Professional research project (Au.D.)--University of South Florida, 2002. / Title from PDF of title page. / Document formatted into pages; contains 24 pages. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Patients with non-compensated vestibular dysfunction frequently complain of the ability to maintain dynamic visual acuity during activities which require the movement of the head. When this occurs the patient is experiencing oscillopsia, which is the symptom resulting from a non-functional vestibulo-ocular reflex (VOR). To measure the presence of oscillopsia, tests of dynamic visual acuity (DVA) may be used.A recent test of DVA has been reported which is administered while patients are walking on a treadmill. Although this test has been shown to be useful in evaluating DVA in patients, there are several disadvantages to treadmill use. These include physical space, cost and accessibility. Additionally, walking at the required treadmill speed to produce sufficient head movement may pose difficulties and be medically contraindicated for patients with certain health risks. The purpose of this study was to evaluate a different method to measure DVA in patients which would not require the use of the treadmill, but instead utilize a volitional head movement to reveal oscillopsia. In this study, patients performed the DVA test in two conditions: (1) walking on a treadmill, and (2) seated on a chair volitionally moving the head.In this study, DVA was tested in both conditions with 15 adults with normal vestibular function, and 16 adults with vestibular impairment. Results revealed that both methods, treadmill walking and volitional head movement, appeared equivalent for measuring DVA in normal subjects and vestibular impaired subjects. The lack of finding a significant main effect of method, and interactions that include method, supports the equivalence of volitional head movement to a treadmill approach for the measurement of DVA. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

The Vestibulo-Ocular Reflex (VOR) produces compensatory eye movements in response to head and body rotations movements, over a wide range of frequencies and in a variety of dimensions. The individual components of the VOR are separated into parallel pathways, each dealing with rotations or movements in individual planes or axes. The Horizontal VOR (hVOR) compensates for eye movements in the Horizontal plane, and comprises a linear and non-linear pathway. The linear pathway of the hVOR provides fast and accurate compensation for rotations, the response being produced through 3-neuron arc, producing a direct translation of detected head velocity to compensatory eye velocity. However, single neurons involved in the middle stage of this 3-neuron arc cannot account for the wide frequency over which the reflex compensates, and the response is produced through the population response of the Medial Vestibular Nucleus (MVN) neurons involved. Population Heterogeneity likely plays a role in the production of high fidelity population response, especially for high frequency rotations. Here we present evidence that, in populations of bio-physical compartmental models of the MVN neurons involved, Heterogeneity across the population, in the form of diverse spontaneous firing rates, improves the response fidelity of the population over Homogeneous populations. Further, we show that the specific intrinsic membrane properties that give rise to this Heterogeneity may be the diversity of certain slow voltage activated Potassium conductances of the neurons. We show that Heterogeneous populations perform significantly better than Homogeneous populations, for a wide range of input amplitudes and frequencies, producing a much higher fidelity response. We propose that variance of Potassium conductances provides a plausible biological means by which Heterogeneity arises, and that the Heterogeneity plays an important functional role in MVN neuron population responses. We discuss our findings in relation to the specific mechanism of Desynchronisation through which the benfits of Heterogeneity may arise, and place those findings in the context of previous work on Heterogeneity both in general neural processing, and the VOR in particular. Interesting findings regarding the emergence of phase leads are also discussed, as well as suggestions for future work, looking further at Heterogeneity of MVN neuron populations.

The semicircular canals detect head rotations and trigger compensatory movements that stabilize gaze and help maintain visual fixation. Mammals with large eyes and high visual acuity presumably require more precise gaze stabilization mechanisms because they experience degradation of spatial resolution at a lower threshold of uncompensated motion. Because semicircular canal radius of curvature is a primary determinant of canal sensitivity, species with large canal radii are expected to be capable of more precise gaze stabilization than species with small canal radii. Here the relationship between semicircular canal radius of curvature, eye size, and visual acuity is examined in a large sample of therian mammals. These results demonstrate that eye size and visual acuity both explain a significant proportion of the variance in mean canal radius of curvature after statistically controlling for the effects of body mass and phylogeny. These findings suggest that interspecific variation in semicircular canal radius of curvature is partly the result of selection for improved gaze stabilization in species with large eyes and acute vision. / text

Thesis (Ph.D.)--McMaster University, 2002. / Includes bibliographical references (leaves 142-143). Also available via World Wide Web.

Simple circuits perform simple tasks. Complex circuits can perform more complicated tasks. This is true for artificial circuits and for brain circuits. As is known from artificial networks, a complexity that makes circuits substantially more powerful is distributing learning across multiple layers. In fact, most brain circuits in vertebrate systems are multi-layer circuits (but for few that perform simple reflexes) in which learning is distributed across layers. Despite the crucial contribution of learning in middle layer neurons to the output of the circuits they are embedded in, there is little understanding of the principles defining this contribution. A very common feature in brain circuits is that middle layer neurons generate two types of signals, known as spikes. These middle layer neurons commonly have long dendrites where they generate dendritic spikes. As well, like most neurons, they generate axonal spikes near the cell body. Neurons exhibiting these two spike types include pyramidal cells in the neo-cortex and the hippocampus, the Purkinje cells in the cerebellum and many more. In this thesis I study another circuit that contains middle layer neurons, the electrosensory lateral lobe (ELL) of the mormyrid fish. The ELL is a tractable brain circuit in which the middle layer neurons generate dendritic and axonal spikes. In this thesis I show that these spike types are not two different expressions of the same inputs. Rather, they have a symbiotic relationship. Instead of all inputs triggering both spikes, some inputs can selectively drive dendritic spikes. The dendritic spikes in return modify the synaptic strength of another set of inputs. The modified inputs are then transmitted to downstream neurons via the axonal spikes, which contributes a desired signal to the output of the circuits. Effectively there is a separation of learning and signaling in the middle layer neurons through the two spike types. Having two types of spikes in the same neuron doing different computations enormously expands the computational power of the neuron. But, being in the same neuron means the separation of function is constrained and needs to be supported by biophysical principles. I have thus built a biophysical model to understand the biophysical principles underlying the separation of function. I show that in the middle layer neurons of the ELL, the axonal spikes are strongly reduced in amplitude as they backpropagate to the apical dendrites, yet they remain crucial in driving dendritic spikes. Critically, modulation of inhibitory inputs can selectively dial up or down the ability of the backpropagating axonal spikes to drive dendritic spikes. Thus, a set of inhibitory modulating inputs can selectively modulate dendritic spikes. Having learning in different layers contributing to the outcome of the circuit, naturally leads to asking how the work is divided across layers and neuron types within the circuit. In this thesis I answer this question in the context of the outcome of the ELL circuit. Finally, another signature of a complex circuit is the ability to integrate many different inputs, usually in middle layer neurons, to generate sophisticated outputs. A goal for scientists studying systems neuroscience is to understand how this integration works. In this thesis I provide a coherent model of a learning behavior called vestibulo occular reflex (VOR) adaptation, that depends on the integration of separate inputs to yield a learned behavior. The VOR is a simple reflex generated in the brain stem. Inputs from the brain stem are also sent to an area in the cerebellar cortex called the flocculus. The flocculus also receives another set of inputs that generate a different behavior, called smooth-pursuit. The integration of VOR inputs with smooth-pursuit inputs in the flocculus generate VOR adaptation. Understanding complex circuits is one of the greatest challenges for today's neuroscientists. In this thesis I tackle two such circuits and hope that through a better understandings of these circuits we gain principles that apply to other circuits and thereby advance our understanding of the brain.

Patients with non-compensated vestibular dysfunction frequently complain of the ability to maintain dynamic visual acuity during activities which require the movement of the head. When this occurs the patient is experiencing oscillopsia, which is the symptom resulting from a non-functional vestibulo-ocular reflex (VOR). To measure the presence of oscillopsia, tests of dynamic visual acuity (DVA) may be used. A recent test of DVA has been reported which is administered while patients are walking on a treadmill. Although this test has been shown to be useful in evaluating DVA in patients, there are several disadvantages to treadmill use. These include physical space, cost and accessibility. Additionally, walking at the required treadmill speed to produce sufficient head movement may pose difficulties and be medically contraindicated for patients with certain health risks. The purpose of this study was to evaluate a different method to measure DVA in patients which would not require the use of the treadmill, but instead utilize a volitional head movement to reveal oscillopsia. In this study, patients performed the DVA test in two conditions: (1) walking on a treadmill, and (2) seated on a chair volitionally moving the head. In this study, DVA was tested in both conditions with 15 adults with normal vestibular function, and 16 adults with vestibular impairment. Results revealed that both methods, treadmill walking and volitional head movement, appeared equivalent for measuring DVA in normal subjects and vestibular impaired subjects. The lack of finding a significant main effect of method, and interactions that include method, supports the equivalence of volitional head movement to a treadmill approach for the measurement of DVA.

No description available.

Diss. Linköping : Univ., 2003. / Härtill 4 uppsatser.

Le récepteur aux hydrocarbures aromatiques (AhR) est un facteur de transcription de la famille bHLH/PAS, activé par différents ligands exogènes dont les hydrocarbures aromatiques polycycliques ou halogénés (dioxines). A ce titre, il est décrit historiquement comme un récepteur de xénobiotiques dont le principal rôle est l’élimination de ces composés via la régulation des enzymes du métabolisme des xénobiotiques. Toutefois, des études récentes menées à l’aide de modèles souris invalidées pour le AhR, suggèrent indirectement que cette protéine régule des fonctions endogènes, notamment dans le système nerveux de mammifères dans lequel aucun rôle du AhR n’a jusqu’à présent été démontré. Nous avons donc utilisé le modèle de souris C57BL/6J AhR-/- pour mener à la fois des études comportementales et mécanistiques afin de déterminer ce rôle. Tout d’abord, nous avons identifié un défaut oculomoteur chez les souris AhR-/-, caractérisé par des mouvements spontanés horizontaux. En étudiant l’ensemble des circuits neurosensoriels potentiellement impliqués dans ce nystagmus pendulaire, nous avons montré que son origine est liée à des déficits du système visuo-moteur. De plus, en caractérisant et comparant les profils d’expression génique des cervelets de souris AhR+/+ et AhR-/- traitées ou non par de la 2,3,7,8 TétraChloroDibenzo-p-Dioxine (TCDD), nous avons montré que ce polluant, ligand du AhR, perturbait les fonctions endogènes du récepteur. Cet effet de « perturbation endogène » a été retrouvé dans un autre organe et est associé à une toxicité (fibrose hépatique). Cette étude a permis d’identifier de nouvelles fonctions physiologiques du AhR dans le système nerveux des souris, de caractériser un nouveau modèle animal de nystagmus pendulaire et ouvre de nouvelles perspectives de travail en neurotoxicologie. / The AhR is a basic helix-loop-helix Per/ARNT/Sim family (bHLH-PAS) transcription factor which is activated by many diverse compounds including polyphenols and aromatic hydrocarbons such as 2,3,7,8 TétraChloroDibenzo-p-Dioxin (TCDD). Initially, the AhR was described as a ubiquitous xenobiotic-activated transcription factor which promotes the elimination of xenobiotics by regulating the expression of genes involved in xenobiotic metabolism. However, mouse AhR knockout models have demonstrated that the AhR also regulates other normal physiological functions. In particular, functioning of the nervous system of mammals, previously unexplored in this respect, might depend upon the activity of the AhR. We, thus, performed behavioral and gene expression studies in AhR-/- mice to discover these functions. We, first, found that AhR-/- mice exhibit an oculomotor deficit which is characterized by spontaneous horizontal pendular eye movements that are probably due to a deficit in the visuo-motor circuitry. Second, we found that the cerebellar gene expression profiles of AhR-/- as compared to AhR+/+ mice resembled those of AhR+/+ mice treated with TCDD (the ligand with the highest affinity for the AhR). This suggests that TCDD disrupts some normal physiological functions of the AhR in the nervous system. Third, AhR-/- mice and AhR+/+ treated with TCDD both develop liver fibrosis. This further suggests a role for the AhR in normal liver function. In conclusion, this study reveals new physiological functions for the AhR in the mouse nervous system and describes a new model of pendular nystagmus. Moreover, the results also provide novel research perspectives in the field of neurotoxicology.

Athletic potential is one of the most complex human traits. An elite athlete is produced from a complex interaction of an innumerable number of traits exhibited by the athlete. However, it’s not clear whether these traits are innate, allowing the athlete to excel, or, alternatively, are a consequence of practice. To be successful, athletes rely heavily on sensory information from the visual and vestibular systems. This study investigated the relationship eye movement control has with innate athleticism by comparing the saccadic and Vestibulo-Ocular Reflex (VOR) responses of former, no longer practicing, elite athletes against their age and gender matched counterparts who were non-elite or non-athletes. Results showed subjects who participated in athletic activities longer (regardless of type or level achieved), showed both significantly better VOR suppression capabilities, as well as higher head velocities while suppressing their VOR. Although, these results are correlative in nature, they do not support the potential that VOR suppression is a learned trait of athletes. A longitudinal study would be required to assess this relationship fully.