Human optokinetic nystagmus: a stochastic analysis.

Optokinetic nystagmus (OKN) is a fundamental gaze-stabilizing response found in almost all vertebrates, in which eye movements attempt to compensate for the optic flow caused by self-motion. It is an alternating sequence of slow compensatory eye movements made in the direction of stimulus motion and fast eye movements made predominantly in the opposite direction. The timing and amplitude of these slow phases (SPs) and quick phases (QPs) are remarkably variable, and the cause of this variability is poorly understood. In this study principal components analysis was performed on OKN data to illustrate that the variability in correlation matrices across individuals and recording sessions reflected changes in the noise in the system while the linear relationships between variables remained predominantly the same. Three components were found that could explain the variance in OKN data, and only variables from within a single cycle contributed highly to any given component. A linear stochastic model of OKN was developed based on these results that describes OKN as a triple first order Markov process, with three sources of noise affecting SP velocity, the QP trigger, and QP amplitude. This model was used to predict the degree of signal dependent noise in the system, the duration of the transient state of SP velocity, and an apparent undershoot bias to the QP target location

Saccade-like behavior in the fast-phases of optokinetic nystagmus: An illustration of the emergence of volitional actions from automatic reflexes

As a potential exemplar for understanding how volitional actions emerged from reflexes, we studied the relationship between an ancient reflexive gaze stabilization mechanism (optokinetic nystagmus [OKN]) and purposeful eye movements (saccades) that target an object. Traditionally, these have been considered distinct (except in the kinematics of their execution) and have been studied independently. We find that the fast-phases of OKN clearly show properties associated with saccade planning: (a) They are characteristically delayed by irrelevant distractors in an indistinguishable way to saccades (the saccadic inhibition effect), and (b) horizontal OKN fast-phases produce curvature in vertical targeting saccades, just like a competing saccade plan. Thus, we argue that the saccade planning network plays a role in the production of OKN fast-phases, and we question the need for a strict distinction between eye movements that appear to be automatic or volitional. We discuss whether our understanding might benefit from shifting perspective and considering the entire “saccade” system to have developed from an increasingly sophisticated OKN system

Volition and automaticity in the interactions of optokinetic nystagmus, infantile nystagmus, saccades and smooth pursuit

Volitional target-selecting eye movements, such as saccades or smooth pursuit, are frequently considered distinct and separate from automatic gaze-stabilising eye movements like optokinetic nystagmus or the vestibulo-ocular reflex. This difference is regularly mapped onto brain anatomy, with distinctions made between subcortical, automatic processes; and cortical, volitional ones. However gaze-stabilising and target-selecting eye movements must work together when a moving observer views natural scenes. Yet such co-ordination would not be possible if automatic and volitional actions are sharply divided. This thesis focuses upon interactions between gaze-stabilising and target-selecting eye movements, and how these interactions can aid our understanding of the relationship between automatic and volitional processes. For a saccade executed during optokinetic nystagmus to accurately land on target, it must compensate for the ongoing optokinetic movement. It was found that targeting saccades can partially compensate for concomitant optokinetic nystagmus. The degree of compensation during optokinetic nystagmus was indistinguishable from compensation due to voluntary smooth pursuit displacements. A subsequent experiment found that locations are similarly misperceived during optokinetic nystagmus and smooth pursuit. Furthermore, saccade end-points are subject to the same perceptual mislocalisations. The next experiment established that fast-phases of optokinetic nystagmus can act like competitive saccades and cause curvature in targeting saccades. Moreover, optokinetic nystagmus fast-phases are delayed by irrelevant visual distractors in the same way as saccades (the saccadic inhibition effect). Lastly, it was established that the fast-phases of Infantile Nystagmus Syndrome also show the saccadic inhibition effect. In conclusion, target-selecting and gaze-stabilising eye movements show substantial co-ordination. Furthermore these results demonstrate considerable commonalties between ‘automatic’ and ‘volitional’ eye movements. Such commonalities provide further evidence there is no sharp distinction between automatic and volitional processes. Instead it is likely there are substantial interconnections between automatic and volitional mechanisms, and volition has a graded influence upon behaviour

Binocular integration using stereo motion cues to drive behavior in mice

The visual system presents an opportunity to study how two signals converge to generate a novel representation of the world: depth. The slight difference in positions between the two eyes means that different images are encoded by the left and right eyes by generating disparity signals. Another way to generate depth signals is by presenting different motion signals to the two eyes. Even though the binocular visual system has been studied for a long time, the mechanisms behind binocular integration when objects move in depth are largely unknown. In this dissertation, I demonstrate a new model for studying motion-in-depth signals using mice. Mice are an attractive animal to study the binocular visual system not only because they share common visual pathway as primates and other mammals, but also because there are genetic tools that can be used to study the underlying circuitry for binocular integration during motion-in-depth cues. Thus far there have been very few studies regarding binocularity in mice. This dissertation will focus on the behavioral output during stereoscopic motion-in-depth signals in mice and investigate visual areas involved in these behaviors. In the first section, I investigate whether mice discriminate motion-in-depth signals like primates, using disparity and motion signals presented to each eye. I find that mice are able to discriminate towards and away stimuli and that the binocular neurons in the visual cortex were critical for the computation of this signal. In the second section we measured optokinetic eye movement generated by motion-in-depth stimulus. I found that vergence eye movement in mice is driven primarily by the motion signals presented in each eye. This phenomenon can be explained largely by the summation of monocular motor signals of the two eyes that happens subcortically. These two experiments both show clear behavioral output that can be only generated when presented with binocular motion-in-depth signals. I find both cortical and subcortical components of binocular integration that are responsible for the generation of these behavior outputs which demonstrates the complicated nature of binocular integration associated with motion-in-depth signals. My work in this dissertation provides the foundation for studying binocular integration in rodentsNeuroscienc

The Effects of Cigarette Smoking During Acute Alcohol Intoxication

The consumption, and often abuse, of alcohol is frequently accompanied by cigarette smoking. Between eighty and ninety-five percent of alcoholics also smoke cigarettes, a rate more than four times higher than in the general population. The mechanisms underlying this association remain poorly understood. A general class of explanation is that smoking might affect the acutely intoxicating effects of alcohol. The relationships could take several forms, none of which is necessarily exclusive of another. These could include 1) synergism of effects, especially reward-related feelings of stimulation and positive affect, 2) additive effects, whereby the stimulating effects of nicotine could offset the depressant effects of alcohol, and 3) smoking-related desensitization to the effects of alcohol, by a mechanism of cross-tolerance. The latter proposal, that smoking (i.e., nicotine) leads to cross-tolerance to alcohol, provides a guiding hypothesis for the research described here. Such a proposal is supported by an extensive body of evidence from animal studies that is consistent with an interpretation in terms of cross-tolerance between nicotine and alcohol, such that nicotine consumption diminishes sensitivity to the acute intoxicating effects of alcohol (on multiple measures). It has been hypothesized that the reduced sensitivity to the effects of alcohol could lead, in turn, to increased consumption and risk of addiction. This research examines systematically the acute effects of moderate doses of alcohol and cigarette smoking alone and in combination, on several measures in a controlled laboratory environment. Principal focus is on measures of postural control, which are emphasized because of their known sensitivity to alcohol at moderate doses, and the role they have played in prior studies of individual differences in sensitivity to acute alcohol. Additionally, measures were obtained of subjective effects, oculomotor control, and cognitive functioning. Eight participants (four female) were tested in four counterbalanced sessions involving alcohol only, cigarette only, alcohol with cigarette, and alcohol placebo only. During all sessions measures were obtained at baseline and at repeated intervals after dosing. Consistent with indications of cross-tolerance between alcohol and nicotine, smoking during the experimental sessions diminished selected effects of alcohol on key measures of postural and, to lesser extent, oculomotor control and subjective effects. The specific cognitive tasks chosen for study proved to be ineffective at detecting effects of alcohol or cigarette smoking. Results are discussed in terms of the physiological and psychological changes associated with the development of acute cross-tolerance, and other forms of interaction between alcohol and nicotine

First International Microgravity Laboratory experiment descriptions

Brief descriptions of the experiments for the First International Microgravity Laboratory (IML-1) which is scheduled for launch from KSC aboard the Orbiter Discovery in early 1992 are presented