Control of Joint Rotations in Overarm Throws of Different Speeds Made by Dominant and Nondominant Arms

We tested the hypothesis that dominant and nondominant overarm throws of different speeds are made by time-scaling of joint rotations, i.e., by joint rotations that have the same positions and amplitudes but that are scaled in time. Eight skilled subjects stood and made overarm throws with both their dominant and nondominant arms. Six joint rotations were computed from recordings of arm segments made with the search-coil technique. Throws made with nondominant arms were less accurate and had lower ball speeds. In contrast to the hypothesis, dominant arms showed large and consistent differences between fast and slow throws in six-dimensional angular position joint space. These same throws showed similar hand angular paths when these were time-scaled based on ball speed. Nondominant arms showed only small differences in angular position joint space in fast and slow throws. It is concluded that a joint space pattern resembling that predicted by time-scaling occurs in nondominant arm throwing when it is unskilled. However, time-scaling does not occur in dominant arm throwing, i.e., a skilled fast throw is not simply a skilled slow throw whose joint positions and amplitudes remain constant but whose joint velocities are sped-up. We hypothesize for future study that, when subjects first learn to throw at different speeds with their dominant arms, they use time-scaling of joint rotations that involves compensating for interaction torques; then as they become skilled at throwing fast, time-scaling is superseded by a more complex pattern of interjoint coordination that involves exploiting interaction torques.

Kinematic Comparison of Spontaneously Generated Blinks and Voluntary Blinks in Normal Adult Subjects

Purpose: Spontaneous and voluntary blinks share a common final neuromuscular pathway but have totally different purposes and have come under different evolutionary pressures. The purpose of this study is to compare quantitative blink kinematics (down-phase duration, amplitude, peak velocity, and lid-closure duration) of spontaneously generated blinks with voluntary blinks in normal adults. Methods: Spontaneous and voluntary (tone generated) blinks of the right eye of 7 subjects were studied by a modified scleral search coil technique. Automated analysis of each blink kinematic was performed and statistical analysis of pooled data was undertaken. Results: All kinematics of voluntary blinks were greater than those of spontaneous blinks: duration (77.6 ± 10.0 milliseconds) was 17.6% longer, amplitude (43.1 ± 7.9°) was 40.8% larger, peak velocity (1288.6 ± 358.5°/ms) was 47.3% faster, and mean lid-closure duration (13.4 ± 4.7 milliseconds) was 61.4% longer (P < .001 in all cases). Conclusion: Kinematics of voluntary and spontaneous blinks are significantly different, reflecting their different supranuclear control. This quantitative study confirms previous qualitative observations and clearly separates these categories of eyelid movement as distinct, with spontaneous blinks serving a purely physiologic function and voluntary blinks being a part of facial expressivity.

Psychophysical Investigation of an Auditory Spatial Illusion in Cats: The Precedence Effect

The precedence effect (PE) describes several spatial perceptual phenomena that occur when similar sounds are presented from two different locations and separated by a delay. The mechanisms that produce the effect are thought to be responsible for the ability to localize sounds in reverberant environments. Although the physiological bases for the PE have been studied, little is known about how these sounds are localized by species other than humans. Here we used the search coil technique to measure the eye positions of cats trained to saccade to the apparent locations of sounds. To study the PE, brief broadband stimuli were presented from two locations, with a delay between their onsets; the delayed sound meant to simulate a single reflection. Although the cats accurately localized single sources, the apparent locations of the paired sources depended on the delay. First, the cats exhibited summing localization, the perception of a “phantom” sound located between the sources, for delays < ±400 μs for sources positioned in azimuth along the horizontal plane, but not for sources positioned in elevation along the sagittal plane. Second, consistent with localization dominance, for delays from 400 μs to about 10 ms, the cats oriented toward the leading source location only, with little influence of the lagging source, both for horizontally and vertically placed sources. Finally, the echo threshold was reached for delays >10 ms, where the cats first began to orient to the lagging source on some trials. These data reveal that cats experience the PE phenomena similarly to humans.

Scleral Search Coils Influence Saccade Dynamics

The scleral search coil technique is commonly used for recording eye movements. The goal of this paper is to investigate to what extent the placement of scleral search coils onto the eyes influences the kinematics of saccades. To that end saccadic eye movements of human subjects were recorded with an infrared video system, while they wore coils and we compared the main sequence properties with recordings in which no coils were mounted on the eyes. It was found that saccades last longer (by about 8%) and become slower (by about 5%) when both eyes wear coils. This is truly due to the fact that the coils are on the eyes and not due to other factors that are part of this method, such as the scleral anesthesia. The influence of coils in both eyes was also observed when one coil was mounted on one eye only. Therefore the effect that the coils have on the eye movements cannot be attributed to purely mechanical factors, such as inertial load on the eyeball or increased friction. Rather the coils appear to change the oculomotor command signals that drive the saccadic eye movements.