Biomedical subjects
Publications and source records attributed to D J Weir.
Selected surface characteristics of screw-type titanium dental implants from four different manufacturers were evaluated. Considerable differences in surface and near-surface contaminants were demonstrated. The fixture treated with radiofrequency glow discharge (plasma) demonstrated the thinnest titanium oxide layer and the cleanest surface.
Monkeys trained to track a continuously moving visual target with a joystick do so by making a series of intermittent positional corrections rather than in a single smooth movement. The amplitude of each correction is highly correlated both with the error between the target and joystick positions, and with the velocity of the target, measured at movement onset. This velocity estimate is used to predict where the target will be by the end of each movement, and thus helps to set its amplitude correctly. To do this successfully, the monkey must know in advance how long his next movement will take. But, confusingly, the eventual duration of each movement is also highly correlated with its amplitude. So it appears that the monkeys need to simultaneously know the amplitude and duration of a movement, but cannot determine one without prior knowledge of the other. We have examined two possible solutions to this problem; only one agrees with our data. The monkeys seem to select the amplitude of their movements by scaling target velocity by a standard time constant which gives the additional distance the target will move. They then add this to the positional error estimated at or near to the start of each movement, to get the final movement amplitude. The velocity scaling value that gives the best fit to the observed amplitudes is very close to the average duration of all the monkeys movements. We therefore propose that the monkeys use a standard time constant for the purpose of calculating how far the target will move during each of their positional corrections.(ABSTRACT TRUNCATED AT 250 WORDS)
Two monkeys were trained to track a continuously moving target using a joystick. One then had a cooling probe implanted in nucleus interpositus of the cerebellum ipsilateral to his tracking arm. The other had a cannula implanted in the ipsilateral cortex of the lateral cerebellum through which local anaesthetic could be infused. Both monkeys showed similar tracking deficits during temporary inactivation of the cerebellum. The main effects seen were an increase in the peak velocity of their intermittent corrective tracking movements, and a decrease in the accuracy of these movements. Linear regression analyses were undertaken of the peak velocity and amplitude of each corrective movement against a number of possible control signals (target velocity, target position, error, error velocity etc.). The initially strong correlation of the amplitude of each movement made with target velocity was severely reduced during cerebellar inactivation, and movement amplitude became better predicted by the error between target and joystick positions. The peak velocity of movements became more strongly correlated with movement amplitude and less correlated with target velocity than in the intact animal. These results are consistent with the hypothesis that intermittent tracking is achieved by the production of 'primitive' movements, that are then adjusted to the correct amplitude and velocity required to catch up with the moving target. Our findings suggest that the cerebellum may normally be responsible for these adjustments, using visual and memorised cues about the target. The velocity of each movement may be reduced, and its amplitude adjusted, by combining measures of the current error with estimates of target speed and direction. We conclude that the cerebellum has an inhibitory role in tuning movements during visuo-motor tasks and that optimal tuning using feedforward measurements of target motion cannot be made without it.
Monkeys were trained to track moving visual targets using a hand-held joystick. The overall frequency response of their visuomotor system was determined using sinusoidal target waveforms. Their responses could be approximately represented by a linear feedforward model consisting of a 0.9 Hz low-pass filter with an additional 150 ms time delay. However, the monkeys normally tracked the target by making intermittent movements of the joystick. Thus, their responses were more realistically modelled as a non-linear sampled feedback model with a loop delay of 250-280 ms. Intermittency allows the monkeys to achieve a good frequency response and maintain tracking stability despite an irreducible visuomotor loop delay of 250-300 ms. When tracking pseudorandom waveforms the monkeys' movements were mainly controlled by positional error. But when tracking predictable sinusoids the amplitude and velocity of each movement was not solely determined by positional error. Instead the monkeys made use of target feedforward, and also internal models of the target waveform, in order to improve their tracking performance. Feedforward control dominated feedback control at high target frequencies, suggesting that the monkeys cannot model targets with long cycle periods.
A rhesus monkey and five human subjects used a hand-held joystick to track unpredictable continuously moving targets. Both monkey and human respond by making discrete ("step-and-hold") corrections of positional error, at an average frequency of 1.33 and 2.26 movements/second, respectively. By delaying visual feedback of joystick position, we could reduce these frequencies in a predictable manner. These results imply that the primate visuomotor system probably does not operate as a "sampled-data mechanism" governed by an asynchronous clock, but that inevitable delays in visuomotor feedback control determine the frequency of corrective movements.
The following conclusions were made after statistical analysis. There was no significant difference between racemic epinephrine cord and aluminum sulfate cord. Both racemic epinephrine cord and aluminum sulfate cord were more effective than nonmedicated cord. There was no difference between dry cord and water-saturated cord. Hemorrhage control with a cord saturated in Hemodent was more effective than water-saturated or dry cords. The racemic epinephrine cord was not superior to the aluminum sulfate cord, so the dentist could eliminate a cord with potential systemic effects and substitute the more innocuous cord. Hemodent approximately doubled the hemostasis success of any cord used in this study.