Strategies of eye-head coordination

Short-Latency Covert Saccades - The Explanation for Good Dynamic Visual Performance After Unilateral Vestibular Loss?

Background: Functional head impulse test (fHIT) tests the ability of the vestibulo-ocular reflex (VOR) to allow visual perception during head movements. Our previous study showed that active head movements to the side with a vestibular lesion generated a dynamic visual performance that were as good as during movements to the intact side. Objective: To examine the differences in eye position during the head impulse test when performed with active and passive head movements, in order to better understand the role of the different saccade properties in improving visual performance. Method: We recruited 8 subjects with complete unilateral vestibular loss (4 men and 4 women, mean age 47 years) and tested them with video Head Impulse Test (vHIT) and Functional Head Impulse Test (fHIT) during passive and active movements while looking at a target. We assessed the mean absolute position error of the eye during different time frames of the head movement, the peak latency and the peak velocity of the first saccade, as well as the visual performance during the head movement. Results: Active head impulses to the lesioned side generated dynamic visual performances that were as good as when testing the intact side. Active head impulses resulted in smaller position errors during the visual perception task (p = 0.006) compared to passive head-impulses and the position error during the visual perception time frame correlated with shorter latencies of the first saccade (p < 0.001). Conclusion: Actively generated head impulses toward the side with a complete vestibular loss resulted in a position error within or close to the margin necessary to obtain visual perception for a brief period of time in patients with chronic unilateral vestibular loss. This seems to be attributed to the appearance of short-latency covert saccades, which position the eyes in a more favorable position during head movements.

Hierarchical control of two-dimensional gaze saccades

Coordinating the movements of different body parts is a challenging process for the central nervous system because of several problems. Four of these main difficulties are: first, moving one part can move others; second, the parts can have different dynamics; third, some parts can have different motor goals; and fourth, some parts may be perturbed by outside forces. Here, we propose a novel approach for the control of linked systems with feedback loops for each part. The proximal parts have separate goals, but critically the most distal part has only the common goal. We apply this new control policy to eye-head coordination in two-dimensions, specifically head-unrestrained gaze saccades. Paradoxically, the hierarchical structure has controllers for the gaze and the head, but not for the eye (the most distal part). Our simulations demonstrate that the proposed control structure reproduces much of the published empirical data about gaze movements, e.g., it compensates for perturbations, accurately reaches goals for gaze and head from arbitrary initial positions, simulates the nine relationships of the head-unrestrained main sequence, and reproduces observations from lesion and single-unit recording experiments. We conclude by showing how our model can be easily extended to control structures with more linked segments, such as the control of coordinated eye on head on trunk movements.

Neural mechanisms for predictive head movement strategies during sequential gaze shifts

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

Optimal control of gaze shifts

To explore the visible world, human beings and other primates often rely on gaze shifts. These are coordinated movements of the eyes and head characterized by stereotypical metrics and kinematics. It is possible to determine the rules that the effectors must obey to execute them rapidly and accurately and the neural commands needed to implement these rules with the help of optimal control theory. In this study, we demonstrate that head-fixed saccades and head-free gaze shifts obey a simple physical principle, "the minimum effort rule." By direct comparison with existing models of the neural control of gaze shifts, we conclude that the neural circuitry that implements the minimum effort rule is one that uses inhibitory cross talk between independent eye and head controllers.

Head control strategies during whole-body turns

Recent reports have described the motion of individual body segments during turns toward visual targets. During a whole-body turn, the head's trajectory in space is determined by both body-in-space rotation and head-on-body rotation. To inform subsequent investigations of head control strategies during turns, we provide a kinematic description of head and pelvis rotation in 20 healthy human subjects as they performed step turns toward a visible target or in the direction of a previously seen target. At the time of peak head velocity in space, the head was moving faster than the pelvis by approximately 54 degrees s(-1) when turning toward a visible target, and approximately 25 degrees s(-1) when turning in a remembered direction. Peak head velocities were slower in the absence of a visual target, but pelvis velocities were not significantly different. The pattern of relative motion between the head and pelvis followed a temporal sequence. Early in the turn, the head rotated with respect to the pelvis in the same direction as the pelvis was rotating in space. During the mid portions of the turn, en bloc rotation of the head and pelvis predominated. Later in the turn, head-in-space velocity was lower than pelvis-in-space velocity, and was thus relatively stabilized. This pattern of head movement during turns is quite similar to eye-in-head movements during large eye-head gaze shifts. This suggests that in addition to saccadic and stabilization mechanisms, a specific control strategy to move segments together should be incorporated into models of gaze reorienting behavior.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.

Optimal contributions of head and eye positions to spatial accuracy in man tested by visually directed pointing

Encoding of visual target location in extrapersonal space requires convergence of at least three types of information: retinal signals, information about orbital eye positions, and the position of the head on the body. Since the position of gaze is the sum of the head position and the eye position, inaccuracy of spatial localization of the target may result from the sum of the corresponding three levels of errors: retina, ocular and head. In order to evaluate the possible errors evoked at each level, accuracy of target encoding was assessed through a motor response requiring subjects to point with the hand towards a target seen under foveal vision, eliminating the retinal source of error. Subjects had first to orient their head to one of three positions to the right (0, 40, 80 degrees) and maintain this head position while orienting gaze and pointing to one of five target positions (0, 20, 40, 60, 80 degrees). This resulted in 11 combinations of static head and eye positions, and corresponded to five different gaze eccentricities. The accuracy of target pointing was tested without vision of the moving hand. Six subjects were tested. No systematic bias in finger pointing was observed for eye positions ranging from 0 to 40 degrees to the right or left within the orbit. However, the variability (as measured by a surface error) given by the scatter of hand pointing increased quadratically with eye eccentricity. A similar observation was made with the eye centered and the head position ranging from 0 to 80 degrees, although the surface error increased less steeply with eccentricity. Some interaction between eye and head eccentricity also contributed to the pointing error. These results suggest that pointing should be most accurate with a head displacement corresponding to 90% of the gaze eccentricity. These results explain the systematic hypometry of head orienting towards targets observed under natural conditions: thus the respective contribution of head and eye to gaze orientation might be determined in order to optimize accuracy of target encoding.

Reticulo-spinal neurons participating in the control of synergic eye and head movements during orienting in the cat. I. Behavioral properties

The activity of 24 reticulo-spinal neurons (RSN) identified by antidromic stimulation at the C1-C2 level has been recorded intra-axonally in the pons of alert head-fixed cats during spontaneous gaze shifts and orienting towards novel targets. Relationship of neuronal discharge to saccadic eye movements, positions of fixation and EMG of dorsal neck muscles were analysed. The present report describes behavioral properties of a group of 14 RSN showing similar types of correlations with motor parameters during eye-head synergies. These "eye-neck" RSN (EN-RSN) generate bursts in synchrony with phasic components of ipsilateral neck EMG and leading ipsiversive saccades by a variable lead time. Bursts are followed by a prolonged discharge whose frequency decays even when eccentric eye position is maintained constant and accompanied by sustained neck muscle activity. The firing rate of EN-RSNs depends on eye position: they are silent with saccades in their ON-direction when the eyes are deviated towards the contralateral half of the oculomotor range and the ipsilateral neck muscles are relaxed. When the eyes cross the vertical meridian, the frequency of phasic and tonic components related to eye-head synergies increase proportionally to ipsilateral eye position. Ten of the 14 EN-RSNs, located in the pontine reticular formation, received monosynaptic input from the contralateral superior colliculus. Two were labeled by intra-axonal injection of HRP which revealed extensive branching in the abducens, facial, medial and lateral vestibular, prepositus and intercalatus nuclei and in the caudal pontine and bulbar reticular formation. It is concluded that the caudal pontine tegmentum, including the region just anterior to the abducens nucleus, contains RSNs whose signals seem appropriate to control phasic neck muscle activity and which also project to structures related to ocular and facial movements. Comparisons with the perisaccadic activity of tectal neurons projecting in the predorsal bundle reveals a profound transformation of the descending signal at the level of EN-RSNs which represent first order relay neurons of the tecto-reticulo-spinal pathway.