Visual information processing in older adults: reaction time and motor unit pool modulation

Abstract

Presently, there is no evidence that magnification of visual feedback has motor implications beyond impairments in force control during a visuomotor task. We hypothesized that magnification of visual feedback would increase visual information processing, alter the muscle activation, and exacerbate the response time in older adults. To test this hypothesis, we examined whether magnification of visual feedback during a reaction time task alters the premotor time and the motor unit pool activation of older adults. Participants responded as fast as possible to a visual stimulus while they maintained a steady ankle dorsiflexion force (15% maximum) either with low-gain or high-gain visual feedback of force. We quantified the following: 1) response time and its components (premotor and motor time), 2) force variability, and 3) motor unit pool activity of the tibialis anterior muscle. Older adults exhibited longer premotor time and greater force variability than young adults. Only in older adults, magnification of visual feedback lengthened the premotor time and exacerbated force variability. The slower premotor time in older adults with high-gain visual feedback was associated with increased force variability and an altered modulation of the motor unit pool. In conclusion, our findings provide novel evidence that magnification of visual feedback also exacerbates premotor time during a reaction time task in older adults, which is correlated with force variability and an altered modulation of motor unit pool. Thus these findings suggest that visual information processing deficiencies in older adults could result in force control and reaction time impairments. NEW & NOTEWORTHY It is unknown whether magnification of visual feedback has motor implications beyond impairments in force control for older adults. We examined whether it impairs reaction time and motor unit pool activation. The findings provide novel evidence that magnification of visual feedback exacerbates reaction time by lengthening premotor time, which implicates time for information processing in older adults, which is correlated with force variability and an altered modulation of motor unit pool.

Keywords: aging; electromyogram decomposition; force variability; motor unit activity; reaction time.

Fig. 1.

A: experimental setup for testing the dorsiflexion of left ankle. B: visual display of task with low-gain visual feedback (left) and high-gain visual feedback (right). The participants were presented with unanticipated visual stimulus in the middle of the screen while performing the constant force task with the ankle dorsiflexion. This stimulus appeared with illumination of a green background. Participants were asked to respond to the stimulus as quickly as possible by dorsiflexing the ankle with adequate force, to show the response clearly. Subjects were guided that this response force should be more than the target force (15% maximum voluntary contraction) of the constant isometric force task. The duration of the task was 31 s. dEMG, electromyogram decomposition.

Fig. 2.

A: decomposition of motor units. Force and motor unit data were analyzed over a segment of 10 s before response to the stimulus (arrow). B: quantification of motor units. Single motor unit (MU) activity (left) and multiple motor unit activity (right) are shown. Interspike interval (ISI) was calculated from spike train from each motor unit (accumulated spike train for multiple motor unit). Each discrete number of interspike interval from spike train was interpolated to generate continuous signal to yield power spectrum density.

Fig. 3.

A: total response time. Older adults (●) exhibited significantly (*P < 0.05) longer total response time with both low-gain and high-gain visual feedback compared with young adults (○). In addition, the change in response time from low-gain to high-gain visual feedback was significantly (#P < 0.05) greater in older adults. In contrast, young adults exhibited similar response time for both visual gain conditions. B: premotor response time. Older adults (●) exhibit significantly (*P < 0.05) longer premotor response time with both low-gain and high-gain visual feedback compared with young adults. C: association between total response time and premotor response time. Longer total response time for older adults was associated with a slower premotor response time, especially in older adults. D: changes in total response time vs. changes in premotor response time. Longer total response time with high-gain visual feedback was associated with a slower premotor response time, especially in older adults.

Fig. 4.

A: SD of force. Older adults (●) exhibited significantly (*P < 0.05) greater SD of force than young adults (○) only with high-gain visual feedback and not with low-gain visual feedback. In addition, the change in SD of force from low-gain to high-gain visual feedback was significantly (#P < 0.05) greater in older adults than young adults. B: root mean square error (RMSE). Independent of visual gains, older adults exhibited significantly (*P < 0.05) greater RMSE than young adults. C: association between premotor response time and SD of force. Slower premotor response time for older adults was associated with an increase in force variability. D: changes in premotor response time vs. changes in SD of force. Slower premotor response time with high-gain visual feedback was associated with an increase in force variability, especially in older adults.

Fig. 5.

Correlation of changes in premotor response time and the modulation of multiple motor units. Increase in premotor response time with high-gain visual feedback was associated with lesser power from 4–10 Hz (A: r = −0.627, P = 0.001) and greater relative power from 10–35 Hz (B: r = 0.625, P = 0.001) in the modulation of multiple motor units.