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.

Reaction time to peripheral visual stimuli during exercise under hypoxia

The purpose of this study was to test the hypothesis that decrease in cerebral oxygenation compromises an individual's ability to respond to peripheral visual stimuli during exercise. We measured the simple reaction time (RT) to peripheral visual stimuli at rest and during and after cycling at three different workloads [40%, 60%, and 80% peak oxygen uptake (V̇o2)] under either normoxia [inspired fraction of oxygen (FiO2) = 0.21] or normobaric hypoxia (FiO2 = 0.16). Peripheral visual stimuli were presented at 10° to either the right or the left of the midpoint of the eyes. Cerebral oxygenation was monitored during the RT measurement over the right frontal cortex with near-infrared spectroscopy. We used the premotor component of RT (premotor time) to assess effects of exercise on the central process. The premotor time was significantly longer during exercise at 80% peak V̇o2 (normoxia: 214.2 ± 33.0 ms, hypoxia: 221.5 ± 30.1 ms) relative to that at rest (normoxia: 201.0 ± 27.2 ms, hypoxia: 202.9 ± 29.7 ms) ( P < 0.01). Under normoxia, cerebral oxygenation gradually increased up to 60% peak V̇o2 and then decreased to the resting level at 80% peak V̇o2. Under hypoxia, cerebral oxygenation progressively decreased as exercise workload increased. We found a strong correlation between increase in premotor time and decrease in cerebral oxygenation ( r2 = 0.89, P < 0.01), suggesting that increase in premotor time during exercise is associated with decrease in cerebral oxygenation. Accordingly, exercise at high altitude may compromise visual perceptual performance.

Local Muscular Fatigue and Attentional Processes in a Fencing Task

Study of the effects of brief exercise on mental processes by Tomporowski and Ellis (1986) has shown that moderate muscular tension improves cognitive performance while low or high tension does not. Improvements in performance induced by exercise are commonly associated with increase in arousal, while impairments are generally attributed to the effects of muscular or central fatigue. To test two hypotheses, that (1) submaximal muscular exercise would decrease premotor time and increase motor time in a subsequent choice-RT task and (2) that submaximal muscular exercise would increase the attentional and preparatory effects observed in premotor time 9 men, aged 20 to 30 years, performed an isometric test at 50% of their maximum voluntary contraction between blocks of a 3-choice reaction-time fencing task. Analysis showed (1) physical exercise did not improve postexercise premotor time, (2) muscular fatigue induced by isometric contractions did not increase motor time, (3) there was no effect of exercise on attentional and preparatory processes involved in the postexercise choice-RT task. The invalidation of hypotheses was mainly explained by disparity in directional effects across subjects and by use of an exercise that was not really fatiguing.

Fractionated Premotor, Motor, and Ankle Dorsiflexion Reaction Times in Hemiplegia

Ten hemiplegic subjects completed 20 rapid dorsiflexions of their afflicted and nonafflicted limbs. Electrodes were attached to the tibialis anterior and the gastrocnemius muscles and electromyograms were recorded for their premotor time, motor time, and simple reaction time during ankle dorsiflexion and plantar flexion of their lower limbs. The fractionated components of reaction time, namely, premotor time and motor time, of both legs were statistically compared. It was found that the premotor time of the subject's stroke-affected limb was significantly slower than the premotor time of the nonaffected limb (control), with no differences between their associated mean motor times. These results supported the hypothesis that a stroke has a deleterious affect upon the central, premotor time processing centers and has no disruptive influence upon the peripheral motor time. Comparing the fractionated components of reaction time (premotor time and motor time), with simple reaction time, the former provided a more sensitive and valid method to detect possible injurious side effects of a stroke upon the brain's neuromotor transmission centers and subcenters, and their peripheral, stimulus, response network.

Fractionated Reflex and Reaction Times in Children with Developmental Coordination Disorder

The patellar tendon reflex (PTR) and simple visual reaction time (VRT) were fractionated and compared in 40 subjects with developmental coordination disorder (DCD) and normal coordination (NC) in two age groups. Four equal groups of subjects, 6 years DCD (6DCD), 6 years NC (6NC), 9 years DCD (9DCD), and 9 years NC (9NC) were compared using ANOVA for the main effects of coordination and age. PTR and its components of reflex latency and motor time were not significantly affected by the level of coordination; however, a significant coordination by age interaction (p< .05) revealed an increased motor time in the 6DCD group. VRT, premotor time, and motor time were all significantly (p< .05) increased in children with DCD; the increased VRT and premotor time support earlier findings, whereas the increased motor time has not previously been found. These findings suggest that the processing of reflexive and volitional responses by children with DCD differs from that of their NC peers.

Fractionated Reaction Time as a Function of Magnitude of Force in Simple and Choice Conditions

The present study examined whether varying magnitude of force required to perform an isometric response influences fractionated reaction time in simple and choice conditions and whether reaction time and premotor time to initiate the response are shorter when force is selected freely by the subject than when it is selected by the experimenter. 20 subjects were required to react and produce a designated peak force as quickly and accurately as possible by squeezing a handle after a reaction signal. Four different magnitudes of force were 30, 50, and 70% of the maximum grip strength of the subjects and subject-selected magnitude of force. Reaction time and premotor time did not change across the range of forces examined in both simple and choice reaction-time conditions regardless of whether a desired force was selected by the experimenter or by the subject These findings suggest that programming an isometric response may require a constant amount of time.