Instrument considerations in measuring fast eye movements

The dynamic limitations of eye movement recorders can distort the measurement of fast eye movements such as saccades and nystagmic quick phases. In this paper, the effects of the bandwidth and noise of recording methods and the problems incurred by digital sampling are discussed theoretically with respect to the measurement of peak velocity and duration of fast eye movements. As a practical example, a TV-based infrared corneal reflex system is examined and a method for calibrating it for peak velocity measurement is described.     

Selective reaching: distracter effects on movement kinematics as a function of target-distracter separation

The authors investigated distracter effects on the kinematics of reaching movements to determine when during reaching responses (reaction time, time to peak velocity, time after peak velocity, or peak velocity) distracter interference occurred and how target-distracter separation affected the locus of interference. Participants moved a pen on a digitizing tablet toward a target appearing with or without a distracter. With a small target-distracter separation, distracter interference occurred during time after peak velocity (similar amounts of interference from near and far distracters). With a large target-distracter separation, distracter interference occurred during time to peak velocity (more interference from near compared to far distracters). The results demonstrated that target-distracter separation is an important determinant of the locus of distracter interference.     

Controlling velocity in rapid movements

It has often been reported that subjects prefer to use a strategy in which they vary movement velocity and peak amplitude in a linear fashion. In this study, control of velocity and amplitude in rapid reciprocating movements of the interphalangeal joint of the thumb was investigated by examining movement trajectories and patterns of activity in the extensor pollicis longus (EPL) and flexor pollicis longus (FPL) muscles. In controlling either amplitude or peak flexion velocity without constraint, subjects always used a strategy in which peak extension velocity and peak flexion velocity had strong linear correlations with movement amplitude. When they were required to keep either amplitude or peak flexion velocity fixed their movements were still biased toward a strategy in which peak velocity and movement amplitude covaried. It is suggested that the preferred strategy is related to a basic principle of scaling the magnitude and duration of a velocity profile in order to achieve different movement amplitudes.     

Three-dimensional movement trajectories in Fitts' task: Implications for control

According to Fitts (1954), movement time (MT) is a function of the combined effects of movement amplitude and target width (index of difficulty). Aiming movements with the same index of difficulty and MT may have different planning and control processes depending on the specific combination of movement amplitude and target size. Trajectories were evaluated for a broad range of amplitudes and target sizes. A three-dimensional motion recording system (WATSMART) monitored the position of a stylus during aiming movements. MT results replicated Fitts' Law. Analysis of the resultant velocity profiles indicated the following significant effects: As amplitude of movement increased, so did the time to peak resultant velocity; peak resultant velocity increased slightly with target size, and to a greater extent with increases in the amplitude of movement; the time after peak resultant velocity was a function of both amplitude and target size. Resultant velocity profiles were normalized in the time domain to look for scalar relation in the trajectory shape. This revealed that: the resultant velocity profiles were not symmetrical; the proportion of time spent prior to and after peak speed was sensitive to target size only, i.e. as target size decreased, the profiles became more skewed to the right, indicating a longer decelerative phase; for a given target size, a family of curves might be defined and scaled on movement amplitude. These results suggest that a generalized program (base trajectory representation) exists for a given target width and is parameterized or scaled according to the amplitude of movement.     

Kinematics of Goal-Directed Arm Movements in Neglect: Control of Hand Velocity

Do patients with unilateral neglect exhibit direction-specific deficits in the control of movement velocity when performing goal-directed arm movements? Five patients with left-sided neglect performed unrestrained three-dimensional pointing movements to visual targets presented at body midline, the left and right hemispace. A group of healthy adults and a group of patients with right-hemispheric brain damage but no neglect served as controls. Pointing was performed under normal room light or in darkness. Time-position data of the hand were recorded with an opto-electronic camera system. We found that compared to healthy controls, movement times were longer in both patient groups due to prolonged acceleration and deceleration phases. Tangential peak hand velocity was lower in both patient groups, but not significantly different from controls. Single peak, bell-shaped velocity profiles of the hand were preserved in all right hemispheric patients and in three out of five neglect patients. Most important, the velocity profiles of neglect patients to leftward targets did not differ significantly from those to targets in the right hemispace. In summary, we found evidence for general bradykinesia in neglect patients, but not for a direction-specific deficit in the control of hand velocity. We conclude that visual neglect induces characteristic changes in exploratory behavior, but not in the kinematics of goal-directed movements to objects in peripersonal space.     

Development of muscle fatigue assessed by using superposition of evoked and volitional myoelectric potentials.

We studied muscle fatigue development using evoked myoelectrical potentials superimposed on volitional ones. The instantaneous frequency of superim posed M-waves and mean power frequencies of volitional electromyography (EMG) declined during sustained contraction, indicating that fatigue progressed. We divided fatigue into 3 phases, with 20 frames in each fatigue phase, corresponding to one-third of the total sample. The instantaneous frequency of superimposed M-waves and mean power frequencies of volitional EMG were correlated during early intensive isometric voluntary contractions and became increasingly uncorrelated as contraction proceeded. The coefficient between the mean power frequency and instantaneous frequency correlation was also greater at the first peak than at the second peak of the superimposed M-wave, indicating that the motor unit action potential was distorted. Distortion in the motor unit action potential shape depends on elongation of the depolarization zone of muscle fiber, because the superimposed M-wave is a peripheral indicator elicited by electrical stimulation. These results suggest that muscle fatigue develops based on the reduction of the conduction velocity of muscle fiber and on the elongation of the depolarization zone of muscle fiber.     

Contribution of stereopsis,vergence, and accommodative function to the performance of a precision grasping and placement task in typically developing children age 8–14 years

Upper limb reaching and grasping movements are performed more efficiently during binocular viewing; however, the distinct contribution of stereopsis, fusional vergence, and accommodation (binocular facility, amplitude and accuracy) has not been examined in typically developing children. This study examined binocular visual function in a cohort of 57 typically developing children, 8 to 14 years old. Hand kinematics were recorded using a motion capture camera while children performed a prehension task involving threading a bead onto a needle. Results showed that different aspects of binocular vision contribute to the control of distinct phases of upper limb movements. Specifically, fusional vergence was associated with higher peak reach velocity, stereoacuity was associated with shorter grasp execution, and accommodation was associated with shorter placement duration. These findings suggest that different aspects of binocular vision play an important role in optimizing the control of distinct phases of prehension movements during development.     

Peak velocity as a cue in audiovisual synchrony perception of rhythmic stimuli

This study investigated audiovisual synchrony perception in a rhythmic context, where the sound was not consequent upon the observed movement. Participants judged synchrony between a bouncing point-light figure and an auditory rhythm in two experiments. Two questions were of interest: (1) whether the reference in the visual movement, with which the auditory beat should coincide, relies on a position or a velocity cue; (2) whether the figure form and motion profile affect synchrony perception. Experiment 1 required synchrony judgment with regard to the same (lowest) position of the movement in four visual conditions: two figure forms (human or non-human) combined with two motion profiles (human or ball trajectory). Whereas figure form did not affect synchrony perception, the point of subjective simultaneity differed between the two motions, suggesting that participants adopted the peak velocity in each downward trajectory as their visual reference. Experiment 2 further demonstrated that, when judgment was required with regard to the highest position, the maximal synchrony response was considerably low for ball motion, which lacked a peak velocity in the upward trajectory. The finding of peak velocity as a cue parallels results of visuomotor synchronization tasks employing biological stimuli, suggesting that synchrony judgment with rhythmic motions relies on the perceived visual beat.     

Estimating peak velocity of rapid eye movements from video recordings

Although video offers many advantages for recording human eye orientation, it involves such low temporal resolution (60 Hz) that it seems an unpromising method for evaluating the dynamics of rapid (saccadic) eye movements. This study demonstrates, nevertheless, that such measurements can provide surprisingly reliable estimates of the peak velocity of larger saccades. Simulations of 60-Hz sampling of eye position during idealized saccades provided replicated estimates of “apparent peak velocity.” The results indicate that when saccadic amplitude is about 10° or larger, estimates of peak velocity would on average be biased downward by less than 10%, with standard deviations due to measurement timing of less than 5%. Experimental data (from recordings of 10° and 20° saccades with customized video) demonstrate that these theoretical sources of uncertainty are considerably smaller than the trialto- trial variability in performance of real saccades. Reliability of video recording, however, rapidly deteriorates when saccades become smaller than about 10°.     

Space-time behavior of single and bimanual rhythmical movements: data and limit cycle model

How do space and time relate in rhythmical tasks that require the limbs to move singly or together in various modes of coordination? And what kind of minimal theoretical model could account for the observed data? Earlier findings for human cyclical movements were consistent with a nonlinear, limit cycle oscillator model (Kelso, Holt, Rubin, & Kugler, 1981) although no detailed modeling was performed at that time. In the present study, kinematic data were sampled at 200 samples/second, and a detailed analysis of movement amplitude, frequency, peak velocity, and relative phase (for the bimanual modes, in phase and antiphase) was performed. As frequency was scaled from 1 to 6 Hz (in steps of 1 Hz) using a pacing metronome, amplitude dropped inversely and peak velocity increased. Within a frequency condition, the movement's amplitude scaled directly with its peak velocity. These diverse kinematic behaviors were modeled explicitly in terms of low-dimensional (nonlinear) dissipative dynamics, with linear stiffness as the only control parameter. Data and model are shown to compare favorably. The abstract, dynamical model offers a unified treatment of a number of fundamental aspects of movement coordination and control.     

Near,Far, or In Between?—Target Edges and the Transport Component of Prehension

It is well known that during visually guided prehension movements the peak velocity of the arm is scaled for object distance (e.g., Gentilucci et al., 1991; Jakobson & Goodale, 1991; Servos, Goodale, & Jakobson, 1992). Those movements are being directed not to single points in space, however, but rather to objects with extent. Thus, object distance must be computed relative to some particular point on the object. Whether that point corresponds to the location of a particular edge, for example, has not been clearly demonstrated. In the present study, subjects (N = 9) were presented with a series of oblong blocks positioned at different locations. Peak velocity increased with object size for reaches in which different-sized objects had their near edges lined up; in contrast, the peak velocities of reaches directed to objects of different sizes did not differ when the far edges of the objects were lined up. The present study, therefore, provided confirmation that subjects calibrate the peak velocity of their reaches relative to the far edge of a target object.     

Relationship of induced motion and apparent straight-ahead shifts to optokinetic stimulus velocity

Induced motion (IM) of a fixated spot stimulus and shifts of the apparent straight-ahead (ASA) from the objective median plane were studied as a function of the velocity of a full-field optokinetic background stimulus. Both IM and ASA were influenced similarly by changes in stimulus velocity. The magnitude of both responses, averaged across subjects, increased to a peak level with background velocities of 40-80 deg/sec and decreased at higher velocities. Individual subjects differed with respect to the precise functions by which IM and ASA shifts were related to stimulus velocity. However, for individual subjects, the effects of velocity on IM and ASA shifts were typically highly correlated. Although IM is correlated with shifts of ASA in the opposite direction, the magnitude of the ASA shift is insufficient to account for the observed IM.     

Effects of Auditory Feedback on Movement Time in a Fitts Task

The purpose of the present experiment was to investigate the role of auditory feedback and its impact on movement time in a standard Fitts task. Feedback was given at the moment of target acquisition. A 2-way analysis of variance found significant differences between feedback groups at all three indexes of difficulty (F(2, 40) = 156.02, p < .001). Results from a mixed-model multivariate analysis of variance for kinematic factors show significant differences in peak velocity and the location of peak velocity when comparing feedback groups. In general, the addition of auditory feedback decreased the task ID by .5.     

Response timing variability: coherence of kinematic and EMG parameters

A detailed kinematic and electromyographic (EMG) analysis of single degree of freedom timing responses is reported to (a) determine the coherence of kinematic and EMG variability to the reduced timing error variability exhibited with amplitude increments within a given criterion movement time and (b) understand the temporal organization of various movement parameters in simple responses. The data reveal that the variability of kinematic (time to peak acceleration, duration of acceleration phase, time to peak deceleration) and EMG (duration of agonist burst, duration of antagonist burst, time to antagonist burst) timing parameters decreased with increments of average velocity in a manner consistent with the variable timing error. In addition, the coefficient of variation for peak acceleration, peak deceleration, and integrated EMG of the agonist burst followed the same trend. Increasing average movement velocity also led to decreases in premotor and motor reaction times. Overall, the findings suggest a strong coherence between the variability of response outcome, kinematic, and EMG parameters.     

Response Timing Variability

A detailed kinematic and electromyographic (EMG) analysis of single degree of freedom timing responses is reported to (a) determine the coherence of kinematic and EMG variability to the reduced timing error variability exhibited with amplitude increments within a given criterion movement time and (b) understand the temporal organization of various movement parameters in simple responses. The data reveal that the variability of kinematic (time to peak acceleration, duration of acceleration phase, time to peak deceleration) and EMG (duration of agonist burst, duration of antagonist burst, time to antagonist burst) timing parameters decreased with increments of average velocity in a manner consistent with the variable timing error. In addition, the coefficient of variation for peak acceleration, peak deceleration, and integrated EMG of the agonist burst followed the same trend. Increasing average movement velocity also led to decreases in premotor and motor reaction times. Overall, the findings suggest a strong coherence between the variability of response outcome, kinematic, and EMG parameters.     

Comparison of upper limb kinematics in two activities of daily living with different handling requirements

IntroductionRecently, kinematic analysis of the drinking task (DRINK) has been recommended to assess the quality of upper limb (UL) movement after stroke, but the accomplishment of this task may become difficult for poststroke patients with hand impairment. Therefore, it is necessary to study ADLs that involve a simpler interaction with a daily life target, such as the turning on a light task (LIGHT). As the knowledge of movement performed by healthy adults becomes essential to assess the quality of movement of poststroke patients, the main goal of this article was to compare the kinematic strategies used by healthy adults in LIGHT with those that are used in DRINK.Methods63 adults, aged 30 to 69 years old, drank water and turned on a light, using both ULs separately, while seated. The movements of both tasks were captured by a 3D motion capture system. End-point and joint kinematics of reaching and returning phases were analysed. A multifactorial analysis of variance with repeated measures was applied to the kinematic metrics, using age, sex, body mass index and dominance as main factors.ResultsMean and peak velocities, index of curvature, shoulder flexion and elbow extension were lower in LIGHT, which suggests that the real hand trajectory was smaller in this task. In LIGHT, reaching was less smooth and returning was smoother than DRINK. The instant of peak velocity was similar in both tasks. There was a minimal anterior trunk displacement in LIGHT, and a greater anterior trunk displacement in DRINK. Age and sex were the main factors which exerted effect on some of the kinematics, especially in LIGHT.ConclusionThe different target formats and hand contact in DRINK and LIGHT seem to be responsible for differences in velocity profile, efficiency, smoothness, joint angles and trunk displacement. Results suggest that the real hand trajectory was smaller in LIGHT and that interaction with the switch seems to be less demanding than with the glass. Accordingly, LIGHT could be a good option for the assessment of poststroke patients without grasping ability. Age and sex seem to be the main factors to be considered in future studies for a better match between healthy and poststroke adults.     

Investigating the efficacy of a tactile feedback system to increase the gait speed of older adults

The objective of this study was to determine (1) if a novel haptic feedback system could increase the walking speed of older adults while it is being employed during overground walking and (2) whether the frequency at which this feedback was presented would have a differential impact on the ability of users to change walking speed while it was present. Given that peak thigh extension has been found to be a biomechanical surrogate for stride length, and consequently gait speed, vibrotactile haptic feedback was provided to the participants' thighs as a cue to increase peak thigh extension while the effect on gait speed was monitored. Ten healthy community-dwelling older adults (68.4 ± 4.1 years) participated. Participants' peak thigh extension, cadence, normalized stride length and velocity, along with their coefficients of variation (COV) were compared across baseline normal and fast walking (with no feedback) and three different frequency of feedback conditions. The findings indicated that, compared to self-selected normal and fast walking speeds, peak thigh extension was significantly increased when feedback was present and after it was withdrawn in a post-test. An increase in thigh extension led to an increase in stride length and, consequently, an increase in stride velocity compared to normal speed. There were no significant differences in the gait parameters as a function of feedback frequency during its application. In conclusion, while present, the haptic feedback system increased thigh extension and walking speed in older adults regardless of the feedback frequency and when the feedback was withdrawn, participants could maintain an increase in those parameters.     

Visual awareness and the on-line modification of action.

An influential theory of visually guided action proposes that (a) conscious perception of target displacement disrupts on-line action and (b) small target perturbations are inconsequential, provided the participant is unaware of them. This study examined these claims in a study of rapid aiming movements to targets. Novel features included on-line verbal reports of target displacement, and the factorial combination of small versus large displacements occurring near peak saccadic velocity or 100 ms later. Although awareness of target displacement had no effect on movement kinematics, even small target displacements near peak saccadic velocity affected kinematic measures. These results support both a strong view of visual stream separation in the on-line control of action and richer spatial coding by unconscious processes than has previously been acknowledged.     

Comparison of ankle kinematics and ground reaction forces between prospectively injured and uninjured collegiate cross country runners

Biomechanical comparative studies on running-related injuries have included either currently or retrospectively injured runners. The purpose of this study was to prospectively compare ankle joint and ground reaction force variables between collegiate runners who developed injuries during the cross country season and those who did not. Running gait analyses using a motion capture system and force platform were conducted on 19 collegiate runners prior to the start of their cross country season. Ten runners sustained running-related injuries and 9 remained healthy during the course of the season. Strike index, peak loading rate of the vertical ground reaction force, dorsiflexion range of motion (ROM), eversion ROM, peak eversion angle, peak eversion velocity, and eversion duration from the start of the season were compared between injury groups. Ankle eversion ROM and peak eversion velocity were greater in uninjured runners while peak eversion angle was greater in injured runners. Greater ankle eversion ROM and eversion velocity with lower peak eversion angle may be beneficial in reducing injury risk in collegiate runners. The current data may only be applicable to collegiate cross country runners with similar training and racing schedules and threshold magnitudes of ankle kinematic variables to predict injury risk are still unknown.     

Swimming constraints and arm coordination

Following Newell's concept of constraint (1986), we sought to identify the constraints (organismic, environmental and task) on front crawl performance, focusing on arm coordination adaptations over increasing race paces. Forty-two swimmers (15 elite men, 15 mid-level men and 12 elite women) performed seven self-paced swim trials (race paces: as if competitively swimming 1500m, 800m, 400m, 200m, 100m, 50m, and maximal velocity, respectively) using the front crawl stroke. The paces were race simulations over 25m to avoid fatigue effects. Swim velocity, stroke rate, stroke length, and various arm stroke phases were calculated from video analysis. Arm coordination was quantified in terms of an index of coordination (IdC) based on the lag time between the propulsive phases of each arm. This measure quantified three possible coordination modes in the front crawl: opposition (continuity between the two arm propulsions), catch-up (a time gap between the two arm propulsions) and superposition (an overlap of the two arm propulsions). With increasing race paces, swim velocity, stroke rate, and stroke length, the three groups showed a similar transition in arm coordination mode at the critical 200m pace, which separated the long- and mid-pace pattern from the sprint pace pattern. The 200m pace was also characterized by a stroke rate close to 40strokemin(-1). The finding that all three groups showed a similar adaptation of arm coordination suggested that race paces, swim velocity, stroke rate and stroke length reflect task constraints that can be manipulated as control parameters, with race paces (R(2)=.28) and stroke rate (R(2)=.36) being the best predictors of IdC changes. On the other hand, only the elite men reached a velocity greater than 1.8ms(-1) and a stroke rate of 50strokemin(-1). They did so using superposition of the propulsion phases of the two arms, which occurred because of the great forward resistance created when these swimmers achieved high velocity, i.e., an environmental constraint. Conversely, the elite women and mid-level men had shorter stroke lengths and maintained a time gap between the propulsions of the two arms throughout the increase in paces, with gender and expertise explaining 9% and 8.3% of the IdC changes, respectively. These results indicate that arm coordination cannot be interpreted solely from the IdC value but should be considered from the perspective of task, environmental, and organismic constraints. These constraints can serve as control parameters in experiments aimed at gaining insight into changes in arm coordination during the front crawl. In this context, catch-up coordination, which is often considered as a mistake, was seen to be an adaptation to a relative constraint.