Predictive Mechanisms Control Grip Force after Impact in Self-Triggered Perturbations

ABSTRACT

Impulsive loadings during object grasping are common in everyday life. In predictable conditions, the grip force (GF) increases before the impact to anticipate the perturbation and reaches a maximum after the perturbation. In the present study, the authors addressed the predictive or reactive nature of this late GF component. The load of a handheld object was briskly increased by dropping a mass attached to the object (impact trials). The drop was self-induced, but for one third of the trials, the mechanism was blocked and no impact occurred (blank trials). Evidence that the late GF component is programmed as a predictive action emerged from a systematic comparison between impact and blank trials. The authors conclude that the GF increase occurring after a predictable impulsive loading is essentially of a predictive nature.     

Compensatory Coordination of Release Parameters in a Throwing Task

The consistency and coordination of release parameters in ball-throwing movements were investigated. The authors used a newly developed index of coordination for release parameters (ICRP) that quantifies the degree of improvement of performance consistency caused by compensatory relationships among parameters (i.e., not caused by consistency of parameters). Eight participants practiced for 150 trials, with the nondominant hand, a ball-throwing task aimed at a stationary target. The magnitude of the ball-release velocity vector, among release parameters, as well as the performance was found to become consistent with practice. The ICRP score suggested that the release parameters were complementarily coordinated with one another, and that the coordination improved with practice. Those results indicate that compensatory relationships among varying release parameters contribute to reducing the variability of performance in a ball-throwing task whose goal is accuracy.     

Numerosity and Rhythmicity in Stimulus-Response Compatibility

When people must respond discriminatively to 1 or 2 stimuli by making 1 or 2 taps of a response key, they initiate the response more rapidly when the correct number of taps matches the number of stimuli (compatible condition) than when it mismatches (incompatible condition; J. O. Miller, S. G. Atkins, & F. Van Nes, 2005). Miller et al. sometimes found an effect of compatibility on response execution time, as reflected in the interresponse intervals between successive taps. The authors report 2 further experiments (N=8 participants) in which they generalized the numerosity compatibility effects on response-initiation time and interresponse intervals to 2- versus 3-stimulus sequences. In addition, they varied gap length between stimuli to see whether the rhythm of the stimulus would influence that of the response. Weak rhythmicity effects were repeatedly found, but those were too small to suggest a plausible alternative explanation for the numerosity compatibility effect on response-initiation time.     

Discrete Visual Samples May Control Locomotor Equilibrium and Foot Positioning in Man

The static or dynamic visual cues required for equilibrium as well as for foot guidance in visually guided locomotion in man were studied using a variety of locomotion supports and illumination and visual conditions. Stroboscopic illumination (brief flashes) and intermittent lighting (longer flashes) were used to control and to vary the visual sampling frequency of static (positional/orientational) visual cues. There were three main findings: First, visual control of foot positioning during locomotion over discontinuous terrain depends mainly upon static visual cues with a low sampling frequency (about 3 Hz); second, visual control of dynamic equilibrium during locomotion over a narrow support depends mainly upon the availability of high frequency static visual cues (up to about 12 Hz); and third, static visual cues required for equilibrium control are extracted from both the peripheral and the central visual field.

Assuming that discrete demands for feedback occur, a simple probabilistic model was proposed, according to which the mean time that elapses following presentation of static visual cues about positions or changes of position accounts for the differences in the difficulty of the various illumination conditions.     

Perturbations of Human Posture

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation.

We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

Perceptions of Submissiveness: Implications for Victimization

Some researchers have suggested that a precondition of affective submissiveness may increase the likelihood of female victimization in sexual assault, whereas others have suggested that criminal offenders use perceptions of vulnerability when selecting a victim. In this study, based on American college students, men (decoders) rated videotaped women (encoders) dominant versus submissive using a semantic differential instrument. Cue evaluators analyzed the body language and appearance of the videotaped women using a Likert instrument. The results suggest that (a) men form differentiated perceptions of dominant versus submissive women, (b) such perceptions substantially rely on nonverbal cues, (c) dominant and submissive women display visually different behaviors and appearances, and (d) men tend to select submissive females for exploitation.     

The neuronal basis of bimanual coordination: recent neurophysiological evidence and functional models

Recent physiological studies of the neuronal processes underlying bimanual movements provide new tests for earlier functional models of bimanual coordination. The recently acquired data address three conceptual areas: the generalized motor program (GMP), intermanual crosstalk and dynamic systems models. To varying degrees, each of these concepts has aspects that can be reconciled with experimental evidence. The idea of a GMP is supported by the demonstration of abstract neuronal motor codes, e.g. bimanual-specific activity in motor cortex. The crosstalk model is consistent with the facts that hand-specific coding also exists and that interactions occur between the motor commands for each arm. Uncrossed efferent projections may underlie crosstalk on an executional level. Dynamic interhemispheric interactions through the corpus callosum may provide a high-level link at the parametric programming level, allowing flexible coupling and de-coupling. Flexible neuronal interactions could also underlie adaptive large-scale systems dynamics that can be formalized within the dynamic systems theory approach.

The correspondence of identified neuronal processes with functions of abstract models encourages the development of realistic computational models that can predict bimanual behavior on the basis of neuronal activity.     

Speech motor programming in hypokinetic and ataxic dysarthria

It is widely accepted that the cerebellar and basal ganglia control circuits contribute to the programming of movement. Converging evidence from neuroimaging, limb control, and neuropsychological studies suggests that (1) people with cerebellar disease have reduced ability to program movement sequences in advance of movement onset and (2) people with Parkinson's disease are unable to maintain a programmed response or to rapidly switch between responses. Despite a substantial supporting literature, no studies have addressed these potential areas of speech programming disruption for speakers with ataxic and hypokinetic dysarthria. Control participants and adults with dysarthria completed speech reaction time protocols designed to capture these aspects of utterance preparation. Results provided initial support for processing deficits in speakers with ataxic and hypokinetic dysarthria that are separable from motor execution impairments.     

How do children coordinate simultaneous upper and lower extremity tasks? The development of dual motor task coordination

When performing simultaneous clapping with walking or galloping, adults adopt coupled, consistent and stable dual motor task coordination; do developmental trends in this coordination exist? In this study, we measured and compared coupling characteristics, consistency across trials and variability of phasing in 4-, 6-, 8-, and 10-year-olds (n=44) as they also performed the same dual motor task. For walk/clap, children adopted specific coupling patterns like adults by 8 years and with the same consistency by 10 years. Across age, children became less variable in clap and step movements separately and as coupled together. In the gallop/clap, children did not resemble adults in coupling patterns by 10 years but all measures were becoming more consistent across age. We discuss dual motor task coordination as a function of age and task complexity using a "dynamic" perspective within a developmental context.