Variability and noise in continuous force production

In the present 3 experiments, the authors examined the hypothesis, derived from information theory, that increases in the variability of motor responses result from increases in perceptual-motor noise. Three different groups of participants (Ns = 10, 9, and 10, respectively, in Experiments 1, 2, and 3) produced continuous isometric force under either low, intermediate, or high target force levels. When considered together, the results showed that force variability (SD) increased exponentially as a function of force level. However, an index of information transmission (M/SD), as well as measures of noise in both the time (approximate entropy) and the frequency (power spectrum) domains, changed according to an inverted-U-shaped function over the range of force levels. The findings provide further evidence that increased information transmission is related to increases, and not to decreases, in the noisiness of the structure of force output.     

Visuomotor contribution to force variability in the plantarflexor and dorsiflexor muscles

The visual correction employed during isometric contractions of large proximal muscles contributes variability to the descending command and alters fluctuations in muscle force. This study explored the contribution of visuomotor correction to isometric force fluctuations for the more distal dorsiflexor (DF) and plantarflexor (PF) muscles of the ankle. Twenty-one healthy adults performed steady isometric contractions with the DF and PF muscles both with (VIS) and without (NOVIS) visual feedback of the force. The target forces exerted ranged from 2.5% to 80% MVC. The standard deviation (SD) and coefficient of variation (CV) of force was measured from the detrended (drift removed) VIS and NOVIS steadiness trials. Removal of VIS reduced the CV of force by 19% overall. The reduction in fluctuations without VIS was significant across a large range of target forces and was more consistent for the PF than the DF muscles. Thus, visuomotor correction contributes to the variability of force during isometric contractions of the ankle dorsiflexors and plantarflexors.     

Quantification of manual force control and tremor

Isometric impulse frequencies associated with active tremor and force regulation were examined in 10 patients with idiopathic Parkinson's disease (PD) and in 10 older adults (OAs) who performed an isometric tracing task. The authors decoupled and analyzed the data to determine whether PD-related tremor in the thumb and in the index finger during isometric force control are related and whether PD impairs the performance of volitional force control beyond the errors contributed by tremor. After decoupling, there were clear and robust differences in PD patients' control of isometric force that could not be attributed to action-tremor error. Those errors, which occurred in the absence of movement, suggest impairment in coordinated recruitment and derecruitment of motor units during a fine-motor task.     

Motor limitation in dual-task processing with different effectors

According to the extended bottleneck model, dual-task interference does not arise only from a central bottleneck but also from processing limitations at the motor stage. Evidence for this assumption has previously been found only for same-effector tasks but not for different-effector tasks. In order to examine the existence of motor interference with different effectors, we used the psychological refractory period paradigm and employed response sequences of different length in Task 1 (R1 sequence length). Experiment 1 incorporated vocal response sequences in Task 1 and a manual response in Task 2. In Experiment 2, the assignment of the effectors to the two tasks was reversed. In both experiments, the long R1 sequence prolonged reaction time for Task 2 (RT2), and this effect was reduced with decreasing temporal overlap of the two tasks. Thus, the present experiments demonstrate motor interference between different-effector tasks. This interference may be due to on-line programming or to central response monitoring.     

Intermittent visual information and the multiple time scales of visual motor control of continuous isometric force production

In an experiment, we examined the effect of intermittency (from 25.6 Hz to 0.2 Hz) of visual information on continuous isometric force production as a function of force level (5%, 10%, 25%, and 50% of maximal voluntary contraction [MVC]). The amount of force variability decreased and the irregularity of force output increased as a function of increased visual intermittency rate. Vision was found to have an influence on the frequency structure of force output up to 12 Hz, and the 25% MVC force level had more high-frequency modulations with higher rates of visual information. The effective use of intermittent visual information is mediated nonlinearly by force level, and there are multiple time scales of visual control (range, approximately 0 - 12 Hz) that are postulated to be a function of both feedback and feedforward control processes.     

The effects of age and musculoskeletal pain on force variability among manual workers

The present study investigated the influence of age and musculoskeletal pain on force variability during a continuous isometric handgrip force task performed at 30% of maximal voluntary contraction carried out until failure. We recruited 96 male manual workers aged 51–72 years. The participants were stratified according to their age (50–59 and 60+ years) and by pain status (no pain, acute pain and chronic pain). The amplitude and structure of variability expressed as respectively standard deviation (SD) and coefficient of variation (CV), and sample entropy (SaEn) were calculated from the endurance task. The oldest group had an approximately 18% longer endurance time than the youngest group. No between-group differences were found in SD or CV, whereas a significant interaction between age and pain stage was found for SaEn. The youngest group showed lower SaEn than the oldest for both those with chronic pain and those without pain, indicating less force complexity, whereas a tendency for the opposite was found in the acute pain group. Within the pain stage groups, workers with acute pain had higher SaEn compared with both the no pain and chronic pain groups. These findings suggest that age and musculoskeletal pain differentially affects the structure of force variability in manual workers.     

Temporal production and visuospatial processing

Current models of prospective timing hypothesize that estimated duration is influenced either by the attentional load or by the short-term memory requirements of a concurrent nontemporal task. In the present study, we addressed this issue with four dual-task experiments. In Exp. 1, the effect of memory load on both reaction time and temporal production was proportional to the number of items of a visuospatial pattern to hold in memory. In Exps. 2, 3, and 4, a temporal production task was combined with two visual search tasks involving either pre-attentive or attentional processing. Visual tasks interfered with temporal production: produced intervals were lengthened proportionally to the display size. In contrast, reaction times increased with display size only when a serial, effortful search was required. It appears that memory and perceptual set size, rather than nonspecific attentional or short-term memory load, can influence prospective timing.     

Lateralization in haptic processing: An apparatus for analyzing manual strategies

We describe an apparatus for testing laterality in haptic processing in the discrimination of nonsense shapes in humans or monkeys. The system, which permits either mono- or dichhaptic discrimination, automatically provides data on the measurement of accuracy along with information on hand exploratory strategies.     

Levels of processing in speech production

When subjects repeated short alphabetic sequences at experimenter-controlled rates while retaining consonant trigrams, subsequent recall was unrelated to speech rate if single letters or a long sequence was repeated, but recall was inversely related to speech rate for sequences three or five letters in length. The latter effect was predicted by a model in which sequences were organized as single response units in a central phonetic processor but independently executed by means of mechanisms not requiring the services of the central processor. Quantitative predictions were generated from this model by estimating organization time for the sequences and assuming that the probability of forgetting would be equal to the organization time/execution time ratio. All obtained recall probabilities were as predicted for sequences three of five letters in length with both experimenter- and subject-controlled speech rates. Repetition of a nine-letter sequence from the end of the alphabet produced forgetting patterns clearly out of accord with any organization time/execution time ratio, but a similar sequence from the beginning of the alphabet produced evidence of being a marginal case with unstable organization.     

The effects of divided attention on information processing in manual tracking.

Six subjects performed a manual tracking task concurrently uith each of two secondary tasks: in input task (auditory signal detection) and an output task (application of a constant force). A feedback-control analysis of tracking performance was utilized to analyze the time-sharing decrements observed in mean squared errer, in terms of components due to processing delay, addition of internal processing noise, and change in response bias (tracking gain). The results indicated that only the parameters measuring noise and gain were sensitive to time-sharing conditions, and these only to concurrent performance of the force application task. It is concluded that limits of attention in dual-task performance are more severe in output than in input stages of processing, but that these limits are not necessarily those of a single-channel bottleneck. Instead, a broader conception of attention is proposed: one that included changes in processing noise and shifts in response bias, as attention-related phenomena.     

Temporal interval production and processing in working memory

Short-term memory or working memory has been proposed as a cognitive structure contributing to time estimation. Thus, in a previous experiment, retrieving a stored item during a temporalinterval production lengthened the interval in proportion to the number of items in the memory set. In the present study, this issue was analyzed further by testing whether the proportional lengthening is induced by the load itself (i.e., the number of items) or by comparing the probe with memorized items. In a first experiment, a memory set was maintained during a temporal production, and the comparison of the probe with memorized items was postponed until the end of time production. Varying the number of items in the memory set had no effect on temporal intervals produced during its retention, suggesting that mental comparison was the source of the lengthening of time intervals. In succeeding experiments, tasks requiring processing in working memory but involving no memory load were combined with temporal production. In Experiment 2, increasing the number of syllables in a rhyme-judgment task proportionally lengthened temporal intervals that were produced simultaneously. In Experiment 3, increasing the amount of mental rotation in a task involving visuospatial processing also lengthened simultaneous temporal production. This interference between processing in working memory and time estimation suggests that working memory, defined as a work space for active processing of current information, contributes to time estimation.     

Movement errors and levels of processing in sentence production

From a corpus of 3,530 slips of the tongue in Spanish, a sample of 753 cases of movement errors was analyzed, comprising those tokens that could be unambiguously assigned to the major categories of anticipations, perseverations, exchanges, and shifts. The analysis was performed according to two main criteria: (a) the degree of correspondence between the linguistic elements interacting in an error, and (b) the distance between such elements in terms of the type and number of the intervening linguistic boundaries. The results of this analysis converge with those obtained in English, supporting a model of sentence planning with different levels of representation and processing. Furthermore, Spanish provides a clear case to attest the role of syllabic structure in production processes, the constraints set by word boundaries in sublexical errors, and the contribution of inflectional suffixes to the assignment of grammatical category to the root morphemes.     

Age differences in noise and variability of isometric force production.

This study examined whether age-related improvements observed in the motor performance of children result from a reduction of noise in the output of the sensori-motor system. Children ages 6, 8, and 10 years and young adults (N = 48, 12 per group) performed continuous, constant isometric force contractions with the index finger at four different force levels with and without visual feedback. The results revealed that: (a) performance improved with increases in age, (b) the force output signal exhibited increased irregularity and a more broadband frequency profile with increases in age under conditions with feedback, and (c) there were no age differences in the irregularity of the force signal and smaller age differences in the frequency profiles under conditions without feedback. It is proposed that the age-related enhancements in performance throughout childhood are primarily due to a more appropriate mapping of the organization of the sensori-motor system to the task constraints rather than to reduction of system noise.     

The effects of force and direction uncertainty of choice reaction time in an isometric force production task.

The two experiments reported examined the temporal organization of force and direction motor-programming processes in a step-input tracking type task. Both experiments observed a reduction in reaction time in the direction-uncertain conditions compared to the direction-certain ones. Thus it seems as though the direction decision does not have to precede the selection of the proper amount of forces. Experiment 2 observed an under-additive interaction between levels of direction uncertainty (certain or uncertain) and levels of force uncertainty (certain or uncertain). This interaction was interpreted as support for the programming of force and direction and thus, strongly supports the parallel model of programming recently proposed by Klapp (1977a,b).     

Syllables as processing units in handwriting production

This research focused on the syllable as a processing unit in handwriting. Participants wrote, in uppercase letters, words that had been visually presented. The interletter intervals provide information on the timing of motor production. In Experiment 1, French participants wrote words that shared the initial letters but had different syllable boundaries. In Experiment 2, French- and Spanish-speaking participants wrote cognates and pseudowords with a letter sequence that was always intrasyllabic in French and intersyllabic in Spanish. In Experiment 3, French-Spanish bilinguals wrote the cognates and pseudowords with the same type of sequences. In the 3 experiments, the critical interletter intervals were longer between syllables than within syllables, indicating that word syllable structure constrains motor production both in French and Spanish.     

Characteristics of finger force production during one- and two-hand tasks

Relations among finger forces were studied during one-hand and two-hand isometric maximal force production tasks in right- and left-handers. We particularly focused on the phenomena of force deficit during one-hand multi-finger tasks and of bilateral force deficit during two-hand tasks. Ten healthy subjects (five of them left-handed) performed maximal voluntary force production tasks with different finger combinations involving fingers of one of the hands or of both hands together. In one-hand tasks, finger enslaving (forces produced by fingers that were not instructed to produce force) was larger in the dominant hand, while force deficit (drop in individual finger peak force during multi-finger tasks) showed no differences between the hands. An additional drop in finger forces was seen in two-hand tests (bilateral deficit). The magnitude of the bilateral deficit for a hand was larger for tasks involving fewer fingers within the hand and more fingers in the other hand, with a ceiling effect. Smaller bilateral deficit was seen in tasks involving symmetrical finger combinations. In two-hand tasks that could potentially lead to the generation of large total moments in the frontal plane, the hand that was expected to generate larger moments showed larger bilateral deficit, so that the magnitude of the total moment was reduced. These observations suggest that force deficit within a hand and bilateral deficit have different origins but their effects are combined at a certain level of the multi-finger control hierarchy. Bilateral deficit may display task dependence reflecting, in particular, the principle of minimization of secondary moments. A double-representation, mirror-image hypothesis is suggested to provide a neurophysiological basis for the observed patterns of bilateral deficit.     

Visuomotor memory for target location in near and far reaching spaces

The authors investigated systematic error associated with endpoints of memory-guided actions performed in near and far reaching spaces. To accomplish that objective, the authors instructed 12 participants to initiate open-loop and memory-guided reaches (0, 2,000, and 5,000 ms of visual delay) from a common start position to remembered midline targets in near (i.e., a backward reach) and far (i.e., a forward reach) reaching spaces. The results indicated that near and far reaches, respectively, over- and undershot veridical target location, and the direction-specific nature of the error was amplified in the memory-guided conditions. The latter finding represents an important aspect of the present research because it suggests that the direction-specific error identified here is related to factors arising within the sensory component of the task rather than mechanical differences in reaching direction. The authors propose that stored target information serving memory guided actions is susceptible to a compression of visual space in memory such that the egocentric distance of a remembered target is underestimated.