Task Performance Accuracy Research Articles

Interactions between Affective and Cognitive Processing Systems in Problematic Gamblers: A Functional Connectivity Study

BackgroundMotivational and cognitive abnormalities are frequently reported in pathological gambling. However, studies simultaneously investigating motivational and cognitive processing in problematic gamblers are lacking, limiting our understanding of the interplay between these systems in problematic gambling. Studies in non-clinical samples indicate that interactions between dorsal “executive” and ventral “affective” processing systems are necessary for adequate responses in various emotive situations.MethodsWe conducted a generalized Psycho-Physiological Interaction (gPPI) analysis to assess the influence of affective stimuli on changes in functional connectivity associated with response inhibition in 16 treatment seeking problematic gamblers (PRGs) and 15 healthy controls (HCs) using an affective Go-NoGo fMRI paradigm including neutral, gambling-related, positive and negative pictures as neutral and affective conditions.ResultsAcross groups, task performance accuracy during neutral inhibition trials was positively correlated with functional connectivity between the left caudate and the right middle frontal cortex. During inhibition in the gambling condition, only in PRGs accuracy of task performance was positively correlated with functional connectivity within sub-regions of the dorsal executive system. Group interactions showed that during neutral inhibition, HCs exhibited greater functional connectivity between the left caudate and occipital cortex than PRGs. In contrast, during inhibition in the positive condition, PRGs compared to HCs showed greater functional connectivity between the left caudate and occipital cortex. During inhibition trials in the negative condition, a stronger functional connectivity between the left caudate and the right anterior cingulate cortex in PRGs compared to HCs was present. There were no group interactions during inhibition in the gambling condition.ConclusionsDuring gamble inhibition PRGs seem to benefit more from functional connectivity within the dorsal executive system than HCs, because task accuracy in this condition in PRGs is positively correlated with functional connectivity, although the groups show similar connectivity patterns during gamble inhibition. Greater functional connectivity between the ventral affective system and the dorsal executive system in PRGs in the affective conditions compared to HCs, suggests facilitation of the dorsal executive system when affective stimuli are present specifically in PRGs.

The test–retest reliability of gait-related dual task performance in community-dwelling fallers and non-fallers

Gait-related dual task tests have been used to assess differences between fallers and non-fallers, without a thorough assessment of reliability. This study investigated the test–retest reliability of eight gait-related dual and one triple task tests in forty-four community-dwelling older adults (twenty with and twenty-four without a history of falls). The reliability of single, dual and change in performance (percentage change) from single to dual task conditions was assessed. The results showed that single and dual task walking time had fair to excellent reliability (ICC=0.53–0.92) across tasks. The percentage change in walking time (dual-task decrement) showed only poor to good reliability (ICC=−0.11 to 0.75). Cognitive task performance speed and accuracy showed poor to good reliability in single and dual task conditions (ICC=−0.16 to 0.83). The difference between the two conditions in speed and accuracy showed poor to fair reliability (ICC=−0.40 to 0.51). A secondary motor task (carrying a cup of water) ranged from slightly to moderately reliable in dual task conditions and when change in performance was measured (kappa=0.18–0.57). This study showed that simple dual task tests are reliable in the single and dual conditions but measures of change in performance, which are recommended for dual task assessment, are less reliable and is something which needs to be considered in future research. Of the nine examined, only one test, the walking while talking test, showed good reliability on primary and secondary tasks.

Music meets surgery: two sides to the art of “healing”

The effect of music in the operating room is not fully understood. Through a systematic review the authors aim to give a conceptual presentation of the effect that music has on the pre- and postoperative course of surgical patients and on the effectiveness of the surgical work performed by both physicians and staff. The search was conducted both on the basis of the Medical Subject Headings (MeSH) tree and as a text search using the Medline database (1946 to December 2011). The main search heading was "music in operating room" with the accessory keyword "surgery." The selection criteria specified the English language and the availability of abstracts or full-text articles. From 85 articles listed with the corresponding search, 28 were relevant and enrolled for the review. Patients exhibit lower anxiety levels before and during surgery when hearing music and a significant reduction in analgesia and sedation requirements has been observed. Music was found to reduce the heart rate, blood pressure, and muscle effort of surgeons while at the same time increasing the accuracy of surgical tasks. Surgeons who played a musical instrument were found to perform surgical tasks faster. On the other hand, anesthesiologists report that music is associated with difficulties communicating and offering a stable level of sedation. The most appropriate music in the operating room seems to be the classical type. Music in the operating room can have beneficial effects on patients by decreasing stress, anxiety, and the demand for analgesic and anesthetic drugs. For the surgical staff, music is considered to be distracting. For the surgeon, music can increase the speed and accuracy of task performance.

The effects of circadian phase, time awake, and imposed sleep restriction on performing complex visual tasks: Evidence from comparative visual search

Cognitive performance not only differs between individuals, but also varies within them, influenced by factors that include sleep-wakefulness and biological time of day (circadian phase). Previous studies have shown that both factors influence accuracy rather than the speed of performing a visual search task, which can be hazardous in safety-critical tasks such as air-traffic control or baggage screening. However, prior investigations used simple, brief search tasks requiring little use of working memory. In order to study the effects of circadian phase, time awake, and chronic sleep restriction on the more realistic scenario of longer tasks requiring the sustained interaction of visual working memory and attentional control, the present study employed two comparative visual search tasks. In these tasks, participants had to detect a mismatch between two otherwise identical object distributions, with one of the tasks (mirror task) requiring an additional mental image transformation. Time awake and circadian phase both had significant influences on the speed, but not the accuracy of task performance. Over the course of three weeks of chronic sleep restriction, speed but not accuracy of task performance was impacted. The results suggest measures for safer performance of important tasks and point out the importance of minimizing the impact of circadian phase and sleep-wake history in laboratory vision experiments.

After-training emotional interference may modulate sequence awareness in a serial reaction time task

The purpose of the present experiment was to investigate the effects of emotional interference on consolidation of sequential learning. In different sessions, 6 groups of subjects were initially trained on a serial reaction time task (SRTT). To modulate consolidation of the newly learned skill, subjects were exposed, after the training, to 1 of 3 (positive, negative or neutral) different classes of emotional stimuli which consisted of a set of emotional pictures combined with congruent emotional musical pieces or neutral sound. Emotional intervention for each subject group was done in 2 different time intervals (either directly after the training session or 6 h later). After a 72 h post-training interval, each group was retested on the SRTT. Re-test performance was evaluated in terms of response times and accuracy during execution of a target sequence. Emotional intervention did not influence either response times or accuracy of re-testing SRTT target task performance, both variables sensitive to implicit knowledge acquired during SRTT training. However, explicit awareness of sequence knowledge after 72 h was enhanced when negative stimuli had been applied at 0 h after training. These findings suggest that consolidation of explicit aspects of procedural learning may be more responsive toward emotional interference than implicit aspects.

The COMT Val158 allele is associated with impaired delayed-match-to-sample performance in ADHD

BackgroundThis study explored the association between three measures of working memory ability and genetic variation in a range of catecholamine genes in a sample of children with ADHD.MethodsOne hundred and eighteen children with ADHD performed three working memory measures taken from the CANTAB battery (Spatial Span, Delayed-match-to-sample, and Spatial Working Memory). Associations between performance on working memory measures and allelic variation in catecholamine genes (including those for the noradrenaline transporter [NET1], the dopamine D4 and D2 receptor genes [DRD4; DRD2], the gene encoding dopamine beta hydroxylase [DBH] and catechol-O-methyl transferase [COMT]) were investigated using regression models that controlled for age, IQ, gender and medication status on the day of test.ResultsSignificant associations were found between performance on the delayed-match-to-sample task and COMT genotype. More specifically, val/val homozygotes produced significantly more errors than did children who carried a least one met allele. There were no further associations between allelic variants and performance across the other working memory tasks.ConclusionsThe working memory measures employed in the present study differed in the degree to which accurate task performance depended upon either the dynamic updating and/or manipulation of items in working memory, as in the spatial span and spatial working memory tasks, or upon the stable maintenance of representations, as in the delay-match–to-sample task. The results are interpreted as evidence of a relationship between tonic dopamine levels associated with the met COMT allele and the maintenance of stable working memory representations required to perform the delayed-match-to-sample-task.

KINEMATIC PARAMETER IDENTIFICATION OF PARALLEL ROBOTS FOR SEMI-PHYSICAL SIMULATION PLATFORM OF SPACE DOCKING MECHANISM

Because of errors in the kinematic parameters of parallel robots, it is necessary to identify them to improve the pose accuracy for accurate task performance of semiphysical simulation platform. In this paper, a methodical way of kinematic parameter identification is introduced. It requires measurements of all legs and the pose information provided by a coordinate measuring machine. Nonlinear least squares algorithm is employed to determine the kinematic parameters. The measurement errors in the leg sensors are considered during kinematic modeling and kinematic parameter identification. Simulations and experimental studies on a Stewart parallel robot built in the Institute of Electrohydraulic Servo Simulation & Test System of Harbin Institute of Technology reveal the convenience and effectiveness of the proposed robot parameter identification algorithm and calibration method for parallel robots.http://dx.doi.org/10.5755/j01.mech.17.5.731

Low-frequency oscillations of the neural drive to the muscle are increased with experimental muscle pain

We investigated the influence of nociceptive stimulation on the accuracy of task execution and motor unit spike trains during low-force isometric contractions. Muscle pain was induced by infusion of hypertonic saline into the abductor digiti minimi muscle of 11 healthy men. Intramuscular EMG signals were recorded from the same muscle during four isometric contractions of 60-s duration at 10% of the maximal force [maximal voluntary contraction (MVC)] performed before injection (baseline), after injection of isotonic (control) or hypertonic saline (pain), and 15 min after pain was no longer reported. Each contraction was preceded by three 3-s ramp contractions from 0% to 10% MVC. The low-frequency oscillations of motor unit spike trains were analyzed by the first principal component of the low-pass filtered spike trains [first common component (FCC)], which represents the effective neural drive to the muscle. Pain decreased the accuracy of task performance [coefficient of variation (CoV) for force: baseline, 2.8 ± 1.8%, pain, 3.9 ± 1.8%; P < 0.05] and reduced motor unit discharge rates [11.6 ± 2.3 pulses per second (pps) vs. 10.7 ± 1.7 pps; P < 0.05]. Motor unit recruitment thresholds (2.2 ± 1.2% MVC vs. 2.4 ± 1.6% MVC), interspike interval variability (18.4 ± 4.9% vs. 19.1 ± 5.4%), strength of motor unit short-term synchronization [common input strength (CIS) 1.02 ± 0.44 vs. 0.83 ± 0.22], and strength of common drive (0.47 ± 0.08 vs. 0.47 ± 0.06) did not change across conditions. The FCC signal was correlated with force (R = 0.45 ± 0.06), and the CoV for FCC increased in the painful condition (5.69 ± 1.29% vs. 7.83 ± 2.61%; P < 0.05). These results indicate that nociceptive stimulation increased the low-frequency variability in synaptic input to motoneurons.

Dysfunctional Prefrontal Cortical Network Activity and Interactions following Cannabinoid Receptor Activation

Coordinated activity spanning anatomically distributed neuronal networks underpins cognition and mediates limbic-cortical interactions during learning, memory, and decision-making. We used CP55940, a potent agonist of brain cannabinoid receptors known to disrupt coordinated activity in hippocampus, to investigate the roles of network oscillations during hippocampal and medial prefrontal cortical (mPFC) interactions in rats. During quiet wakefulness and rest, CP55940 dose-dependently reduced 0.1-30 Hz local field potential power in CA1 of the hippocampus while concurrently decreasing 30-100 Hz power in mPFC; these contrasting population-level effects were paralleled by differential effects on underlying single-unit activity in the two structures. During decision-making phases of a spatial working memory task, CP5540-induced deficits in hippocampal theta and prefrontal gamma oscillations were observed alongside disrupted theta-frequency coherence between the two structures. These changes in coordinated limbic-cortical network activities correlated with (1) reduced accuracy of task performance, (2) impaired phase-locking of prefrontal single-unit spiking to the local gamma and hippocampal theta rhythms, and (3) impaired task-dependent activity in a subset of mPFC units. In addition to highlighting the importance of CA1-mPFC network oscillations for cognition, these results implicate disrupted theta-frequency coordination of CA1-mPFC activity in the cognitive deficits caused by exogenous activation of brain cannabinoid receptors.

Altered brain activation during response inhibition in children with primary nocturnal enuresis: An fMRI study

Nocturnal enuresis is a common developmental disorder in children, and primary nocturnal enuresis (PNE) is the dominant subtype. The main purpose of this study was to investigate brain functional abnormalities specifically related to motor response inhibition in children with PNE using fMRI in combination with a Go/NoGo task. Twenty-two children with PNE and 22 healthy children, group-matched for age and sex, took part in this experiment. Although no significant between-group differences in task performance accuracy were observed, PNE patients showed significantly longer response times on average. There were several brain regions with reduced activation during motor response inhibition in children with PNE: the bilateral inferior frontal gyri, right superior and middle frontal gyri, right inferior parietal lobe, bilateral cingulate gyri and insula. Our data indicate that response inhibition in children with PNE is associated with a relative lack of or delay in the maturation of prefrontal cortex circuitry that is known to suppress inappropriate responses. This result might give clues to understanding the pathophysiology of PNE.

Traumatic brain injury alters the functional brain network mediating working memory

Primary objective: Investigation of the impact of traumatic brain injury (TBI) on the functional brain network that mediates working memory function.Research design: Functional magnetic resonance imaging (fMRI) during an n-back working memory task in nine chronic-stage patients with TBI and nine age-matched healthy controls. In addition to classical analyses investigating regional activity, the authors examined functional connectivity of the brain regions critical to working memory performance using psychophysiological interaction (PPI) analyses.Main outcomes and results: Patients with TBI made a greater percentage of errors than controls at high working memory load conditions. The fMRI data showed that the activation of the left inferior parietal gyrus (LIPG) was significantly reduced, whereas the activation of the right inferior frontal gyrus (RIFG) was significantly increased in patients compared with controls. Task performance accuracy was significantly associated with the activation of the LIPG in controls and the activation of the RIFG in patients. PPI analyses on fMRI data further suggested that the functional connectivity between the RIFG and LIPG was compromised in patients.Conclusion: The abnormal functional connectivity between LIPG and RIFG may underlie the observed working memory deficits and abnormal brain activation pattern in patients.

EEG correlates of fatigue during administration of a neuropsychological test battery

Objective Mental fatigue, a poorly understood symptom of sports-related concussion, ideally requires assessment across multiple modalities. Our study aimed to examine mental fatigue effects among 10 neurologically normal, athletically active students undergoing typical concussion testing. It is our intention to ultimately address the question whether fatigue effects due to mild traumatic brain injury (mTBI) may become confounded with fatigue effects due to testing effort. Methods Fourteen athletically active and neurologically normal volunteers were initially recruited from Penn State University. Self-reported fatigue, neuropsychological performance, and electroencephalographic (EEG) activity were measured throughout the whole testing duration. EEG measures in frequency domain (e.g., relative power of theta, alpha, and beta bands) were examined over the course of neuropsychological (NP) test administration. Results Predicted fatigue effects over the course of testing included: (a) increased self-reported fatigue; (b) increased errors on the Stroop Interference Test; (c) significantly increased relative power of theta activity during the Stroop Interference Test in frontal–central and parietal regions; and (d) migration of alpha activation from the occipital to anterior (left parietal and pre-central) regions during the Stroop Interference task administered at the beginning compared with the end of testing. Conclusions Results supported predictions related to subjective fatigue and cognitive performance and offered partial support for predictions related to EEG activation patterns over the course of administering the NP testing. Significance Neurologically intact and athletically active college students demonstrate effects related to fatigue after undergoing a typical sports concussion assessment battery, including an increase in subjectively experienced fatigue, a decrease in cognitive task performance accuracy and associated modulations in EEG activity. This finding should be considered by clinical practitioners while evaluating the symptoms of concussion and making a decision regarding the return-to-sport participation.

Ketamine anesthesia during the first week of life can cause long-lasting cognitive deficits in rhesus monkeys

Previously our laboratory has shown that ketamine exposure (24 h of clinically relevant anesthesia) causes significant increases in neuronal cell death in perinatal rhesus monkeys. Sensitivity to this ketamine-induced neurotoxicity was observed on gestational days 120–123 ( in utero exposure via maternal anesthesia) and on postnatal days (PNDs) 5–6, but not on PNDs 35–37. In the present study, six monkeys were exposed on PND 5 or 6 to intravenous ketamine anesthesia to maintain a light surgical plane for 24 h and six control animals were unexposed. At 7 months of age all animals were weaned and began training to perform a series of cognitive function tasks as part of the National Center for Toxicological Research (NCTR) Operant Test Battery (OTB). The OTB tasks used here included those for assessing aspects of learning, motivation, color discrimination, and short-term memory. Subjects responded for banana-flavored food pellets by pressing response levers and press-plates during daily (M–F) test sessions (50 min) and were assigned training scores based upon their individual performance. As reported earlier ( Paule et al., 2009) beginning around 10 months of age, control animals significantly outperformed (had higher training scores than) ketamine-exposed animals for approximately the next 10 months. For animals now over 3 and one-half years of age, the cognitive impairments continue to manifest in the ketamine-exposed group as poorer performance in the OTB learning and color and position discrimination tasks, as deficits in accuracy of task performance, but also in response speed. There are also apparent differences in the motivation of these animals which may be impacting OTB performance. These observations demonstrate that a single 24-h episode of ketamine anesthesia, occurring during a sensitive period of brain development, results in very long-lasting deficits in brain function in primates and provide proof-of-concept that general anesthesia during critical periods of brain development can result in subsequent functional deficits. Supported by NICHD, CDER/FDA and NCTR/FDA.

Functional MR imaging evidence of altered functional activation in metabolic syndrome.

MetS is a cluster of risk factors associated with significant cardiovascular morbidity and mortality and diminished cognitive function. Given that little is known about the early signs of brain vulnerability related to persistent metabolic dysfunction, we set out to determine whether cognitively healthy middle-aged individuals with MetS exhibit an altered cerebrovascular response to a cognitive challenge relative to those without MetS. Forty neurologically healthy adults aged 40-60 years (19 with MetS and 21 healthy controls) performed a 2-back verbal working memory task during fMRI. We compared BOLD responses between the 2 groups in 8 a priori regions of interest previously shown to be associated with the 2-back in patients with cardiovascular disease. Age, education level, sex distribution, cognitive and emotional functioning, and task performance (accuracy and reaction time) were not different between the groups. Compared with healthy controls, individuals with MetS demonstrated a lower 2-back-related BOLD response in the right superior frontal gyrus, right superior parietal lobule, and left inferior parietal lobule. This study provides preliminary evidence that cognitively intact middle-aged individuals with MetS exhibit significant alterations in cerebrovascular response to a cognitive challenge. Our results also demonstrate that fMRI may identify early brain changes associated with MetS.

Neural Correlates of Skill Acquisition with a Cortical Brain–Machine Interface

ABSTRACT Research into the development of brain–machine interfaces (BMIs) has led to demonstrations of rodents, nonhuman primates, and humans controlling prosthetic devices in real time through modulation of neural signals. In particular, cortical BMI studies have shown that improvements in performance require learning and are associated with changes in neuronal tuning properties. These studies have further shown evidence of long-term improvements in performance with practice. The authors conducted experiments to understand long-term skill acquisition with BMIs and to characterize the neural correlates of improvements in task performance. They specifically assessed long-term acquisition of neuroprosthetic skill (i.e., accurate task performance readily recalled across days). In 2 monkeys performing a center–out task using a brain-controlled (BC) computer cursor, they closely monitored daily performance trends and the neural correlates under different conditions. Importantly, they assessed BC performance using a continuous-control multistep task. The authors first conducted experiments that mimicked experimental conditions commonly used. Specifically, a large set of neurons was incorporated with daily recalibration of the transform of neural activity to BC. Under such conditions, they found evidence of variable daily performance. In contrast, when a fixed transform was applied to stable recordings from an ensemble of neurons across days, there was consistent evidence of long-term skill acquisition. Such skill acquisition was associated with the crystallization of a cortical map for prosthetic control. Taken together, the results suggest that the primate motor cortex can achieve skilled control of a neuroprosthetic device through consolidation of a cortical representation.

Effects of Task Complexity on Coordination of Inter-Limb and Within-Limb Forces in Static Bimanual Manipulation

Coordination of the hand grip (G; acting normally to the grasping surface) and load forces (L; acting in parallel) in bimanual static tasks was studied. L symmetry (either the magnitude or direction) and frequency were manipulated in healthy participants (N = 14). More complex tasks (i.e., the higher frequency and/or asymmetric ones) revealed expected deterioration in both the task performance (accuracy of the prescribed L force profiles) and force coordination (G/L ratio and G-L correlation) suggesting importance of L frequency and symmetry in prehension activities. However, the same tasks revealed a more prominent deterioration of interlimb than the within-limb force coordination. This could be interpreted by two partly different and noncompeting neural control mechanisms where the coordination of interlimb forces may be based on ad-hoc and task-specific muscle coordination (often referred to as muscle synergies) while the within-limb coordination of G and L could be based on more stable and partly reflex mechanisms.

Attention in High Precision Tasks and Perceptual Learning

At any moment, the world presents far more information than the brain can process. Visual attention allows the effective selection of information relevant for high priority processing, and is often more easily focused on one object than two. Both spatial selection and object attention have important consequences for the accuracy of task performance. Such effects are historically assessed primarily for relatively “easy” lower-precision tasks, yet the role of attention can depend critically on the demand for fine, high precision judgments. High precision task performance generally depends more upon attention and attention affects performance across all contrasts with or without noisy stimuli. Low precision tasks with similar processing loads generally show effects of attention only at intermediate contrasts and may be restricted to noisy display conditions. Perceptual learning can reduce the costs of inattention. The different roles of attention and task precision are accounted for within the context of an elaborated perceptual template model of the observer showing distinct functions of attention, and providing an integrated account of performance as a function of attention, task precision, external noise and stimulus contrast. Taken together, these provide a taxonomy of the functions and mechanisms of visual attention.

The possibility of determination of accuracy of performance just before the onset of a reaching task using movement-related cortical potentials

This study aimed to examine the relationships among electroencephalogram parameters as physiological indices, ballistic movement as an index of operation, and accuracy of task performance. Experiments were conducted using a "reaching" task, in which subjects touched the target appearing 300 pixels away from a start point in a vertical direction on a touch-sensitive screen with the forefinger. During experiments, EEG, EMG as trigger, high-speed camera images, and task efficiency were acquired. Significant differences between the high- and low-performance groups were clearly confirmed on the slope of NS (negative slope) in movement-related cortical potentials (MRCPs) acquired from Fz (p < 0.05) and Cz (p < 0.05). Furthermore, a difference between these groups was confirmed in duration of ballistic movement. Based on our findings, we discuss whether it is possible to extract MRCP rapidly and automatically without using signal averaging and also to estimate accuracy just before a motion is executed.

Manipulation of Objects with and without the Use of the Index Finger: Implications for Digital Amputations

Study Design Clinical Measurement. Purpose To evaluate changes in temporal and amplitude movement accuracy with tasks requiring fine motor manipulation with and without the use of the index finger (WIF). Participants Twenty right-handed participants (10 males, 10 females, aged 24–47 years) were recruited. Methods Three objects, ranging in weight and size, that required the use of 2 or 3 fingers were selected for this study. Motor performance was quantified during manipulation of a pen, cork, and wine glass using a computerized visual guided tracking task. The miniBird (Ascension Technology, Burlington, VT, USA) miniature motion tracking sensor was attached to each object to measure and record the 3D linear and angular motion. Results Task performance and temporal accuracy of the pen task in the normal condition was more accurate ( P = .033). During the WIF condition there was significantly more motion performing the wine task ( P < .001). Conclusions The protocol directly measures the ability of the hand to coordinate movement in response to a visual tracking target. Both temporal accuracy and amplitude consistency can be objectively evaluated. The current study evaluates the ability of the hand to manipulate 3 objects used in fine motor manipulation, using motion analysis and visual tracking. Level of Evidence 3b.

Experimental kinematic calibration of parallel manipulators using a relative position error measurement system

Because of errors in the geometric parameters of parallel robots, it is necessary to calibrate them to improve the positioning accuracy for accurate task performance. Traditionally, to perform system calibration, one needs to measure a number of robot poses using an external measuring device. However, this process is often time-consuming, expensive and difficult for robot on-line calibration. In this paper, a methodical way of calibration of parallel robots is introduced. This method is performable only by measuring joint variable vector and positioning differences relative to a constant position in some sets of configurations that the desired positions in each set are fixed, but the moving platform orientations are different. In this method, measurements are relative, so it can be performed by using a simple measurement device. Simulations and experimental studies on a Hexaglide parallel robot built in the Sharif University of Technology reveal the convenience and effectiveness of the proposed robot calibration method for parallel robots.