Search PubMed⌕ Search

Biomedical subjects

J F Stein

Publications and source records attributed to J F Stein.

At least 73 records · Page 4Linked to original sources
PubMed1993

Ocular limit cycles induced by delayed retinal feedback.

Lisberger's and Robinson's models of smooth pursuit predict very different results from altering retinal feedback delay. We have therefore investigated the effects of increasing the retinal feedback time delay in three normal human subjects by means of an artificial feedback paradigm. When additional delays were incorporated into the retinal feedback path a threshold was reached beyond which the eye exhibited sustained self-excited oscillations or "limit cycles". The oscillation period increased linearly (as the added delay was increased) with slopes ranging from 1.41 to 1.6 with zero-delay intercepts of between 212 and 306 ms. Contrary to our experimental findings the Robinson and Lisberger models predict that the plot of oscillation period against added delay should have a slope of 3.4 and 2.7 and an intercept of 479 and 554 ms, respectively. Neither model produced comparable limit cycles, both being unstable at delays greater than 280 ms. Our results imply that the models of smooth pursuit need to incorporate predictive control.

Adult↗
PubMed1993

Intermittency in human manual tracking tasks.

We confirm Craik's (1947) observation that the human manual1y tracking a visual target behaves like an intermittent servo-control1er. Such tracking responses are indicative of "sampled" negative-feedback control but could be the result of other, continuous, mechanisms. Tracking performance therefore was recorded in a task in which visual feedback of the position of the hand-held joystick could be eliminated. Depriving the subjects of visual feedback led to smoother tracking and greatly reduced the signal power of their responses between 0.5-1.8 Hz. Their responses remained intermittent when they used feedback of their own position but not of the target to track a remembered (virtual) target. Hence, intermittency in tracking behavior is not exclusively a signature of visual feedback control but also may be a sign of feedback to memorized waveforms. Craik's (1947) suggestion that the intermittency is due to a refractory period following each movement was also tested. The errors measured at the start of each intermittent response, during tracking of slow waveforms, showed evidence of a small error deadzone (measuring 0.7 cm on the VDU screen or 0.80 degrees at the eye). At higher target speeds, however, the mean size of starting errors increased, and the upper boundary of the distribution of starting error was close to that expected of a refractory delay of approximately 170 ms between responses. We consider a model of the control system that can fit these results by incorporating an error deadzone within a feedback control loop. We therefore propose that the initiation of intermittent tracking responses may be limited by a positional error deadzone and that evidence for a refractory period between successive corrective movements can be satisfied without evoking an explicit timing or sampling mechanism.

Journal Article↗
PubMed1993

Is the cerebellum a smith predictor?

The motor system may use internal predictive models of the motor apparatus to achieve better control than would be possible by negative feedback. Several theories have proposed that the cerebellum may form these predictive representations. In this article, we review these theories and try to unify them by reference to an engineering control model known as a Smith Predictor. We suggest that the cerebellum forms two types of internal model. One model is a forward predictive model of the motor apparatus (e.g., limb and muscle), providing a rapid prediction of the sensory consequences of each movement. The second model is of the time delays in the control loop (due to receptor and effector delays, axonal conductances, and cognitive processing delays). This model delays a copy of the rapid prediction so that it can be compared in temporal register with actual sensory feedback from the movement. The result of this comparison is used both to correct for errors in performance and as a training signal to learn the first model. We discuss evidence that the cerebellum could form both of these models and suggest that the cerebellum may hold at least two separate Smith Predictors. One, in the lateral cerebellum, would predict the movement outcome in visual, egocentric, or peripersonal coordinates. Another, in the intermediate cerebellum, would predict the consequences in motor coordinates. Generalization of the Smith Predictor theory is discussed in light of cerebellar involvement in nonmotor control systems, including autonomic functions and cognition.

Journal Article↗
PubMed1992

Covert orienting to central visual cues and sport practice relations in the development of visual attention.

This study examines the development of controlled covert orienting of visual attention, according to the age and the level of performance of tennis players. Practicers and nonpracticers age 13, 16, and 25 participated in a covert orienting task. On each trial, subjects responded to a target which appeared in one of four locations arrayed horizontally across the display. Three central cues were used: neutral, the cue did not specify in which location the target would appear; valid, the target was present in the cued location on 80% of the trials; invalid, the target was present in one of the uncued locations on the remaining 20% of the trials. The results showed that (1) practicers were not faster than nonpracticers in processing the signals, (2) observers of all ages oriented attention voluntarily to the cued locations, (3) central cues had a smaller effect on older and practiced subjects, and (4) developmental and sport practice factors had similar effects on orienting efficiency. The implications of these findings for theories of attentional development are discussed.

Adolescent↗
PubMed1992

Evidence for an error deadzone in compensatory tracking.

Humans and monkeys show intermittent arm movements while tracking moving targets. This intermittency has been explained by postulating either a psychological refractory period after each movement and/or an error deadzone, an area surrounding the target within which movements are not initiated. We present a technique to detect and quantify the size of this deadzone, using a compensatory tracking paradigm that distinguishes it from a psychological refractory period. An artificial deadzone of variable size was added around a visual target displayed on a computer screen. While the subject was within this area, he received visual feedback that showed him to be directly on target. The presence of this artificial deadzone could affect tracking performance only if it exceeded the size of his intrinsic deadzone. Therefore, the size of artificial deadzone at which performance began to be affected revealed the size of the intrinsic deadzone. Measured at the subjects' eye, the deadzone was found to vary between 0.06 and 0.38 degrees, depending on the tracking task and viewing conditions; on the screen, this range was 1.3 mm to 3.3 mm. It increased with increasing speed of the target, with increasing viewing distance, and when the amplitude of the movement required was reduced. However, the deadzone size was not significantly correlated with the subjects' level of performance. We conclude that an intrinsic deadzone exists during compensatory tracking, and we suggest that its size is set by a cognitive process not simply related to the difficulty of the tracking task.

Journal Article↗
PubMed1992

Does attention modulate the perception of luminance changes?

In a previous study assessing the modulation of visual processing by attention, Bonnel, Possamî, and Schmitt showed that, when discriminating line-length, subjects precisely shared processing resources between two pairs of lines presented to the left and right of fixation. In a close replication requiring the detection of luminance increments instead of line-length differences, subjects were unable to follow the instructions and to allocate attention differentially, thus supporting the claim that light detection is fundamentally different from shape discrimination. In a subsequent experiment, we tested and rejected the possibility that luminance perception was not open to modulation by attention due to its physical nature. Replacing brightness detection by brightness identification allowed voluntary control on the quality of processing to be evidenced. The similarity between the latter results and the data from line-length discrimination suggests that task requirements may be crucial in determining the distribution of attention.

Attention↗
PubMed1992

Effect of potassium on ventilation in the rhesus monkey.

Increasing the concentration of arterial plasma K+ to 6-8 mM increased ventilation in two sedated analgesic-treated rhesus monkeys who had their end-tidal CO2 held constant during euoxia (arterial oxygen pressure, Pa,O2, ca 100 Torr) and hypoxia (Pa,O2, ca 40 Torr). During euoxia and hypoxia, hyperkalaemia increased ventilation up to 40 and 250%, respectively. This effect was reduced in euoxia and virtually abolished in hypoxia following an abrupt switch to 100% oxygen. Thus the ventilatory response of this primate to hyperkalaemia is at least as sensitive as that of the cat and if hypoxia is added the two stimuli generate a powerful drive to breathing.

Animals↗
PubMed1991

Yellow spectacles to improve vision in children with binocular amblyopia.

Yellow spectacles were given to 20 children who had binocular amblyopia. Immediately, their vision improved. Furthermore, in the younger children, after they had worn the yellow lenses for up to nine months, this improvement persisted even when they were not wearing the spectacles. Yellow filters may help these children with binocular amblyopia because they remove the blue fringes that are due to chromatic aberration of the eye.

Adolescent↗
PubMed1990

Specialization of the right hemisphere for visuomotor control.

The accuracy of saccades directed towards the remembered positions of targets in left (LVF) or right (RVF) visual hemifield was measured. The majority of right-handed subjects were found to be more accurate at directing their gaze to locations in the LVF than in the RVF, suggesting that the right hemisphere is superior to the left in oculomotor control. Even after completion of a corrective saccade following the primary saccade, subjects systematically undershot target direction and overshot target depth, suggesting that visual feedback normally plays an important role in the fine guidance of gaze after the completion of a primary saccade.

Adult↗
PubMed1990

Neuronal activity in the lateral cerebellum of trained monkeys, related to visual stimuli or to eye movements.

1. The responses of neurones in the lateral cerebellar cortex to visual stimuli and to eye movements were recorded in rhesus monkeys trained to perform visually guided arm and eye movements in a tracking task. 2. Twenty-two of 134 units recorded (16%) modulated their discharge in response to a bright Xenon flash. They were mainly located in the dorsal paraflocculus. Among those identified as Purkinje cells both simple spike and climbing fibre responses to the flash were seen. (72% of the units were related to arm movements; these were centred in the paramedian lobule, and have been described fully in Marple-Horvat & Stein (1987).) 3. The visual responsiveness of one of the units varied according to the phase of the monkey's task. Around the time that the target stepped, which was the monkey's cue to move, its sensitivity to other stimuli disappeared. 4. Only two neurones responded to the movements of the tracking target. These responses were conditional upon the monkey using visual signals to guide his movements; they did not respond to the target step if he moved before the target did. 5. Fourteen units (10%) located in crus I and II and lobulus simplex correlated strongly with the velocity of horizontal eye movements. Only one of these also responded to visual stimuli. 6. Thus most neurones were found to carry only visual, or eye movement, or limb movement information rather than combinations of these signals; they were located in different but overlapping regions of lateral cerebellar cortex. Visually responsive neurones are probably involved in planning the visual goal of movements, while eye and arm movement neurones probably help to create co-ordinative structures for executing voluntary eye and arm movements.

Action Potentials↗
PubMed1990

Spatial discrimination in children with poor vergence control.

In a 1987 study of children with reading difficulties, Stein, Riddell, and Fowler observed a relationship between poor vergence control, as indicated on a synoptophore vergence test, and the children's problems with learning to read. They suggested that poor vergence control led to impaired accuracy of spatial localisation and that this impeded learning to read. Here we have compared the accuracy of spatial localisation on a nonlinguistic computer game by children having good and poor vergence control. The children with poor vergence control made significantly more errors when locating targets than children with good vergence control. These results lend further support to the hypothesis that some children do not learn to read because they are unable to determine accurately the positions of letters in words.

Attention↗
PubMed1989

Cues and control strategies in visually guided tracking.

Detailed quantitative models are required to investigate the neurological basis of motor behavior. Previous studies of visually guided manual tracking have either identified a variety of control signals (cues) for planning tracking movements or analyzed how a single cue is used (i.e., one-tracking strategy). A systematic, quantitative analysis of the effects and interactions of cues in terms of human manual-tracking performance is presented here together with measurements of concomitant eye movements. These measurements help define the routes by which information reaches the CNS, and the analysis elucidates how the control signals are processed and combined. The results quantify not only the large improvement in performance observed when the target waveform being tracked is predictable but also the extent to which this improvement depends on the availability of current information about target movements and positional error. Target information is shown to provide short-term prediction independent of the error signals used in on-line negative feedback control.

Journal Article↗