Capture of fixation by rotational flow; a deterministic hypothesis regarding scaling and stochasticity in fixational eye movements.

Visual scan paths exhibit complex, stochastic dynamics. Even during visual fixation, the eye is in constant motion. Fixational drift and tremor are thought to reflect fluctuations in the persistent neural activity of neural integrators in the oculomotor brainstem, which integrate sequences of transient saccadic velocity signals into a short term memory of eye position. Despite intensive research and much progress, the precise mechanisms by which oculomotor posture is maintained remain elusive. Drift exhibits a stochastic statistical profile which has been modeled using random walk formalisms. Tremor is widely dismissed as noise. Here we focus on the dynamical profile of fixational tremor, and argue that tremor may be a signal which usefully reflects the workings of oculomotor postural control. We identify signatures reminiscent of a certain flavor of transient neurodynamics; toric traveling waves which rotate around a central phase singularity. Spiral waves play an organizational role in dynamical systems at many scales throughout nature, though their potential functional role in brain activity remains a matter of educated speculation. Spiral waves have a repertoire of functionally interesting dynamical properties, including persistence, which suggest that they could in theory contribute to persistent neural activity in the oculomotor postural control system. Whilst speculative, the singularity hypothesis of oculomotor postural control implies testable predictions, and could provide the beginnings of an integrated dynamical framework for eye movements across scales

A Self-Organizing Neural Model of Motor Equivalent Reaching and Tool Use by a Multijoint Arm

This paper describes a self-organizing neural model for eye-hand coordination. Called the DIRECT model, it embodies a solution of the classical motor equivalence problem. Motor equivalence computations allow humans and other animals to flexibly employ an arm with more degrees of freedom than the space in which it moves to carry out spatially defined tasks under conditions that may require novel joint configurations. During a motor babbling phase, the model endogenously generates movement commands that activate the correlated visual, spatial, and motor information that are used to learn its internal coordinate transformations. After learning occurs, the model is capable of controlling reaching movements of the arm to prescribed spatial targets using many different combinations of joints. When allowed visual feedback, the model can automatically perform, without additional learning, reaches with tools of variable lengths, with clamped joints, with distortions of visual input by a prism, and with unexpected perturbations. These compensatory computations occur within a single accurate reaching movement. No corrective movements are needed. Blind reaches using internal feedback have also been simulated. The model achieves its competence by transforming visual information about target position and end effector position in 3-D space into a body-centered spatial representation of the direction in 3-D space that the end effector must move to contact the target. The spatial direction vector is adaptively transformed into a motor direction vector, which represents the joint rotations that move the end effector in the desired spatial direction from the present arm configuration. Properties of the model are compared with psychophysical data on human reaching movements, neurophysiological data on the tuning curves of neurons in the monkey motor cortex, and alternative models of movement control.National Science Foundation (IRI 90-24877); Office of Naval Research (N00014-92-J-1309); Air Force Office of Scientific Research (F49620-92-J-0499); National Science Foundation (IRI 90-24877

Aerospace medicine and biology. A continuing bibliography (supplement 231)

This bibliography lists 284 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1982

Individual Differences in Coping with Large Multimodal Conflicts in a Spatial Orientation Paradigm

This study examined how humans respond to large 180º disparities between internally generated self-motion cues and external landmarks in a navigation task. Subjects learned the locations of 4 objects in a virtual environment, navigating back to these sites in a testing phase that probed their sense of direction at dead-ends. In select (incongruent) trials, subjects’ virtual rotations were mirrored relative to their physical turns, forcing them to navigate along separate virtual and physical trajectories. On these trials, subjects were either instructed to navigate using their memory of the required turn sequence (proprioceptive instructions) or the external environment (visual instructions). Subjects fell into two categories based on whether they required clarification about how they should be orienting at endpoints in incongruent trials. The clarification group achieved higher accuracy under congruent conditions and appeared to prefer the visual modality in incongruent trials. This group performed similarly regardless of pathway geometry, and was more prone to modality discounting. The other group yielded higher errors in congruent conditions, performed differentially depending on pathway geometry, tended to place more weight on the proprioceptive modality, and was prone to modality averaging. Both groups modulated their modality weighting based on navigation instructions. This study supports the position that human individuals employ different strategies when navigating under cue conflict conditions. It demonstrates that individuals who rely on visual landmarks commit smaller errors (and are less sensitive to trajectory shape) than individuals who rely on internal self-motion feedback when orienting, who also demonstrated greater propensity for cue averaging. These findings suggest that there may be individual differences in the internal representation of space, including how it is accessed and encoded online

The Fourier analysis of saccadic eye movements

This thesis examines saccadic eye movements in the frequency domain and develops sensitive tools for characterising their dynamics. It tests a variety of saccade models and provides the first strong empirical evidence that saccades are time-optimal. By enabling inferences on the neural command, it also allows for better clinical differentiation of abnormalities and the evaluation of putative mechanisms for the development of congenital nystagmus. Chapters 3 and 4 show how Fourier transforms reveal sharp minima in saccade frequency spectra, which are robust to instrument noise. The minima allow models based purely on the output trajectory, purely on the neural input, or both, to be directly compared and distinguished. The standard, most commonly accepted model based on bang-bang control theory is discounted. Chapter 5 provides the first empirical evidence that saccades are time-optimal by demonstrating that saccade bandwidths overlap across amplitude onto a single slope at high frequencies. In Chapter 6, the overlap also allows optimal (Wiener) filtering in the frequency domain without a priori assumptions. Deconvolution of the aggregate neural driving signal is then possible for current models of the oculomotor plant. The final two chapters apply these Fourier techniques to the quick phases of physiological (optokinetic) nystagmus and of pathological (congenital) nystagmus. These quick phases are commonly assumed to be saccadic in origin. This assumption is thoroughly tested and found to hold, but with subtle differences implying that the smooth pursuit system interacts with the saccade system during the movement. This interaction is taken into account in Chapter 8 in the assessment of congenital nystagmus quick phases, which are found to be essentially normal. Congenital nystagmus models based on saccadic abnormalities are appraised

Investigating the interplay of the human attentional and vestibular systems using transcranial magnetic stimulation

The aim of this doctoral thesis was to investigate the relationship between the processing of vestibular information, on the one hand, and higher cognitive functions such as visual (spatial) attention and perceptual decision making, on the other. In order to draw causal inference about the role of specific cortical regions in this interplay, two experimental studies were conducted which combined psychophysical task designs using verticality judgment tasks with transcranial magnetic stimulation (TMS). The first study employed a simultaneous TMS-EEG approach to examine the role of the right intraparietal sulcus (IPS) within the dorsal parietal cortex in verticality judgments – a cortical area that has repeatedly been associated with both the visual attention and vestibular systems. Facilitatory effects of right IPS TMS on the bias of verticality perception were reported and mirrored by EEG results, which pointed to a normalization of individual perceptual biases reflected in a fronto-central ERP component following the stimulation. In contrast, no effects of left IPS TMS on either behavioural or electrophysiological measures were observed and right IPS TMS did not modulate performance in a control task that used the same set of stimuli (vertical Landmark task). These findings point to a causal role of the right IPS in the neuronal implementation of upright perception and strengthen the notion of vestibular-attentional coupling. In the second study verticality judgments had to be made under different levels of perceptual demand to address the question of how perceptual decision making interacts with vestibular processing. Stimuli adapted from those used in the first study were presented in a visual search setting, which required perceptual and response switches, in a way that varied attentional demands. This task was combined with offline theta-burst TMS applied to the dorsal medial frontal cortex (dMFC). The dMFC has been found to crucially contribute to perceptual decision making and is connected to core parts of the vestibular cortical network. Analysis of distinct features of behavioural performance before as compared to following dMFC TMS revealed a specific involvement of the dMFC in establishing the precision and accuracy of verticality judgments, particularly under conditions of high perceptual load. In summary, the results of the two studies support the idea of a functional link between the processing of vestibular information, (spatial) attention, and perceptual decision making, giving rise to higher vestibular cognition. Moreover, they suggest that on a cortical level this interplay is achieved within a network of multimodal processing regions such as the parietal and frontal cortices

IDENTIFICATION AND CHARACTERIZATION OF THE GENETIC AETIOLOGY OF A RARE DISEASE KNOWN AS ORAL-FACIAL-DIGITAL SYNDROME TYPE II OR MOHR SYNDROME

Ph.DDOCTOR OF PHILOSOPH

Psychophysical and Psychological Factors Affecting Recovery from Acute Balance Disorders

Patients with acute vestibular neuritis are traditionally investigated with caloric or rotational examination of the vestibular-ocular reflex. However, clinical outcome is poorly predicted by such vestibular reflex assessments. We hypothesised that symptomatic recovery would depend upon higher order visuo-vestibular compensatory mechanisms. Thirty-one patients were studied in the acute and recovery phases of vestibular neuritis (median 2 days and 10 weeks, respectively). Patients underwent examination of vestibulo-ocular and vestibular-perceptual responses, at threshold and supra-threshold levels. Supra-threshold stimuli (90°/s velocity step rotations) allowed quantification of vestibulo-ocular and vestibulo-perceptual time constants. Additional measures of visual dependency (rod-and-disc task), dizziness symptom load (Vertigo Symptom Scale and Dizziness Handicap Inventory) and psychological factors (including - autonomic arousal, anxiety, depression, fear of bodily sensations) were obtained. Vestibulo-perceptual and vestibulo-ocular thresholds were raised and asymmetric acutely and remained slightly elevated and asymmetric at recovery. Acutely, supra-threshold vestibulo-ocular time constants were shortened and asymmetric. In contrast, perceptual responses were reduced but notably symmetrical. At recovery, vestibulo-ocular supra-threshold responses remained abnormal but perceptual supra-threshold responses normalised. Visual dependency was significantly elevated above normals in both acute and recovery stages. Vertigo symptom recovery was significantly predicted by acute levels of visual dependency (p=0.002), autonomic anxiety (p=0.004). A number of measures were associated with vertigo symptoms at recovery, in addition to visual dependency (p=0.012) and autonomic anxiety (p<0.001), including - anxiety and depression (p<0.003), fear of body sensations (p=0.033), vestibular perceptual thresholds (p=0.017) and caloric canal paresis (p=0.001). Factor Analysis revealed a strong association between clinical outcome, visual dependency and psychological factors, all loading on a single component accounting for 59.15% of the variance. The bilateral suppression of supra-threshold vestibular perception observed acutely represents a hitherto unrecognised central adaptive ‘anti-vertiginous’ mechanism. However, poor symptomatic recovery is best predicted by increased visual dependency and psychological factors. The findings show that long term recovery from unilateral vestibular deficit is mediated by central compensatory mechanisms, including multi-sensory integration and psychological processing