Dynamic adaptation of the blind pointing characteristic to stepwise lateral tilts of body, head, and trunk

Assessment of Upper Limb Neuromuscular Control via a Target-Pointing Task: A Reliability Study

Objective:

To examine the accuracy and reliability of upper limb target pointing among normal subjects.

Design:

Prospective observational.

Setting:

Community sports-medicine practice.

Participants:

28 male and female normal right-handed volunteers age 22–35 years.

Intervention:

8-point target-pointing task completed with both upper limbs.

Main Outcome Measures:

Accuracy of point reproduction (cm error) and reliability over time (ICC2,1).

Results:

Target-pointing errors were 4.8–9.9 cm. Subject error and reach height explained 88% of performance variability. Error was greater when pointing to the lower half of the target (P< .05) and to ipsilateral points (P< .05). Gender, test day, reach length, and arm dominance did not affect accuracy. Test–retest reliability ranged from .30 to .71.

Conclusion:

Target-pointing tasks might be useful to assess upper limb neuromuscular control. Points with lower errors and greater reliabilities might be useful to differentiate normal vs abnormal performances, whereas a battery of reliable points over a spectrum of errors might be useful to document changes over time.

A Self-Organizing Neural Model of Motor Equivalent Reaching and Tool Use by a Multijoint Arm

This paper describes a self-organizing neural model for eye-hand coordination. Called the DIRECT model, it embodies a solution of the classical motor equivalence problem. Motor equivalence computations allow humans and other animals to flexibly employ an arm with more degrees of freedom than the space in which it moves to carry out spatially defined tasks under conditions that may require novel joint configurations. During a motor babbling phase, the model endogenously generates movement commands that activate the correlated visual, spatial, and motor information that are used to learn its internal coordinate transformations. After learning occurs, the model is capable of controlling reaching movements of the arm to prescribed spatial targets using many different combinations of joints. When allowed visual feedback, the model can automatically perform, without additional learning, reaches with tools of variable lengths, with clamped joints, with distortions of visual input by a prism, and with unexpected perturbations. These compensatory computations occur within a single accurate reaching movement. No corrective movements are needed. Blind reaches using internal feedback have also been simulated. The model achieves its competence by transforming visual information about target position and end effector position in 3-D space into a body-centered spatial representation of the direction in 3-D space that the end effector must move to contact the target. The spatial direction vector is adaptively transformed into a motor direction vector, which represents the joint rotations that move the end effector in the desired spatial direction from the present arm configuration. Properties of the model are compared with psychophysical data on human reaching movements, neurophysiological data on the tuning curves of neurons in the monkey motor cortex, and alternative models of movement control.

Vestibular-neck interaction of human ocular counterroll

Compensatory rolling of the eye about the line of sight (ocular counterroll, OCR) was studied in four normal human subjects during 10 min of lateral tilt (20 degrees to the left/right) of body, head and trunk. OCR was objectively measured at 1-min intervals by determining the rotation of a circular sample of iris pattern using a videocamera and digital image processing. Body and head tilt caused symmetrical OCR (range 2.4 to 9.5 degrees) to either side, whereas trunk tilt induced no significant change in torsional eye position. No adaptation in OCR was found during the time spent in tilted orientation in neither paradigm though unsystematic torsional fluctuation was a regular finding in all subjects. These findings are taken as evidence that only the non-adapting otolith system, as opposed to the proprioceptive system of the neck, contributes to the generation and maintenance of OCR.

Control of arm movements in a 2-dimensional pointing task

The present study analyses in humans the control principles of sequential, unpracticed pointing movements in a 2-dimensional space. Our data reveal that variable pointing errors add up within such sequences. This finding supports the hypothesis that movement amplitude rather than position is the controlled variable of the investigated movements.

Interaction of visual and non-visual signals in the initiation of smooth pursuit eye movements in primates

The initiation of smooth pursuit eye movements (PEM) by visual and non-visual signals was analysed in humans and monkeys. While PEM latency ranged around 150 ms when a purely visual target was provided, it often dropped to about 0 ms, or even became negative, when target movement was coupled to the subject's arm; this suggests that signals about the intention to move the arm can be evaluated for PEM control. Eye movements always started in the visually correct direction, independent of the sign of coupling between arm and target; from this we conclude that intentional signals are not mere triggers, but also convey directional information. Short-latency PEM trials were intermixed with those characterized by normal latencies, which often resulted in bimodal latency distributions; this suggests that visual and intentional signals compete for the control of PEM.