Visuomotor Accuracy Research Articles

Short-term interference in learning ballistic motor tasks: Refining the notion of specificity

Introduction Learning of an isometric ballistic force task (BT) can be impaired if it is followed by the learning of a visuomotor accuracy task (VMT) involving isometric contractions of the same muscles (Lundbye-Jensen et al., 2011). This phenomenon, known as retrograde interference, is thought to occur because the memory trace created after learning the BT is affected by the activation of similar neural circuits involved in the VMT (Lundbye-Jensen et al., 2011). However, it remains unclear how the contraction type involved in the two motor tasks influences potential interference effects. The aim of this study was therefore to compare the influence of a VMT involving dynamic position control or isometric force control on short-term retrograde interference in a previously learned isometric BT involving the same muscles. Methods 45 participants were randomly assigned to one of 3 groups. Participants started by learning an isometric BT involving the wrist flexors of the non-dominant hand. Participants in the first two groups then learned a VMT involving dynamic (VMTDynamic group) or isometric (VMTIsometric group) contractions of the same muscles, while participants in the third group took a break (control group). Finally, participants completed the BT retention test. Each BT training consisted of 35 trials in which participants were asked to achieve the highest rate of force development (RFDmax). The VMT training consisted of 50 trials in which participants were asked to follow a curve with a cursor. Position of the cursor was linked to the wrist flexion/extension angle (VMTDynamic) or the isometric force produced (VMTIsometric). Interference was quantified by comparing RFDmax between the end of the initial training (POST) and the start of the retention test (RET). Results A mixed design ANOVA revealed a significant interaction effect (F(2, 40) = 3.87, p = 0.029). Post hoc corrected paired t-tests showed a significant decrease in RFDmax for the VMTIsometric group between POST (2,524 ± 1,144 N/s) and RET (1,960 ± 921 N/s). In contrast, performance of the other two groups did not change (VMTDynamic group: 2,679 ± 1,321 vs. 2,499 ± 1,278 N/s; control group: 2,489 ± 1,005 vs. 2,334 ± 1,013 N/s). Discussion/Conclusion Interference was only observed in the VMTIsometric group, suggesting that the same type of contraction must be involved in the VMT and BT for short-term retrograde interference to occur. It is known that the neural control of isometric contractions is different from dynamic contractions (Duchateau & Enoka, 2016). It can therefore be speculated that the resulting reduction in neural competition between the motor tasks was sufficient to prevent interference in the VMTDynamic group. In conclusion, for retrograde interference to occur, not only must the same muscles be involved in both motor tasks, but they must also perform the same type of contraction.

The effects of a therapeutic program based on motor learning in the recovery of apraxia of speech in children

The purpose of the present study is to analyze comparatively the effects of some therapeutic interventions to improve the symptoms of apraxia of speech in children (CAS). In this sense, three different therapeutic programs were implemented, each lasting 12 months, in three groups of children. Two measurements were made, before and after the children's participation in the programs. P1 included therapy based on augmentative and alternative communication, P2 included therapy based on the classic speech therapy model and therapeutic speech massage, P3 included therapy based on verbal motor learning, VML. 106 children diagnosed with CAS participated in the study, aged between 5 and 7 years, M = 5.75, SD = .75, of which 81 boys (76%) and 25 girls (24%), were randomly divided into three groups. CAS severity was measured using a grid from the Dynamic Assessment of Motor Speech Skills, measuring articulatory accuracy, vowel scoring, and prosody. Visuomotor accuracy, visuospatial processing, phonemic hearing, narrative memory, and attention were measured with a series of tests from the PEDb and PEDa packages (Cognitrom, 2012). Although all three therapeutic programs registered significant effects on the improvement of CAS symptomatology, the most notable effects were obtained by the VML method. The practical implications of the study are discussed, as well as future research directions.

Acute exercise performed before and after motor practice enhances the positive effects on motor memory consolidation

Performing a single bout of exercise can enhance motor learning and long-term retention of motor skills. Parameters such as the intensity and when the exercise bout is performed in relation to skill practice (i.e., timing) likely influence the effectiveness. However, it is still not fully understood how exercise should be administered to maximize its effects and how exercise interacts with distinct components of skill learning. Here, we expand this knowledge by investigating the potential synergistic effects of performing acute exercise both prior to and following motor practice. Sixty-four, able-bodied, young adult male participants practiced a sequential visuomotor accuracy tracking (SVAT) task requiring rapid and accurate force modulation and high levels of precision control using intrinsic hand muscles. The task also contained a repeated pattern of targets that allowed sequence-specific skill improvements. Sequential and non-sequential motor performance was assessed at baseline, immediately after motor practice, and again seven days later. One group performed moderate-intensity exercise before practice (PREMO), a second group performed high-intensity exercise after practice (POSTHI), a third group exercised both before and after practice (PREMO + POSTHI), and a fourth group did not exercise during these periods (CON). Regardless of the exercise condition, acute exercise improved long-term retention of the skill by countering performance decay between experimental sessions (i.e., a 7-day interval). Furthermore, exercising both before and after motor practice led to the greatest improvements in skilled performance over time. We found that the effects of exercise were not specific to the practiced sequence. Namely, the effects of exercise generalized across sequential and non-sequential target positions and orders. This suggests that acute exercise works through mechanisms that promote general aspects of motor memory (e.g., lasting improvements in fast and accurate motor execution). The results demonstrate that various exercise protocols can promote the stabilization and long-term retention of motor skills. This effect can be enhanced when exercise is performed both before and after practice.

Visuomotor skill learning in young adults with Down syndrome

BackgroundIndividuals with Down syndrome (DS) have impaired general motor skills compared to typically developed (TD) individuals. AimsTo gain knowledge on how young adults with DS learn and retain new motor skills. Methods and ProceduresA DS-group (mean age = 23.9 ± 3 years, N = 11), and an age-matched TD-group (mean age 22.8 ± 1.8, N = 14) were recruited. The participants practiced a visuomotor accuracy tracking task (VATT) in seven blocks (10.6 min). Online and offline effects of practice were assessed based on tests of motor performance at baseline immediate and 7-day retention. Outcomes and ResultsThe TD-group performed better than the DS-group on all blocks (all P < 0.001). Both groups improved VATT-performance online from baseline to immediate retention, (all P < 0.001) with no difference in online effect between groups. A significant between-group difference was observed in the offline effect (∆TD - ∆DS, P = 0.04), as the DS-group’s performance at 7-day retention was equal to their performance at immediate retention (∆DS, P > 0.05), whereas an offline decrease in performance was found in the TD-group (∆TD, P < 0.001). Conclusions and ImplicationsVisuomotor pinch force accuracy is lower for adults with DS compared to TD. However, adults with DS display significant online improvements in performance with motor practice similar to changes observed for TD. Additionally, adults with DS demonstrate offline consolidation following motor learning leading to significant retention effects.

Influence of Heat Exposure on Motor Control Performance and Learning as Well as Physiological Responses to Visuomotor Accuracy Tracking Task.

This study aimed to determine whether heat exposure attenuates motor control performance and learning, and blunts cardiovascular and thermoregulatory responses to visuomotor accuracy tracking (VAT) tasks. Twenty-nine healthy young adults (22 males) were divided into two groups performing VAT tasks (5 trials × 10 blocks) in thermoneutral (NEUT: 25 °C, 45% RH, n = 14) and hot (HOT: 35 °C, 45% RH, n = 15) environments (acquisition phase). One block of the VAT task was repeated at 1, 2, and 4 h after the acquisition phase (retention phase). Heat exposure elevated skin temperature to ~3 °C with a marginally increased core body temperature. VAT performance (error distance of curve tracking) was more attenuated overall in HOT than in NEUT in the acquisition phase without improvement in magnitude alteration. Heat exposure did not affect VAT performance in the retention phase. The mean arterial blood pressure and heart rate, but not for sweating and cutaneous vascular responses to VAT acquisition trials, were more attenuated in HOT than in NEUT without any retention phase alternations. We conclude that skin temperature elevation exacerbates motor control performance and blunts cardiovascular response during the motor skill acquisition period. However, these alternations are not sustainable thereafter.

Frequency-Dependent Effects of Cerebellar Repetitive Transcranial Magnetic Stimulation on Visuomotor Accuracy.

The cerebellum plays a critical role in acquiring visuomotor skills. Visuomotor task mastery requires improving both visuomotor accuracy and stability; however, the cerebellum’s contribution to these processes remains unclear. We hypothesized that repetitive transcranial magnetic stimulation (rTMS) of the cerebellum exerts frequency-dependent modulatory effects on both accuracy and stability in subjects performing a visuomotor coordination task (i.e., pursuit rotor task). We recruited 43 healthy volunteers and randomly assigned them to the high-frequency (HF), low-frequency (LF), and sham rTMS groups. We calculated changes in performance of the pursuit rotor task at the highest rotation speed and the minimum distance from target as indices of accuracy. We also calculated the intertrial variability (standard deviations) of time on target and distance from target as indices of stability. Visuomotor accuracy was significantly enhanced in the HF group and disrupted in the LF group compared to the sham group, indicating frequency-dependent effects of rTMS. In contrast, both HF and LF rTMS demonstrated no significant change in visuomotor stability. Surprisingly, our findings demonstrated that the accuracy and stability of visuomotor performance may be differentially influenced by cerebellar rTMS. This suggests that visuomotor accuracy and stability have different underlying neural mechanisms and revealed the possibility of training strategies based on cerebellar neuromodulation.

Chapter 13 - Bálint syndrome

This chapter starts by reviewing the various interpretations of Bálint syndrome over time. We then develop a novel integrative view in which we propose that the various symptoms, historically reported and labeled by various authors, result from a core mislocalization deficit. This idea is in accordance with our previous proposal that the core deficit of Bálint syndrome is attentional (Pisella et al., 2009, 2013, 2017) since covert attention improves spatial resolution in visual periphery (Yeshurun and Carrasco, 1998); a deficit of covert attention would thus increase spatial uncertainty and thereby impair both visual object identification and visuomotor accuracy. In peripheral vision, we perceive the intrinsic characteristics of the perceptual elements surrounding us, but not their precise localization (Rosenholtz et al., 2012a,b), such that without covert attention we cannot organize them to their respective and recognizable objects; this explains why perceptual symptoms (simultanagnosia, neglect) could result from visual mislocalization. The visuomotor symptoms (optic ataxia) can be accounted for by both visual and proprioceptive mislocalizations in an oculocentric reference frame, leading to field and hand effects, respectively. This new pathophysiological account is presented along with a model of posterior parietal cortex organization in which the superior part is devoted to covert attention, while the right inferior part is involved in visual remapping. When the right inferior parietal cortex is damaged, additional representational mislocalizations across saccades worsen the clinical picture of peripheral mislocalizations due to an impairment of covert attention.

Acute Exercise Protects Newly Formed Motor Memories Against rTMS-induced Interference Targeting Primary Motor Cortex

Acute cardiovascular exercise can promote motor memory consolidation following motor practice, and thus long-term retention, but the underlying mechanisms remain sparsely elucidated. Here we test the hypothesis that the positive behavioral effects of acute exercise involve the primary motor cortex and the corticospinal pathway by interfering with motor memory consolidation using non-invasive, low frequency, repetitive transcranial magnetic stimulation (rTMS). Forty-eight able-bodied, young adult male participants (mean age = 24.8 y/o) practiced a visuomotor accuracy task demanding precise and fast pinch force control. Following motor practice, participants either rested or exercised (20 min total: 3 × 3 min at 90% VO2peak) before receiving either sham rTMS or supra-threshold rTMS (115% RMT, 1 Hz) targeting the hand area of the contralateral primary motor cortex for 20 min. Retention was evaluated 24 h following motor practice, and motor memory consolidation was operationalized as overnight changes in motor performance. Low-frequency rTMS resulted in off-line decrements in motor performance compared to sham rTMS, but these were counteracted by a preceding bout of intense exercise. These findings demonstrate that a single session of exercise promotes early motor memory stabilization and protects the primary motor cortex and the corticospinal system against interference.

Exercise-heat stress with and without water replacement alters brain structures and impairs visuomotor performance.

Effects of exercise‐heat stress with and without water replacement on brain structure and visuomotor performance were examined. Thirteen healthy adults (23.6 ± 4.2 years) completed counterbalanced 150 min trials of exercise‐heat stress (45°C, 15% RH) with water replacement (EHS) or without (~3% body mass loss; EHS‐DEH) compared to seated rest (CON). Anatomical scans and fMRI Blood‐Oxygen‐Level‐Dependent responses during a visuomotor pacing task were evaluated. Accuracy decreased (P < 0.05) despite water replacement during EHS (−8.2 ± 6.8% vs. CON) but further degraded with EHS‐DEH (−8.3 ± 6.4% vs. EHS and −16.5 ± 10.2% vs. CON). Relative to CON, EHS elicited opposing volumetric changes (P < 0.05) in brain ventricles (−5.3 ± 1.7%) and periventricular structures (cerebellum: 1.5 ± 0.8%) compared to EHS‐DEH (ventricles: 6.8 ± 3.4, cerebellum: −0.7 ± 0.7; thalamus: −2.7 ± 1.3%). Changes in plasma osmolality (EHS: −3.0 ± 2.1; EHS‐DEH: 9.3 ± 2.1 mOsm/kg) were related (P < 0.05) to thalamus (r = −0.45) and cerebellum volume (r = −0.61) which, in turn, were related (P < 0.05) to lateral (r = −0.41) and fourth ventricle volume (r = −0.67) changes, respectively; but, there were no associations (P > 0.50) between structural changes and visuomotor accuracy. EHS‐DEH increased neural activation (P < 0.05) within motor and visual areas versus EHS and CON. Brain structural changes are related to bidirectional plasma osmolality perturbations resulting from exercise‐heat stress (with and without water replacement), but do not explain visuomotor impairments. Negative impacts of exercise‐heat stress on visuomotor tasks are further exacerbated by dehydration.

Acute Exercise and Motor Memory Consolidation: The Role of Exercise Intensity.

A single bout of high intensity aerobic exercise (~90% VO2peak) was previously demonstrated to amplify off-line gains in skill level during the consolidation phase of procedural memory. High intensity exercise is not always a viable option for many patient groups or in a rehabilitation setting where low to moderate intensities may be more suitable. The aim of this study was to investigate the role of intensity in mediating the effects of acute cardiovascular exercise on motor skill learning. We investigated the effects of different exercise intensities on the retention (performance score) of a visuomotor accuracy tracking task. Thirty six healthy male subjects were randomly assigned to one of three groups that performed either a single bout of aerobic exercise at 20 min post motor skill learning at 45% (EX45), 90% (EX90) maximal power output (Wmax) or rested (CON). Randomization was stratified to ensure that the groups were matched for relative peak oxygen consumption (ml O2/min/kg) and baseline score in the tracking task. Retention tests were carried out at 1 (R1) and 7 days (R7) post motor skill learning. At R1, changes in performance scores were greater for EX90 compared to CON (p<0.001) and EX45 (p = 0.011). The EX45 and EX90 groups demonstrated a greater change in performance score at R7 compared to the CON group (p = 0.003 and p<0.001, respectively). The change in performance score for EX90 at R7 was also greater than EX45 (p = 0.049). We suggest that exercise intensity plays an important role in modulating the effects that a single bout of cardiovascular exercise has on the consolidation phase following motor skill learning. There appears to be a dose-response relationship in favour of higher intensity exercise in order to augment off-line effects and strengthen procedural memory.

Simulated motion negatively affects motor task but not neuromuscular performance

The effects of long duration simulated motion on motor task and neuromuscular performance along with time frames required to recover from these effects are relatively unknown. This study aimed to determine (1) how simulated motion affects motor task and neuromuscular performance over one hour of motion and (2) the time course of recovery from any decrements. The dependent variables that were measured included: reaction time; visuomotor accuracy tracking; maximal voluntary contractions; voluntary activation; evoked contractile properties and biceps brachii electromyography of the elbow flexors. Reaction times and error rates of the visuomotor accuracy tracking task were compromised in motion, but maximal force, voluntary activation, evoked contractile properties and rmsEMG responses of the biceps brachii were unaffected by motion. It is concluded that motion causes an increase in attention demands, which have a greater effect on motor task rather than neuromuscular performance.Practitioner Summary: Minor delays or mistakes can separate life and death at sea. The safety and productivity of most vessels rely on error-free performance of motor tasks. This study demonstrates that human ability to perform motor tasks is compromised by ship motions and may aid in developing training and safety guidelines for seafarers.

Cross-Limb Interference during Motor Learning

It is well known that following skill learning, improvements in motor performance may transfer to the untrained contralateral limb. It is also well known that retention of a newly learned task A can be degraded when learning a competing task B that takes place directly after learning A. Here we investigate if this interference effect can also be observed in the limb contralateral to the trained one. Therefore, five different groups practiced a ballistic finger flexion task followed by an interfering visuomotor accuracy task with the same limb. Performance in the ballistic task was tested before the training, after the training and in an immediate retention test after the practice of the interference task for both the trained and the untrained hand. After training, subjects showed not only significant learning and interference effects for the trained limb but also for the contralateral untrained limb. Importantly, the interference effect in the untrained limb was dependent on the level of skill acquisition in the interfering motor task. These behavioural results of the untrained limb were accompanied by training specific changes in corticospinal excitability, which increased for the hemisphere ipsilateral to the trained hand following ballistic training and decreased during accuracy training of the ipsilateral hand. The results demonstrate that contralateral interference effects may occur, and that interference depends on the level of skill acquisition in the interfering motor task. This finding might be particularly relevant for rehabilitation.

Implications of 25-Year Follow-Up of White Matter Integrity and Neurocognitive Function of Childhood Leukemia Survivors: A Wake-Up Call

Improved treatment via new drug development, combination therapy, and methods of treatment have resulted in 73% to 80% of children treated for acute lymphoblastic leukemia (ALL) being considered long-term survivors who are cured. This survival rate has not come without a cost, with long-term consequences (late effects) occurring in reproductive capacity, cardiac function, neurocognitive function, endocrine function, pulmonary function, and second malignancy related to treatment. As children survive longer into adulthood, an additional late effect is beginning to be recognized: an onset of diseases associated with aging occurring earlier than in the general population. Though much is known about neurocognitive late effects in children and adolescents during school-age and adolescence, nothing is known about the long-term effects of treatment received during childhood on the survivors of ALL 20 to 30 years after diagnosis. In the article accompanying this editorial, Schuitema et al describe one of the first studies in this area, and the results are consistent with other late-effects studies related to early aging concerns. Ninety-three young adults treated between 1978 and 1990 for childhood ALL with either cranial radiation therapy (CRT) or CNS prophylaxis with chemotherapy were evaluated using magnetic resonance imaging (MRI) and standardized tests of neurocognitive function. Survivors who were treated with CRT had significantly reduced white-matter integrity, as measured by MRI fractional anisotropy (FA), and poorer neurocognitive function (lower intelligence quotient, poorer visuomotor accuracy, stability, and sequential working memory, and poorer work flow during sustained attention). Decreases in FA were observed in chemotherapy-only patients, but those decreases were more moderate than in CRT-treated survivors. Consistent with other reports describing white-matter integrity during childhood and adolescence, younger patient age at the time of CRT and higher dose were associated with worse white-matter integrity. Steep declines in FA in the frontal and parietal white matter were observed related to age of assessment, suggesting an accelerated pattern of aging. To emphasize this point, the authors note anatomic similarities between these long-term ALL survivors and aging patients with cognitive decline or dementia. Accelerated aging of the CNS is a concern of late-effects investigators. Evidence is growing for persistent neurocognitive impairment after both CRT and systemic chemotherapy in survivors of cancers involving treatment of the CNS with CRT, chemotherapy, or combination CRT and chemotherapy. Impairment is most frequently seen in the areas of executive function, processing speed, and short-term or working memory. There is additional risk for increased severity in other areas of cognitive function in children who are younger at time of treatment or who receive higher doses of CRT. For children, school performance is often negatively affected; little is known about daily-function challenges in adult survivors. Treatment of childhood ALL has changed dramatically over the last 30 years, particularly CNS prophylaxis. Before 1985, with the introduction of Pediatric Oncology Group 8602 study, children with B-lineage or T-lineage ALL were typically treated with craniospinal radiation therapy, sometimes alone and sometimes in combination with intrathecal methotrexate. The dose of CRT was typically 24 Gy or 18 Gy. The Pediatric Oncology Group 8602 study eliminated CRT for the majority of children treated for ALL, replacing it with intrathecal methotrexate or triple intrathecal chemotherapy (methotrexate, cytarabine, and hydrocortisone). This strategy reduced some of the neuroendocrine and growth delays that had been associated with CRT, but neurocognitive deficits were still observed 3 or more years after diagnosis. Treatment for ALL since the early 1990s has included a more aggressive use of systemic methotrexate during the consolidation phase of therapy, increasing from 1 gm/m to 5 gms/m. This change, without CRT, has also resulted in reports of increased neurocognitive impairment and computed tomography and MRI evidence of primarily calcification in the CNS. For children with CNS tumors, CRT remains the backbone of therapy after surgical resection. CRT doses are typically much higher than those used in the historical and current treatment of ALL (30 Gy to 65 Gy v 18Gy or less). There have been efforts dating back to the mid-1980s to reduce the dose of CRT, delay its use, or eliminate it from treatment, but most children with high-grade malignant CNS tumors will likely be treated with CRT, either photon-beam or proton-beam. JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 31 NUMBER 27 SEPTEMBER 2

Long-term effects of cranial irradiation and intrathecal chemotherapy in treatment of childhood leukemia: a MEG study of power spectrum and correlated cognitive dysfunction

BackgroundProphylaxis to prevent relapses in the central nervous system after childhood acute lymphoblastic leukemia (ALL) used to consist of both intrathecal chemotherapy (CT) and cranial irradiation (CRT). CRT was mostly abolished in the eighties because of its neurotoxicity, and replaced with more intensive intrathecal CT. In this study, a group of survivors treated with CRT before 1983 and another group treated without CRT thereafter are investigated 20–25 years later, giving a much stronger perspective on long-term quality of life than previous studies. The outcomes will help to better understand these groups’ current needs and will aid in anticipating late effects of prophylactic CRT that is currently applied for other diseases. This study evaluates oscillatory neuronal activity in these long-term survivors. Power spectrum deviations are hypothesized to correlate with cognitive dysfunction.MethodsResting state eyes-closed magnetoencephalography (MEG) recordings were obtained from 14 ALL survivors treated with CT + CRT, 18 treated with CT alone and 35 controls. Relative spectral power was calculated in the δ, θ, α1, α2, β and γ frequency bands. The Amsterdam Neuropsychological Tasks (ANT) program was used to assess cognition in the executive functions domain. MEG data and ANT scores were correlated.ResultsIn the CT + CRT group, relative θ power was slightly increased (p = 0.069) and α2 power was significantly decreased (p = 0.006). The CT + CRT group performed worse on various cognitive tests. A deficiency in visuomotor accuracy, especially of the right hand, could be clearly associated with the deviating regional θ and α2 powers (0.471 < r < 0.697). A significant association between decreased regional α2 power and less attentional fluctuations was found for CT + CRT patients as well as controls (0.078 < r < 0.666). Patients treated with CT alone displayed a power spectrum similar to controls, except for a significantly increased level of left frontal α2 power (p = 0.030).ConclusionsThe tendency towards global slowing of brain oscillatory activity, together with the fact that dementia has been reported as a late effect of CRT and the neuropsychological deficiencies currently present, suggest that the irradiated brain might be aging faster and could be at risk for early‐onset dementia. The CT group showed no signs of early aging.

Exposure to alternating electromagnetic fields and effects on the visual and visuomotor systems

Acute effects on the visual and visuo-motor systems by exposure to electromagnetic fields (EMFs) at a frequency and amplitude similar to those produced by MR imaging gradient coils were assessed. 40 volunteers were exposed in random order to three, time varying, magnetic field gradients (0, 20 and 10 mT m(-1)r.m.s.). The waveform was 50 cycles of a 490 Hz sinusoidal waveform repeated every second with a total duration of 10 min for each trial. The EMFs were generated using an in-house designed and built magnetic gradient coil and waveform generator. During each trial, a test battery assessing the visual sensory (FACT) and visuo-motor (Pursuit Aiming II and visual tracking) neurobehavioral domains was completed by all volunteers. The sequence of these tests was assigned at random for each volunteer. Performance in these tests was analysed using linear mixed effects models adjusted for confounding factors collected in a pre-trial questionnaire. Variability of the estimates was assessed using a delete-1 jack-knife procedure. There was a trend for visuo-motor accuracy to be reduced (p = 0.06) by 1% during high exposure, but not at medium exposure. There was a weaker trend for visual contrast sensitivity to be improved by 12% and 21% during medium and high exposure, respectively, compared with the non-exposed sessions (p = 0.08). These effects did not reach 5% statistical significance within a population of 40 volunteers, but also the magnitude of these effects did not depend on single "extreme" observations.

Humans Trade Off Viewing Time and Movement Duration to Improve Visuomotor Accuracy in a Fast Reaching Task

Previous research has shown that the brain uses statistical knowledge of both sensory and motor accuracy to optimize behavioral performance. Here, we present the results of a novel experiment in which participants could control both of these quantities at once. Specifically, maximum performance demanded the simultaneous choices of viewing and movement durations, which directly impacted visual and motor accuracy. Participants reached to a target indicated imprecisely by a two-dimensional distribution of dots within a 1200 ms time limit. By choosing when to reach, participants selected the quality of visual information regarding target location as well as the remaining time available to execute the reach. New dots, and consequently more visual information, appeared until the reach was initiated; after reach initiation, no new dots appeared. However, speed accuracy trade-offs in motor control make early reaches (much remaining time) precise and late reaches (little remaining time) imprecise. Based on each participant's visual- and motor-only target-hitting performances, we computed an "ideal reacher" that selects reach initiation times that minimize predicted reach endpoint deviations from the true target location. The participant's timing choices were qualitatively consistent with ideal predictions: choices varied with stimulus changes (but less than the predicted magnitude) and resulted in near-optimal performance despite the absence of direct feedback defining ideal performance. Our results suggest visual estimates, and their respective accuracies are passed to motor planning systems, which in turn predict the precision of potential reaches and control viewing and movement timing to favorably trade off visual and motor accuracy.

THE EFFECT OF WALKING WITH AN ASSISTIVE DEVICE AND USING A WHEELCHAIR ON SCHOOL PERFORMANCE IN STUDENTS WITH MYELOMENINGOCELE

An alternating-condition, single-subject research design was used to examine the effect of ambulation on three measures of school performance in three students with myelomeningocele. The subjects, aged 9, 10, and 15 years, had a physiological cost index greater than 1.00 beats per meter when walking with crutches or a walker. Subjects propelled a wheelchair at school for 5 days, ambulated with crutches or a walker for 5 days, and propelled a wheelchair for an additional 5 days. Performance in reading fluency, visuomotor accuracy, and manual dexterity was assessed at the end of each school day. Results were graphed and analyzed using the two-standard-deviation band method. All subjects had significantly lower visuomotor accuracy scores during the assistive-device ambulation phase than during the wheelchair phases. Performance in manual dexterity during the assistive-device ambulation and wheelchair phases varied among the subjects. Reading fluency was not affected by method of mobility. The results suggest that the high energy cost of walking may have a negative effect on certain aspects of the subjects' school performance. [Franks CA, Palisano RJ, Darbee JC. The effect of walking with an assistive device and using a wheelchair on school performance in students with myelomeningocele.

The Effect of Walking with an Assistive Device and Using a Wheelchair on School Performance in Students with Myelomeningocele

An alternating-condition, single-subject research design was used to examine the effect of ambulation on three measures of school performance in three students with myelomeningocele. The subjects, aged 9, 10, and 15 years, had a physiological cost index greater than 1.00 beats per meter when walking with crutches or a walker. Subjects propelled a wheelchair at school for 5 days, ambulated with crutches or a walker for 5 days, and propelled a wheelchair for an additional 5 days. Performance in reading fluency, visuomotor accuracy, and manual dexterity was assessed at the end of each school day. Results were graphed and analyzed using the two-standard-deviation band method. All subjects had significantly lower visuomotor accuracy scores during the assistive-device ambulation phase than during the wheelchair phases. Performance in manual dexterity during the assistive-device ambulation and wheelchair phases varied among the subjects. Reading fluency was not affected by method of mobility. The results suggest that the high energy cost of walking may have a negative effect on certain aspects of the subjects' school performance. [Franks CA, Palisano RJ, Darbee JC. The effect of walking with an assistive device and using a wheelchair on school performance in students with myelomeningocele.

Exposure-response relationship between styrene exposure and central nervous functions.

For the study of the relationship between styrene exposure and symptoms and signs of central nervous dysfunctions, 98 male workers occupationally exposed to styrene were given clinical, neurophysiological and psychological examinations; also a symptom survey was made. Urinary mandelic acid concentrations, measured once a week during five consecutive weeks, were used to express the exposure intensity. Different unexposed groups were used for reference. No exposure-response relationship was observed between symptoms of ill health and the urinary mandelic acid concentration, although the exposed group as a whole expressed significantly more symptoms than the reference group. The occurrence of abnormal electroencephalograms was about 10% in the group of workers with mandelic acid concentrations below 700 mg/l, but it was 30% among those whose mandelic acid concentration exceeded 700 mg/l, a level corresponding to the 8-h time-weighted average (TWA) of styrene exposure of about 30 ppm. With regard to psychological functions, the first change in visuomotor accuracy became discernible when the urinary mandelic acid concentration exceeded 800 mg/l. A more pronounced decrement appeared in both visuomotor accuracy and psychomotor performance when the mandelic acid concentration exceeded 1,200 mg/l, which corresponds to an 8-h TWA of styrene exposure of about 55 ppm.

Disturbances in psychological functions of workers occupationally exposed to styrene.

The relationship between long-term occupational styrene exposure and disturbances in psychological functions was studied. The exposed group consisted of 98 male workers exposed to styrene in the manufacture of polyester plastic products, and the comparison group comprised 43 concrete reinforcement workers. The duration of exposure in years and the mean mandelic acid concentration in urine (five determinations during 5 weeks before the clinical examination) and the cumulative dose of exposure were used as the indicators of exposure. The psychological functions studied were intelligence, visuomotor speed, visuomotor accuracy, memory, vigilance, and psychomotor performance. Some personality test variables were also included. In a group comparison two variables (visuomotor inaccuracy and poor psychomotor performance) out of 20 showed statistically significant differences and characterized the styrene exposed group. These findings were confirmed when workers with low and high styrene exposure were compared. One variable measuring visuomotor speed and another measuring visual memory were related to the duration of exposure (R = 0.28, p less than 0.05). In a multiple regression analysis disturbances in visuomotor accuracy, poor psychomotor performance, and lowered vigilance proved to have some connection with a high mandelic acid concentration (R = 0.49, p less than 0.01). In addition workers with disturbances in visuomotor accuracy and slight disturbances in virumotor speed had higher mandelic acid concentrations when compared with those workers without any deterioration. The results indicate that disturbances in visuomotor accuracy and, to a lesser degree, in psychomotor performance are the main findings measured by test methods used in this study and related to one indicator of styrene exposure, mandelic acid concentration. The psychological methods used revealed subclinical symptoms related to exposure, and they should aid in making group diagnoses, e.g., when additional information is needed for the determination of the threshold limit value. The deterioration found in visuomotor accuracy could be related to lowered safety in work and traffic.