Effects of kainic acid lesions of the nucleus reticularis tegmenti pontis on fast and slow phases of vestibulo-ocular and optokinetic reflexes in the pigmented rat

Nucleus reticularis tegmenti pontis: a bridge between the basal ganglia and cerebellum for movement control

Neural processing in the basal ganglia is critical for normal movement. Diseases of the basal ganglia, such as Parkinson's disease, produce a variety of movement disorders including akinesia and bradykinesia. Many believe that the basal ganglia influence movement via thalamic projections to motor areas of the cerebral cortex and through projections to the cerebellum, which also projects to the motor cortex via the thalamus. However, lesions that interrupt these thalamic pathways to the cortex have little effect on many movements, including limb movements. Yet, limb movements are severely impaired by basal ganglia disease or damage to the cerebellum. We can explain this impairment as well as the mild effects of thalamic lesions if basal ganglia and cerebellar output reach brainstem motor regions without passing through the thalamus. In this report, we describe several brainstem pathways that connect basal ganglia output to the cerebellum via nucleus reticularis tegmenti pontis (NRTP). Additionally, we propose that widespread afferent and efferent connections of NRTP with the cerebellum could integrate processing across cerebellar regions. The basal ganglia could then alter movements via descending projections of the cerebellum. Pathways through NRTP are important for the control of normal movement and may underlie deficits associated with basal ganglia disease.

Neurophysiology of the optokinetic system

This chapter provides a review of early studies into the neural substrate for optokinetic-vestibular responses. Properties and connections of retinal and brainstem neurons contributing to optokinetic responses in the afoveate rabbit are summarized. Electrophysiological and lesion studies provide support for confluence of optokinetic and vestibular signals in the vestibular nucleus to provide the brain's estimate of self-rotation. Evidence for optokinetic-vestibular symbiosis in humans comes from the observation that individuals who have lost vestibular function show no optokinetic after-nystagmus in darkness, following full-field stimulus motion. An anatomical scheme for brainstem elaboration of optokinetic responses is proposed and cerebellar contributions are reviewed.

Visuomotor cerebellum in human and nonhuman primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula-nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed.

Nicotine-induced nystagmus correlates with midpontine activation

The pathomechanism of nicotine-induced nystagmus (NIN) is unknown. The aim of this study was to delineate brain structures that are involved in NIN generation. Eight healthy volunteers inhaled nicotine in darkness during a functional magnetic resonance imaging (fMRI) experiment; eye movements were registered using video-oculography. NIN correlated with blood oxygen level-dependent (BOLD) activity levels in a midpontine site in the posterior basis pontis. NIN-induced midpontine activation may correspond to activation of the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis, structures known to participate in the generation of multidirectional saccades and smooth pursuit eye movements.

Anatomical pathways that link perception and action

Pathways linking action to perception are generally presented as passing from sensory pathways, through the thalamus, and then to a putative hierarchy of corticocortical links to motor outputs or to memory. Evidence for more direct sensorimotor links is now presented to show that cerebral cortex rarely, if ever, receives messages representing receptor activity only; thalamic inputs to cortex also carry copies of current motor instructions. Pathways afferent to the thalamus represent the primary input to neocortex. Generally they are made up of branching axons that send one branch to the thalamus and another to output centers of the brain stem or spinal cord. The information transmitted through the classical "sensory" pathways to the thalamus represents not only information about the environment and the body, but also about instructions currently on their way to motor centers. The proposed hierarchy of direct corticocortical connections of the sensory pathways is not the only possible hierarchy of cortical connections. There is also a hierarchy of the corticofugal pathways to motor centers in the midbrain, and there are transthalamic corticocortical pathways that may show a comparable hierarchy. The extent to which these hierarchies may match each other, and relate to early developmental changes are poorly defined at present, but are important for understanding mechanisms that can link action and perception in the developing brain.

The thalamus as a monitor of motor outputs

Many of the ascending pathways to the thalamus have branches involved in movement control. In addition, the recently defined, rich innervation of 'higher' thalamic nuclei (such as the pulvinar) from pyramidal cells in layer five of the neocortex also comes from branches of long descending axons that supply motor structures. For many higher thalamic nuclei the clue to understanding the messages that are relayed to the cortex will depend on knowing the nature of these layer five motor outputs and on defining how messages from groups of functionally distinct output types are combined as inputs to higher cortical areas. Current evidence indicates that many and possibly all thalamic relays to the neocortex are about instructions that cortical and subcortical neurons are contributing to movement control. The perceptual functions of the cortex can thus be seen to represent abstractions from ongoing motor instructions.

Chronic recording of the vestibulo-ocular reflex in the restrained rat using a permanently implanted scleral search coil

A technique is described which allows accurate long-term monitoring of eye movements in the rat using permanently implanted scleral search coils. Search coils permanently sutured around the sclera yield vestibulo-ocular reflex (VOR) gain and phase values which are comparable to those reported previously in the literature using acutely implanted coils or electrooculographic electrodes. Considerations related to strain, sex and surgical procedures which permit measurement of responses in the chronically restrained rat are described. VOR gain and phase show a time course to their recovery following the implant surgery, with asymptotic performance typically attained approximately 10 days post-surgically. This technique, with the ability to monitor eye movements over weeks to months, appears ideal for development of rodent models of reflex adaptation which require observation of reflex behavior over extended periods of time. Development of a chronic procedure for monitoring eye movement in rodents is especially important given their initial response to restraint (extensive struggling). Finally, adaptation of this technique to smaller species (e.g., mouse) appears technically feasible which should permit the application of transgenic and knockout techniques to the determination of various vestibular reflex functions requiring long-term monitoring.

Inertial, substratal and landmark cue control of hippocampal CA1 place cell activity

Hippocampal 'place cells' discharge when a rat occupies a location that is fixed in relation to environmental landmarks. A principal goal of this study was to determine whether hippocampal place cell activity could be influenced by inertial cues. Water-deprived rats were trained in a square-walled open field in a dark room. The behavioral task required alternating visits to water reservoirs in the centre and in the four corners of the arena. The rat and arena were rotated in total darkness through +/-90, 180 or 270 degrees C. The next water reward was then presented in the corner at the same position relative to the outside room as before the rotation. A cue card was later illuminated in this corner as a visual cue for the extra-arena (room) reference frame. Fifteen out of 97 recorded hippocampal CA1 complex spike cells had spatially selective discharges in non-central parts of the arena. After arena rotations, the firing fields of three units shifted between corners of the arena to maintain a fixed orientation relative to the room. This indicates that the hippocampus updated its representation of the position and heading direction of the rat using vestibular-derived inputs concerning rotation angle. Other spatially selective discharges were guided to landmark cues (cue card or position of the reward: two units) or arena-locked 'substratal' cues (eight units). In six cells, place cell activity suddenly ceased or appeared following rotations. These results provide evidence for contributions of inertial as well as substratal and landmark information to hippocampal spatial representations.

The role of compensatory eye and head movements in the rat for image stabilization and gaze orientation

Compensatory horizontal eye movements of head restrained rats were compared with compensatory horizontal eye-head movements of partially restrained rats (head movements limited to the horizontal plane). Responses were evoked by constant velocity optokinetic and vestibular stimuli (10-60 degrees/s) and recorded with search coils in a rotating magnetic field. Velocity and position components of eye and head responses were analysed. The velocity gains of optokinetic and vestibular responses of partially restrained and of head restrained rats were similarly high (between 0.8 and 1.0). Eye movements in partially restrained rats also contributed most (about 80%) to the velocity components of the responses. At stimulus velocities above 10 degrees/s, the "beating field" of the evoked optokinetic and vestibular nystagmus was shifted transiently in the direction of ocular quick phases. The amplitude of this shift of the line of sight was about 3-10 degrees in head restrained and about 20-30 degrees in partially head restrained rats. Most of this large, transient gaze shift (about 80%) was accomplished by head movements. We interpret this gaze shift as an orienting response, and conclude that the recruitment of the ocular and the neck motor systems can be independent and task specific: head movements are primarily used to orient eye, ear and nose towards a sector of particular relevance, whereas eye movements provide the higher frequency dynamics for image stabilization and vergence movements.

Spatial Organization of the Maculo-Ocular Reflex of the Rat: Responses During Off-Vertical Axis Rotation

Pigmented, head restrained rats were rotated on a turntable about a tilted axis (off-vertical axis rotation; OVAR) in darkness. Evoked eye movements in the horizontal, vertical and torsional planes were recorded simultaneously with a dual search coil in a magnetic field, horizontal response components of both eyes were recorded with a coil on either eye. OVAR resulted in a persisting horizontal, unidirectional ocular nystagmus, compensatory in direction for the rotation of head in space. Superimposed upon this nystagmus were slower cyclic responses of the eye in the vertical and torsional movement planes, that were tightly phase locked with changing head positions in space: ocular depression/elevation with right ear up/down and ocular intorsion/extorsion with nose up/down. Simultaneous recordings of horizontal response components from both eyes revealed phase and gain differences between the horizontal movement components of both eyes, that resulted in a cyclic modulation of the vergence angle. Convergence of the lines of sight during nose up and divergence during nose down, adequate compensatory responses in light for changes in the viewing distance, were actually observed in darkness. Thus the utricular maculo-ocular reflex takes part of the visual consequences of a translational gaze shift into account. It reduces expected retinal disparities by appropriate and rapid vertical, torsional and vergence response components in the same way as canal-ocular reflexes 'compensate' for direction and velocity of expected retinal image slip during head rotation.