ISSN:0006-8977    

DOI:10.1159/000115899

出版商:S. Karger AG    

年代:1989

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000115900

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

According to the ‘equilibrium trajectory'' hypothesis, the generation of multi-joint arm movements by the CNS involves the gradual shifting of the hand equilibrium position between the movement end points. This work presents a model which suggests that reaching movements are explicitly planned in terms of spatially and temporally invariant hand equilibrium trajectories. Implementing the model in computer simulations, and using stiffness parameters which were measured during arm posture, arm trajectories were simulated and compared to measured trajectories. The success of the predicted behavior in capturing the fine kinematic details of measured movements supports the validity of the proposed model for biological trajectory contro

ISSN:0006-8977    

DOI:10.1159/000115901

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

These experiments examine how human subjects use information from a target to trigger a response and to specify its trajectory. We first determined if response initiation is predicated on the prior specification of response amplitude by examining the latencies and trajectories of impulses of isometric elbow flexion aimed to one of three visual targets. We varied target predictability (simple versus choice), the urgency with which the response was required, and the level of practice. With practice, subjects could respond to unpredictable targets with the same latency as to predictable ones; the range of response amplitudes was, however, always constricted. This central tendency bias disappeared when subjects were allowed long latencies to respond to the target, suggesting that with urgency, subjects can respond before specification is complete. To determine the time course of specification, the subjects were trained to initiate force impulses in synchrony with the last of a series of predictable tones. They also attempted to match the amplitude of their force impulses to one of three unpredictable visual targets presented at randomly varying times (50–400 ms) prior to the synchronizing tone. At the shortest stimulus-response intervals, before target information could be processed, the amplitudes of responses to all targets were clustered around that of the middle-sized target. Then, as the stimuli-response interval increased, response amplitudes gradually converged upon their specific targets. Specification started at stimulus-response intervals of about 100 ms and extended until about 350 ms. We conclude that (1) subjects prepare an adaptive or default response prior to the occurrence of the target; (2) the specification of response amplitude is a gradual process by which the dimensions of the default are adjusted according to target information, and (3) amplitude specification begins earlier and terminates later than the time needed to initiate the motor response or reaction tim

ISSN:0006-8977    

DOI:10.1159/000115902

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

The dorsolateral basilar pontine grey is assumed to play an essential role in a cortico-ponto-cerebello-pontine pathway subserving smooth-pursuit eye movements. The dorsolateral basilar pontine grey interconnects those cerebral and cerebellar cortical areas known to be involved in the generation of smooth-pursuit eye movements. In the present study three categories of neurons presumably contributing to smooth pursuit eye movements were recorded: visual-only neurons, visual-tracking neurons, and neurons combining both properties. Preference for directions of visual stimulation could be either iso- or antidirectional.

ISSN:0006-8977    

DOI:10.1159/000115903

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

In most normal visual search situations, the environment is filled with a variety of stimuli, and selection among possible saccade targets is necessary. Current theories of visuomotor mechanisms must take into account not only the classical ''retinal error'' input signal, but also perceptual and decisional factors as well as task-specific strategies adopted by the subjects. Experiments are reported in which eye movements are recorded in tasks requiring ocular saccades to be made onto target letters indicated by a peripherally visible mark or embedded within lines of homogeneous background letters. The results show how nonsensory factors interact with visual determinants in the preparation of the exploratory saccades. Expectations concerning the visibility of the sought-for target influence the spatial and temporal parameters of the eye movement which will be executed.

ISSN:0006-8977    

DOI:10.1159/000115904

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

Previous studies indicate that in response to a step-ramp visual target movement, the saccade amplitude approximates target displacement 100 ms before saccade onset. This study examines whether the saccadic system takes target motion into consideration when computing saccadic amplitude, if target movement is seen by the subject before he is requested to make a saccade. In the first experiment, while the subject fixated at the target (laser dot) and maintained fixation, the target jumped to the left and moved to the right at a fixed velocity. At some predetermined site the target jumped a step to the right and continued to move in that direction. After the target step, the subject had to make a saccade and follow target motion. In the second experiment, while the subject fixated at the target and maintained fixation, the target jumped to the right and up and moved down at a fixed velocity. At some predetermined site, an auditory signal was given, and the subject had to make a saccade and follow target motion. Results in both experiments showed that the eye position was statistically different from the target displacement at 100 ms before saccade onset, indicating that the saccadic system uses target velocity in computing saccade amplitude.

ISSN:0006-8977    

DOI:10.1159/000115905

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

A quantitative model for the generation of saccades is presented which is based on the accepted notion that saccade metrics are coded in the deeper layers of the superior colliculus by the population of recruited movement-related cells. The model has important features derived from electrophysiological data in the monkey: a nonhomogeneous and anisotropic afferent-mapping function relating the outside visual world to collicular coordinates and a dome-shaped activity profile in the deeper layers describing the spatial extent of the recruited movement-related cell population. It is proposed that each cell generates a small movement contribution which is determined both by the neuron''s location in the collicular map and its firing rate. The individual cell contributions are then added vectorially to yield the total saccade. The model can generate saccades to single visual stimuli in all directions and simulate decimal stimulation experiments. To account for some nonlinear properties of the saccadic system in the computation of saccade metrics, a nonlinear version of the linear summation model is discussed.

ISSN:0006-8977    

DOI:10.1159/000115906

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

Cats were tested for their ability to follow a dot target which periodically jumps horizontally from side to side. They can learn to perform saccadic tracking of various stimulus frequencies, and often they produce saccades predicting the target displacements. The preferred stimulus frequency for response to synchronize was in the 0.5-Hz range. Cats tend to undershoot the target. They also tend to adopt a strategy of locking predominantly to one direction of displacement, but they can be trained to reverse this preferred side or to lock to both sides. Saccades to the preferred side are more accurate, and longer and slower in comparison to saccades to the other side, thus demonstrating a velocity-accuracy trade-off phenomenon in the cat responses. Saccadic characteristics as expressed by the main sequence are apparently a stable property independent of the stimulus frequency.

ISSN:0006-8977    

DOI:10.1159/000115907

出版商:S. Karger AG    

年代:1989

数据来源: Karger

摘要:

Coherent motion of random dot pattern across a stationary stereograting at 1–5.5 deg/s causes an impairment in perceiving the stereograting which is associated with optokinetic nystagmus. This study was aimed towards understanding the cause of the impairment. We tested two alternative hypotheses: (1) that the impairment is caused by efferent-afferent interactions and (2) that it is due to a temporal integration process, the effect of which is expressed at the temporal resolution limit of the stereoscopic mechanism to disparity alternation. The first hypothesis was rejected on the basis of modified displays and experimental conditions which clearly showed that in these displays stereopsis was not impaired in the presence of optokinetic nystagmus. In testing the second hypothesis, we first determined, for the original display, the threshold values of spatial frequency and angular velocity at which stereopsis ceased. We found for these values spatial frequency × angular velocity = 7.2 cycles/s, i.e., a constant limiting alternation rate for all angular velocity values tested. The averaging effect at the critical alternation rate was demonstrated by bisecting the display, each part having a different disparity value. The perceived depth levels of the two parts were different and are in accordance with an averaging process explanation. It is, therefore, argued that the cause of the impairment is a temporal integration process which averages the alternating disparity values of the moving do

ISSN:0006-8977    

DOI:10.1159/000115908

出版商:S. Karger AG    

年代:1989

数据来源: Karger