Dynamic and static violations of Hering's law of equal innervation

Simultaneous recordings of ocular microtremor and microsaccades with a piezoelectric sensor and a video-oculography system

Our eyes are in continuous motion. Even when we attempt to fix our gaze, we produce so called "fixational eye movements", which include microsaccades, drift, and ocular microtremor (OMT). Microsaccades, the largest and fastest type of fixational eye movement, shift the retinal image from several dozen to several hundred photoreceptors and have equivalent physical characteristics to saccades, only on a smaller scale (Martinez-Conde, Otero-Millan & Macknik, 2013). OMT occurs simultaneously with drift and is the smallest of the fixational eye movements (∼1 photoreceptor width, >0.5 arcmin), with dominant frequencies ranging from 70 Hz to 103 Hz (Martinez-Conde, Macknik & Hubel, 2004). Due to OMT's small amplitude and high frequency, the most accurate and stringent way to record it is the piezoelectric transduction method. Thus, OMT studies are far rarer than those focusing on microsaccades or drift. Here we conducted simultaneous recordings of OMT and microsaccades with a piezoelectric device and a commercial infrared video tracking system. We set out to determine whether OMT could help to restore perceptually faded targets during attempted fixation, and we also wondered whether the piezoelectric sensor could affect the characteristics of microsaccades. Our results showed that microsaccades, but not OMT, counteracted perceptual fading. We moreover found that the piezoelectric sensor affected microsaccades in a complex way, and that the oculomotor system adjusted to the stress brought on by the sensor by adjusting the magnitudes of microsaccades.

Static and dynamic properties of vergence-induced reduction of ocular counter-roll in near vision

We have examined the characteristics of vergence-induced reduction of ocular counter-roll in near vision. Monkeys were trained to make convergent and divergent refixations with the head and body either upright or in various roll orientations. During near viewing requiring 17 degrees horizontal vergence, we found that static binocular torsion was suppressed by about 68% (averaged over both eyes, two monkeys and both near target locations). This result is in accordance with a previous study in which binocular torsion was quantified based on the displacement planes of eye positions in far and near viewing. Latency and duration of the change in torsional eye position depended (for each eye differently) on body roll and the depth plane of fixation. For instance, during convergent refixations in left-ear-down orientations, the latencies of the left eye were smaller and the durations were longer than those of the right eye. However, both eyes reached their final positions required to fixate the second visual target at roughly the same time. The different dynamics of the two eyes is explained by the fact that each eye rotated temporally when the eyes converged, a pattern named binocular extension of Listing's law. Coming from or aiming at a common torsional value (normal ocular counter-roll) in convergent or divergent refixations, the required torsion differs in the two eyes. The brain compensates for these differences by adjusting the dynamics of each eye's movement.

Neural basis of disjunctive eye movements

New evidence has challenged a widely accepted interpretation of Hering's law of equal innervation, which states that disjunctive saccades are produced by the linear addition of conjugate and vergence innervation commands produced by independent oculomotor subsystems. We hypothesize, instead, that saccades are produced by a monocular premotor control network. A model, based on this hypothesis and consistent with known brain-stem anatomy, simulates realistic disjunctive saccades including initial and late slow vergence movements.

The influence of repetitive eye movements on vergence performance

We measured the peak velocity of convergence eye movement responses in four normal subjects before and after a large number of either repetitive vergence or repetitive saccadic eye movements. A 20% decrease in the mean value of peak velocity was observed in vergence responses after 100 repetitive step vergence eye movements. However, 100 cycles of slow sinusoidal vergence tracking did not induce any notable change in vergence dynamics. Five hundred repetitive saccadic eye movements also caused an approximately 20% decrease in peak velocity. The reduction in peak velocity was related to the number of repetitions for both vergence and saccadic fatiguing stimuli. The frequency of occurrence of double-vergences was also used as an index to monitor the influence of repetitive eye movements on convergence performance. Results showed that repetitive step convergence movements could double, or even triple, the frequency of the occurrence of double-vergence responses, while slow sinusoidal vergence tracking or repetitive saccades had no influence on the frequency of response doubles.

New ideas about binocular coordination of eye movements: is there a chameleon in the primate family tree?

Many animals with laterally placed eyes, such as chameleons, move their eyes independently of one another. In contrast, primates with frontally placed eyes and binocular vision must move them together so that both eyes are aimed at the same point in visual space. Is binocular coordination an innate feature of how our brains are wired, or have we simply learned to move our eyes together? This question sparked a controversy in the 19(th) century between two eminent German scientists, Ewald Hering and Hermann von Helmholtz. Hering took the position that binocular coordination was innate and vigorously challenged von Helmholtz's view that it was learned. Hering won the argument and his hypothesis, known as Hering's Law of Equal Innervation, became generally accepted. New evidence suggests, however, that similar to chameleons, primates may program movements of each eye independently. Binocular coordination is achieved by a neural network at the motor periphery comprised of motoneurons and specialized interneurons located near or in the cranial nerve nuclei that innervate the extraocular muscles. It is assumed that this network must be trained and calibrated during infancy and probably throughout life in order to maintain the precise binocular coordination characteristic of primate eye movements despite growth, aging effects, and injuries to the eye movement neuromuscular system. Malfunction of this network or its ability to adaptively learn may be a contributing cause of strabismus.

Dynamic asymmetries in disparity convergence eye movements

Vergence eye movements have traditionally been considered the product of a single neural control center and are usually studied by combining the movements of each eye into a single 'vergence' response. In the present experiment, disparity-driven eye movements were produced by symmetrical step stimuli, and the dynamic properties of each eye movement were analyzed separately. Although the final positions of the two eyes were symmetrical, large dynamic asymmetries often occurred. The timing between the two eyes showed fair synchrony as they attained maximum velocity at approximately the same time. Since the final static positions were symmetrical, asymmetries occurring during the initial dynamic component must necessarily be compensated by offsetting asymmetries in the latter portion of the response.

Treacher Collins prize essay. The significance of nystagmus

Nystagmus occurs in a very wide range of circumstances, each type showing characteristic clinical, pathological and electrophysiological features, and analogies between them can be identified by comparing and contrasting nystagmus of different kinds. The effect of altered visual and vestibular conditions on nystagmus, and the features of its waveform, indicate the relationship between eye movements and vision, and the influence of visual and vestibular input in stabilising steady fixation. Ultimately, the significance of nystagmus is that it indicates the state of the mechanisms underlying this stabilisation: in physiological nystagmus they are operating successfully, and in pathological nystagmus they are disturbed. More than this, investigation of nystagmus has shown that the visual system is not divided in a clear-cut way into sensory and motor poles, but that between them there exists a neural region where a 'copy' of the visual world is matched with a programme of potential eye movements, and where sensorimotor information exists indivisibly. Long feedback loops, involving occipital cortex and extraocular muscle proprioceptors, and short ones within the cerebellum and integrator, emphasise the great precision involved in eye movement control, enabling the visual cortex to make optimal use of the resolution capabilities of the fovea. Nystagmus always reflects an asymmetry in the output of the eye movement generators, and it has been shown that the inappropriate movement which is responsible for pathological nystagmus is the slow movement. This may arise because of an intrinsic defect in that part of the generator called the neural integrator, or because of 'tonic imbalance' in its input. Nystagmus occurring with identifiable acquired central nervous pathology can, to some extent, be understood mechanistically, but idiopathic congenital nystagmus poses greater difficulties. Analysis of its waveform suggests that an intrinsic fault in the integrator can explain the clinical and electrophysiological findings in CN, but the cause of the high gain instability in the integrator remains to be explained. The integrator is adaptable, or self-tuning, adjusting its output by visual feedback. Circumstantial evidence suggests that the original disorder in idiopathic CN may occur higher than the integrator, detuning it by conveying an inappropriately organised visual input. In particular if the organisation of the visual system into fields is defective, the gaze generators, whose output is orientated according to field, will have a less accurate 'copy' of the world from which to formulate their movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of mechanical disturbance on oculomotor behavior

Using a binocular search coil technique, we measured oculomotor behavior during gentle pressing with a finger on the outer canthus of one eye. With a steadily pressed viewing eye, and the fellow eye occluded, the occluded eye deviates while the pressed eye does not. If the eye is rapidly pressed and released, the compensation of rotational force in the pressed eye becomes incomplete, so that both eyes move. At high frequencies of press (greater than 1 Hz) the pressed eye is deviated and the contralateral eye no longer moves. In darkness the pressed eye always rotates but the contralateral eye never does, demonstrating that any inflow from proprioceptors sensing rotation of the pressed eye does not affect oculomotor posture as measured in the fellow eye. With binocular viewing the results are more variable. On some trials neither eye moves, while on others both move. The results can be interpreted as a Hering's law controlled attempting to reconcile disparate inputs from the two eyes. The results confirm and extend, with objective measures, earlier conclusions from subjective experiments.

Binocular micromovements in normal persons

Under physiological viewing conditions, binocular micromovements in normal subjects showed multiple saccadic formations which, in their vertical and horizontal components, combined to produce different forms of overshoot which were usually large. On comparing the right and left eyes, micromovements were considerably incongruous, though rough direction identity and absolute synchronism of saccades and drifts were given. Vertical, horizontal and overshoot components of saccades show good correlation in their amplitude/velocity relationship, as seen in voluntary large saccades. Formation, frequency and direction of saccades showed intra-individual similarity rather than dependence on viewing conditions. From our results, we concluded that a central generating process rather than the primary retinal error signals are the source of micromovements during fixation.