Directional asymmetry in smooth ocular tracking in the presence of visual background in young and adult primates

Human estimates of descending objects' motion are more accurate than those of ascending objects regardless of gravity information

Humans can accurately estimate and track object motion, even if it accelerates. Research shows that humans exhibit superior estimation and tracking performance for descending (falling) than ascending (rising) objects. Previous studies presented ascending and descending targets along the gravitational and body axes in an upright posture. Thus, it is unclear whether humans rely on congruent information between the direction of the target motion and gravity or the direction of the target motion and longitudinal body axes. Two experiments were conducted to explore these possibilities. In Experiment 1, participants estimated the arrival time at a goal for both upward and downward motion of targets along the longitudinal body axis in the upright (both axes of target motion and gravity congruent) and supine (both axes incongruent) postures. In Experiment 2, smooth pursuit eye movements were assessed while tracking both targets in the same postures. Arrival time estimation and smooth pursuit eye movement performance were consistently more accurate for downward target motion than for upward motion, irrespective of posture. These findings suggest that the visual experience of seeing an object moving along an observer's leg side in everyday life may influence the ability to accurately estimate and track the descending object's motion.

More precise tracking of horizontal than vertical target motion with both the eyes and hand

When tracking targets moving in various directions with one's eyes, horizontal components of pursuit are more precise than vertical ones. Is this because horizontal target motion is predicted better or because horizontal movements of the eyes are controlled more precisely? When tracking a visual target with the hand, the eyes also track the target. We investigated whether the directional asymmetries that have been found during isolated eye movements are also present during such manual tracking, and if so, whether individual participants' asymmetry in eye movements is accompanied by a similar asymmetry in hand movements. We examined the data of 62 participants who used a joystick to track a visual target with a cursor. The target followed a smooth but unpredictable trajectory in two dimensions. Both the mean gaze-target distance and the mean cursor-target distance were about 20% larger in the vertical direction than in the horizontal direction. Gaze and cursor both followed the target with a slightly longer delay in the vertical than in the horizontal direction, irrespective of the target's trajectory. The delays of gaze and cursor were correlated, as were their errors in tracking the target. Gaze clearly followed the target rather than the cursor, so the asymmetry in both eye and hand movements presumably results from better predictions of the target's horizontal than of its vertical motion. Altogether this study speaks for the presence of anisotropic predictive processes that are shared across effectors.

Retinal Stabilization Reveals Limited Influence of Extraretinal Signals on Heading Tuning in the Medial Superior Temporal Area

Heading perception in primates depends heavily on visual optic-flow cues. Yet during self-motion, heading percepts remain stable, even though smooth-pursuit eye movements often distort optic flow. According to theoretical work, self-motion can be represented accurately by compensating for these distortions in two ways: via retinal mechanisms or via extraretinal efference-copy signals, which predict the sensory consequences of movement. Psychophysical evidence strongly supports the efference-copy hypothesis, but physiological evidence remains inconclusive. Neurons that signal the true heading direction during pursuit are found in visual areas of monkey cortex, including the dorsal medial superior temporal area (MSTd). Here we measured heading tuning in MSTd using a novel stimulus paradigm, in which we stabilize the optic-flow stimulus on the retina during pursuit. This approach isolates the effects on neuronal heading preferences of extraretinal signals, which remain active while the retinal stimulus is prevented from changing. Our results from 3 female monkeys demonstrate a significant but small influence of extraretinal signals on the preferred heading directions of MSTd neurons. Under our stimulus conditions, which are rich in retinal cues, we find that retinal mechanisms dominate physiological corrections for pursuit eye movements, suggesting that extraretinal cues, such as predictive efference-copy mechanisms, have a limited role under naturalistic conditions.SIGNIFICANCE STATEMENT Sensory systems discount stimulation caused by an animal's own behavior. For example, eye movements cause irrelevant retinal signals that could interfere with motion perception. The visual system compensates for such self-generated motion, but how this happens is unclear. Two theoretical possibilities are a purely visual calculation or one using an internal signal of eye movements to compensate for their effects. The latter can be isolated by experimentally stabilizing the image on a moving retina, but this approach has never been adopted to study motion physiology. Using this method, we find that extraretinal signals have little influence on activity in visual cortex, whereas visually based corrections for ongoing eye movements have stronger effects and are likely most important under real-world conditions.

Smooth Pursuit Eye Movement of Monkeys Naive to Laboratory Setups With Pictures and Artificial Stimuli

When animal behavior is studied in a laboratory environment, the animals are often extensively trained to shape their behavior. A crucial question is whether the behavior observed after training is part of the natural repertoire of the animal or represents an outlier in the animal’s natural capabilities. This can be investigated by assessing the extent to which the target behavior is manifested during the initial stages of training and the time course of learning. We explored this issue by examining smooth pursuit eye movements in monkeys naïve to smooth pursuit tasks. We recorded the eye movements of monkeys from the 1st days of training on a step-ramp paradigm. We used bright spots, monkey pictures and scrambled versions of the pictures as moving targets. We found that during the initial stages of training, the pursuit initiation was largest for the monkey pictures and in some direction conditions close to target velocity. When the pursuit initiation was large, the monkeys mostly continued to track the target with smooth pursuit movements while correcting for displacement errors with small saccades. Two weeks of training increased the pursuit eye velocity in all stimulus conditions, whereas further extensive training enhanced pursuit slightly more. The training decreased the coefficient of variation of the eye velocity. Anisotropies that grade pursuit across directions were observed from the 1st day of training and mostly persisted across training. Thus, smooth pursuit in the step-ramp paradigm appears to be part of the natural repertoire of monkeys’ behavior and training adjusts monkeys’ natural predisposed behavior.

Clinical application of eye movement tasks as an aid to understanding Parkinson's disease pathophysiology

Parkinson's disease (PD) is a progressive neurodegenerative disorder of the basal ganglia. Most PD patients suffer from somatomotor and oculomotor disorders. The oculomotor system facilitates obtaining accurate information from the visual world. If a target moves slowly in the fronto-parallel plane, tracking eye movements occur that consist primarily of smooth-pursuit interspersed with corrective saccades. Efficient smooth-pursuit requires appropriate target selection and predictive compensation for inherent processing delays. Although pursuit impairment, e.g. as latency prolongation or low gain (eye velocity/target velocity), is well known in PD, normal aging alone results in such changes. In this article, we first briefly review some basic features of smooth-pursuit, then review recent results showing the specific nature of impaired pursuit in PD using a cue-dependent memory-based smooth-pursuit task. This task was initially used for monkeys to separate two major components of prediction (image-motion direction memory and movement preparation), and neural correlates were examined in major pursuit pathways. Most PD patients possessed normal cue-information memory but extra-retinal mechanisms for pursuit preparation and execution were dysfunctional. A minority of PD patients had abnormal cue-information memory or difficulty in understanding the task. Some PD patients with normal cue-information memory changed strategy to initiate smooth tracking. Strategy changes were also observed to compensate for impaired pursuit during whole body rotation while the target moved with the head. We discuss PD pathophysiology by comparing eye movement task results with neuropsychological and motor symptom evaluations of individual patients and further with monkey results, and suggest possible neural circuits for these functions/dysfunctions.

Learning the trajectory of a moving visual target and evolution of its tracking in the monkey

An object moving in the visual field triggers a saccade that brings its image onto the fovea. It is followed by a combination of slow eye movements and catch-up saccades that try to keep the target image on the fovea as long as possible. The accuracy of this ability to track the "here-and-now" location of a visual target contrasts with the spatiotemporally distributed nature of its encoding in the brain. We show in six experimentally naive monkeys how this performance is acquired and gradually evolves during successive daily sessions. During the early exposure, the tracking is mostly saltatory, made of relatively large saccades separated by low eye velocity episodes, demonstrating that accurate (here and now) pursuit is not spontaneous and that gaze direction lags behind its location most of the time. Over the sessions, while the pursuit velocity is enhanced, the gaze is more frequently directed toward the current target location as a consequence of a 25% reduction in the number of catch-up saccades and a 37% reduction in size (for the first saccade). This smoothing is observed at several scales: during the course of single trials, across the set of trials within a session, and over successive sessions. We explain the neurophysiological processes responsible for this combined evolution of saccades and pursuit in the absence of stringent training constraints. More generally, our study shows that the oculomotor system can be used to discover the neural mechanisms underlying the ability to synchronize a motor effector with a dynamic external event.

Smooth pursuit eye movements in children with strabismus and in children with vergence deficits

Purpose

The objective of our study was to examine horizontal smooth pursuit performance in strabismic children and in children with vergence deficits, and to compare these data with those recorded in a group of control age-matched children.

Methods

Binocular eye movements were recorded by video-oculography in ten strabismic children (mean age: 9.8±0.8) and seven children with vergence deficits (mean age: 10.8±0.6). Data were compared to that of age-matched control children (mean age: 9.8±0.8 years).

Results

Catch-up saccades amplitude in strabismic children and in children with vergence deficits were significantly higher than in control age-matched children. Moreover, in strabismic children the amplitude of catch-up saccades was significantly higher in rightward than in leftward direction. The number of catch-up saccades was also significantly higher in rightward than in leftward direction. The gain value of pursuits in rightward direction was significantly higher in the right eye than in the left one; for the right eye, the gain value was significantly higher in rightward than in leftward direction. Binocular coordination of pursuit was better in control age-matched children than in children with vergence deficits and than in strabismic children.

Conclusions

Binocular coordination of pursuit is abnormal in children with vergence deficits and worse in strabismic children. Binocular vision plays an important role in improving binocular coordination of pursuit.

Vestibular-related frontal cortical areas and their roles in smooth-pursuit eye movements: representation of neck velocity, neck-vestibular interactions, and memory-based smooth-pursuit

Smooth-pursuit eye movements are voluntary responses to small slow-moving objects in the fronto-parallel plane. They evolved in primates, who possess high-acuity foveae, to ensure clear vision about the moving target. The primate frontal cortex contains two smooth-pursuit related areas; the caudal part of the frontal eye fields (FEF) and the supplementary eye fields (SEF). Both areas receive vestibular inputs. We review functional differences between the two areas in smooth-pursuit. Most FEF pursuit neurons signal pursuit parameters such as eye velocity and gaze-velocity, and are involved in canceling the vestibulo-ocular reflex by linear addition of vestibular and smooth-pursuit responses. In contrast, gaze-velocity signals are rarely represented in the SEF. Most FEF pursuit neurons receive neck velocity inputs, while discharge modulation during pursuit and trunk-on-head rotation adds linearly. Linear addition also occurs between neck velocity responses and vestibular responses during head-on-trunk rotation in a task-dependent manner. During cross-axis pursuit-vestibular interactions, vestibular signals effectively initiate predictive pursuit eye movements. Most FEF pursuit neurons discharge during the interaction training after the onset of pursuit eye velocity, making their involvement unlikely in the initial stages of generating predictive pursuit. Comparison of representative signals in the two areas and the results of chemical inactivation during a memory-based smooth-pursuit task indicate they have different roles; the SEF plans smooth-pursuit including working memory of motion-direction, whereas the caudal FEF generates motor commands for pursuit eye movements. Patients with idiopathic Parkinson's disease were asked to perform this task, since impaired smooth-pursuit and visual working memory deficit during cognitive tasks have been reported in most patients. Preliminary results suggested specific roles of the basal ganglia in memory-based smooth-pursuit.

Memory-based smooth pursuit: neuronal mechanisms and preliminary results of clinical application

Using a memory-based smooth-pursuit task, macaque monkeys were trained to pursue (i.e., go) or not pursue (i.e., no-go), a cued direction, based on the memory of visual motion-direction and a go/no-go instruction. Task-related neuronal activity was examined in the supplementary eye fields, caudal frontal eye fields, cerebellar floccular region, dorsal vermis lobules VI-VII, and caudal fastigial nuclei. Different cerebral and cerebellar areas carried distinctly different signals during memory-based smooth pursuit. Chemical inactivation of these areas produced effects consistent with the differences in signals represented in each area. This task was applied to patients with idiopathic Parkinson's disease (PD), because impaired visual working memory has been reported during cognitive tasks in PD. None of the PD patients tested exhibited impaired working memory of motion-direction and/or go/no-go selection, but they had difficulty in preparing for and executing smooth-pursuit eye movements, suggesting a selective motor-related disturbance in Parkinson's disease.

Development of eye-movement control

Cognitive control of behavior continues to improve through adolescence in parallel with important brain maturational processes including synaptic pruning and myelination, which allow for efficient neuronal computations and the functional integration of widely distributed circuitries supporting top-down control of behavior. This is also a time when psychiatric disorders, such as schizophrenia and mood disorders, emerge reflecting a particularly vulnerability to impairments in development during adolescence. Oculomotor studies provide a unique neuroscientific approach to make precise associations between cognitive control and brain circuitry during development that can inform us of impaired systems in psychopathology. In this review, we first describe the development of pursuit, fixation, and visually-guided saccadic eye movements, which collectively indicate early maturation of basic sensorimotor processes supporting reflexive, exogenously-driven eye movements. We then describe the literature on the development of the cognitive control of eye movements as reflected in the ability to inhibit a prepotent eye movement in the antisaccade task, as well as making an eye movement guided by on-line spatial information in working memory in the oculomotor delayed response task. Results indicate that the ability to make eye movements in a voluntary fashion driven by endogenous plans shows a protracted development into adolescence. Characterizing the transition through adolescence to adult-level cognitive control of behavior can inform models aimed at understanding the neurodevelopmental basis of psychiatric disorders.

Discharge of pursuit-related neurons in the caudal part of the frontal eye fields in juvenile monkeys with up-down pursuit asymmetry

The smooth-pursuit system uses retinal image-slip-velocity information of target motion to match eye velocity to actual target velocity. The caudal part of the frontal eye fields (FEF) contains neurons whose activity is related to direction and velocity of smooth-pursuit eye movements (pursuit neurons), and these neurons are thought to issue a pursuit command. During normal pursuit in well-trained adult monkeys, a pursuit command is usually not differentiable from the actual eye velocity. We examined whether FEF pursuit neurons signaled the actual eye velocity during pursuit in juvenile monkeys that exhibited intrinsic differences between upward and downward pursuit capabilities. Two, head-stabilized Japanese monkeys of 4 years of age were tested for sinusoidal vertical pursuit of target motion at 0.2-1.2 Hz (+/-10 degrees, peak target velocity 12.5-75.0 degrees/s). Gains of downward pursuit were 0.8-0.9 at 0.2-1.0 Hz, and peak downward eye velocity increased up to approximately 60 degrees/s linearly with target velocity, whereas peak upward eye velocity saturated at 15-20 degrees/s. The majority of downward FEF pursuit neurons increased the amplitude of their discharge modulation almost linearly up to 1.2 Hz. The majority of upward FEF pursuit neurons also increased amplitude of modulation nearly linearly as target frequency increased, and the regression slope was similar to that of downward pursuit neurons despite the fact that upward peak eye velocity saturated at approximately 0.5 Hz. These results indicate that the responses of the majority of upward FEF pursuit neurons did not signal the actual eye velocity during pursuit. We suggest that their activity reflected primarily the required eye velocity.

Directional asymmetry in vertical smooth-pursuit and cancellation of the vertical vestibulo-ocular reflex in juvenile monkeys

Young primates exhibit asymmetric eye movements during vertical smooth-pursuit across a textured background such that upward pursuit has low velocity and requires many catch-up saccades. The asymmetric eye movements cannot be explained by the un-suppressed optokinetic reflex resulting from background visual motion across the retina during pursuit, suggesting that the asymmetry reflects most probably, a low gain in upward eye commands (Kasahara et al. in Exp Brain Res 171:306-321, 2006). In this study, we examined (1) whether there are intrinsic differences in the upward and downward pursuit capabilities and (2) how the difficulty in upward pursuit is correlated with the ability of vertical VOR cancellation. Three juvenile macaques that had initially been trained only for horizontal (but not vertical) pursuit were trained for sinusoidal pursuit in the absence of a textured background. In 2 of the 3 macaques, there was a clear asymmetry between upward and downward pursuit gains and in the time course of initial gain increase. In the third macaque, downward pursuit gain was also low. It did not show consistent asymmetry during the initial 2 weeks of training. However, it also exhibited a significant asymmetry after 4 months of training, similar to the other two monkeys. After 6 months of training, these two monkeys (but not the third) still exhibited asymmetry. As target frequency increased in these two monkeys, mean upward eye velocity saturated at approximately 15 degrees /s, whereas horizontal and downward eye velocity increased up to approximately 40 degrees /s. During cancellation of the VOR induced by upward whole body rotation, downward eye velocity of the residual VOR increased as the stimulus frequency increased. Gain of the residual VOR during upward rotation was significantly higher than that during horizontal and downward rotation. The time course of residual VOR induced by vertical whole body step-rotation during VOR cancellation was predicted by addition of eye velocity during pursuit and VOR x1. These results support our view that the directional asymmetry reflects the difference in the organization of the cerebellar floccular region for upward and downward directions and the preeminent role of pursuit in VOR cancellation.

Developmental asymmetries between horizontal and vertical tracking

The development of the asymmetry between horizontal and vertical eye tracking was investigated longitudinally at 5, 7, and 9 months of age. The target moved either on a 2D circular trajectory or on a vertical or horizontal 1D sinusoidal trajectory. Saccades, smooth pursuit, and head movements were measured. Vertical tracking was found to be inferior to horizontal tracking at all age levels. The results also show that the mechanisms responsible for horizontal and vertical tracking mutually influence one another in the production of 2D visual pursuit. Learning effects were observed within-trials but no transfer between trials was found.

Role of Vestibular Signals in the Caudal Part of the Frontal Eye Fields in Pursuit Eye Movements in Three-Dimensional Space

For accurate visual information about objects of interest moving slowly in three-dimensional (3D) space, primates with binocular fields use both frontal smooth-pursuit (frontal-pursuit) and vergence eye movements (i.e., depth pursuit) to maintain the images of the objects precisely on the foveae of left and right eyes. Moreover, during head or whole-body movement, both frontal- and depth-pursuit systems must interact with the vestibular system to minimize slip of the retinal images that degrades image quality considerably. The caudal part of the frontal eye fields (FEF) contains many frontal-pursuit neurons. Previous studies have shown that a majority of pursuit neurons there discharge for both frontal pursuit and vergence and carry pursuit-in-3D signals. To understand how vestibular inputs interact with pursuit-in-3D signals, three different experiments that examined the nature of vestibular signals in the caudal FEF are described in this review. A majority of caudal FEF pursuit neurons responded to whole-body rotation with preferred directions similar to frontal-pursuit directions and carried frontal gaze (eye-in-space) velocity signals. They were activated in association with adaptive pursuit eye movements induced by cross-axis pursuit-vestibular interactions. During fore/aft and right/left translation in complete darkness, they were also modulated with preferred directions of many neurons similar to pursuit-preferred directions. Previous studies showed that caudal FEF pursuit neurons also receive visual signals about target motion. Taken together, these results suggest that the caudal FEF coordinates its various inputs to provide signals for accurate eye-movement-in-space commands.

Further evidence for selective difficulty of upward eye pursuit in juvenile monkeys: Effects of optokinetic stimulation, static roll tilt, and active head movements

The smooth-pursuit system moves the eyes in space accurately to track slowly moving objects of interest despite visual inputs from the moving background and/or vestibular inputs during head movements. Recently, our laboratory has shown that young primates exhibit asymmetric eye movements during vertical pursuit across a textured background; upward eye velocity gain is reduced. To further understand the nature of this asymmetry, we performed three series of experiments in young monkeys. In Experiment 1, we examined whether this asymmetry was due to an un-compensated downward optokinetic reflex induced by the textured background as it moves across the retina in the opposite direction of the pursuit eye movements. For this, we examined the monkeys' ability to fixate a stationary spot in space during movement of the textured background and compared it with vertical pursuit across the stationary textured background. We also examined gains of optokinetic eye movements induced by downward motion of the textured background during upward pursuit. In both task conditions, gains of downward eye velocity induced by the textured background were too small to explain reduced upward eye velocity gains. In Experiment 2, we examined whether the frame of reference for low-velocity, upward pursuit was orbital or earth vertical. To test this, we first applied static tilt in the roll plane until the animals were nearly positioned on their side in order to dissociate vertical or horizontal eye movements in the orbit from those in space. Deficits were observed for upward pursuit in the orbit but not in space. In Experiment 3, we tested whether asymmetry was observed during head-free pursuit that requires coordination between eye and head movements. Asymmetry in vertical eye velocity gains was still observed during head-free pursuit although it was not observed in vertical head velocity. These results, taken together, suggest that the asymmetric eye movements during vertical pursuit are specific for upward, primarily eye pursuit in the orbit.

The development of two-dimensional tracking: a longitudinal study of circular pursuit

We investigated 6- to 12-month-old infants' ability to track an object moving on circular trajectories, using a longitudinal design. Consistent predictive gaze tracking was not found before 8 months of age. These results indicate that infants' horizontal and vertical components of circular tracking are less mature than expected from previous studies of one-dimensional horizontal tracking. Vertical components are especially immature, particularly during high velocity tracking (approximately 20 degrees /s). The results also suggest that horizontal and vertical tracking are mutually dependent during early development. Saccades were predictive (average lag >-125 ms) from 6 months onwards.

Structural neural networks subserving oculomotor function in first-episode schizophrenia

BACKGROUND

Smooth pursuit and antisaccade abnormalities are well documented in schizophrenia, but their neuropathological correlates remain unclear.

METHODS

In this study, we used statistical parametric mapping to investigate the relationship between oculomotor abnormalities and brain structure in a sample of first-episode schizophrenia patients (n = 27). In addition to conventional volumetric magnetic resonance imaging, we also used magnetization transfer ratio, a technique that allows more precise tissue characterization.

RESULTS

We found that smooth pursuit abnormalities were associated with reduced magnetization transfer ratio in several regions, predominantly in the right prefrontal cortex. Antisaccade errors correlated with gray matter volume in the right medial superior frontal cortex as measured by conventional magnetic resonance imaging but not with magnetization transfer ratio.

CONCLUSIONS

These preliminary results demonstrate that specific structural abnormalities are associated with abnormal eye movements in schizophrenia.

'Nurturing the brain' as an emerging research field involving child neurology

'Nurturing the brain' is an emerging research field integrating brain science, child care and education, which also involves child neurology. It has emerged from the recent remarkable progress in brain science and strong social demands for improvements in child care and education. This article reviews the current status of three major research themes in this field. First, developmental disorders represented by attention deficit/hyperactivity disorder, autism and Asperger syndrome often introduce difficulties in child care and education, which are to be addressed by appropriate assessment and treatment of affected children based on new knowledge of the pathogenesis of these disorders. Second, recent progress in research on the critical/sensitive periods of development of brain structures and functions promises useful advice for teachers and parents regarding optimal timing and ways of teaching various subjects. Third, the development of the brain throughout infancy, childhood and adolescence is paralleled by the growth and maturation of the mind. Neuronal mechanisms underlying the theory of mind, mirror neurons, internal model, cognitive control, and cognitive emotion regulation are important themes that bridge our understandings of the brain and the mind.

Pursuit-Related Neurons in the Supplementary Eye Fields: Discharge During Pursuit and Passive Whole Body Rotation

The primate frontal cortex contains two areas related to smooth-pursuit: the frontal eye fields (FEFs) and supplementary eye fields (SEFs). To distinguish the specific role of the SEFs in pursuit, we examined discharge of a total of 89 pursuit-related neurons that showed consistent modulation when head-stabilized Japanese monkeys pursued a spot moving sinusoidally in fronto-parallel planes and/or in depth and with or without passive whole body rotation. During smooth-pursuit at different frequencies, 43% of the neurons tested (17/40) exhibited discharge amplitude of modulation linearly correlated with eye velocity. During cancellation of the vestibulo-ocular reflex and/or chair rotation in complete darkness, the majority of neurons tested (91% = 30/33) responded. However, only 17% of the responding neurons (4/30) were modulated in proportion to gaze (eye-in-space) velocity during pursuit-vestibular interactions. When the monkeys fixated a stationary spot, 20% of neurons tested (7/34) responded to motion of a second spot. Among the neurons tested for both smooth-pursuit and vergence tracking ( n = 56), 27% (15/56) discharged during both, 62% (35/56) responded during smooth-pursuit only, and 11% (6/56) during vergence tracking only. Phase shifts (relative to stimulus velocity) of responding neurons during pursuit in frontal and depth planes and during chair rotation remained virtually constant (≤1 Hz). These results, together with the robust vestibular-related discharge of most SEF neurons, show that the discharge of the majority of SEF pursuit-related neurons is quite distinct from that of caudal FEF neurons in identical task conditions, suggesting that the two areas are involved in different aspects of pursuit-vestibular interactions including predictive pursuit.