Ocular limit cycles induced by delayed retinal feedback

Interlimb differences of directional biases for stroke production

Directional preferences during center-out horizontal shoulder-elbow movements were previously characterized for the dominant arm. These preferences were attributed to a tendency to actively accelerate one joint, while exploiting largely passive motion at the other joint. Since the non-dominant arm is known for inefficient coordination of inter-segmental dynamics, here we hypothesized that directional preferences would differ between the arms. A center-out free-stroke drawing task was used that allowed freedom in the selection of movement directions. The task was performed both with and without a secondary cognitive task that has been shown to increase directional biases of the dominant arm. Mirror-symmetrical directional preferences were observed in both arms, with similar bias strength and secondary task effects. The preferred directions were characterized by maximal exploitation of interaction torques for movement production, but only in the dominant arm. The non-dominant arm failed to benefit from interaction torques. The results point to a hierarchical architecture of control. At the higher level, a movement capable to perform the task while satisfying preferences in joint control is specified through forward dynamic transformations. This process is mediated for both arms from a common neural network adapted to the dominant arm and, specifically, to its ability to exploit interaction torques. Dynamic transformations that determine actual control commands are specified at the lower level of control. An alternative interpretation that strokes might be planned evenly across directions, and biases emerge during movement execution due to anisotropic resistance of intrinsic factors that do not depend on arm dominance is also discussed.

Nystagmus and saccadic intrusions

We review current concepts of nystagmus and saccadic oscillations, applying a pathophysiological approach. We begin by discussing how nystagmus may arise when the mechanisms that normally hold gaze steady are impaired. We then describe the clinical and laboratory evaluation of patients with ocular oscillations. Next, we systematically review the features of nystagmus arising from peripheral and central vestibular disorders, nystagmus due to an abnormal gaze-holding mechanism (neural integrator), and nystagmus occurring when vision is compromised. We then discuss forms of nystagmus for which the pathogenesis is not well understood, including acquired pendular nystagmus and congenital forms of nystagmus. We then summarize the spectrum of saccadic disorders that disrupt steady gaze, from intrusions to flutter and opsoclonus. Finally, we review current treatment options for nystagmus and saccadic oscillations, including drugs, surgery, and optical methods. Examples of each type of nystagmus are provided in the form of figures.

Evidence for object permanence in the smooth-pursuit eye movements of monkeys

We recorded the smooth-pursuit eye movements of monkeys in response to targets that were extinguished (blinked) for 200 ms in mid-trajectory. Eye velocity declined considerably during the target blinks, even when the blinks were completely predictable in time and space. Eye velocity declined whether blinks were presented during steady-state pursuit of a constant-velocity target, during initiation of pursuit before target velocity was reached, or during eye accelerations induced by a change in target velocity. When a physical occluder covered the trajectory of the target during blinks, creating the impression that the target moved behind it, the decline in eye velocity was reduced or abolished. If the target was occluded once the eye had reached target velocity, pursuit was only slightly poorer than normal, uninterrupted pursuit. In contrast, if the target was occluded during the initiation of pursuit, while the eye was accelerating toward target velocity, pursuit during occlusion was very different from normal pursuit. Eye velocity remained relatively stable during target occlusion, showing much less acceleration than normal pursuit and much less of a decline than was produced by a target blink. Anticipatory or predictive eye acceleration was typically observed just prior to the reappearance of the target. Computer simulations show that these results are best understood by assuming that a mechanism of eye-velocity memory remains engaged during target occlusion but is disengaged during target blinks.

Rabbit classical eyeblink conditioning is altered by brief cerebellar cortical stimulation

A pair of studies examined how cortical intracerebellar stimulation (ICS) affects eyeblink conditioning in the rabbit. Rabbits were implanted with chronic bipolar stimulating electrodes in the cell body layers of cerebellar lobule H-VI. Brief (40 ms) trains of intracranial stimulation (100 Hz, 250 microA) were delivered during training trials [forward pairings of a tone-conditioned stimulus (CS) with an air puff unconditioned stimulus (US)]. In Experiment 1, the onset of ICS varied randomly within sessions. US-onset-coincident ICS proved detrimental to the maintenance of conditioning [measured as the percentage of trials on which conditioned responses (CRs) were made] compared to ICS that ended 60 ms before US onset. Based on these findings, a second experiment compared a group trained with ICS consistently delivered at US onset to groups trained with ICS consistently delivered either at CS onset or between the two stimuli, as well as to unstimulated control subjects. Animals receiving CS- or US-coincident ICS learned slowest, whereas animals receiving middle stimulation learned more quickly than all other groups. In both Experiments 1 and 2, highly trained animals produced blinks in direct response to the stimulation. These data are discussed in terms of a new hypothesis concerning interactions between cerebellar cortex and the deep cerebellar nuclei during eyeblink conditioning--a rebound from inhibition hypothesis.

Task-dependent changes in visual feedback control: a frequency analysis of human manual tracking

Prominent components in the frequency spectrum of human manual tracking responses are thought to reflect the visual feedback control loop and have been used in estimations of the visual feedback loop delay. The frequency structure of human tracking was therefore examined here in two tasks: visually guided tracking of slow and fast pseudorandom targets. Visually related frequency components were identified by testing, in each condition, the effect of adding additional feedback delays on the frequency spectrum. The major frequency components of the responses consisted of a fundamental component and its odd harmonics. These components were related to the visual feedback loop delay and shifted in concert toward lower frequencies as the feedback delay was increased. Furthermore, there were no differences in responses between 3 normal subjects and 1 subject with peripheral sensory loss. This implies that the frequency structure is dominated by the visual feedback control loop, without significant influence from proprioceptive control loops. However, the feedback-loop delay was shown to decrease from around 341 to 264 ms as the task speed doubled. Thus the estimates of visual feedback delays are influenced by the target being followed, and this suggests that the subjects can lquot;tunerquot: their feedback system to suit the demands of the tracking task.