Disconjugate adaptation of the vertical oculomotor system

A 1-Diopter Vertical Prism Induces a Decrease of Head Rotation: A Pilot Investigation

Clinical studies in non-specific chronic arthralgia and back pain seem to indicate an association between vertical heterophoria (VH - latent vertical retinal misalignment) and asymmetrical head rotation. Such clinical observations suggest a link between VH and head rotation, but this was never tested. The purpose of this study was to simulate a VH in healthy subjects and examine its influence on the amplitude of active head rotation during 3D motion capture in upright stance. Subjects were asked to rotate their head three times from the straight ahead position and then to the right, back to straight ahead, to the left, and back to the straight ahead again. Three randomized conditions were run: normal viewing, with a 1-diopter prism base down on the dominant eye, or the non-dominant eye. The most important finding is that the experimental VH whichever the eye with the prism induces a significant decrease in the mean angle of head rotation compared to the normal viewing condition. This decrease was significant for rotation to the left. We suggest that the prism-induced VH modifies the reference posture and thereby affects head rotation; further studies are needed to confirm this effect and to extend to other types of dynamic activities.

A bi-hemispheric neuronal network model of the cerebellum with spontaneous climbing fiber firing produces asymmetrical motor learning during robot control

To acquire and maintain precise movement controls over a lifespan, changes in the physical and physiological characteristics of muscles must be compensated for adaptively. The cerebellum plays a crucial role in such adaptation. Changes in muscle characteristics are not always symmetrical. For example, it is unlikely that muscles that bend and straighten a joint will change to the same degree. Thus, different (i.e., asymmetrical) adaptation is required for bending and straightening motions. To date, little is known about the role of the cerebellum in asymmetrical adaptation. Here, we investigate the cerebellar mechanisms required for asymmetrical adaptation using a bi-hemispheric cerebellar neuronal network model (biCNN). The bi-hemispheric structure is inspired by the observation that lesioning one hemisphere reduces motor performance asymmetrically. The biCNN model was constructed to run in real-time and used to control an unstable two-wheeled balancing robot. The load of the robot and its environment were modified to create asymmetrical perturbations. Plasticity at parallel fiber-Purkinje cell synapses in the biCNN model was driven by error signal in the climbing fiber (cf) input. This cf input was configured to increase and decrease its firing rate from its spontaneous firing rate (approximately 1 Hz) with sensory errors in the preferred and non-preferred direction of each hemisphere, as demonstrated in the monkey cerebellum. Our results showed that asymmetrical conditions were successfully handled by the biCNN model, in contrast to a single hemisphere model or a classical non-adaptive proportional and derivative controller. Further, the spontaneous activity of the cf, while relatively small, was critical for balancing the contribution of each cerebellar hemisphere to the overall motor command sent to the robot. Eliminating the spontaneous activity compromised the asymmetrical learning capabilities of the biCNN model. Thus, we conclude that a bi-hemispheric structure and adequate spontaneous activity of cf inputs are critical for cerebellar asymmetrical motor learning.

Effects of ocular dominance on the vertical vergence induced by a 2-diopter vertical prism during standing

This study examined the eye movement responses to vertical disparity induced by a 2-diopter vertical prism base down while in standing position. Vertical vergence movements are known to be small requiring accurate measurement with the head stabilized, and was done with the EyeLink 2. The 2-diopter vertical prism, base down, was inserted in front of either the non-dominant eye (NDE) or dominant eye (DE) at 40 and 200 cm. The results showed that vertical vergence was stronger and excessive relative to the required value (i.e. 1.14 degrees ) when the prism was on the NDE for both distances, but more appropriate when the prism was on the DE. The results suggest that sensory disparity process and vertical vergence responses are modulated by eye dominance.

Effects of a two-diopter vertical prism on posture

Postural control in upright stance requires the central integration of visual, vestibular, somatosensory (as cutaneous receptors) and proprioceptive (as joint receptors) inputs. Clinical studies seem to indicate an association between vertical heterophoria (VH) and balance control. The purpose of the study was to simulate a VH and examine its influence on body stabilisation in quiet stance. We studied 15 healthy subjects (25.6+/-3.0 years). The postural stability was measured with a platform under the following conditions: normal viewing, with a two-diopter prism base down placed on the dominant eye (PDE) or the non-dominant eye (PNDE). Both eyes were open in all conditions. All conditions were run at two distances: 200 and 40 cm. The results showed: (i) PNDE increased the antero-posterior body sway for both distances; this result could be related to sensory processing of disparity and/or to inappropriate eye movement response to the disparity induced by the prism; (ii) PDE improved the postural stability only at far distance (reduction of the center-of-pressure excursion area and of the lateral body sway). Such positive result could be due to appropriate sensory processing of disparity and/or eye movement response; the latter would reduce vertical disparity and modify the dynamic and tonic eye muscle activity relative to normal viewing at far distance. We conclude that: (i) experimentally induced vertical phoria does indeed influence postural control; (ii) vertical prisms, even of small power, can have complex effects, positive or negative, depending on the eye wearing it and at the distance fixated.

The coordination of binocular eye movements: vertical and torsional alignment

Precise binocular alignment of the visual axes is of utmost importance for good vision. The fact that so few of us ever experience diplopia is evidence of how well the oculomotor system performs this function in the face of changes due to development, disease and injury. The capacity of the oculomotor system to adapt to visual stimuli that mimic alignment deficits has been extensively explored in laboratory experiments. While the present paper reviews many of those studies, the primary focus is on issues involved in maintaining good vertical and torsional alignment in everyday viewing situations where the parsing of muscle forces may vary for the same horizontal and vertical eye positions due to changes in horizontal vergence and head posture.

Disconjugate vertical memory-guided saccades to disparate targets

We studied the binocular coordination of normal memory-guided saccades and the possibility of inducing memory-based disconjugate learning. First, we report that normal vertical memory-guided saccades to non-disparate targets are yoked vertically in the two eyes as well as visually guided vertical saccades. To induce disconjugate vertical learning, at each trial we flashed a target that was disparate (i.e. 8% more elevated or more depressed for one eye); the memory delay was 1 s. Memory-guided vertical saccades developed a vertical disconjugacy that was appropriate for the disparity of the remembered target. After 15 min of repetition, this vertical disconjugacy persisted even when the target to be remembered was no longer disparate; this indicates disconjugate vertical learning based on short-term memory. However, this was observed only for a few individual cases and its amplitude was small. This contrasts with prior findings on horizontal saccades associated with horizontal disparities. We conclude that vertical memory-based disconjugate learning is possible but very limited. Together with other studies, this study suggests that the natural vertical conjugacy of vertical saccades relies little on rapid learning mechanisms. Rather it seems to be built-in, and this is consistent with earlier electrophysiological findings.

Proprioceptive and retinal afference modify postsaccadic ocular drift

Drift of the eyes after saccades produces motion of images on the retina (retinal slip) that degrades visual acuity. In this study, we examined the contributions of proprioceptive and retinal afference to the suppression of postsaccadic drift induced by a unilateral ocular muscle paresis. Eye movements were recorded in three rhesus monkeys with a unilateral weakness of one vertical extraocular muscle before and after proprioceptive deafferentation of the paretic eye. Postsaccadic drift was examined in four visual states: monocular viewing with the normal eye (4-wk period); binocular viewing (2-wk period); binocular viewing with a disparity-reducing prism (2-wk period); and monocular viewing with the paretic eye (2-wk period). The muscle paresis produced vertical postsaccadic drift in the paretic eye, and this drift was suppressed in the binocular viewing condition even when the animals could not fuse. When the animals viewed binocularly with a disparity-reducing prism, the drift in the paretic eye was suppressed in two monkeys (with superior oblique pareses) but generally was enhanced in one animal (with a tenotomy of the inferior rectus). When drift movements were enhanced, they reduced the retinal disparity that was present at the end of the saccade. In the paretic-eye-viewing condition, postsaccadic drift was suppressed in the paretic eye and was induced in the normal eye. After deafferentation in the normal-eye-viewing state, there was a change in the vertical postsaccadic drift of the paretic eye. This change in drift was idiosyncratic and variably affected the amplitude and velocity of the postsaccadic drift movements of the paretic eye. Deafferentation of the paretic eye did not affect the postsaccadic drift of the normal eye nor did it impair visually mediated adaptation of postsaccadic drift. The results demonstrate several new findings concerning the roles of visual and proprioceptive afference in the control of postsaccadic drift: disconjugate adaptation of postsaccadic drift does not require binocular fusion; slow, postsaccadic drift movements that reduce retinal disparity but concurrently increase retinal slip can be induced in the binocular viewing state; postsaccadic drift is modified by proprioception from the extraocular muscles, but these modifications do not serve to minimize retinal slip or to correct errors in saccade amplitude; and visually mediated adaptation of postsaccadic drift does not require proprioceptive afference from the paretic eye.