controversial. We wanted to test in humans whether a remapping of
motion adaptation occurs in visual perception.
The motion aftereffect (MAE) occurs after viewing of a moving stimulus as an
apparent movement to the opposite direction. We designed a saccade paradigm
suitable for revealing pre-saccadic remapping of the MAE. Indeed, a transfer of
motion adaptation from pre-saccadic to post-saccadic position could be observed
when subjects prepared saccades. In the remapping condition, the strength of the
MAE was comparable to the effect measured in a control condition
(33±7% vs. 27±4%). Contrary, after a saccade or
without saccade planning, the MAE was weak or absent when adaptation and test
stimulus were located at different retinal locations, i.e. the effect was
Regarding visual cognition, our study reveals for the first time predictive
remapping of the MAE but no spatiotopic transfer across saccades. Since the
cortical sites involved in motion adaptation in primates are most likely the
primary visual cortex and the middle temporal area (MT/V5) corresponding to
human MT, our results suggest that pre-saccadic remapping extends to these
areas, which have been associated with strict retinotopy and therefore with
classical RF organization. The pre-saccadic transfer of visual features
demonstrated here may be a crucial determinant for a stable percept despite
When we applied a single pulse of transcranial magnetic stimulation (TMS) to any part of the human head during a saccadic eye movement, the ongoing eye velocity was reduced starting as early as 45ms after the TMS, and lasted around 32ms. The perturbation to the saccade trajectory was not due to a mechanical effect of the lid on the eye (e.g., from blinks). When the saccade involved coordinated movements of both the eyes and the lids, e.g., in vertical saccades, TMS produced a synchronized inhibition of the motor commands to both eye and lid muscles. The TMS induced perturbation of the eye trajectory did not show habituation with repetition, and was present in both pro- and anti-saccades. Despite the perturbation, the eye trajectory was corrected within the same saccade with compensatory motor commands that guided the eyes to the target. This within-saccade correction did not rely on visual input, suggesting that the brain monitored the oculomotor commands as the saccade unfolded, maintained a real time estimate of the position of the eyes, and corrected for the perturbation. TMS disrupted saccades regardless of the location of the coil on the head, suggesting that the coil discharge engages a non-habituating startle-like reflex system. This system affects ongoing motor commands upstream of the oculomotor neurons, possibly at the level of the superior colliculus or omnipause neurons. Therefore, a TMS pulse centrally perturbs saccadic motor commands, which are monitored possibly via efference copy, and are corrected via internal feedback.
saccade accuracy; pause; TMS; startle; omnipause neuron; forward model
Prefrontal neurons exhibit saccade-related activity and pre-saccadic memory-related activity often encodes the directions of forthcoming eye movements, in line with demonstrated prefrontal contribution to flexible control of voluntary eye movements. However, many prefrontal neurons exhibit post-saccadic activity that is initiated well after the initiation of eye movement. Although post-saccadic activity has been observed in the frontal eye field, this activity is thought to be a corollary discharge from oculomotor centers, because this activity shows no directional tuning and is observed whenever the monkeys perform eye movements regardless of goal-directed or not. However, prefrontal post-saccadic activities exhibit directional tunings similar as pre-saccadic activities and show context dependency, such that post-saccadic activity is observed only when monkeys perform goal-directed saccades. Context-dependency of prefrontal post-saccadic activity suggests that this activity is not a result of corollary signals from oculomotor centers, but contributes to other functions of the prefrontal cortex. One function might be the termination of memory-related activity after a behavioral response is done. This is supported by the observation that the termination of memory-related activity coincides with the initiation of post-saccadic activity in population analyses of prefrontal activities. The termination of memory-related activity at the end of the trial ensures that the subjects can prepare to receive new and updated information. Another function might be the monitoring of behavioral performance, since the termination of memory-related activity by post-saccadic activity could be associated with informing the correctness of the response and the termination of the trial. However, further studies are needed to examine the characteristics of saccade-related activities in the prefrontal cortex and their functions in eye movement control and a variety of cognitive functions.
prefrontal cortex; saccadic eye movement; post-saccadic activity; context dependency; directional selectivity; frontal eye field
The paramedian pontine reticular formation contains the premotoneuronal cell groups that constitute the saccadic burst generator and control saccadic eye movements. Despite years of study and numerous investigations, the rostral portion of this area has received comparatively little attention, particularly the cell type known as long-lead burst neurons (LLBNs). Several hypotheses about the functional role of LLBNs in saccade generation have been proposed, although there is little information with which to assess them. To address this issue, I mapped and recorded LLBNs in the rostral pons to measure their discharge characteristics and correlate those characteristics with the metrics of the concurrent saccades. On the basis of their discharge and location, I identified three types of LLBNs in the rostral pons: excitatory (eLLBN), dorsal (dLLBN), and nucleus reticularis tegmenti pontis (nrtp) LLBNs. The eLLBNs, encountered throughout the pons, discharge for ipsilateral saccades in proportion to saccade amplitude, velocity, and duration. The dLLBNs, found at the pontomesencephalic junction, discharge maximally for ipsilateral saccades of a particular amplitude, usually <10°, and are not associated with a particular anatomical nucleus. The nrtp LLBNs, previously described as vector LLBNs, discharge for saccades of a particular direction and sometimes a particular amplitude. The discharge of the eLLBNs suggests they drive motor neurons. The anatomical projections of the nrtp LLBNs suggest that their involvement in saccade production is less direct. The discharge of dLLBNs is consistent with a role in providing the “trigger” signal that initiates saccades.
The countermanding (or stop signal) task probes the control of the initiation of a movement by measuring subjects’ ability to withhold a movement in various degrees of preparation in response to an infrequent stop signal. Previous research found that saccades are initiated when the activity of movement-related neurons reaches a threshold, and saccades are withheld if the growth of activity is interrupted. To extend and evaluate this relationship of frontal eye field (FEF) activity to saccade initiation, two new analyses were performed. First, we fit a neurometric function that describes the proportion of trials with a stop signal in which neural activity exceeded a criterion discharge rate as a function of stop signal delay, to the inhibition function that describes the probability of producing a saccade as a function of stop signal delay. The activity of movement-related but not visual neurons provided the best correspondence between neurometric and inhibition functions. Second, we determined the criterion discharge rate that optimally discriminated between the distributions of discharge rates measured on trials when saccades were produced or withheld. Differential activity of movement-related but not visual neurons could distinguish whether a saccade occurred. The threshold discharge rates determined for individual neurons through these two methods agreed. To investigate how reliably movement-related activity predicted movement initiation; the analyses were carried out with samples of activity from increasing numbers of trials from the same or from different neurons. The reliability of both measures of initiation threshold improved with number of trials and neurons to an asymptote of between 10 to 20 movement-related neurons. Combining the activity of visual neurons did not improve the reliability of predicting saccade initiation. These results demonstrate how the activity of a population of movement-related but not visual neurons in the FEF contributes to the control of saccade initiation. The results also validate these analytical procedures for identifying signals that control saccade initiation in other brain structures.
FRONTAL CORTEX; MOTOR CONTROL; OCULOMOTOR; REACTION TIME; RESPONSE TIME; STOCHASTIC MODELS; STOP SIGNAL; SACCADE LATENCY
In late-onset Tay-Sachs disease (LOTS), saccades are interrupted by one or more transient decelerations. Some saccades reaccelerate and continue on before eye velocity reaches zero, even in darkness. Intervals between successive decelerations are not regularly spaced. Peak decelerations of horizontal and vertical components of oblique saccades in LOTS is more synchronous than those in control subjects. We hypothesize that these decelerations are caused by dysregulation of the fastigial nuclei (FN) of the cerebellum, which fire brain stem inhibitory burst neurons (IBNs).
fastigial nucleus; omnipause neurons; burst neurons; latch circuit
The cerebellum plays an important role in programming accurate saccades. Cerebellar lesions affecting the ocular motor region of the fastigial nucleus (FOR) cause saccadic hypermetria; however, if a second target is presented before a saccade can be initiated (double-step paradigm), saccade hypermetria may be decreased. We tested the hypothesis that the cerebellum, especially FOR, plays a pivotal role in programming sequences of saccades. We studied patients with saccadic hypermetria due either to genetic cerebellar ataxia or surgical lesions affecting FOR and confirmed that the gain of initial saccades made to double-step stimuli was reduced compared with the gain of saccades to single target jumps. Based on measurements of the intersaccadic interval, we found that the ability to perform parallel processing of saccades was reduced or absent in all of our patients with cerebellar disease. Our results support the crucial role of the cerebellum, especially FOR, in programming sequences of saccades.
fastigial nucleus; double-step; saccade; latency; spinocerebellar ataxia; hypermetria; parallel processing
Attenuation of visual activity in the superficial layers (SL) of the superior colliculus during saccades may contribute to reducing perceptual blur during saccades and also may help prevent subsequent unwanted saccades. GABAergic neurons in the intermediate, premotor, layer (SGI) send an inhibitory input to SL. This pathway provided the basis for a model proposing that the SGI premotor cells that project to brainstem gaze centers and discharge before saccades also activate neighboring GABAergic neurons that suppress saccade induced visual activity in SL.
The in vitro method allowed us to test this model. We made whole-cell patch clamp recordings in collicular slices from either rats or GAD67-GFP knock-in mice, in which GABAergic neurons could be identified by their expression of green fluorescence protein (GFP). Antidromic electrical stimulation of SGI premotor cells was produced by applying pulse currents where their axons congregate after exiting the superior colliculus. The stimulation evoked monosynaptic excitatory postsynaptic currents (EPSCs) in SGI GABAergic neurons that project to SL, as would be predicted if these neurons receive excitatory input from the premotor cells. Second, inhibitory postsynaptic currents (IPSCs) were evoked in SL neurons, some of which project to the visual thalamus. These IPSCs were polysynaptically mediated by the GABAergic neurons that were excited by the antidromically activated SGI neurons. These results support the hypothesis that collaterals of premotor neuron axons excite GABAergic neurons that inhibit SL visuosensory cells.
visuomotor integration; inhibitory feedback; in vitro slices; mice; rats
BACKGROUND/AIMS—Abnormalities in the saccadic main sequence are an important finding and may indicate pathology of the ocular motor periphery or central neurological disorders. In young or uncooperative patients it can be difficult eliciting a sufficient number of saccades to measure the main sequence. It is often assumed that the quick phases of optokinetic nystagmus (OKN) are identical to saccades. If this were the case, it would be feasible to use OKN, an involuntary response that is easily evoked, as a simple way of eliciting many saccades. The aim of this study was to determine whether reflexive saccades and the quick phases of OKN are indeed identical, and whether OKN quick phases could have a clinical role in identifying patients with slow saccades.
METHODS—OKN and reflexive saccades were recorded from 10 healthy adults using an infrared limbus eye tracker and bitemporal DC electro-oculography simultaneously. OKN was stimulated by rotating a full field patterned curtain around the subject at 10-50°/s. Reflexive saccades were elicited to red LED targets at 5-20° eccentricity.
RESULTS—OKN quick phases tended to have a longer duration compared to saccades, but these differences were not significant. OKN quick phases had a slightly lower peak velocity compared to saccades, which was statistically significant (p<0.05).
CONCLUSION—The main sequence for duration is the same for reflexive saccades and OKN quick phases. The main sequence for peak velocity is slightly faster for reflexive saccades than OKN quick phases, but the difference is unlikely to be of clinical significance. As an illustration of the potential of this technique, the authors demonstrate that OKN quick phases show similar slowness to saccades in a child with brainstem pathology caused by Gaucher disease type III. It is concluded that recording OKN may be a simple clinical means for approximating the main sequence.
In scrutinizing a scene, the eyes alternate between fixations and saccades. During a fixation, two component processes can be distinguished: visual encoding and selection of the next fixation target. We aimed to distinguish the neural correlates of these processes in the electrical brain activity prior to a saccade onset. Participants viewed color photographs of natural scenes, in preparation for a change detection task. Then, for each participant and each scene we computed an image heat map, with temperature representing the duration and density of fixations. The temperature difference between the start and end points of saccades was taken as a measure of the expected task-relevance of the information concentrated in specific regions of a scene. Visual encoding was evaluated according to whether subsequent change was correctly detected. Saccades with larger temperature difference were more likely to be followed by correct detection than ones with smaller temperature differences. The amplitude of presaccadic activity over anterior brain areas was larger for correct detection than for detection failure. This difference was observed for short “scrutinizing” but not for long “explorative” saccades, suggesting that presaccadic activity reflects top-down saccade guidance. Thus, successful encoding requires local scanning of scene regions which are expected to be task-relevant. Next, we evaluated fixation target selection. Saccades “moving up” in temperature were preceded by presaccadic activity of higher amplitude than those “moving down”. This finding suggests that presaccadic activity reflects attention deployed to the following fixation location. Our findings illustrate how presaccadic activity can elucidate concurrent brain processes related to the immediate goal of planning the next saccade and the larger-scale goal of constructing a robust representation of the visual scene.
saccades; EEG; presaccadic interval; attention; visual encoding; saccade guidance; change detection; heat maps
The oculomotor vermis (OMV) of the cerebellum is necessary for the generation of the accurate rapid eye movements called saccades. Large lesions of the midline cerebellar cortex involving the OMV cause saccades to become hypometric and more variable. However, saccades were not examined immediately after these lesions so the interpretation of the resulting deficits might have been contaminated by some adaptation to the saccade dysmetria. Therefore, to better understand the contribution of the OMV to normal saccades, we impaired its operation locally by injecting small amounts of either an agonist or antagonist of γ-aminobutyric acid (GABA), which is a ubiquitous neurotransmitter throughout the cerebellar cortex. Muscimol, a GABA agonist, inactivated part of the OMV, whereas bicuculline, an antagonist, disinhibited it. Muscimol caused all ipsiversive horizontal saccades from 5 to 30° to become hypometric. In contrast, bicuculline produced an amplitude-dependent dysmetria: ipsiversive horizontal saccades elicited by target steps <10° became hypometric, whereas those in response to larger steps became hypermetric. At the transition target amplitude, saccade amplitudes were quite variable with some being hypo- and others hypermetric. After most injections of either agent, saccades had lower peak velocities and longer durations than pre-injection saccades of the same amplitude. The longer durations were associated with a prolongation of the deceleration phase. Both agents produced inconsistent effects on contraversive saccades. These results establish that the oculomotor vermis helps control the characteristics of normal ipsiversive saccades and that GABAergic inhibitory processes are a crucial part of this process.
monkey; cerebellum; muscimol; bicuculline; saccades; GABA
This review provides a summary of the contributions made by human functional neuroimaging studies to the understanding of neural correlates of saccadic control. The generation of simple visually-guided saccades (redirections of gaze to a visual stimulus or prosaccades) and more complex volitional saccades require similar basic neural circuitry with additional neural regions supporting requisite higher level processes. The saccadic system has been studied extensively in non-human primates (e.g. single unit recordings) and humans (e.g. lesions and neuroimaging). Considerable knowledge of this system’s functional neuroanatomy makes it useful for investigating models of cognitive control. The network involved in prosaccade generation (by definition exogenously-driven) includes subcortical (striatum, thalamus, superior colliculus, and cerebellar vermis) and cortical structures (primary visual, extrastriate, and parietal cortices, and frontal and supplementary eye fields). Activation in these regions is also observed during endogenously-driven voluntary saccades (e.g. antisaccades, ocular motor delayed response or memory saccades, predictive tracking tasks and anticipatory saccades, and saccade sequencing), all of which require complex cognitive processes like inhibition and working memory. These additional requirements are supported by changes in neural activity in basic saccade circuitry and by recruitment of additional neural regions (such as prefrontal and anterior cingulate cortices). Activity in visual cortex is modulated as a function of task demands and may predict the type of saccade to be generated, perhaps via top-down control mechanisms. Neuroimaging studies suggest two foci of activation within FEF - medial and lateral - which may correspond to volitional and reflexive demands, respectively. Future research on saccade control could usefully (i) delineate important anatomical subdivisions that underlie functional differences, (ii) evaluate functional connectivity of anatomical regions supporting saccade generation using methods such as ICA and structural equation modeling, (iii) investigate how context affects behavior and brain activity, and (iv) use multi-modal neuroimaging to maximize spatial and temporal resolution.
The superior colliculus (SC) of the monkey has been shown to be involved in not only initiation of saccades but in the selection of the target to which the saccade can be directed. The present experiments examine whether SC neuronal activity related to target selection is also related to saccade generation. In an asynchronous target task, the monkey was required to make a saccade to the first of two spots of light to appear. Using choice probability analysis over multiple trials, we determined the earliest time at which neurons in the SC intermediate layers indicated target selection. We then determined how closely the neuronal selection was correlated to the saccade onset by using our asynchronous reaction time task, which allowed the monkey to make a saccade to the target as soon as the selection had been made. We found that the selection became evident at widely differing times for different neurons. Some neurons indicated target selection just before the saccade (close to the pre-saccadic burst of activity), others did so at the time of the visual response, and some showed an increase in their activity even before the target appeared. A fraction of this pre-stimulus bias resulted from a priming effect of the previous trial; a saccade to the target in the movement field on the previous trial produced both a higher level of neuronal activity and a higher probability for a saccade to that target on the current trial. We found that most of the neurons (76%) showed a correlation between selection time and reaction time. Furthermore, within this 76% of neurons, many indicated a selection very early during the visual response. There was no evidence of a sequence from target selection first and saccade selection later, but rather that target selection and saccade initiation are intertwined and are probably inseparable.
Target Selection; Superior Colliculus; Saccades; Priming