Background: When reaching towards a visual target, a sudden jump of the target can elicit an implicit motor tracking response with a short latency. This target jump response (TJR) has been considered a fast online control mechanism of the central nervous system, which enables the automatic compensation of a motor error caused by the movement of the target. The effect of aging on TJR has not been studied extensively. Purpose: This study investigated age-related changes in the spatial accuracy of TJR from the perspective of visuomotor coordinate transformation, and relationship between the TJR directional error and endpoint variability. Methods: The participants were 25 healthy elderly [mean+/-SD=70+/-5 years] and 25 young (23+/-4 years) adults. The right hand was used to reach and touch a visual target presented in the center of a screen located 0.5min front of the participant. After the initiation of reach motion, the target presented in the center either shifted (5degree right, left, upward, or downward the screen center, 36 trials for each location) or remained at the same spot (48 trials). In the target-shift trial, the participants were required to correct their reaching trajectory towards a new target location as quickly as possible. Arm kinematics during reaching was monitored using a three-dimensional motion capture system (500 Hz), and its velocity was computed subsequently. The directional error of TJR was obtained by the angle between the direction of target-shift and the vector of hand velocity computed from the time of initial TJR to the TJR peak time before the onset of voluntary correction movement. Results: For the mean directional error of TJR in absolute value, ANOVA (group x direction) revealed significant effect of direction while neither group nor group x direction interaction was significant. For the mean inter-trial variability of the directional error, main effects of group and direction were significant. The elderly group had greater inter-trial variability of directional error than the young group. This directional error variability was significantly correlated with the endpoint's inter-trial variability for the young group, whereas that for the elderly group was non-significant. Conclusion(s): Healthy elderly adults were less consistent in making trial to trial directional errors during TJR compared to young controls. Relationship in variability of the TJR's directional error with that of end point error was disrupted with aging. The effective use of fast and reflexive visuomotor transformation to slow voluntary guiding of the arm towards the target is thus lost in elderly adults. Implications: The methods developed in this study can be used to investigate modulation of visuomotor transformation in stroke patients with hemispatial neglect, visual attention disorders, and other neurological problems. It can also be of interest to examine autism children who exhibit deficits in nonverbal skills such as visuao-spatial problem solving, and visuo-motor coordination.
Kimura, D., Kadota, K., Hiramatsu, Y., & Kinoshita, H. (2015). Aging effect on a functional role of reflexive corrective movement during target reaching. Physiotherapy, 101, e755. https://doi.org/10.1016/j.physio.2015.03.3624