Practice of a precision isometric grip-force task by children with spastic cerebral palsy

The ability to produce and sustain a criterion level of precision isometric grip force was studied in a group of 7- to 12-year-old children with cerebral palsy (CP) and in a control group. On-line visual feedback of the forces produced relative to a stationary target was provided for each 5-second trial. Subjects practised 48 trials on each of 3 consecutive days. Measures of accuracy and variability revealed significantly reduced accuracy and increased variability in isometric force production for the group with CP. Both groups demonstrated improvement with practice, with the significantly greater change in the control subjects. There was substantial variability in performance among subjects with CP. Gains associated with practice were sustained over a 5-day retention interval for both groups. The requirement to control proximal segments of the upper extremity was not associated with increased error in grip-force production of the group with CP.

Aging and time to instability in posture

Two experiments are reported that were set up to examine the spatial-temporal boundaries of postural instability in upright stance as a function of age (60-96 years) and postural conditions. Subjects stood on a force platform under different experimental conditions (vision/no vision and arms up/down) so that the effect of age on key dynamic properties of postural stability could be determined. The findings showed that the ratio of the area of the motion of the center of pressure to the area within the stability boundary increased with age. Also, the virtual time-to-contact with the postural stability boundary decreased with age. Collectively, the findings show that the margins to the spatial-temporal boundaries of postural stability decrease with advancing age in the elderly. These reduced margins of dynamic stability may be a factor contributing to the progressive instability of posture with aging in the elderly.

The Sequential Structure of Movement Outcome in Learning a Discrete Timing Task

The sequential structure of discrete movement outcomes in an elbow-flexion movement task was examined with a crossed design of 2 movement-time (125 and 500 ms) and 2 range-of-motion (5° and 20°) conditions over sets of 200 trials of practice. Traditional analyses of error score techniques, time-series analyses of the quantitative raw and differenced data, and a symbolic dynamic analysis of qualitative events arising from the data were conducted. The differenced data revealed a consistent order over 3-trial strings that was more apparent with larger steps in the data scores, but quantitative time-series and symbolic dynamic analyses of the raw movement-time data showed weaker relations. There were a few patterns of structure evident in the raw data time-series that were a function of the movement condition and the skill level of the subject. The analyses of the movement-time scores revealed that, in learning the discrete timing task, there is more order apparent in the intrinsic frame of reference of the difference scores than in the extrinsic frame of reference.

Virtual Time-to-Collision and Human Postural Control

This article is a report on 3 experiments designed so that the role of virtual time-to-collision (VTC), which specifies the spatiotemporal proximity of the center of pressure to the postural stability boundary in the regulation of posture in upright stances, could be examined. Virtual time-to-collision was estimated for normal upright stance with different bases of support, and for postural oscillations in which the speed of movement and instructional constraints on the coordination mode used were manipulated. The results showed that virtual time-to-collision was predictably reduced as (a) the base of support was reduced, (b) the speed of the postural oscillation was increased, and (c) the number of biomechanical degrees of freedom regulated in the coordination mode increased. Over a range of task conditions, the coefficients of variation of the VTC time-series were significantly lower than the coefficients of variation for the velocity and acceleration time-series of the center of pressure. The absolute values of VTC increased with the increment of the ground reaction forces a performer generated to avoid falling while approaching the stability boundary. These findings are consistent with the proposition that VTC may serve as an organizing informational control parameter for posture.

Akathisia in adults with mental retardation: development of the Akathisia Ratings of Movement Scale (ARMS)

We developed an akathisia rating scale for use with persons who have mental retardation and screened for the occurrence of akathisia in three samples: 66 adults receiving maintenance neuroleptic treatment, 20 adults not receiving neuroleptics, and 8 adults undergoing neuroleptic dose reduction. The scale had an acceptable level of interrater reliability and validly measured group differences related to neuroleptic treatment status. Using an empirically derived cut-off-score, we estimated the prevalence rate for akathisia to be 5% in neuroleptic-free subjects, 17% in neuroleptic-maintenance subjects, and 25% in neuroleptic-reduction subjects. Akathisia, dyskinesia, and stereotypy manifested as qualitatively different movement topograhies. The occurrence of dyskinesia stereotyped movement disorder was associated significantly with an increased occurrence of akathisia.

Stochastic processes in postural center-of-pressure profiles

The stochastic processes of postural center-of-pressure profiles were examined in 3- and 5-year-old children, young adult students (mean 20 years), and an elderly age group (mean 67 years). Subjects stood still in an upright bipedal stance on a force platform under vision and nonvision conditions. The time evolutionary properties of the center-of-pressure dynamic were examined using basic stochastic process models. The amount of motion of the center of pressure decreased with increments of age from 3 to 5 years to young adult but increased again in the elderly age group. The availability of vision decreased the amount of motion of the center of pressure in all groups except the 3-year-old group, where there was less motion of the center of pressure with no vision. The stochastic properties of the center-of-pressure dynamic were assessed using both a two-process, random-walk model of Collins and De Luca and an Ornstein-Uhlenbeck model that is linear and has displacement governed only by a single stiffness term in the random walk. The two-process open- and closed-loop model accounted for about 96% and the Ornstein-Uhlenbeck model 92% of the variance of the diffusion term. Diffusion parameters in both models showed that the data were correlated and that they varied with age in a fashion consistent with developmental accounts of the changing regulation of the degrees of freedom in action. The findings suggest that it is premature to consider the trajectory of the center-of-pressure as a two-process, open- and closed-loop random-walk model given that: (a) the linear Ornstein-Uhlenbeck dynamic equation with only two parameters accommodates almost as much of the variance of the random walk; and (b) the linkage of a discontinuity in the diffusion process with the transition of open- to closed-loop processes is poorly founded. It appears that the nature of the stochastic properties of the random walk of the center-of-pressure trajectory in quiet, upright standing remains to be elucidated.

Dyskinetic movement disorder among adults with mental retardation: phenomenology and co-occurrence with stereotypy

We screened for the occurrence of dyskinetic and stereotypic movement disorders using item-independent screening protocols to determine whether these forms of movement disorder can be distinguished among adults with mental retardation. Stereotypies and dyskinesias were reliably distinguished in terms of topography. Tardive dyskinesia occurred in 18.2% of a cohort of individuals receiving chronic neuroleptic treatment. Stereotypic movement disorder was associated with increased dyskinesia scores and increased prevalence of tardive dyskinesia. Increased dyskinesia scores were also found for subjects exhibiting stereotypy who had been free of neuroleptic treatment for 3 years. Results indicate that dyskinesia and stereotypy are discriminable movement disorders and provide preliminary support for the hypothesis that they may be related by common mechanisms.

Inter- and intra-limb coordination in arm tremor

Inter- and intra-limb coordination in arm tremor was examined in adult subjects under vision and no vision conditions using accelerometery techniques. The accelerometer data were analyzed using standard time and frequency domain analyses and the regularity of the acceleration time series was determined using an approximate entropy (Ap En) measure. The data analysis was structured to examine the hypothesis that there is a functional compensatory relation between the motion (tremor) of the limb segments in the arm coordination postural pointing task. The results showed that the level of acceleration increased in a proximal to distal direction within a single arm and was symmetrical across homologous arm segments. The frequency analysis showed the established power spectral profiles for each limb segment in postural tremor tasks, but the finger motion included (beyond the normal 8-12 Hz and 20 Hz tremor) a third slower peak at around 2-3 Hz, due possibly to the reactive forces of the other arm links. There was no effect of vision on the level or frequency patterns of accleration in the limb segments. The coordination analysis showed that there was no linkage between the arms in either the time or frequency domain in the execution of this postural task. This result would tend to suggest that the neuronal commands underlying normal tremor are not derived from a common central oscillator within the central nervous system but are organized in a parallel fashion. The strength of the coupling of intra-limb coordination varied according to the particular adjacent limb links. There were significant correlations in the time domain and coherence in the frequency domain in the acceleration signals between upper arm and forearm, and between hand and finger. The phase lag of the arm units within each of these respective segment pairs was close to in phase or 0 deg. Significant coherence in the frequency domain was also evident between upper arm and hand motion, with the phase lag between these segments being close to 180 deg out of phase. The Ap En analysis of the acceleration signals revealed that there was more regularity to the upper arm and hand accelerometer signals than the forearm and finger signals. The findings show that the intra-limb coordination of the arm links in a two-limb postural pointing task is effected by a compensatory synergy organized about the action of the wrist and shoulder joints. This compensatory synergy reduces the coordination of the 4 within-limb degrees of freedom (arm links) to, in effect, a single degree of freedom arm control task that is not coupled in organization to the motion of the other limb or the torso. It is proposed that this coordination solution reduces the degrees of freedom independently regulated for realization of the task goal but preserves independent body segment control in critical degrees of freedom for potential adaptation to postural perturbations.

The informational role of knowledge of results in motor learning

An experiment is reported that was set-up to examine the informational role of knowledge of results (KR) in the learning of a single-limb movement timing task. A group with KR practiced 200 trials a day for 5 days prior to receiving a sixth day of practice without KR. The performance of this group was contrasted to another group that practiced 200 trials without KR for one day. Traditional movement error and time series analyses revealed that KR serves to calibrate the movement outcome to the task demands and modulate the performance outcome relation between trials. The degree of systematic trial-to-trial modulation was strongly dependent upon the degree of error exhibited on any given trial, and was enhanced under no-KR conditions. Information in KR has both immediate and persistent influences on learning and performance that are dependent upon the task constraints and the skill level of the performer.

Facial stereotypic movements and tardive dyskinesia in a mentally retarded population

The facial stereotypies of adults diagnosed as having mental retardation and tardive dyskinesia were examined through a kinematic analysis of video-taped lip and tongue motions. A control group of healthy adult subjects without mental retardation was also examined in the production of preferred rates of lip and tongue oscillatory motions to provide a basis to assess the degree of movement variability in the stereotypies. The inter- and intraindividual variability of the movement form characteristics of the lip and tongue stereotypic motions was higher in the subjects with mental retardation. Results suggest that the low variability of discrete properties of movement kinematics may not be a defining feature of stereotypies. The concept of invariance in stereotypies may relate only to the topological kinematic properties of the movement sequence that provide the basis to infer that the same stereotypic movement sequence was reproduced from observation to observation.

The dynamical structure of tremor in tardive dyskinesia

The movement disorder syndrome of tardive dyskinesia arises as a consequence of prolonged regimens of neuroleptic medication, and is characterized, although not exclusively, by jerky and sometimes rhythmical stereotypical motions in a wide range of muscle systems. It is well established that the degree and variability of tremor in tardive dyskinesia is greater than that in normal age-matched subjects. The findings from the current experiment show that the dimension of the tardive dyskinetic finger tremor time series is systematically lower than that evident in normal finger tremor. Furthermore, the variability of finger motion in both groups is inversely related to the dimension of the respective attractor dynamic. The neuroleptic medication appears to constrain the degrees of freedom regulated in organization of the motor system. (c) 1995 American Institute of Physics.

Perceptual-motor exploration as a function of biomechanical and task constraints

Four experiments are reported that were designed to examine perceptual-motor exploration employed in determining the solution to a dual-axis positioning task under various biomechanical and task constraints. Experiments 1, 2, and 3 used the two elbow joints to examine the impact of varying several geometric features of the relation between visual information and action in this task. Experiment 4 examined the use of within-limb, between-limb, and within-joint axes of motion in a similar task. The exploratory process was analyzed using a symbolic dynamic defined over nominal categories of visual information and actions elicited by the performer. The search strategy used to improve task performance was consistent across all the experimental manipulations imposed. The frequency pattern of nominal action categories demonstrated a preference for single-axis activity except in the within-joint condition which exhibited a preference for dual-axis activity. The pattern of preferred transitions among these action categories was also consistent across conditions, and lag sequential analysis revealed a robust tendency for cyclical activity in that opposite actions were often coupled in sequence. The topologically equivalent (extrinsic geometry) task spaces led to qualitatively similar search strategies when considered at the level of action-information interaction (intrinsic geometry). The physical implementation of this strategy was strongly influenced by the biomechanical constraints of the action system, while the manipulations of the geometric features of the action-information relation served only to influence the quantitative properties of performance outcome.

The coordination and control of human creeping with increases in speed

Changes were documented in the inter-limb and intra-limb timing of well trained adult males creeping at increasing speed on a motor-driven treadmill. A power function (y = axb) showed a good fit to hand and foot stride and support duration (y) and speed (x), a finding that parallels the changes expected in normal upright walking. Similarly, limbs of the same girdle maintained a half-cycle phase lag as speed increased. Swing duration, however, showed cyclical changes with increasing speed, a phenomenon not previously reported either for quadrupedal gait or normal upright gait. Ipsilateral hands and feet abruptly changed their phase-lags at a constant percentage of maximum speed in a manner similar to gait transitions observed in quadrupedal species. Explanations for these transitions were suggested, such as physical constraints on the speed of limb movement and the organism's biomechanical adaptation to increased metabolic energy expenditure.

Energy expenditure and motor performance relationships in humans learning a motor task

The ability of human subjects to learn minimum energy-demanding variants of biological motion was examined in three adult males trained to walk on hands and feet (creep) on a motor-driven treadmill at constant speed (0.64 m/s) for 16 3-min trials. Two subjects systematically decreased oxygen consumption and heart rate over trials. Following this acquisition phase, subjects completed walking and creeping trials at positive and negative treadmill grades and selected a freely chosen creeping grade that felt "most comfortable." One subject selected a grade that was more efficient than those imposed. Oxygen-consumption curves for walking and creeping converged with increasing positive grade, indicating that increased grade influences the metabolic energy viability of the task (creeping or walking). The acquisition data provide empirical support for the "principle of least effort" and lend support to the concept of a "comfort mode" in the execution of motor tasks.

Quantification of postural sway patterns in tardive dyskinesia

An assessment was made of the orientation and variability in postural center of pressure patterns in individuals with tardive dyskinesia (TD) and/or developmental disability during quiet standing. Postural patterns were compared and contrasted between four groups of individuals: those with (a) TD and developmentally disabled (severely and profoundly retarded); (b) developmental disability only; (c) TD but of normal intelligence; and (d) a healthy control group. The center of pressure displacements were derived from the lateral, vertical, and anterior-posterior force and moment components of force platform measurements. Analyses demonstrated that individuals with TD in combination with developmental disability had a different center of pressure orientation and variability compared to healthy individuals and individuals suffering only from developmental disability or TD. The center of pressure pattern in the developmentally disabled TD group was characterized by a more prominent lateral orientation, whereas in the other three groups, it had a more predominant anterior-posterior orientation. In addition, the variability in these orientation components was much smaller in the developmentally disabled TD group, indicating a more regular pattern of sway in the center of pressure during quiet standing in these individuals. These findings show that assessments of postural center of pressure profile orientation and variability may be useful indicators for investigating TD, especially in distinguishing between developmental disability and TD.

Arm tremor, tardive dyskinesia, and mental retardation

The arm tremor of adults diagnosed as having mental retardation and/or tardive dyskinesia was examined through an analysis of the acceleration properties of several arm postures. The degree of arm acceleration was increased in all groups compared to a control group without mental retardation. The tardive dyskinesia and/or mentally retarded groups also showed a shift to a lower modal frequency of physiological tremor. Results showed that both time domain and frequency properties of the tremor acceleration signal are necessary to distinguish between groups. The neuroleptic medication appears to engender a dual and opposing influence on performance in the arm tremor task for the mentally retarded group. The medication reduces behavioral variability as indexed by analyses of arm acceleration over time (time domain) but lowers the modal frequency of physiological tremor.

Body scale and infant grip configurations

This study examined whether hand/object size ratios define common boundaries to the grip configuration patterns of infants and adults. A group of 5- to 8-month-old infants and a group of adults engaged in a displacement grasping task with inverted cups that varied in size. The findings showed that infant and adult grip configurations varied systematically with object size: More digits were brought into the contact grip configurations with increasing object size. Furthermore, when object size was scaled to hand size, common dimensionless ratios defined the grasping patterns and transitions between grasping patterns in a similar manner for both adults and infants. Consistent with a dynamical view of the development of coordination, the strong role of body scale on the developmental prehensile coordination pattern was observed for a given set of task constraints.

Impulse variability in isometric tasks

An isometric elbow flexion task was used in two experiments that examined the influence of force-production characteristics on impulse variability. Impulse size was held constant while peak force, time to peak force, rate of force, and, hence, the shape of the criterion force-time curve were manipulated. The results indicated that changes in the force-time curve under conditions of equal impulse bring about systematic changes in impulse variability, and this effect is more pronounced for larger impulse conditions. The inability of existing functions to account for the peak force variability findings led to the generation of a new predicted force variability function. The proposed function accounts for changes in the standard deviation and coefficient of variation of peak force, impulse, and rate of force over a range of force-time conditions.

Space-time accuracy of rapid movements

The space-time accuracy of an elbow flexion movement task was examined in two experiments over a range of motion extents (1 degrees through 100 degrees ) and short-duration movement times (100, 125, 150, and 400 ms). Nonlinear speed-accuracy functions emerged for both spatial and temporal error over all the movement conditions examined. The results showed that the timing error and spatial error had a high degree of complementarity as predicted by a space-time model of the speed-accuracy relation (Hancock & Newell, 1985). The findings confirm that the frame of reference for measuring movement error determines in part the error functions observed.

Correlations between kinematic and rating scale measures of tardive dyskinesia in a developmentally disabled population

This paper reports a study that examines the relations between Dyskinesia Identification System: Condensed User Scale (DISCUS) scores and a battery of postural and movement kinematic measures in a group of adults diagnosed as being developmentally disabled and screened as having tardive dyskinesia. The results showed that finger tremor measures correlated with the tongue tremor and pill rolling items of DISCUS, whereas the postural stability scores correlated with the toe movement item of DISCUS and the total DISCUS score. There was also a high stability in subject kinematic performance from trial to trial over the postural and movement tests. The pattern of correlations between the DISCUS items and movement kinematic measures is consistent with the proposition that tremor is a centrally rather than peripherally driven phenomenon, although many factors contribute to emergent tremors. These findings provide construct and content validity for the DISCUS as a screening device for tardive dyskinesia and suggest that certain posture and movement kinematic measures could be sensitive measuring methods for tardive dyskinesia in developmentally disabled populations.

The Interaction of Criterion and Feedback Information in Learning a Drawing Task

Experiments were designed to examine the influence of criterion and feedback information in the learning of a two-dimensional drawing task. Experiment 1 showed that when the task criterion is well known to the subject, the combined presentation of criterion information and information feedback facilitates the rate of acquisition of the skill but not its overall performance level of achievement. Experiment 2 showed that when the task criterion information is not well known to the subject, presentation of criterion information facilitates both the rate of acquisition and the overall performance level and, furthermore, is essential if configuration information feedback is to be utilized effectively. Experiment 3 showed that it is the combined presentation of criterion and configuration information feedback rather than the isolate presentation of either type of information alone, that facilitates learning and performance. Collectively, the findings from the three experiments suggest an interactive effect of prior knowledge by the learner and type of augmented information in facilitating the acquisition of skill, according to the constraints imposed in the task. The data are consistent with the proposal that the degrees of freedom in the information available to support motor skill learning must match the degrees of freedom to be constraint in the perceptual-motor workspace.

Preload and Isometric Force Variability

Two experiments are reported, which examined the relative contributions of preload (resting force level), change of force, and the time taken to achieve the force in determining isometric force variability. The findings showed that change of force is the strongest determiner of peak force variability but that preload and time to peak force have smaller though systematic effects. A formula that predicts peak force variability is proposed, with preload as an additive effect to the ratio between the change of force level and the square root of time to peak force. These findings confirm that these three impulse variables are significant in predicting force variability and that the impact of rate of force on peak force variability is nonlinear.

Task constraints and infant grip configurations

The prehensile grip configurations of infants aged 4 through 8 months were examined as they grasped objects that varied in size and shape. The findings revealed that infants as young as 4 months systematically differentiate grip configurations as a function of the object properties in essentially the same way that 8-month-old infants do. However, the younger 4-month-old infants predominantly used the haptic system in addition to the visual system for information pick-up regarding object properties, whereas 8-month-old infants predominantly used information from the visual system alone to differentiate grip configurations according to the object properties. Infants apparently perceive the same action-relevant information through different emphases of the sensory modes to drive the action system with a similar grip configuration for a given object. It is proposed that the traditional description of an orderly sequence to the development of infant prehension (e.g., Halverson, 1931) is too conservative and inflexible to capture the functionally adaptive prehensile behavior of infants to changing task constraints.

The Effects of Practice on Limb Kinematics in a Throwing Task

The effect of practice on limb kinematics in a dart-throwing task was examined to test three current hypotheses regarding limb control: trajectory formation; end-point control; and coordinated joint-space control. Practice was given to both the relatively well-practiced dominant ant the relatively unpracticed nondominant limbs of 5 male subjects to permit analysis of the early phase of coordination acquisition. The nondominant limb demonstrated high absolute joint cross-correlations with high variability throughout practice and consistency in the hand trajectory. The dominant limb exhibit a significant decrease in wrist-elbow and wrist-shoulder cross-correlations over practice while also maintaining a consistent hand trajectory and significantly higher scoring performance. The findings demonstrate that practice effects can be seen in both coordination mode and variability of various parameters of limb motion, but the changing relationship between the variables suggests that control cannot be ascribed to any one of the three hypotheses advanced. It is proposed that the observed invariance or variance in limb trajectories, end-point control, and coordinated joint angles are a reflection of more global parameters emerging from the flow field properties of the organism, environment, and task interaction.

On Task and Theory Specificity

One of the significant limitations of the motor control and skill acquisition domain is that the theories, models, and hypotheses are, in most cases, task specific. Many lines of theorizing fail to hold up under even small changes in task constraints, although clearly the field does have some robust phenomena. It is proposed that a broader consideration of the role of task constraints, which is grounded in the methodology of nonlinear dynamics, may help to formulate a more general action theory of coordination and control.

Body scale and the development of prehension

This study examined whether common dimensionless ratios define the critical point in the shifts in the grip coordination pattern of preschoolers and adults engaged in a displacement grasping activity with cubes that varied in object size. The findings indicated that there was an interaction between task constraints and organismic constraints in determining the grasping pattern utilized. However, when the object size is scaled to hand size there are common dimensionless ratios that correspond to the grasping patterns and the limb orientations employed across the age range utilized. Furthermore, 5 grip configurations accounted for the majority of grip variance in both age groups. The findings suggest a strong role for the impact of body scale on the development of coordination and provide preliminary evidence for the view that the development of prehension is a reflection of the constraints imposed on action.

Force variability in isometric responses

In the present study we examined the contribution of different impulse parameters to peak force variability in an isometric task. Five experiments are reported that each held constant a different impulse parameter while allowing the other impulse parameters to vary. The results indicate that change in force level is the parameter that has the greatest effect on peak force variability, although time to peak force and preload also systematically influence response variability. A formula that accommodates the relation between impulse parameters and force variability is proposed. The data suggest that even in isometric tasks, it is the force-time properties of the impulse, rather than discrete parameters such as peak force, that determine the outcome variability.

On the Relationship Between Peak Force and Peak Force Variability in Isometric Tasks

An experiment is reported documenting the relationship between peak force and peak force variability with a fixed criterion time to peak force for an isometric task requiring activation of the elbow flexors. The results show that maximum peak force increases with increments in time to peak force and that peak force variability increases with increments of peak force in an exponential type function. Furthermore, despite the presence of peak force and time to peak force feedback, subjects systematically shifted time to peak force as a function of the percentage of peak force being produced. This temporal modulation changes the percentage of peak force represented by any given peak force criterion. When peak force is made proportional to the degree of departure from the criterion time to peak force, a linear relationship is found between peak force and peak force variability. These findings suggest that time to peak force and rate of force production are parameters that influence veridical estimates of the force variability function.

Training reaction and movement times of moderately and severely mentally retarded persons in aiming movements

The effect of information feedback of movement components on the reaction time (RT) and movement time (MT) of mentally retarded adults performing a discrete movement aiming task was investigated. We anticipated that specific training and informational feedback on the RT portion of the RT-MT task would more effectively reduce RT, despite aiming requirements of the movement. Results suggested that specific training and feedback on RT decreased both RT and MT components of the task, whereas specific training on MT reduced only MT. The data were discussed with respect to the differential effects of training on subject populations at varying levels of intellectual capacity.

Forgotten Moments

It is proposed that reliance on only the mean and standard deviation of a distribution to describe response frequency may lead to erroneous inferences concerning such distributions when skewness and kurtosis are present. After defining the first four moments of a distribution, it is demonstrated analytically that skewness and kurtosis may vary to systematically influence the mean and standard deviation of a set of related distributions. The significance of these relationships for the interpretation of differing response distributions is advanced through examples gleaned from the movement control literature. in addition, it is suggested that the use of bandwidths to select scores from a distribution for subsequent data analysis can further compound the problems of both descriptive and explanatory inference, particularly when skewness and kurtosis are features of such distribution(s). Whether or not inferential statistics are invoked, a veridical perspective of distributions is essential to meaningful data analysis.

Speed and accuracy of compensatory responses to limb disturbances

This study examines the speed and accuracy of compensatory responses to flexion-extension perturbations of the wrist in the horizontal plane. In Experiments 1 and 2 the subjects were required to establish an initial flexion or extension force of approximately 15% maximum at a prescribed initial muscle length. The perturbations changed the load force by +/-5% in both simple and choice reaction protocols. The results showed that the latencies to compensate for the perturbation were longer when the direction of disturbance was unknown (i.e., choice effect) and when the perturbation unloaded the muscle (i.e., directional effect). Accuracy constraints on the compensatory response increased movement time and reduced the variability of latency without affecting mean latency. In Experiment 3, a visual stimulus generated a comparable choice effect on latency to that produced by the perturbations, but no directional effect in relation to the preload was apparent. Our behavioral analysis of compensatory responses triggered by wrist perturbations confirms that these responses are susceptible to variables that influence the initiation of voluntary movements. Our analysis also demonstrates a directional preload effect that is stimulus specific.

Visual control of discrete aiming movements

An experiment is reported which investigated the visual control of discrete rapid arm movements. Subjects were required to move as rapidly as possible to several target width-movement distance combinations under both visual and non-visual conditions. The movement time (MT) data were supportive of Fitts' Law in that MT was linearly related and highly correlated to the Index of Difficulty (ID). MT was also similar for different target width-distance combinations sharing the same ID value. The error rate analysis, which compared visual to non-visual Performance, indicated that vision was only used, and to varying degrees, when MT exceeded 200 ms (3.58 ID level). There was some evidence that vision was differentially used within target width-distance combinations sharing the same ID. Estimates of endpoint variability generally reflected the results of the error rate analysis. These results do not support the discrete correction model of Fitts' Law proposed by Keele (1968).

Speed and accuracy of compensatory responses to limb disturbances

This study examines the speed and accuracy of compensatory responses to flexion-extension perturbations of the wrist in the horizontal plane. In Experiments 1 and 2 the subjects were required to establish an initial flexion or extension force of approximately 15% maximum at a prescribed initial muscle length. The perturbations changed the load force by +/-5% in both simple and choice reaction protocols. The results showed that the latencies to compensate for the perturbation were longer when the direction of disturbance was unknown (i.e., choice effect) and when the perturbation unloaded the muscle (i.e., directional effect). Accuracy constraints on the compensatory response increased movement time and reduced the variability of latency without affecting mean latency. In Experiment 3, a visual stimulus generated a comparable choice effect on latency to that produced by the perturbations, but no directional effect in relation to the preload was apparent. Our behavioral analysis of compensatory responses triggered by wrist perturbations confirms that these responses are susceptible to variables that influence the initiation of voluntary movements. Our analysis also demonstrates a directional preload effect that is stimulus specific.

Coincident timing behavior of young mentally retarded workers under varying conditions of target velocity and exposure

The degree to which response complexity affects the ability of moderately mentally retarded adults to perform on a coincident timing task was examined. In Experiment 1 subjects were required to depress a key to estimate accurately the arrival of a moving target at a predetermined coincident point. Analyses of both constant and variable error scores suggested that subjects had difficulty inhibiting their responses and that an optimal exposure distance for the task existed. In Experiment 2 subjects were required to make a movement response to the target. Subjects made more constant errors in this experiment but improved when provided maximum target preview. These mixed findings are discussed in terms of subjects' impulsivity and their sensitivity to the characteristics of the display.

Training moderately and severely mentally retarded adults to improve reaction and movement times

Two experiments were conducted to examine the degree to which mentally retarded adults can be trained to improve their reaction time (RT) and movement time (MT). A discrete aiming task that had both RT and MT components was utilized in Experiment 1. Only MT improved significantly across the 15 training days, and this performance level was maintained after 5 months without practice. In Experiment 2 the extent to which RT can be trained in the absence of the spatial-temporal constraints of the aiming response was examined. Reaction time decreased significantly for both the training and training-plus-feedback groups over a 10-day training period. The findings demonstrate the role of response complexity in response-initiation processes for retarded adult workers.

Velocity as a Factor in Movement Timing Accuracy

Our experiments examined the boundaries within which movement timing error decreases in timing responses as a function of increases in movement velocity. Experiment 1 investigated the lower end of the movement-velocity continuum and showed a curvilinear decrease in movement error as average velocity increased from 5 to 25 cm/sec. Experiment 2 also revealed a decrease in movement error as velocities increased from 67 to 95% (98-320 cm/sec) of maximum velocity for the amplitudes employed. These findings were confirmed in Experiment 3 which examined the full range of the velocity continuum in a completely within-subject design. Absolute timing error, expressed as a percentage of movement time, was a logarithmic function of average velocity, with error decreasing as velocity increased. Overall, the findings demonstrate the generalizability of the movement-velocity effect on timing error. The discussion focuses on explanations for this phenomenon which currently appear far from clear-cut.

Temporal anticipation of response initiation by mentally retarded persons

The ability of retarded persons to anticipate a signal to initiate discrete movements of varying complexity was investigated in two experiments. The results revealed an interaction between temporal anticipation and response complexity. Experiment 1 showed no reduction in reaction time (RT) in discrete aiming movements when the foreperiod was changed from a random interval to a fixed interval (1 or 3 seconds). Reaction time did not vary with length of the interval or the difficulty of the movement. Experiment 2 showed a reduction of RT to a simple release response at a fixed 1-second warning interval. The findings reflect the difficulty retarded persons have in programming more complex movements and the interaction of temporal anticipation in this response-initiation process.

Movement Time and Velocity as Determinants of Movement Timing Accuracy

Three experiments investigated the effect of movement time (MT) and movement velocity on the accuracy and initiation of linear timing movements. MTs of 100, 200, 500, 600, and 1000 msec were examined over various distances; timing accuracy decreased with longer MTs and slower average velocities. The velocity effect was independent of MT and occurred when the velocities were above and below about 15 cm/sec. Self-paced initiation times to movement increased directly with MT and inversely as a function of movement velocity. The latency data complement the MT findings in suggesting that average velocity is a key parameter in the initiation and control of discrete timing movements and, that there is some lower velocity below which movement control breaks down.

Coordinating visual and kinaesthetic memory codes

Four experiments examined the Connolly & Jones (1970) model which postulates that translation between modalities in the cross-modal paradigm occurs before storage in short-term memory. In general, the results provided no support for the translation notion. Delaying until the end of the retention interval knowledge of the reproduction mode failed to produce a matching performance decrement. Subjects were able to maintain the code of original presentation through the retention interval even when they did not expect reproduction to be in this mode. In addition, the asymmetry in the cross-modal matching of visual (V) and kinaesthetic (K) information, whereby K-V performance is more accurate than V-K performance, was found to occur only under certain visual display conditions. The implications of these findings for general models of cross-modal translation were discussed.

On The Structure of Motor Recognition Memory

Two experiments determined the influence of the range and number of auditory sensory consequences, associated with a rapid timing task, on the development of motor recognition. Experiment 1 observed no beneficial effect upon subsequent movement-transfer performance from experience with the criterion-movement-time sound compared to experience with either a narrow or a wide range of sounds that bracketed the criterion sound (not including the criterion); 60 sound trials prior to transfer did not produce better transfer than did six sounds. The second experiment examined transfer outside of the range of previous listening experience by having subjects transfer to one of two possible criterion movement times after having received either constant, or one of two types of variable, listening experience. Transfer performance was influenced by the amount of variability in listening experience. These results were seen as support for a schematic representation for motor recognition memory (Schmidt, 1975)