Effect of the distance of optokinetic stimuli from the eyes on certain parameters of the optokinetic nystagmus.

The above effect was studied in 65 subjects with normal vision (mean age 20 years) in investigations in which the following factors were successively changed: distance of optokinetic stimuli from the eyes; this distance and angular velocity of stimuli; distance and frequency of stimuli or finally distance and accommodation level. The angular velocity of the pursuit nystagmus phase was found to be by far the highest and simultaneously the closest to the angular velocity of optokinetic stimuli when the latter are 1.5m from the eyes. With shorter distances, the velocity of the pursuit movements lags steadily behind that of stimulus velocity. This change is conditioned by changes in OKN amplitude since its frequency as a whole does not change. Even though the accommodation level significantly affects the velocity of the pursuit nystagmus phase, the dependence on the distance of optokinetic stimuli from the eyes persists even after atropinization. The interpretation of these findings must take into account sepcifically the demands on accommodation, convergence, and on visual attention which are increased with shorter distances.     

The fast phase of optokinetic nystagmus in the locust

ABSTRACT. When their visual environment is rotated about them, freely moving locusts make the typical slow (following) and fast (flick back) phases of optokinetic nystagmus. The fast phase was examined and is compared with that of Crustacea and vertebrates. It is unusual in the locust in that it is symmetrical about the body midline. Mechanoreceptors play a significant role in maintaining the fast phase reset function.     

Development of optokinetic neuronal responses in the pretectum and horizontal optokinetic nystagmus in unilaterally enucleated rats

Responses of single units to constant-velocity rotations of the visual surround (0.25-10 degrees/s) were studied in the pretectum of unilateral enucleated rats at different ages. Enucleation was performed either in the first postnatal week ("early" enucleated rats) or in the adult stage ("late" enucleated rats). Pretectal unitary responses were recorded in early enucleated animals at postnatal day 20-21, 36-49 and, in both experimental groups, in the adult stage. Optokinetic ocular nystagmus was studied in early and late enucleated rats in the adult stage. Gain of optokinetic nystagmus in temporo-nasal stimulus direction was not changed for visual surround rotations of up to 20 degrees/s compared to controls in monocular viewing conditions. At higher stimulus velocities, however, the gain dropped. In naso-temporal stimulus direction, optokinetic nystagmus was improved in gain for optokinetic pattern motions of up to 5-10 degrees/s. There were only minor differences in the gain behaviour of optokinetic nystagmus obtained from early or late enucleated rats. The optokinetic responses of pretectal neurons obtained from early and late enucleated rats were reduced in sensitivity by more than 50%. The response patterns of neurons recorded in the contralateral pretectum relative to the intact eye were shifted by a large amount from directional selective to directional nonselective response types. No such changes were obtained in the ipsilateral pretectum. In contrast to normal rats, there were very few directional selective units responding to temporo-nasal pattern motion. On the other hand, a large proportion of directional selective units responded to naso-temporal pattern motion. These latter units were found in both early and late enucleated rats. A similar response type has previously been described for intact young rats but not for adult rats. The velocity tuning curve of pretectal units studied in the adult stage was similar in shape in early and late enucleated rats and resembled that obtained from enucleated or intact young animals. Our results show that response sensitivity, direction and velocity tuning of pretectal units depend crucially on retinal afferent input originating from both eyes. The data suggest that the response characteristics of many of the pretectal units that are considered to be important for mediating optokinetic reflexes depend on interpretectal signal processing using commissural connections. There is very little evidence for an adaptative structural plasticity of the optokinetic system following loss of one eye. The reduced asymmetry observed in gain of optokinetic responses correlated in both early and late enucleated rats with the shifts observed in the distribution of pretectal unitary response patterns.     

A random walk model of fast-phase timing during optokinetic nystagmus

 Most vertebrate animals produce optokinetic nystagmus in response to rotation of their visual surround. Nystagmus consists of an alternation of slow-phase eye rotations, which follow the surround, and fast-phase eye rotations, which quickly reset eye position. The time intervals between fast phases vary stochastically, even during optokinetic nystagmus produced by constant velocity rotation of a uniform surround. The inter-fast-phase interval distribution has a long tail, and intervals that are long relative to the mode become even more likely as constant surround velocity is decreased. This paper provides insight into fast-phase timing by showing that the process of fast-phase generation during constant velocity optokinetic nystagmus is analogous to a random walk with drift toward a threshold. Neurophysiologically, the output of vestibular nucleus neurons, which drive the slow phase, would approximate a random walk with drift because they integrate the noisy, constant surround velocity signal they receive from the visual system. Burst neurons, which fire a burst to drive the fast phase and reset the slow phase, are brought to threshold by the vestibular nucleus neurons. Such a nystagmic process produces stochastically varying inter-fast-phase intervals, and long intervals emerge naturally because, as drift rate (related to surround velocity) decreases, it becomes more likely that any random walk can meander for a long time before it crosses the threshold. The theoretical probability density function of the first threshold crossing times of random walks with drift is known to be that of an inverse Gaussian distribution. This probability density function describes well the distributions of the intervals between fast phases that were either determined experimentally, or simulated using a neurophysiologically plausible neural network model of fast-phase generation, during constant velocity optokinetic nystagmus. Received: 1 June 1995/Accepted in revised form: 15 February 1996     

Random walks, random sequences, and nonlinear dynamics in human optokinetic nystagmus.

Optokinetic nystagmus (OKN) is a reflexive eye movement with target-following slow phases (SP) alternating with oppositely directed fast phases (FP). We measured the following from OKN in three humans: FP beginning and ending positions, amplitudes, and intervals and SP amplitudes and velocities. We sought to predict future values of each parameter on the basis of past values, using state-space representation of the sequence (time-delay embedding) and local second-order approximation of trajectories. Predictability is an indication of determinism: this approach allows us to investigate the relative contributions of random and deterministic dynamics in OKN. FP beginning and ending positions showed good predictability, but SP velocity was less predictable. FP and SP amplitudes and FP intervals had little or no predictability. FP beginnings and endings were as predictable as randomized versions that retain linear autocorrelation; this is typical of random walks. Predictability of FP intervals did not change under random rearrangement, which is characteristic of a random process. Only linear determinism was demonstrated; nonlinear interactions may exist that would not be detected by our present approach.     

Neuronal pathways involved in the optokinetic head nystagmus of the frog

The morphology of projection neurons in the basal optic nucleus and the pretectal area and the interconnections of these brain regions were studied with the aid of the cobalt-filling technique. It was found that the nucleus-sends long descending axons to the medullary reticular formation. The two basal optic nuclei are reciprocally interconnected and do not give a direct descending pathway. The pretectal nuclei and the basal optic nucleus are also reciprocally coupled. It is supposed that the described pathways mediate commands for horizontal and vertical nystagmic head movements.     

On the distribution of fast-phase intervals in optokinetic and vestibular nystagmus

 Histograms of fast-phase intervals in human optokinetic and vestibular nystagmus were generated, and fitted to statistical distributions used in previous studies. The distributions did not depend on stimulation type (optokinetic or vestibular). An inverse Gaussian or a gamma distribution fitted the data better than did a reciprocal Gaussian distribution, but none fitted the data especially well. In some cases, however, the interpretation of these distributions is more physiologically satisfactory than in others. Recommendations are made on which class of models is preferred, and the experiments needed to support the particular models. Our results call into question the validity of previous studies that fit statistical distributions to data sets of a size comparable to ours. Received: 26 June 2001 / Accepted in revised form: 20 February 2002     

The effect of lateral inclinations upon optokinetic and postoptokinetic nystagmus in pigeons

Optokinetic stimulation with the angle velocity 10 degrees/s was carried out in pigeons in differently bent postures. No difference in effects of dynamic and static inclinations upon optokinetic and postoptokinetic nystagmus was found. Neither any rigid connection was found between the inclination direction and the sign of the response change. The data obtained are at variance with the hypothesis of gravity-dependent changes of the "velocity accumulator" time constant as the only cause of changes in the optokinetic and postoptokinetic responses under conditions of dynamic or static inclinations.     

Interactions between GABAergic and cholinergic mechanisms involved in monocular optokinetic nystagmus in frogs]

The systemic administration of atropine, a muscarinic cholinergic antagonist, was found to suppress the Nasal-Temporal (N-T) component of the frog monocular optokinetic nystagmus (OKN), which had appeared following a prior injection of bicuculline and which does not exist in the normal animal. On the contrary, the administration of a nicotinic cholinergic antagonist (D-TC, alpha-BGT, Hexamethonium) following that of bicuculline has prolonged the duration of the induced N-T component. Thus, ACh was shown to attenuate or to reinforce the GABAergic inhibition of the N-T component through muscarinic receptors or nicotinic receptors respectively. These data point to the existence of strong interactions between these two neurotransmission systems involved in frog monocular OKN.