The vergence eye movements induced by radial optic flow: Some fundamental properties of the underlying local-motion detectors

AbstractRadial optic flow applied to large random dot patterns is known to elicit horizontal vergence eye movements at short latency, expansion causing convergence and contraction causing divergence: the Radial Flow Vergence Response (RFVR). We elicited RFVRs in human subjects by applying radial motion to concentric circular patterns whose radial luminance modulation was that of a square wave lacking the fundamental: the missing fundamental (mf) stimulus. The radial motion consisted of successive ¼–wavelength steps, so that the overall pattern and the 4n+1 harmonics (where n =integer) underwent radial expansion (or contraction), whereas the 4n−1 harmonics—including the strongest Fourier component (the 3rd harmonic)—underwent the opposite radial motion. Radial motion commenced only after the subject had fixated the center of the pattern. The initial RFVRs were always in the direction of the 3rd harmonic, e.g., expansion of the mf pattern causing divergence. Thus, the earliest RFVRs were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. If the radial mf stimulus was reduced to just two competing harmonics—the 3rd and 5th—the initial RFVRs showed a nonlinear dependence on their relative contrasts: when the two harmonics differed in contrast by more than about an octave then the one with the higher contrast completely dominated the RFVRs and the one with lower contrast lost its influence: winner-take-all. We suggest that these nonlinear interactions result from mutual inhibition between the mechanisms sensing the motion of the different competing harmonics. If single radial-flow steps were used, a brief inter-stimulus interval resulted in reversed RFVRs, consistent with the idea that the motion detectors mediating these responses receive a visual input whose temporal impulse response function is strongly biphasic. Lastly, all of these characteristics of the RFVR, which we attribute to the early cortical processing of visual motion, are known to be shared by the Ocular Following Response (OFR)—a conjugate tracking (version) response elicited at short-latency by linear motion—and even the quantitative details are generally very similar. Thus, although the RFVR and OFR respond to very different patterns of global motion—radial vs. linear—they have very similar local spatiotemporal properties as though mediated by the same low-level, local-motion detectors, which we suggest are in the striate cortex

Contribution of the subthalamic nucleus to visually guided locomotion

Les ganglions de la base (GB) jouent un rôle important dans le contrôle locomoteur. Ceci est illustré par les troubles locomoteurs dont souffrent les patients atteints de maladies dégénératives qui affectent les GB, telles que la maladie de Parkinson, caractérisées par de petits pas lents et traînants, ainsi qu’un gel de la marche (freezing of gait). Une structure centrale dans les GB est le noyau sous-thalamique (NST), de par son rôle de structure d’entrée et ses projections vers le globus pallidus. Cependant, la nature de la contribution du NST au contrôle de la locomotion, ainsi que les caractéristiques de son activité cellulaire durant la marche, sont peu connues. Afin de mieux comprendre cette contribution, nous avons examiné les propriétés de l’activité neuronale du NST lors de la locomotion non-obstruée et celle sous guidage visuel. Ainsi, nous avons effectué des enregistrements neuronaux chez un chat intact, entraîné à marcher régulièrement sur un tapis roulant et à franchir des obstacles se déplaçant à la même vitesse. Nous avons enregistré 40 cellules montrant une activité reliée au movement du membre antérieur, dont 30 ont montré une activité phasique au cours de la locomotion non obstruée liée aux différentes phases du cycle de la marche, principalement la phase de balancement. Au cours de la modification volontaire de la marche, un groupe de 37/40 cellules, incluant certaines qui étaient modulées pendant la locomotion non-obstruée, ont changé leur fréquence de décharge par rapport à l’obstacle. Ces changement étaient principalement des augmentations de fréquence, mais parfois des diminutions ou une diminution suivie d’une augmentation. Ces modifications se produisaient soit avant l’enjambement de l’obstacle (step-advanced), soit lors de l’enjambement de l’obstacle (step-related). L’activité des cellules step-advanced était indépendante des membres (limb-independent), tandis que celle des cellules step-related était spécifique aux membres (limb-dependent). Cette étude est la première à examiner les caractéristiques de décharge du NST lors de la marche et montre que cette structure contribue au contrôle de la locomotion non obstruée ainsi que la modification volontaire de la marche, en jouant un rôle dans la planification et l’exécution de cette dernière.The Basal ganglia (BG) plays an important role in locomotor control. This is emphasized by the impaired walking of patients with neurodegenerative disorders that affect the BG such as Parkinson’s disease. One important structure in the BG is the subthalamic nucleus (STN), which acts as an input structure for the BG and projects to its output structures. Although the STN has been shown to display movement-related activity during reaching, the nature of its contribution to the control of locomotion, together with the characteristics of its neural activity during locomotion, is poorly known. In order to better understand this contribution, we examined the properties of the neural activity in the STN during unobstructed and visually guided locomotion. To do so, we recorded single neurons in an intact cat trained to walk steadily on a treadmill and to step over obstacles attached to the treadmill belt and moving at the same speed. We recorded 40 neurons which activity was related to the movement of the forelimb during the task. We found that during unobstructed locomotion, many of these cells (30/40) showed phasic step-by-step modulation of their activity pattern, mostly during the swing phase. Most of these swing-related cells discharged throughout the swing phase with no relationship to changes in the pattern of different muscle groups. During voluntary modifications of gait, 37/40 cells, including both cells that were and were not modulated during unobstructed locomotion, changed their firing rate in relationship to the step over the obstacle. The changes observed were mostly increases of activity, but a few cells showed decreases of activity and some showed a decrease followed by an increase of activity. These changes occurred either before the modified step and were classified as step-advanced activity, or they occurred during the modified step and were classified as step-related activity. Step advanced cells mostly showed limb-independent activity, while step related cells showed limb-specific activity. This is the first detailed account of the contribution of the STN to the control of locomotion and our results indicate that the STN is involved in the control of both unobstructed and visually guided locomotion. The results suggest that during unobstructed locomotion, the STN contributes to the general control of the limb trajectory and to both the planning and execution of voluntary changes of gait

A neural network study of precollicular saccadic averaging

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Aerospace medicine and biology, an annotated bibliography. volume xi- 1962-1963 literature

Aerospace medicine and biology - annotated bibliography for 1962 and 196