The Relationship Between Balance Confidence and Cognitive Motor Interference in Individuals With Multiple Sclerosis

The authors examined and compared the development of oral and manual force control in preschool-aged children. In all, 50 typically developing children (aged 3-5 years) performed maximal strength tasks and submaximal visually guided tasks using tongue elevation, power, and precision grips. Dependent measures included strength, rate of force rise, initial force overshoot, force variability, and rate of force release. The authors performed age- and performance-related analyses. Results revealed similar changes for tongue, fingers, and hands across age- and performance-related measures for strength, initial force overshoot, and rate of force release. There were no significant changes in rate of force rise with increasing age. Force variability measures showed effector-specific changes with decreases across age- and performance-related measures for the hands and fingers but not for the tongue. Changes common across effector systems likely reflect biological development coupled with cognitive-strategic development. Effector-specific changes in force variability likely reflect experience gained through functional tasks influencing biological and cognitive-strategic development. Lack of change in force variability of the tongue suggests that fine control of the tongue is activity specific; thus, nonfunctional tasks are not likely to be sensitive to experience-related biological development.     

Noise, information transmission, and force variability.

This study was designed to test the hypothesis derived from information theory that increases in the variability of motor responses result from increases in perceptual-motor noise. Young adults maintained isometric force for extended periods at different levels of their maximum voluntary contraction. Force variability (SD) increased exponentially as a function of force level. However, the signal-to-noise ratio (M/SD), an index of information transmission, as well as measures of noise in both the time (approximate entropy) and frequency (power spectrum) domains, changed according to an inverted U-shaped function over the range of force levels. These findings indicate that force variability is not directly related to noise but that force output noisiness is positively correlated with the amount of information transmitted.     

A further investigation of the filled-duration illusion with a comparison between children and adults

This study investigated the filled duration illusion in children in comparison to adults, using a temporal bisection task with two ranges of anchor durations (1 vs. 4s and 2 vs. 8s). The results revealed a marked filled duration effect when filled and empty durations were presented in the same session (within-subjects condition), with the filled durations being judged to be longer than the empty durations in all age groups. However, the magnitude of this effect appeared to be larger in the children than in the adults. Furthermore, the filled duration effect decreased with the longest duration stimuli, and apparent reversals of this effect were observed with the empty durations being judged longer. In the between-subjects experiment, the filled duration effect disappeared in all age groups. However, in the children, the variability in the temporal judgment appeared to be greater for the empty than for the filled durations, thus indicating a lower sensitivity to empty durations.     

Differential reinforcement of lever-press durations: Effects of deprivation level and reward magnitude

Three experiments investigated the performance of rats on a task involving differential reinforcement of lever-press durations. Experiment 1, which employed a discrete-trials procedure, manipulated deprivation level between subjects and reward magnitude within subjects. The minimum lever-press duration which would result in reward was varied from .4 to 6.4 sec. It was found that low deprivation resulted in longer mean durations and less response variability at the higher criterial values than did high deprivation. The magnitude of reward was not found to affect performance. Experiment 2 manipulated reward magnitude between subjects while holding deprivation level constant, and used the same general procedures as in Experiment 1. Small reward resulted in longer mean lever-press durations and less variability in responding than did large reward at the higher criterial values. The intertrial intervals were omitted in Experiment 3 in which deprivation level was varied between subjects and reinforcement was delivered only for response durations extending between 6.0 and 7.6 sec. Low deprivation resulted in longer mean lever-press durations and less response variability than did high deprivation, but the probability of a rewarded press duration did not differ between groups. The results overall are consistent with the hypothesis that low deprivation and small reward magnitude lead to weaker goal-approach responses and, hence, to less competition with lever holding. The deprivation and reward magnitude manipulations did not appear to influence lever holding performance by affecting the ability of animals to form temporal discriminations.     

Response duration is sensitive to both immediate and delayed reinforcement

We investigated the duration of lever pressing by rats when the delivery of appetitive reinforcers was contingent upon response duration. In the first experiment, response durations increased when duration requirements were imposed, and they decreased when duration requirements were removed. This effect occurred whether reinforcers were immediate or delayed by 8 s. In order to maintain the integrity of the delay intervals, reinforcer delivery was dependent upon both lever depression and release. In a second experiment, lever depression only and a response duration of at least 4 s were required for reinforcer delivery. Compared to immediate reinforcement conditions, delayed reinforcers increased both variability and the length of the maximum response durations. In a third experiment, immediate reinforcers were delivered contingent upon lever depression and release under a variety of duration requirements. Median lever‐press durations tracked the contingencies rapidly. Across all three experiments, rats emitted numerous response durations that were too short to satisfy the reinforcer requirements, and bimodal distributions similar to those produced by differential reinforcement of low rate schedules were evident for most rats. In many aspects, response duration responds to reinforcement parameters in a fashion similar to rate of discrete responding, but an examination of this continuous dimension of behavior may provide additional information about environment–behavior relationships.     

Preference and reinforcer efficacy of high‐ and low‐tech items: A comparison of item type and duration of access

This study examined the interactions of stimulus type (high‐ vs. low‐tech) and magnitude (duration of access) on preference and reinforcer efficacy. Two preference assessments were conducted to identify highly preferred high‐tech and low‐tech items for each participant. A subsequent assessment examined preference for those items when provided at 30‐s and 600‐s durations. We then evaluated reinforcer efficacy for those same items when provided for a range of durations using progressive‐ratio schedules. Results suggested item type and access duration interacted to influence preference and reinforcer efficacy. Participants preferred high‐tech items at longer durations of access and engaged in more responding when the high‐tech item was provided for long durations, but these patterns were reversed for the low‐tech item. In addition, participants engaged in less responding when the high‐tech item was provided for short durations and when the low‐tech item was provided for long durations.     

Syntactic,semantic, and rhythmic influences on children's and adults' motor programming of speech

The influence of linguistic and paralinguistic structure on speech motor programming was investigated for 5-year-olds, 8-year-olds, and adults. Subjects repeated verbal stimuli, each at maximum rate on numerous consecutive trials. It was hypothesized that structure in an utterance would allow a speaker to organize its motor program more efficiently than would be possible for an otherwise identical utterance that lacked such structure, although the types of structure employed might change with age. Differences in the efficiency of motor program organization were expected to be evidenced in the syllable duration and the relative variability of the syllable duration of subjects' repetitions. At all ages, repetition durations were shorter for stimuli with a sentencelike rhythm than for unstructured stimuli. Stimuli that were syntactically structured and contained a sentencelike rhythm were spoken with shorter durations than nonsyntactic stimuli with sentential rhythm but only by 8-year-olds and adults.     

Force Increase in a Repetitive Motor Task Inducing Motor Fatigue

To evaluate task induced motor fatigue in a well-established finger tapping task, we analyzed tapping parameters and included the time course of measures of force. We hypothesized that a decline in tapping force would reflect task induced motor fatigue, defined by a lengthening of inter-tap intervals (ITI). A secondary aim was to investigate the reliability of tapping data acquisition with the force sensor. Results show that, as expected, tapping speed decreased linearly over time, due to both an increase of ITI and tap duration. In contrast, tapping force increased non-linearly over time and was uncorrelated to changes in tapping speed. Force data could serve as a measure to characterize task induced motor fatigue. Force sensors can assess a decline in tapping speed as well as an independent increase of tapping force. We argue that the increase of force reflects central compensation, i.e. perception of fatigue, due to an increase in task effort and difficulty.     

The effects of sound duration on newborns' head orientation

Two experiments assessed the importance of sound duration for eliciting head orientation responses from newborn infants. In Experiment 1, thirty infants turned with equal frequency toward 20-s continuous rattle sounds and 20-s trains of rattle segments. The duration of the rattle segments--14 and 100 ms (2/s), or 500 ms (1/s)--did not influence the likelihood of turning. Response latencies and durations proved quite similar for all stimuli. In Experiment 2, twenty-four infants heard continuous rattle sounds of four different durations: 1, 5, 10, and 20 s. They turned reliably to all stimulus durations; furthermore, the magnitude and temporal characteristics of head orientation responses did not differ for the four stimulus durations. These results suggest that the newborn's head orientation response may reflect a motor program that is initiated by auditory input and then executed in a similar fashion regardless of further stimulation.     

The interaction of stimulus and reinforcer control in complex temporal discrimination.

Six pigeons were trained in a discrete-trials signal-detection procedure to discriminate between a fixed-duration stimulus (5 s or 20 s) and a set of variable durations ranging from 2.5 s to 57.5 s in steps of 5 s. For each fixed-duration stimulus, the ratio of reinforcer frequencies contingent upon reporting the fixed versus the variable stimulus was systematically manipulated. Detection performance was well controlled by both the stimulus value and the reinforcer ratio. Both the discriminability between the fixed duration and the set of variable durations, and the discriminability between the fixed duration and each of the variable durations, were independent of the reinforcer-frequency ratio when discriminability was measured as log d. The sensitivity of response bias to reinforcement-ratio changes was independent of the value of the fixed duration, but was not independent of the discriminability of the variable durations from the fixed durations. Under current models, discriminability measures in complex temporal discrimination may be independent of biasing manipulations, but bias measures are not independent of stimulus values.     

Modulation of motor variability related to experimental muscle pain during elbow-flexion contractions

Experimental muscle pain typically reorganizes the motor control. The pain effects may decrease when the three-dimensional force components are voluntarily adjusted, but it is not known if this could have negative consequences on other structures of the motor system. The present study assessed the effects of acute pain on the force variability during sustained elbow flexion when controlling task-related (one-dimensional) and all (three-dimensional) contraction force components via visual feedback. Experimental muscle pain was induced by bolus injection of hypertonic saline into m. biceps brachii, and isotonic saline was used as control. Twelve subjects performed sustained elbow flexion at different levels of the maximal voluntary contraction (5–30% MVC) before, during, and after the injections. Three-dimensional force components were measured simultaneously with surface electromyography (EMG) from elbow flexors and auxiliary muscles. Results showed that force variability was increased during pain compared to baseline for contractions using one-dimensional feedback (P < .05), but no significant differences were found for three-dimensional feedback. During painful contractions (1) EMG activity from m. trapezius was increased during contractions using both one-dimensional and three-dimensional feedback (P < .05), and (2) the complexity of EMG from m. triceps brachii and m. deltoid was higher for the three-dimensional feedback (P < .05). In conclusion, the three-dimensional feedback reduced the pain-related functional distortion at the cost of a more complex control of synergistic muscles.     

Effects of skill practice on electromyographic activity patterns and frequency spectra

The purpose of this research was to investigate motor skill practice related changes in the activity pattern and mean frequency of the electromyogram and the subsequent motor performance. By using both electromyographic (EMG) activity levels and the frequency spectrum of the EMG signal, it is possible to gain an understanding of the modifications that may occur with skill acquisition. Subject's angular position and bipolar surface EMG activity from the biceps brachii and triceps brachii muscles were recorded and digitized at 1000 samples/second. Movement time and EMG data averaged over early and late practice trials were compared to evaluate the effects of practice on EMG activity patterns (RMS) and the EMG frequency spectra (mean frequency). Changes in EMG activity and frequency patterns observed at both movement velocities reflect modifications in the control of force gradation in relation to skill practice and learning. Although it is not possible to assess specific changes in motor unit recruitment order, it appears that motor units were controlled in a different manner after skill practice.     

The interaction of respiration and visual feedback on the control of force and neural activation of the agonist muscle

The purpose of this study was to compare force variability and the neural activation of the agonist muscle during constant isometric contractions at different force levels when the amplitude of respiration and visual feedback were varied. Twenty young adults (20–32 years, 10 men and 10 women) were instructed to accurately match a target force at 15% and 50% of their maximal voluntary contraction (MVC) with abduction of the index finger while controlling their respiration at different amplitudes (85%, 100% and 125% normal) in the presence and absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8–12 and 16–20 s. Each subject performed three trials with each respiratory condition at each force level. Force variability was quantified as the standard deviation of the detrended force data. The neural activation of the first dorsal interosseus (FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Relative to normal respiration, force variability increased significantly only during high-amplitude respiration (∼63%). The increase in force variability from normal- to high-amplitude respiration was strongly associated with amplified force oscillations from 0 to 3 Hz (R² ranged from .68 to .84, p < .001). Furthermore, the increase in force variability was exacerbated in the presence of visual feedback at 50% MVC (vision vs. no-vision: .97 vs. .87 N) and was strongly associated with amplified force oscillations from 0 to 1 Hz (R² = .82) and weakly associated with greater power from 12 to 30 Hz (R² = .24) in the EMG of the agonist muscle. Our findings demonstrate that high-amplitude respiration and visual feedback of force interact and amplify force variability in young adults during moderate levels of effort.     

Judging multi-minute intervals retrospectively

A total of 50 participants were asked to perform five different cognitive tasks lasting 120, 210, 300, 390 and 480 s, respectively. After completing the series of tasks, they were asked to estimate retrospectively the duration of each one. Psychophysical analyses linking psychological time to physical time revealed that the value of the power law exponent was about .47, but was .79 when the estimate of the total duration of the session was taken into account—a value lower than unity, indicating that shorter durations have been overestimated, and longer durations underestimated. The Weber fraction, or the ratio of variability to time, ranged from .59 (at 120 s) to .21 (at 480 s). Overall, the study shows that it is possible to make certain changes in the traditional retrospective timing method and thus adapt it for further investigations of the mechanisms involved in memory for the duration of past events.     

A recruitment theory of force-time relations in the production of brief force pulses: the parallel force unit model

A theory, the parallel force unit model, is advanced in which the buildup and decline of force in rapid responses of short duration are assumed to reflect variability in timing of several parallel force units. Response force is conceived of as being a summation of a large number of force units, each acting independently of one another. Force is controlled by either the number of recruited force units or the duration each unit contributes its force. Several predictions are derived on the basis of this theory and are shown to be in qualitative agreement with empirical findings about both the mean and variability of brief force impulses. The model also has consequences for the temporal properties of a response. For example, under certain circumstances, it predicts a reciprocal relation between reaction time and response force. Although the theory is proposed as a psychological account, relations between the assumptions and basic principles in neurophysiology are considered. Possible future applications and generalizations of the theory are discussed.     

Changes in Muscle Activity in Response to Assistive Force during Isometric Elbow Flexion

Abstract

This study investigated the muscle activity and force variability in response to perturbation of assistive force during isometric elbow flexion. Sixteen healthy right-handed young men (age: 22.0?±?1.1?years; height: 171.9?±?4.8?cm; weight 68.4?±?11.2?kg) were recruited and the muscle activity of biceps brachii and triceps brachii were assessed using surface electromyography. Workload force and assistive force applied on isometric elbow flexion significantly affected the changes in both biceps and triceps muscle activities. A higher assistive force was shown to result in reduced biceps muscle activity compared to the unassisted period. In contrast, the efficiency of the assistive force acting on the biceps decreased as the assistive force increased. In general, the force variability of the biceps muscle remained approximately the same at lower workload force conditions than that at higher workload force conditions. In conclusion, higher assistive force may not yield a higher performance efficiency in human-assistive force interaction.     

Development of time sensitivity: Duration ratios in time bisection

This study investigated the development of children's abilities to discriminate durations as a function of their ratio and examined whether the ability to discriminate durations that differed by a very difficult ratio is related to the development of attention capacities. Children aged 5 and 8 years, as well as adults, performed a series of temporal bisection tasks with a ratio between the short and the long anchor duration that was changed to control the difficulty of the task (5:6, 2:3, and 1:2) in two duration ranges (<1 s and >3 s). In addition, they completed neuropsychological tests in order to assess their short-term memory, working memory, and visual attention abilities. The results showed that, at ratios of 2:3 and 1:2, most participants were able to discriminate the anchor durations in bisection. However, their sensitivity to time improved, whatever the duration range, both as the distance between the anchor durations increased and with increasing age. For the smallest duration ratio (5:6), few of the children were able to discriminate the anchor durations in the bisection task in comparison to adults. Hierarchical regression analyses performed on the neuropsychological tests revealed that, for the 2:3 ratio between anchor durations, the participants' visual attention scores explained a large part of the variance in time sensitivity. The children's lower temporal sensitivity was probably due to their limited visual attention abilities, thus explaining the difficulty they experienced in discriminating very close durations (5:6).     

The "short-long" reaction time effect in duration programming

The programming processes concerned with response duration were studied in a precueing and in a priming reaction time (RT) paradigm. Participants had to produce a motor response of a specified duration as soon as possible after a response signal (RS) preceded by a warning signal (WS), which could deliver information on 2 response parameters (duration and effector). In Experiment I (precueing; N = 12), 3 effectors (the right hand, the left hand, or the knees) and 3 durations (.7, 2.5, or 5.5 s) were contrasted. Two responses differing in their biomechanical features were required in 2 blocks of trials: Subjects had to accurately time the duration of either a sustained button press or an interval between 2 brief presses. The RT patterns revealed a short-long effect: Shorter RTs were produced before the short duration than before the longer, provided that the duration was not precued. This short-long effect occurred whatever type of response and effector were involved. Two conclusions were reached. First, response duration was included in the motor program elaborated before execution, whatever the biomechanical features of the response; and, second, the program for the short duration was activated on all trials and was used as a basis for programming longer durations when needed. These conclusions were tested in Experiment 2 (priming; N = 12), in which a small proportion of invalid trials concerning duration was provided. Thus, the duration required by the RS differed from that primed by the WS. Two durations (.7 or 2.5 s) and 2 effectors (the index or the middle finger) were involved. In the invalid trials, the responses of short and long durations did not yield any RT differences, thus confirming the particular status of the short duration. This suggests that deprogramming operations (which lengthen the RT) are needed after a RS to produce short response durations but not after a RS to produce long response durations in the invalid trials.     

Stop using time reproduction tasks in a comparative perspective without further analyses of the role of the motor response: The example of children

The temporal reproduction task is often used to investigate inter-individual differences in the ability to perceive time without any further analyses of the contribution of motor responses to temporal performance. The present study examined the role of motor responses in the reproduction of a 2.5 s and a 4.5 s signal duration in children and adults, with the former producing longer motor responses. The results showed that the 2.5 s duration was overestimated, especially by the younger children, whereas the 4.5 s duration was underestimated in all age groups. Further analyses indicated that the developmental differences related to motor response time explained the age-related difference in temporal reproduction for the shorter duration but not for the longer duration. The modelling of our data suggests that, for the shorter signal duration, the children initiated their responses at the same time as the adults, but that they reproduced longer durations because their motor response took more time to complete. In contrast, for the 4.5 s duration, the children initiated their responses earlier than the adults. However, they reproduced duration values close to the target time because their motor responses took longer. In addition, whatever the duration value to be reproduced, the representation of the sample duration was more variable in the younger children.     

The perception of empty and filled time intervals by pigeons

Pigeons were trained in a within-subjects design to discriminate durations of a filled interval (2 s and 8 s of light) and durations of an empty interval (2 s and 8 s bound by two 500-ms light markers). Filled intervals required a response to one set of comparisons (e.g., blue vs. yellow), whereas empty intervals required a response to a different set of comparisons (e.g., red vs. green). Psychophysical testing indicated that empty intervals were judged to be longer than equivalent durations of a filled interval. This finding was replicated when the anchor durations used during training were changed to 1 s and 4 s, or 4 s and 16 s. The difference between the point of subjective equality (PSE) for the empty intervals and the PSE for filled intervals increased as the magnitude of the anchor duration pairs increased. In addition, the difference limens (DL) for empty intervals were smaller than those for filled intervals, and they also increased as the magnitude of anchor duration pairs increased. An analysis of the Weber fractions (WF; i.e., DL/PSE) provided evidence for superimposition of the empty and filled timing functions across the different sets of anchor durations. These results suggest that the accumulation of subjective time was greater for empty intervals than for filled intervals. Within the framework of scalar timing theory, this difference in timing appeared to be the result of a clock rate difference rather than a switch latency difference.