Effects of postural task requirements on the speed–accuracy trade-off

Influence of Target Width and Distance on Postural Adjustments in a Fencing Lunge

The aim of this study was to examine whether target width and target distance influence the planning phase of a fencing lunge (early and anticipatory postural adjustments) as well as the execution phase of a fencing lunge. Eight elite female fencers participated in the study. The displacement of the center of foot pressure, muscle activity of the tibialis anterior, and kinematics of center of mass were recorded using force plates. The results show that target width and distance have no effect on early and anticipatory postural adjustments as well as the acceleration and velocity of the center of mass at the moment of foot-off. However, a greater target distance was associated with a greater max center of mass acceleration and velocity, and larger target width resulted in a greater max center of mass acceleration during lunging (p < 0.05). We suppose that the effect of task parameters on preparing a fencing lunge may be mitigated due to the specific technique adopted by expert fencers and the ballistic nature of a fencing lunge.

Effects of mentally induced fatigue on balance control: a systematic review

The relationship between cognitive demands and postural control is controversial. Mental fatigue paradigms investigate the attentional requirements of postural control by assessing balance after a prolonged cognitive task. However, a majority of mental fatigue research has focused on cognition and sports performance, leaving balance relatively underexamined. The purpose of this paper was to systematically review the existing literature on mental fatigue and balance control. We conducted a comprehensive search on PubMed and Web of Science databases for studies comparing balance performance pre- to post-mental fatigue or between a mental fatigue and control group. The literature search resulted in ten relevant studies including both volitional (n = 7) and reactive (n = 3) balance measures. Mental fatigue was induced by various cognitive tasks which were completed for 20-90 min prior to balance assessment. Mental fatigue affected both volitional and reactive balance, resulting in increased postural sway, decreased accuracy on volitional tasks, delayed responses to perturbations, and less effective balance recovery responses. These effects could have been mediated by the depletion of attentional resources or impaired sensorimotor perception which delayed appropriate balance-correcting responses. However, the current literature is limited by the number of studies and heterogeneous mental fatigue induction methods. Future studies are needed to confirm these postulations and examine the effects of mental fatigue on different populations and postural tasks. This line of research could be clinically relevant to improve safety in occupational settings where individuals complete extremely long durations of cognitive tasks and for the development of effective fall-assessment and fall-prevention paradigms.

Hierarchical Motion Learning for Goal-Oriented Movements With Speed-Accuracy Tradeoff of a Musculoskeletal System

Generating various goal-oriented movements via the flexible muscle model of the musculoskeletal system as fast and accurately as possible is a pressing problem, which is also the basis of most human adaptive behaviors, such as reaching, catching, interception, and pointing. This article focuses on the adaptive motion generation of fast goal-oriented motion on the musculoskeletal system by implementing the speed-accuracy tradeoff (SAT) in a hierarchical motion learning framework. First, we introduce Fitts' Law into the modified basal ganglia circuit-inspired iterative decision-making model for achieving dynamic and adaptive decision making. Then, as a time constraint, the decision is decomposed into a series of supervised terms by the proposed striatal FSI-SPN interneuron circuit-inspired velocity modulator to implement the tradeoff smoothly on the musculoskeletal system. Finally, an improved policy gradient algorithm is suggested to generate the muscle excitations of the modulated motion via the proposed muscle co-contraction policy, which promotes general cooperation between flexor and extensor muscles. In experiments, a redundant musculoskeletal arm model is trained to perform the adaptive quick pointing movements. By combining the muscle co-contraction policy with SAT, our algorithm shows the most efficient training and the best performance in the adaptive motion generation among the other three popular reinforcement learning algorithms on the musculoskeletal model.

Transcranial Direct Current Stimulation (tDCS) modulates motor execution in a limb reaching task

The majority of human activities show a trade-off between movement speed and accuracy. Here we tested 16 participants in a quick pointing action after 20 minutes (2mA) of transcranial Direct Current Stimulation (tDCS) delivered at the Supplementary Motor Area (SMA) in a single-blind crossover design study for testing the feedforward components in the control of action. tDCS stimuli were delivered in three randomized sessions of stimulations as anodal, cathodal and sham as a control. The task performed Pre and Post tDCS stimulation, was to point as fast and as precise as possible with the big toe to targets having different sizes (2 and 8 cm; Width) and positioned at different Distances (20 and 60 cm; Distance). An optoelectronic motion capture system was used to collect the kinematics of movement. Result indicates that individuals after receiving anodal stimulation decreased their movement time and increased their movement speed while the opposite happened after receiving a cathodal stimulation. The scarcity of studies in this area invites us to plan a research that aims at the trade-off especially in the clinical settings.

Variability in the Center of Mass State During Initiation of Accurate Forward Step Aimed at Targets of Different Sizes

Motor control for forward step initiation begins with anticipatory postural adjustments (APAs). During APAs, the central nervous system controls the center of pressure (CoP) to generate an appropriate center of mass (CoM) position and velocity for various task requirements. In this study, we investigated the effect of required stepping accuracy on the CoM and CoP parameters during APA for a step initiation task. Sixteen healthy young participants stepped forward onto the targets on the ground as soon as and as fast as possible in response to visual stimuli. Two target sizes (small: 2 cm square and large: 10 cm square) and two target distances (short: 20% and long: 40% of the body height) were tested. CoP displacement during the APA and the CoM position, velocity, and extrapolated CoM at the timing of the takeoff of the lead leg were compared among the conditions. In the small condition, comparing with the large condition, the CoM position was set closer to the stance limb side during the APA, which was confirmed by the location of the extrapolated center of mass at the instance of the takeoff of the lead leg [small: 0.09 ± 0.01 m, large: 0.06 ± 0.01 m, mean and standard deviation, F _{(1, 15)} = 96.46, p < 0.001, η² = 0.87]. The variability in the mediolateral extrapolated center of mass location was smaller in the small target condition than large target condition when the target distance was long [small: 0.010 ± 0.002 m, large: 0.013 ± 0.004 m, t(15) = 3.8, p = 0.002, d = 0.96]. These findings showed that in the step initiation task, the CoM state and its variability were task-relevantly determined during the APA in accordance with the required stepping accuracy.

Anticipatory and pre-planned actions: A comparison between young soccer players and swimmers

The present study investigated whether a difference exists in reactive and proactive control for sport considered open or closed skills dominated. Sixteen young (11–12 years) athletes (eight soccer players and eight swimmers) were asked to be engaged into two games competitions that required either a reactive and a proactive type of control. By means of kinematic (i.e. movement time and duration) and dynamic analysis through the force platform (i.e. Anticipatory Postural Adjustments, APAs), we evaluated the level of ability and stability in reacting and anticipating actions. Results indicated that soccer players outperformed swimmers by showing higher stability and a smaller number of falls during the competition where proactive control was mainly required. Soccer players were able to reach that result by anticipating actions through well-modulated APAs. On the contrary, during the competition where reactive control was mainly required, performances were comparable between groups. Therefore, the development of specific action control is already established at 11–12 years of age and is enhanced by the training specificity.

Motor Behavior Concepts in the Study of Balance: A Scoping Review

Previous research suggests that using Fitts' law; attentional focus or challenge point framework (CPF) is beneficial in balance control studies. A scoping review was conducted to examine studies that utilized these motor behavior concepts during balance control tasks. An extensive literature search was performed up to January 2018. Two independent reviewers conducted a study selection process followed by data extraction of the search results. Forty-six studies were identified, with 2 studies related to CPF, 12 studies related to Fitts' law and 32 studies related to focus of attention. The CPF appears to be a useful method for designing a progressive therapeutic program. Fitts' law can be used as a tool for controlling the difficulty of motor tasks. Focus of attention studies indicate that adopting an external focus of attention improves task performance. Overall, studies included in this review report benefit when using the selected motor behavior concepts. However, the majority (>80%) of studies included in the review involved healthy populations, with only three clinical trials. In order to ascertain the benefits of the selected motor behavior concepts in clinical settings, future research should focus on using these concepts for clinical trials to examine balance control among people with balance impairments.

Anticipatory postural adjustments during a Fitts' task: Comparing young versus older adults and the effects of different foci of attention

Anticipatory postural adjustments (APAs) are an integral part of standing balance. Previous research with balance control has shown that adopting an external focus of attention, compared to an internal focus of attention, yields better performance during motor skills. Despite the importance of APAs, especially among older adults, and the potential benefits of adopting an external focus of attention, studies investigating methods for improving APAs are limited. The aim of this study was to compare behavioral, kinematic and APAs measures while adopting different foci of attention among young and older adults when performing a lower extremity Fitts' task. Ten young adults (mean age 24 years ± 4.37) and ten older adults (mean age 75 years ± 5.85) performed a lower-extremity reaching task (Fitts' task) while adopting an external focus (focus on target) and an internal focus (focus on limb) in a within-subject design. A motion capture system was used to record participants' movement data. Custom software derived movement time (MT), peak velocity (PV), time to peak velocity (ttPV) and variability at target (SD_T). Electromyography (EMG) was used to determine APAs onset and magnitude. The findings showed that an external focus of attention led to significantly shorter MT, higher PV, shorter ttPV and more accuracy when reaching the target (SD_T) for both age groups. Also, EMG results showed that, with an external focus, APAs onset occurred earlier and APAs magnitude was more efficient. As predicted by Fitts' Law, participants spent more time executing movements to targets with higher indices of difficulty. Older adults compared to young adults were more adversely affected by the increase of difficulty of the Fitts' task, specifically, on measures of APAs. In conclusion, adopting an external focus of attention led to better overall movement performance when performing a lower extremity Fitts' task. The task used in the present study can distinguish between APAs for older and young adults. We recommend that future studies expand on our findings in order to establish a performance-based objective measure of APAs to assess clinical interventions for postural control impairment.

The Organization and Control of Intra-Limb Anticipatory Postural Adjustments and Their Role in Movement Performance

Anticipatory Postural Adjustments (APAs) are commonly described as unconscious muscular activities aimed to counterbalance the perturbation caused by the primary movement, so as to ensure the whole-body balance, as well as contributing to initiate the displacement of the body center of mass when starting gait or whole-body reaching movements. These activities usually create one or more fixation chains which spread over several muscles of different limbs, and may be thus called inter-limb APAs. However, it has been reported that APAs also precede voluntary movements involving tiny masses, like a flexion/extension of the wrist or even a brisk flexion of the index-finger. In particular, such movements are preceded by an intra-limb APA chain, that involves muscles acting on the proximal joints. Considering the small mass of the moving segments, it is unlikely that the ensuing perturbation could threaten the whole-body balance, so that it is interesting to enquire the physiological role of intra-limb APAs and their organization and control compared to inter-limb APAs. This review is focused on intra-limb APAs and highlights a strict correspondence in their behavior and temporal/spatial organization with respect to inter-limb APAs. Hence it is suggested that both are manifestations of the same phenomenon. Particular emphasis is given to intra-limb APAs preceding index-finger flexion, because their relatively simple biomechanics and the fact that muscular actions were limited to a single arm allowed peculiar investigations, leading to important conclusions. Indeed, such paradigm provided evidence that by granting a proper fixation of those body segments proximal to the moving one APAs are involved in refining movement precision, and also that APAs and prime mover activation are driven by a shared motor command.

Is Beauty in the Eyes of the Beholder? Aesthetic Quality versus Technical Skill in Movement Evaluation of Tai Chi

The aim of this study was to compare experts to naïve practitioners in rating the beauty and the technical quality of a Tai Chi sequence observed in video-clips (of high and middle level performances). Our hypothesis are: i) movement evaluation will correlate with the level of skill expressed in the kinematics of the observed action but ii) only experts will be able to unravel the technical component from the aesthetic component of the observed action. The judgments delivered indicate that both expert and non-expert observers are able to discern a good from a mediocre performance; however, as expected, only experts discriminate the technical from the aesthetic component of the action evaluated and do this independently of the level of skill shown by the model (high or middle level performances). Furthermore, the judgments delivered were strongly related to the kinematic variables measured in the observed model, indicating that observers rely on specific movement kinematics (e.g. movement amplitude, jerk and duration) for action evaluation. These results provide evidence of the complementary functional role of visual and motor action representation in movement evaluation and underline the role of expertise in judging the aesthetic quality of movements.

A computational model of altered gait patterns in parkinson's disease patients negotiating narrow doorways

We present a computational model of altered gait velocity patterns in Parkinson's Disease (PD) patients. PD gait is characterized by short shuffling steps, reduced walking speed, increased double support time and sometimes increased cadence. The most debilitating symptom of PD gait is the context dependent cessation in gait known as freezing of gait (FOG). Cowie et al. (2010) and Almeida and Lebold (2010) investigated FOG as the changes in velocity profiles of PD gait, as patients walked through a doorway with variable width. The former reported a sharp dip in velocity, a short distance from the doorway that was greater for narrower doorways. They compared the gait performance in PD freezers at ON and OFF dopaminergic medication. In keeping with this finding, the latter also reported the same for ON medicated PD freezers and non-freezers. In the current study, we sought to simulate these gait changes using a computational model of Basal Ganglia based on Reinforcement Learning, coupled with a spinal rhythm mimicking central pattern generator (CPG) model. In the model, a simulated agent was trained to learn a value profile over a corridor leading to the doorway by repeatedly attempting to pass through the doorway. Temporal difference error in value, associated with dopamine signal, was appropriately constrained in order to reflect the dopamine-deficient conditions of PD. Simulated gait under PD conditions exhibited a sharp dip in velocity close to the doorway, with PD OFF freezers showing the largest decrease in velocity compared to PD ON freezers and controls. PD ON and PD OFF freezers both showed sensitivity to the doorway width, with narrow door producing the least velocity/ stride length. Step length variations were also captured with PD freezers producing smaller steps and larger step-variability than PD non-freezers and controls. In addition this model is the first to explain the non-dopamine dependence for FOG giving rise to several other possibilities for its etiology.

Anticipatory postural adjustments in dart throwing

The aim of this study was to explore the effects of accuracy constraints on the characteristics of anticipatory postural adjustments (APA) in a task that involves a movement consisting of a controlled phase and a ballistic phase. It was hypothesized that APA scaling with task parameters (target size) would be preserved even when the task is performed by muscles that have no direct effects on APA. Sixteen healthy right handed subjects participated in the study. All participants had no prior experience in dart throwing. Subjects’ average age was 24.1 ± 1.9 years. A force platform and a motion capture system were used to register kinetics of the body and kinematics of the throwing arm and throwing accuracy. The experiment consisted of six series of twenty consecutive dart throws to a specified target. Target sizes (T2–T6) were set at 25%, 50%, 75%, 125% and 150% of target 1 (T1) initially set as the spread of the last 20 throws in a 50 throw training session. This allowed to distinguish six indexes of difficulty (ID’s) ranging from 2,9 to 5,9. A one-way ANOVA for repeated measures was used for statistical analysis. Results of ANOVA showed a significant effect of target size at Constant Error but no effect at APA time. There were also no significant differences between hit and miss throws. From a control perspective, it can be stated that changes in central commands did not lead to changes in APA time in the analyzed motor task.

Effects of speed and accuracy strategy on choice step execution in response to the flanker interference task

The purpose of this study is to examine the effects of a speed or accuracy strategy on response interference control during choice step execution. Eighteen healthy young participants were instructed to execute forward stepping on the side indicated by a central arrow (←, left vs. →, right) under task instructions that either emphasized speed or accuracy of response in the neutral condition. In the flanker condition, they were additionally required to ignore the 2 flanking arrows on each side (→→→→→, congruent or →→←→→, incongruent). Errors in the direction of the initial weight transfer (APA errors) and the step execution times were measured from the vertical force data. APA error was increased in response to the flanker task and step execution time was shortened with a speed strategy compared to an accuracy strategy. Furthermore, in response to the visual interference of the flanker task, speed instructions in particular increased APA errors more than other instructions. It may be important to manipulate the level of the speed-accuracy trade-off to improve efficiency and safety. Further research is needed to explore the effects of advancing age and disability on choice step reaction in a speed or accuracy strategy.

Accuracy of pointing movements relies upon a specific tuning between anticipatory postural adjustments and prime mover activation

AIM

Equilibrium-perturbing forces associated with a voluntary upper-limb movement can be strong enough to displace the whole-body centre of mass. In this condition, anticipatory postural adjustments (APAs), developing in muscles other than the prime mover, are essential in maintaining the whole-body balance. Here, we test the hypothesis that APAs preceding an upper-limb target-reaching movement could play a role also in controlling the movement accuracy.

METHODS

Standing subjects (10) were asked to flex the right shoulder and touch with the index fingertip the centre of a target positioned in front of them. The reaching task was also performed while wearing and after doffing prismatic lenses (shifting the eye field rightward). EMGs from different upper- and lower-limb muscles and the mechanical actions to the ground were recorded.

RESULTS

(i) Before wearing prisms, subjects were very accurate in hitting the target, and the pointing movements were accompanied by APAs in quadriceps (Q) and tibialis anterior (TA) of both sides, and in right hamstrings (H) and soleus (SOL). (ii) After donning prisms, rightward pointing errors occurred, associated with a significant APA increase in right Q and TA, but without changes in the recruitment of right anterior deltoid (prime mover) and biceps brachii. (iii) These pointing errors were progressively compensated in about 10 trials, indicating a sensorimotor adaptation, and APAs returned to values recorded before wearing prisms. (iv) After doffing prisms, pointing errors occurred in the opposite direction but changes in APAs did not reach significance.

CONCLUSION

We propose that, besides preserving the whole-body balance, APAs are also tailored to obtain an accurate voluntary movement.

Stepping strategies for regulating gait adaptability and stability

Besides a stable gait pattern, gait in daily life requires the capability to adapt this pattern in response to environmental conditions. The purpose of this study was to elucidate the anticipatory strategies used by able-bodied people to attain an adaptive gait pattern, and how these strategies interact with strategies used to maintain gait stability. Ten healthy subjects walked in a Computer Assisted Rehabilitation ENvironment (CAREN). To provoke an adaptive gait pattern, subjects had to hit virtual targets, with markers guided by their knees, while walking on a self-paced treadmill. The effects of walking with and without this task on walking speed, step length, step frequency, step width and the margins of stability (MoS) were assessed. Furthermore, these trials were performed with and without additional continuous ML platform translations. When an adaptive gait pattern was required, subjects decreased step length (p<0.01), tended to increase step width (p=0.074), and decreased walking speed while maintaining similar step frequency compared to unconstrained walking. These adaptations resulted in the preservation of equal MoS between trials, despite the disturbing influence of the gait adaptability task. When the gait adaptability task was combined with the balance perturbation subjects further decreased step length, as evidenced by a significant interaction between both manipulations (p=0.012). In conclusion, able-bodied people reduce step length and increase step width during walking conditions requiring a high level of both stability and adaptability. Although an increase in step frequency has previously been found to enhance stability, a faster movement, which would coincide with a higher step frequency, hampers accuracy and may consequently limit gait adaptability.

Fitts' Law in early postural adjustments

We tested a hypothesis that the classical relation between movement time and index of difficulty (ID) in quick pointing action (Fitts' Law) reflects processes at the level of motor planning. Healthy subjects stood on a force platform and performed quick and accurate hand movements into targets of different size located at two distances. The movements were associated with early postural adjustments that are assumed to reflect motor planning processes. The short distance did not require trunk rotation, while the long distance did. As a result, movements over the long distance were associated with substantial Coriolis forces. Movement kinematics and contact forces and moments recorded by the platform were studied. Movement time scaled with ID for both movements. However, the data could not be fitted with a single regression: Movements over the long distance had a larger intercept corresponding to movement times about 140 ms longer than movements over the shorter distance. The magnitude of postural adjustments prior to movement initiation scaled with ID for both short and long distances. Our results provide strong support for the hypothesis that Fitts' Law emerges at the level of motor planning, not at the level of corrections of ongoing movements. They show that, during natural movements, changes in movement distance may lead to changes in the relation between movement time and ID, for example when the contribution of different body segments to the movement varies and when the action of Coriolis force may require an additional correction of the movement trajectory.

Inter-joint synergies increase with motor task uncertainty in a whole-body pointing task

The study investigates the effect of task uncertainty on motor synergies and movement time for a whole-body pointing task employing a Fitts' like paradigm. Thirty-three healthy, young adults were asked to hold a 1.5-m long stick and point it as quickly and accurately as possible to the unmarked center of fixed targets on the ceiling at 150% of the subject's height from the ground. Each subject performed fifteen continuous repetitions for each target size (1cm, 2cm, 3cm, 5cm, 8cm, 13cm and 21cm diameters of circles). It was assumed that the task uncertainty increased as the target size increased. Motion capture was used to collect the data for joint angles in the sagittal plane and uncontrolled manifold (UCM) analysis was used in order to investigate synergistic actions of joints. Results from the study revealed that the movement time decreased as task uncertainty increased. The variability within the uncontrolled manifold (V(UCM)) systematically increased with task uncertainty, resulting in an increase in the index of inter-joint synergies (ΔV), although the pointing task errors (V(ORT)) were consistent across different target sizes. The results suggest that the central nervous system systematically modulates the inter-joint synergies with task uncertainty in the whole-body pointing task without affecting motor performance.

Speed-difficulty trade-off in speech: Chinese versus English

This study continues the investigation of the previously described speed-difficulty trade-off in picture description tasks. In particular, we tested a hypothesis that the Mandarin Chinese and American English are similar in showing logarithmic dependences between speech time and index of difficulty (ID), while they differ significantly in the amount of time needed to describe simple pictures, this difference increases for more complex pictures, and it is associated with a proportional difference in the number of syllables used. Subjects (eight Chinese speakers and eight English speakers) were tested in pairs. One subject (the Speaker) described simple pictures, while the other subject (the Performer) tried to reproduce the pictures based on the verbal description as quickly as possible with a set of objects. The Chinese speakers initiated speech production significantly faster than the English speakers. Speech time scaled linearly with ln(ID) in all subjects, but the regression coefficient was significantly higher in the English speakers as compared with the Chinese speakers. The number of errors was somewhat lower in the Chinese participants (not significantly). The Chinese pairs also showed a shorter delay between the initiation of speech and initiation of action by the Performer, shorter movement time by the Performer, and shorter overall performance time. The number of syllables scaled with ID, and the Chinese speakers used significantly smaller numbers of syllables. Speech rate was comparable between the two groups, about 3 syllables/s; it dropped for more complex pictures (higher ID). When asked to reproduce the same pictures without speaking, movement time scaled linearly with ln(ID); the Chinese performers were slower than the English performers. We conclude that natural languages show a speed-difficulty trade-off similar to Fitts' law; the trade-offs in movement and speech production are likely to originate at a cognitive level. The time advantage of the Chinese participants originates not from similarity of the simple pictures and Chinese written characters and not from more sloppy performance. It is linked to using fewer syllables to transmit the same information. We suggest that natural languages may differ by informational density defined as the amount of information transmitted by a given number of syllables.

A logarithmic speed-difficulty trade-off in speech production

We explored the relations between task difficulty and speech time in picture description tasks. Six native speakers of Mandarin Chinese (CH group) and six native speakers or Indo-European languages (IE group) produced quick and accurate verbal descriptions of pictures in a self-paced manner. The pictures always involved two objects, a plate and one of the three objects (a stick, a fork, or a knife) located and oriented differently with respect to the plate in different trials. An index of difficulty was assigned to each picture. CH group showed lower reaction time and much lower speech time. Speech time scaled linearly with the log-transformed index of difficulty in all subjects. The results suggest generality of Fitts' law for movement and speech tasks, and possibly for other cognitive tasks as well. The differences between the CH and IE groups may be due to specific task features, differences in the grammatical rules of CH and IE languages, and possible use of tone for information transmission.

Does movement planning follow Fitts' law? Scaling anticipatory postural adjustments with movement speed and accuracy

We wanted to determine whether movement planning followed Fitts' law by investigating the relationship between movement planning and movement performance in experienced dancers executing a typical classical ballet step in which the big toe was pointed to targets at different distances and of different widths so as to obtain several indices of difficulty (ID). Movement time, velocity and variability at the target were the variables of movement performance kinematics; movement planning was evaluated by analysis of anticipatory postural adjustments (APAs) to assess their modulation at different IDs. Movement time and peak of velocity were found to scale with the ID only when individual movement distance across target widths was entered into the analysis. APA magnitude and duration both scaled according to movement parameters but not in the same way. APA magnitude scaled with movement velocity, while APA duration was sensitive to the amplitude-to-accuracy ratio following the ID for movements performed in the shortest time interval when on-line feedback control is probably not available. Here we show that timing of muscle activation acts as an independent central command that triggers fine-tuning for speed-accuracy trade-off.

How long does it take to describe what one sees? The first step using picture description tasks

The study explored the dependences between quantifiable features of a picture and the time it takes to describe it. Six native English speakers and six bilinguals watched pictures presented on the monitor and described them "as quickly and accurately as possible". The bilingual participants performed the test twice, in English and in their native language. The pictures could contain one to six objects. There were four series of trials that differed in the number of characteristics of the objects the participants were instructed to describe. Reaction time showed a modest, close to linear scaling with the number of objects. Both reaction time and speech time were significantly longer for the bilingual participants performing in English as compared to their performance in the native language and to the English speaking participants. The difference in reaction time did not depend on the number of objects. Speech time showed a close to linear scaling with the number of objects within each of the four series. The linear regression coefficient in this relationship increased linearly with the number of characteristics of the objects across all series. The results are discussed in relation to speed-accuracy trade-off and different strategies of picture description.

Violations of Fitts' law in a ballistic task

The authors explored changes in the postural preparation and movement times during jumps into targets of different sizes placed at different distances from the participant. Both movement and preparation times scaled with movement distance. Neither movement nor preparation time showed an effect of target size, although preparation time showed a tendency to increase for smaller targets. These observations show that the classical Fitts' law can be violated in tasks that involve a ballistic component. The data corroborate a hypothesis that Fitts' law originates at the level of movement planning.

Evidence for slowing as a function of index of difficulty in young adults with Down syndrome

Speed-accuracy trade-offs in persons with Down syndrome and typically developing controls were tested with a Fitts' task. Movement time scaled linearly with index of difficulty in both groups, and there were no accuracy differences. Persons with Down syndrome were slower than typically developing individuals. Regression analysis on movement time and index of difficulty showed a nearly two-fold higher regression coefficient and a nearly three-fold larger intercept value in the Down syndrome group. The dwell time on a target was much longer for Down syndrome persons but scaled with index of difficulty in about the same percentage for participants in both groups. Because of differences primarily related to scaling, we conclude that mechanisms of motor control are similar in Down syndrome and typically developing groups.