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

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

Improvement of Pursuit Eye Movement Alterations after Short Visuo-Attentional Training in ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder without validated and objective diagnostic procedures. Several neurological dysfunctions in the frontal circuit, in the thalamus, and in the cerebellum have been observed in subjects with ADHD. These cortical and subcortical areas are responsible for eye movement control. Therefore, studying eye movements could be a useful tool to better understand neuronal alterations in subjects with ADHD. The aim of the present study was firstly to compare the quality of pursuit eye movements in a group of 40 children with ADHD (age 8.2 ± 1.2) and in a group of 40 sex-, IQ-, age-matched typically developing (TD) children; secondly, we aimed to examine if a short visuo-attentional training could affect pursuit performances in children with ADHD. Findings showed that children with ADHD presented a greater number of catch-up saccade and lower pursuit gain compared to TD children. Differently to TD children, in children with ADHD, the number of catch-up saccades and the pursuit gain were not significantly correlated with children's age. Furthermore, a short visuo-attentional training period can only slightly improve pursuit performance in children with ADHD, leading to a decrease of the occurrence of catch-up saccades only, albeit the effect size was small. The absence of any improvement in pursuit performance with age could be explained by the fact that the prefrontal and fronto-cerebellar circuits responsible for pursuit triggering are still immature. Pursuit eye movements can be used as a useful tool for ADHD diagnosis. However, attentional mechanisms controlled by these cortical structures could be improved by a short visuo-attentional training period. Further studies will be necessary to explore the effects of a longer visuo-attentional training period on oculomotor tasks in order to clarify how adaptive mechanisms are able to increase the attentional capabilities in children with ADHD.

Threat effects on human oculo-motor function

Neuro-anatomical evidence supports the potential for threat-related factors, such as fear, anxiety and vigilance, to influence brainstem motor nuclei controlling eye movements, as well as the vestibular nuclei. However, little is known about how threat influences human ocular responses, such as eye saccades (ES), smooth pursuit eye tracking (SP), and optokinetic nystagmus (OKN), and whether these responses can be facilitated above normal baseline levels with a natural source of threat. This study was designed to examine the effects of height-induced postural threat on the gain of ES, SP and OKN responses in humans. Twenty participants stood at two different surface heights while performing ES (ranging from 8° to 45° from center), SP (15, 20, 30°/s) and OKN (15, 30, 60°/s) responses in the horizontal plane. Height did not significantly increase the slope of the relationship between ES peak velocity and initial amplitude, or the gain of ES amplitude. In contrast height significantly increased SP and OKN gain. Significant correlations were found between changes in physiological arousal and OKN gain. Observations of changes with height in OKN and SP support neuro-anatomical evidence of threat-related mechanisms influencing both oculo-motor nuclei and vestibular reflex pathways. Although further study is warranted, the findings suggest that potential influences of fear, anxiety and arousal/alertness should be accounted for, or controlled, during clinical vestibular and oculo-motor testing.

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.

Multisensory Convergence of Visual and Vestibular Heading Cues in the Pursuit Area of the Frontal Eye Field

Both visual and vestibular sensory cues are important for perceiving one's direction of heading during self-motion. Previous studies have identified multisensory, heading-selective neurons in the dorsal medial superior temporal area (MSTd) and the ventral intraparietal area (VIP). Both MSTd and VIP have strong recurrent connections with the pursuit area of the frontal eye field (FEFsem), but whether FEFsem neurons may contribute to multisensory heading perception remain unknown. We characterized the tuning of macaque FEFsem neurons to visual, vestibular, and multisensory heading stimuli. About two-thirds of FEFsem neurons exhibited significant heading selectivity based on either vestibular or visual stimulation. These multisensory neurons shared many properties, including distributions of tuning strength and heading preferences, with MSTd and VIP neurons. Fisher information analysis also revealed that the average FEFsem neuron was almost as sensitive as MSTd or VIP cells. Visual and vestibular heading preferences in FEFsem tended to be either matched (congruent cells) or discrepant (opposite cells), such that combined stimulation strengthened heading selectivity for congruent cells but weakened heading selectivity for opposite cells. These findings demonstrate that, in addition to oculomotor functions, FEFsem neurons also exhibit properties that may allow them to contribute to a cortical network that processes multisensory heading cues.

[Visual tracking with/without passive whole-body rotation in Parkinson's disease (PD): Dissociation of smooth-pursuit and cancellation of vestibulo-ocular reflex (VOR)]

Although impaired smooth-pursuit in Parkinson's disease (PD) is well known, reports are conflicting on the ability to cancel vestibulo-ocular reflex (VOR) when the target moves with head, requiring gaze-pursuit. To compare visual tracking performance with or without passive whole-body rotation, we examined eye movements of 10 PD patients and 6 age-matched controls during sinusoidal horizontal smooth-pursuit and passive whole-body rotation (0.3 Hz, ± 10°). Three tasks were tested: smooth-pursuit, VOR cancellation, and VORx1 while subjects fixated an earth-stationary spot during whole-body rotation. Mean ± SD eye velocity gains (eye velocities/stimulus velocities) of PD patients during the 3 tasks were 0.32 ± 0.24 0.25 ± 0.22, 0.85 ± 0.20, whereas those of controls were 0.91 ± 0.06, 0.14 ± 0.07, 0.94 ± 0.05, respectively. Difference was significant between the two subject groups only during smooth-pursuit. Plotting eye-velocity gains of individual subjects during VOR cancellation against those during smooth-pursuit revealed significant negative linear correlation between the two parameters in the controls, but no correlation was found in PD patients. Based on the regression equation of the controls, we estimated expected eye velocity gains of individual subjects during VOR cancellation from their smooth-pursuit gains. Estimated gains of PD patients during VOR cancellation were significantly different from their actual gains, suggesting that different neural mechanisms operate during VOR cancellation in the controls and PD.

Plasticity of cerebellar Purkinje cells in behavioral training of body balance control

Neural responses to sensory inputs caused by self-generated movements (reafference) and external passive stimulation (exafference) differ in various brain regions. The ability to differentiate such sensory information can lead to movement execution with better accuracy. However, how sensory responses are adjusted in regard to this distinguishability during motor learning is still poorly understood. The cerebellum has been hypothesized to analyze the functional significance of sensory information during motor learning, and is thought to be a key region of reafference computation in the vestibular system. In this study, we investigated Purkinje cell (PC) spike trains as cerebellar cortical output when rats learned to balance on a suspended dowel. Rats progressively reduced the amplitude of body swing and made fewer foot slips during a 5-min balancing task. Both PC simple (SSs; 17 of 26) and complex spikes (CSs; 7 of 12) were found to code initially on the angle of the heads with respect to a fixed reference. Using periods with comparable degrees of movement, we found that such SS coding of information in most PCs (10 of 17) decreased rapidly during balance learning. In response to unexpected perturbations and under anesthesia, SS coding capability of these PCs recovered. By plotting SS and CS firing frequencies over 15-s time windows in double-logarithmic plots, a negative correlation between SS and CS was found in awake, but not anesthetized, rats. PCs with prominent SS coding attenuation during motor learning showed weaker SS-CS correlation. Hence, we demonstrate that neural plasticity for filtering out sensory reafference from active motion occurs in the cerebellar cortex in rats during balance learning. SS-CS interaction may contribute to this rapid plasticity as a form of receptive field plasticity in the cerebellar cortex between two receptive maps of sensory inputs from the external world and of efference copies from the will center for volitional movements.

Impaired smooth-pursuit in Parkinson's disease: normal cue-information memory, but dysfunction of extra-retinal mechanisms for pursuit preparation and execution

While retinal image motion is the primary input for smooth-pursuit, its efficiency depends on cognitive processes including prediction. Reports are conflicting on impaired prediction during pursuit in Parkinson's disease. By separating two major components of prediction (image motion direction memory and movement preparation) using a memory-based pursuit task, and by comparing tracking eye movements with those during a simple ramp-pursuit task that did not require visual memory, we examined smooth-pursuit in 25 patients with Parkinson's disease and compared the results with 14 age-matched controls. In the memory-based pursuit task, cue 1 indicated visual motion direction, whereas cue 2 instructed the subjects to prepare to pursue or not to pursue. Based on the cue-information memory, subjects were asked to pursue the correct spot from two oppositely moving spots or not to pursue. In 24/25 patients, the cue-information memory was normal, but movement preparation and execution were impaired. Specifically, unlike controls, most of the patients (18/24 = 75%) lacked initial pursuit during the memory task and started tracking the correct spot by saccades. Conversely, during simple ramp-pursuit, most patients (83%) exhibited initial pursuit. Popping-out of the correct spot motion during memory-based pursuit was ineffective for enhancing initial pursuit. The results were similar irrespective of levodopa/dopamine agonist medication. Our results indicate that the extra-retinal mechanisms of most patients are dysfunctional in initiating memory-based (not simple ramp) pursuit. A dysfunctional pursuit loop between frontal eye fields (FEF) and basal ganglia may contribute to the impairment of extra-retinal mechanisms, resulting in deficient pursuit commands from the FEF to brainstem.

No-go neurons in the cerebellar oculomotor vermis and caudal fastigial nuclei: planning tracking eye movements

The cerebellar dorsal vermis lobules VI-VII (oculomotor vermis) and its output region (caudal fastigial nuclei, cFN) are involved in tracking eye movements consisting of both smooth-pursuit and saccades, yet, the exact role of these regions in the control of tracking eye movements is still unclear. We compared the neuronal discharge of these cerebellar regions using a memory-based, smooth-pursuit task that distinguishes discharge related to movement preparation and execution from the discharge related to the processing of visual motion signals or their memory. Monkeys were required to pursue (i.e., go), or not pursue (i.e., no-go) in a cued direction, based on the memory of visual motion direction and go/no-go instructions. Most (>60 %) of task-related vermal Purkinje cells (P-cells) and cFN neurons discharged specifically during the memory period following no-go instructions; their discharge was correlated with memory of no-go instructions but was unrelated to eye movements per se during the action period of go trials. The latencies of no-go discharge of vermal P-cells and cFN neurons were similar, but were significantly longer than those of supplementary eye field (SEF) no-go neurons during an identical task. Movement-preparation signals were found in ~30 % of smooth-pursuit-related neurons in these cerebellar regions and some of them also carried visual memory signals. Our results suggest that no-go neurons are a newly revealed class of neurons, detected using the memory-based pursuit task, in the oculomotor vermis-cFN pathway and that this pathway contributes specifically to planning requiring the working memory of no-go instructions and preparation of tracking eye movements.

Cue-dependent memory-based smooth-pursuit in normal human subjects: importance of extra-retinal mechanisms for initial pursuit

Using a cue-dependent memory-based smooth-pursuit task previously applied to monkeys, we examined the effects of visual motion-memory on smooth-pursuit eye movements in normal human subjects and compared the results with those of the trained monkeys. These results were also compared with those during simple ramp-pursuit that did not require visual motion-memory. During memory-based pursuit, all subjects exhibited virtually no errors in either pursuit-direction or go/no-go selection. Tracking eye movements of humans and monkeys were similar in the two tasks, but tracking eye movements were different between the two tasks; latencies of the pursuit and corrective saccades were prolonged, initial pursuit eye velocity and acceleration were lower, peak velocities were lower, and time to reach peak velocities lengthened during memory-based pursuit. These characteristics were similar to anticipatory pursuit initiated by extra-retinal components during the initial extinction task of Barnes and Collins (J Neurophysiol 100:1135-1146, 2008b). We suggest that the differences between the two tasks reflect differences between the contribution of extra-retinal and retinal components. This interpretation is supported by two further studies: (1) during popping out of the correct spot to enhance retinal image-motion inputs during memory-based pursuit, pursuit eye velocities approached those during simple ramp-pursuit, and (2) during initial blanking of spot motion during memory-based pursuit, pursuit components appeared in the correct direction. Our results showed the importance of extra-retinal mechanisms for initial pursuit during memory-based pursuit, which include priming effects and extra-retinal drive components. Comparison with monkey studies on neuronal responses and model analysis suggested possible pathways for the extra-retinal mechanisms.

Cognitive processes involved in smooth pursuit eye movements: behavioral evidence, neural substrate and clinical correlation

Smooth-pursuit eye movements allow primates to track moving objects. Efficient pursuit requires appropriate target selection and predictive compensation for inherent processing delays. Prediction depends on expectation of future object motion, storage of motion information and use of extra-retinal mechanisms in addition to visual feedback. We present behavioral evidence of how cognitive processes are involved in predictive pursuit in normal humans and then describe neuronal responses in monkeys and behavioral responses in patients using a new technique to test these cognitive controls. The new technique examines the neural substrate of working memory and movement preparation for predictive pursuit by using a memory-based task in macaque monkeys trained to pursue (go) or not pursue (no-go) according to a go/no-go cue, in a direction based on memory of a previously presented visual motion display. Single-unit task-related neuronal activity was examined in medial superior temporal cortex (MST), supplementary eye fields (SEF), caudal frontal eye fields (FEF), cerebellar dorsal vermis lobules VI–VII, caudal fastigial nuclei (cFN), and floccular region. Neuronal activity reflecting working memory of visual motion direction and go/no-go selection was found predominantly in SEF, cerebellar dorsal vermis and cFN, whereas movement preparation related signals were found predominantly in caudal FEF and the same cerebellar areas. Chemical inactivation produced effects consistent with differences in signals represented in each area. When applied to patients with Parkinson's disease (PD), the task revealed deficits in movement preparation but not working memory. In contrast, patients with frontal cortical or cerebellar dysfunction had high error rates, suggesting impaired working memory. We show how neuronal activity may be explained by models of retinal and extra-retinal interaction in target selection and predictive control and thus aid understanding of underlying pathophysiology.

Vestibular responses in the macaque pedunculopontine nucleus and central mesencephalic reticular formation

The pedunculopontine nucleus (PPN) and central mesencephalic reticular formation (cMRF) both send projections and receive input from areas with known vestibular responses. Noting their connections with the basal ganglia, the locomotor disturbances that occur following lesions of the PPN or cMRF, and the encouraging results of PPN deep brain stimulation in Parkinson's disease patients, both the PPN and cMRF have been linked to motor control. In order to determine the existence of and characterize vestibular responses in the PPN and cMRF, we recorded single neurons from both structures during vertical and horizontal rotation, translation, and visual pursuit stimuli. The majority of PPN cells (72.5%) were vestibular-only (VO) cells that responded exclusively to rotation and translation stimuli but not visual pursuit. Visual pursuit responses were much more prevalent in the cMRF (57.1%) though close to half of cMRF cells were VO cells (41.1%). Directional preferences also differed between the PPN, which was preferentially modulated during nose-down pitch, and cMRF, which was preferentially modulated during ipsilateral yaw rotation. Finally, amplitude responses were similar between the PPN and cMRF during rotation and pursuit stimuli, but PPN responses to translation were of higher amplitude than cMRF responses. Taken together with their connections to the vestibular circuit, these results implicate the PPN and cMRF in the processing of vestibular stimuli and suggest important roles for both in responding to motion perturbations like falls and turns.