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

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.

Motor control differs for increasing and releasing force

Control of the motor output depends on our ability to precisely increase and release force. However, the influence of aging on force increase and release remains unknown. The purpose of this study, therefore, was to determine whether force control differs while increasing and releasing force in young and older adults. Sixteen young adults (22.5 ± 4 yr, 8 females) and 16 older adults (75.7 ± 6.4 yr, 8 females) increased and released force at a constant rate (10% maximum voluntary contraction force/s) during an ankle dorsiflexion isometric task. We recorded the force output and multiple motor unit activity from the tibialis anterior (TA) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) mean discharge rate and variability of discharge rate of multiple motor units; and 3) power spectrum of the multiple motor units from 0-4, 4-10, 10-35, and 35-60 Hz. Participants exhibited greater force variability while releasing force, independent of age (P < 0.001). Increased force variability during force release was associated with decreased modulation of multiple motor units from 35 to 60 Hz (R(2) = 0.38). Modulation of multiple motor units from 35 to 60 Hz was further correlated to the change in mean discharge rate of multiple motor units (r = 0.66) and modulation from 0 to 4 Hz (r = -0.64). In conclusion, these findings suggest that force control is altered while releasing due to an altered modulation of the motor units.