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1.  Origins of submovements in movements of elderly adults 
Slowness is a well-recognized feature of movements in aging. One of the possible reasons for slowness suggested by previous research is production of corrective submovements that compensate for shortened primary submovement to the target. Here, we re-examine this traditional interpretation and argue that the majority of submovements in older adults may be a consequence rather than the cause of slowness.
Pointing movements in young and older adults were recorded. Conditions for submovement emergence were manipulated by using small and large targets and three movement modes: discrete (required stopping on the target), reciprocal (required reversal on the target), and passing (required crossing the target and stopping after that). Movements were parsed into a primary and secondary submovement based on zero-crossings of velocity (type 1 submovements), acceleration (type 2 submovements), and jerk (type 3 submovements). In the passing mode, secondary submovements were analyzed only after crossing the target to exclude that they were accuracy adjustments.
Consistent with previous research, the primary submovement was shortened and total secondary submovement incidence was increased in older adults. However, comparisons across conditions suggested that many submovements were non-corrective in both groups. Type 1 submovements were non-corrective because they were more frequent for large than small targets. They predominantly emerged due to arm stabilization and energy dissipation during motion termination in the discrete and passing mode. Although type 2 and 3 submovements were more frequent for small than large targets, this trend was also observed in the passing mode, suggesting that many of these submovements were non-corrective. Rather, they could have been velocity fluctuations associated predominantly with low speed of movements to small targets.
The results question the traditional interpretation of frequent submovements in older adults as corrective adjustments. Rather, the increased incidence of submovements in older adults is directly related to low movement speed observed in aging, whereas the relationship between submovement incidence and target size is a result of speed-accuracy trade-off. Aging-related declines in muscular control that may contribute to the disproportional increases in submovement incidence during slow movements of older adults are discussed.
PMCID: PMC2628348  PMID: 19014548
2.  Age-related changes in speed and accuracy during rapid targeted center of pressure movements near the posterior limit of the base of support 
Backward falls are often associated with injury, particularly among older women. An age-related increase occurs in center of pressure variability when standing and leaning. So, we hypothesized that, in comparison to young women, older women would display a disproportionate decrease of speed and accuracy in the primary center of pressure submovements as movement amplitude increases.
Ground reaction forces were recorded from thirteen healthy young and twelve older women while performing rapid, targeted, center of pressure movements of small and large amplitude in upright stance. Measures included center of pressure speed, the number of center of pressure submovements, and the incidence rate of primary center of pressure submovements undershooting the target.
In comparison to young women, older women used slower primary submovements, particularly as movement amplitude increased (P < 0.01). Even though older women achieved similar endpoint accuracy, they demonstrated a 2 to 5-fold increase in the incidence of primary submovement undershooting for large-amplitude movements (P < 0.01). Overall, posterior center of pressure movements of older women were 41% slower and exhibited 43% more secondary submovements than in young women (P < 0.01).
We conclude that the increased primary submovement undershoots and secondary center of pressure submovements in the older women reflect the use of a conservative control strategy near the posterior limit of their base of support.
PMCID: PMC3444664  PMID: 22770467
Balance; Center of pressure submovement; Postural control; Aging
3.  Characterization of Information-Based Learning Benefits with Submovement Dynamics and Muscular Rhythmicity 
PLoS ONE  2013;8(12):e82920.
For skill advancement, motor variability must be optimized based on target information during practice sessions. This study investigated structural changes in kinematic variability by characterizing submovement dynamics and muscular oscillations after practice with visuomotor tracking under different target conditions. Thirty-six participants were randomly assigned to one of three groups (simple, complex, and random). Each group practiced tracking visual targets with trajectories of varying complexity. The velocity trajectory of tracking was decomposed into 1) a primary contraction spectrally identical to the target rate and 2) an intermittent submovement profile. The learning benefits and submovement dynamics were conditional upon experimental manipulation of the target information. Only the simple and complex groups improved their skills with practice. The size of the submovements was most greatly reduced by practice with the least target information (simple > complex > random). Submovement complexity changed in parallel with learning benefits, with the most remarkable increase in practice under a moderate amount of target information (complex > simple > random). In the simple and complex protocols, skill improvements were associated with a significant decline in alpha (8–12 Hz) muscular oscillation but a potentiation of gamma (35–50 Hz) muscular oscillation. However, the random group showed no significant change in tracking skill or submovement dynamics, except that alpha muscular oscillation was reduced. In conclusion, submovement and gamma muscular oscillation are biological markers of learning benefits. Effective learning with an appropriate amount of target information reduces the size of submovements. In accordance with the challenge point hypothesis, changes in submovement complexity in response to target information had an inverted-U function, pertaining to an abundant trajectory-tuning strategy with target exactness.
PMCID: PMC3867443  PMID: 24367568
4.  Human basal ganglia and the dynamic control of force during on-line corrections 
Natural movements are corrected in part by the generation of submovements, occuring early in a movement such that they amend an ongoing action. Submovements are associated with activity of the basal ganglia, implying a role for the structures in error correction. In parallel, the basal ganglia are linked to the generation and control of force amplitude, change and duration. Here we tested if activity in human basal ganglia is associated with submovements generally, or was specific to a condition where the submovements only occurred in the face of unexpected proprioceptive error. Submovements were induced by introducing unexpected and variable viscous loads (augmenting the need for trial specific grip forces) or by reducing target size (augmenting the need for visually guided on-line control) in a 1-D target capture task. In both cases, subjects compensated for the increased task difficulty by generating corrective submovements, which were closely matched in frequency and type. Activity in the internal segment of the globus pallidus and subthalamic nucleus correlated strongly with the number of submovements during the viscous challenge but not with the target challenge. The effects could not be explained by kinematic differences, i.e. movement amplitude or average number of submovements. The results support a specific role for the basal ganglia in error correction under conditions of variable load where there is a need for the dynamic control of force within an ongoing movement.
PMCID: PMC3037279  PMID: 21289168
Motor control; on-line correction; error; adaptation; basal ganglia; human
5.  Submovement Composition of Head Movement 
PLoS ONE  2012;7(11):e47565.
Limb movement is smooth and corrections of movement trajectory and amplitude are barely noticeable midflight. This suggests that skeletomuscular motor commands are smooth in transition, such that the rate of change of acceleration (or jerk) is minimized. Here we applied the methodology of minimum-jerk submovement decomposition to a member of the skeletomuscular family, the head movement. We examined the submovement composition of three types of horizontal head movements generated by nonhuman primates: head-alone tracking, head-gaze pursuit, and eye-head combined gaze shifts. The first two types of head movements tracked a moving target, whereas the last type oriented the head with rapid gaze shifts toward a target fixed in space. During head tracking, the head movement was composed of a series of episodes, each consisting of a distinct, bell-shaped velocity profile (submovement) that rarely overlapped with each other. There was no specific magnitude order in the peak velocities of these submovements. In contrast, during eye-head combined gaze shifts, the head movement was often comprised of overlapping submovements, in which the peak velocity of the primary submovement was always higher than that of the subsequent submovement, consistent with the two-component strategy observed in goal-directed limb movements. These results extend the previous submovement composition studies from limb to head movements, suggesting that submovement composition provides a biologically plausible approach to characterizing the head motor recruitment that can vary depending on task demand.
PMCID: PMC3489904  PMID: 23139749
6.  Primary and submovement control of aiming in C6 tetraplegics following posterior deltoid transfer 
Upper limb motor control in fast, goal-directed aiming is altered in tetraplegics following posterior-deltoid musculotendinous transfer. Specifically, movements have similar end-point accuracy but longer duration and lower peak velocity than those of age-matched, neurotypical controls. Here, we examine in detail the interplay between primary movement and submovement phases in five C6 tetraplegic and five control participants.
Aiming movements were performed in two directions (20 cm away or toward), with or without vision. Trials that contained a submovement phase (i.e., discontinuity in velocity, acceleration or jerk) were identified. Discrete kinematic variables were then extracted on the primary and submovements phases.
The presence of submovements did not differ between the tetraplegic (68%) and control (57%) groups, and almost all submovements resulted from acceleration and jerk discontinuities. Tetraplegics tended to make a smaller amplitude primary movement, which had lower peak velocity and greater spatial variability at peak velocity. This was followed by a larger amplitude and longer duration secondary submovement. Peak velocity of primary movement was not related to submovement incidence. Together, the primary and submovement phases of both groups were equally effective in reducing end-point error.
C6 tetraplegic participants exhibit some subtle differences in measures of motor behaviour compared to control participants, but importantly feedforward and feedback processes work effectively in combination to achieve accurate goal-directed aiming.
PMCID: PMC4127436  PMID: 25055852
Tetraplegia; Aiming; Submovement; Upper-limb control; Muscle transfer
7.  A Common Structure Underlies Low-Frequency Cortical Dynamics in Movement, Sleep, and Sedation 
Neuron  2014;83(5):1185-1199.
Upper-limb movements are often composed of regular submovements, and neural correlates of submovement frequencies between 1 and 4 Hz have been found in the motor cortex. The temporal profile of movements is usually assumed to be determined by extrinsic factors such as limb biomechanics and feedback delays, but another possibility is that an intrinsic rhythmicity contributes to low frequencies in behavior. We used multielectrode recordings in monkeys performing an isometric movement task to reveal cyclic activity in primary motor cortex locked to submovements, and a distinct oscillation in premotor cortex. During ketamine sedation and natural sleep, cortical activity traversed similar cycles and became synchronized across areas. Because the same cortical dynamics are coupled to submovements and also observed in the absence of behavior, we conclude that the motor networks controlling the upper limb exhibit an intrinsic periodicity at submovement frequencies that is reflected in the speed profile of movements.
•Local field potentials (LFPs) and neural firing are phase locked to submovements•Movement kinematics can be decoded from the areal velocity of LFP trajectories•The same dynamic patterns are seen during free reaching, sleep, and sedation•An intrinsic periodicity in motor circuits imposes temporal structure on behavior
Hall et al. find that a common 3 Hz oscillation in the motor cortex of monkeys explains both the speed profile of movements and slow-wave activity during sleep and sedation. These results reveal how intrinsic network dynamics shape upper-limb behaviors.
PMCID: PMC4157580  PMID: 25132467
8.  Dynamic Primitives of Motor Behavior 
Biological cybernetics  2012;106(0):727-739.
We present in outline a theory of sensorimotor control based on dynamic primitives, which we define as attractors. To account for the broad class of human interactive behaviors—especially tool use—we propose three distinct primitives: submovements, oscillations and mechanical impedances, the latter necessary for interaction with objects. Due to fundamental features of the neuromuscular system, most notably its slow response, we argue that encoding in terms of parameterized primitives may be an essential simplification required for learning, performance, and retention of complex skills. Primitives may simultaneously and sequentially be combined to produce observable forces and motions. This may be achieved by defining a virtual trajectory composed of submovements and/or oscillations interacting with impedances. Identifying primitives requires care: in principle, overlapping submovements would be sufficient to compose all observed movements but biological evidence shows that oscillations are a distinct primitive. Conversely, we suggest that kinematic synergies, frequently discussed as primitives of complex actions, may be an emergent consequence of neuromuscular impedance. To illustrate how these dynamic primitives may account for complex actions, we briefly review three types of interactive behaviors: constrained motion, impact tasks, and manipulation of dynamic objects.
PMCID: PMC3735361  PMID: 23124919
Discrete; submovement; rhythmic; oscillation; impedance; primitive
9.  The effect of age, movement direction, and target size on the maximum speed of targeted COP movements in healthy women 
Human movement science  2012;31(5):1213-1223.
Rapid center of pressure (COP) movements are often required to avoid falls. Little is known about the effect of age on rapid and accurate volitional COP movements. We hypothesized that COP movements to a target would be slower and exhibit more submovements in older versus younger adults, particularly in posterior versus anterior movements. Healthy older (N = 12, mean age = 76 years) and young women (N = 13, mean age = 23 years) performed anterior and posterior lean movements while standing on a force plate, and were instructed to move their COP ‘as fast and as accurately as possible’ using visual feedback. The results show that rapid posterior COP movements were slower and had an increased number of submovements and ratio of peak-to-average velocity, in comparison to anterior movements (p < .005). Moreover, older compared to younger adults were 27% slower and utilized nearly twice as many compensatory submovements (p < .005), particularly when moving posteriorly (p < .05). Older women also had higher ratios of peak-to-average COP velocity than young (p < .05). Thus, despite moving more slowly, older women needed to take more frequent submovements to maintain COP accuracy, particularly posteriorly, thereby providing evidence of a compensatory strategy that may be used for preventing backward falls.
PMCID: PMC3330159  PMID: 22225924
Balance; Center of pressure; Speed-accuracy trade-offs; Gerontology
10.  Characterizing and Predicting Submovements during Human Three-Dimensional Arm Reaches 
PLoS ONE  2014;9(7):e103387.
We have demonstrated that 3D target-oriented human arm reaches can be represented as linear combinations of discrete submovements, where the submovements are a set of minimum-jerk basis functions for the reaches. We have also demonstrated the ability of deterministic feed-forward Artificial Neural Networks (ANNs) to predict the parameters of the submovements. ANNs were trained using kinematic data obtained experimentally from five human participants making target-directed movements that were decomposed offline into minimum-jerk submovements using an optimization algorithm. Under cross-validation, the ANNs were able to accurately predict the parameters (initiation-time, amplitude, and duration) of the individual submovements. We also demonstrated that the ANNs can together form a closed-loop model of human reaching capable of predicting 3D trajectories with VAF >95.9% and RMSE ≤4.32 cm relative to the actual recorded trajectories. This closed-loop model is a step towards a practical arm trajectory generator based on submovements, and should be useful for the development of future arm prosthetic devices that are controlled by brain computer interfaces or other user interfaces.
PMCID: PMC4110007  PMID: 25057968
11.  Catching a Ball at the Right Time and Place: Individual Factors Matter 
PLoS ONE  2012;7(2):e31770.
Intercepting a moving object requires accurate spatio-temporal control. Several studies have investigated how the CNS copes with such a challenging task, focusing on the nature of the information used to extract target motion parameters and on the identification of general control strategies. In the present study we provide evidence that the right time and place of the collision is not univocally specified by the CNS for a given target motion; instead, different but equally successful solutions can be adopted by different subjects when task constraints are loose. We characterized arm kinematics of fourteen subjects and performed a detailed analysis on a subset of six subjects who showed comparable success rates when asked to catch a flying ball in three dimensional space. Balls were projected by an actuated launching apparatus in order to obtain different arrival flight time and height conditions. Inter-individual variability was observed in several kinematic parameters, such as wrist trajectory, wrist velocity profile, timing and spatial distribution of the impact point, upper limb posture, trunk motion, and submovement decomposition. Individual idiosyncratic behaviors were consistent across different ball flight time conditions and across two experimental sessions carried out at one year distance. These results highlight the importance of a systematic characterization of individual factors in the study of interceptive tasks.
PMCID: PMC3285177  PMID: 22384072
12.  Multifaceted effects of noisy galvanic vestibular stimulation on manual tracking behavior in Parkinson’s disease 
Parkinson’s disease (PD) is a neurodegenerative movement disorder that is characterized clinically by slowness of movement, rigidity, tremor, postural instability, and often cognitive impairments. Recent studies have demonstrated altered cortico-basal ganglia rhythms in PD, which raises the possibility of a role for non-invasive stimulation therapies such as noisy galvanic vestibular stimulation (GVS). We applied noisy GVS to 12 mild-moderately affected PD subjects (Hoehn and Yahr 1.5–2.5) off medication while they performed a sinusoidal visuomotor joystick tracking task, which alternated between 2 task conditions depending on whether the displayed cursor position underestimated the actual error by 30% (‘Better’) or overestimated by 200% (‘Worse’). Either sham or subthreshold, noisy GVS (0.1–10 Hz, 1/f-type power spectrum) was applied in pseudorandom order. We used exploratory (linear discriminant analysis with bootstrapping) and confirmatory (robust multivariate linear regression) methods to determine if the presence of GVS significantly affected our ability to predict cursor position based on target variables. Variables related to displayed error were robustly seen to discriminate GVS in all subjects particularly in the Worse condition. If we considered higher frequency components of the cursor trajectory as “noise,” the signal-to-noise ratio of cursor trajectory was significantly increased during the GVS stimulation. The results suggest that noisy GVS influenced motor performance of the PD subjects, and we speculate that they were elicited through a combination of mechanisms: enhanced cingulate activity resulting in modulation of frontal midline theta rhythms, improved signal processing in neuromotor system via stochastic facilitation and/or enhanced “vigor” known to be deficient in PD subjects. Further work is required to determine if GVS has a selective effect on corrective submovements that could not be detected by the current analyses.
PMCID: PMC4313776
Parkinson’s disease; vestibular system; GVS; manual tracking; discriminant analysis
13.  Dynamic primitives in the control of locomotion 
Humans achieve locomotor dexterity that far exceeds the capability of modern robots, yet this is achieved despite slower actuators, imprecise sensors, and vastly slower communication. We propose that this spectacular performance arises from encoding motor commands in terms of dynamic primitives. We propose three primitives as a foundation for a comprehensive theoretical framework that can embrace a wide range of upper- and lower-limb behaviors. Building on previous work that suggested discrete and rhythmic movements as elementary dynamic behaviors, we define submovements and oscillations: as discrete movements cannot be combined with sufficient flexibility, we argue that suitably-defined submovements are primitives. As the term “rhythmic” may be ambiguous, we define oscillations as the corresponding class of primitives. We further propose mechanical impedances as a third class of dynamic primitives, necessary for interaction with the physical environment. Combination of these three classes of primitive requires care. One approach is through a generalized equivalent network: a virtual trajectory composed of simultaneous and/or sequential submovements and/or oscillations that interacts with mechanical impedances to produce observable forces and motions. Reliable experimental identification of these dynamic primitives presents challenges: identification of mechanical impedances is exquisitely sensitive to assumptions about their dynamic structure; identification of submovements and oscillations is sensitive to their assumed form and to details of the algorithm used to extract them. Some methods to address these challenges are presented. Some implications of this theoretical framework for locomotor rehabilitation are considered.
PMCID: PMC3689288  PMID: 23801959
discrete; submovement; rhythmic; oscillation; impedance; primitive; locomotion; rehabilitation
14.  Movement structure in young and elderly adults during goal-directed movements of the left and right arm 
Brain and cognition  2008;69(1):30-38.
Elderly adults often exhibit performance deficits during goal-directed movements of the dominant arm compared with young adults. Recent studies involving hemispheric lateralization have provided evidence that the dominant and non-dominant hemisphere-arm systems are specialized for controlling different movement parameters and that hemispheric specialization may be reduced during normal aging. The purpose was to examine age-related differences in the movement structure for the dominant (right) and non-dominant (left) during goal-directed movements. Young and elderly adults performed 72 aiming movements as fast and as accurately as possible to visual targets with both arms. The findings suggest that previous research utilizing the dominant arm can be generalized to the non-dominant arm because performance was similar for the two arms. However, as expected, the elderly adults showed shorter relative primary submovement lengths and longer relative primary submovement durations, reaction times, movement durations, and normalized jerk scores compared to the young adults.
PMCID: PMC2663567  PMID: 18556103
Aging; Feedback; Hand; Laterality; Submovement
15.  Influence of cueing on the preparation and execution of untrained and trained complex motor responses 
This study investigated the influence of cueing on the performance of untrained and trained complex motor responses. Healthy adults responded to a visual target by performing four sequential movements (complex response) or a single movement (simple response) of their middle finger. A visual cue preceded the target by an interval of 300, 1000, or 2000 ms. In Experiment 1, the complex and simple responses were not previously trained. During the testing session, the complex response pattern varied on a trial-by-trial basis following the indication provided by the visual cue. In Experiment 2, the complex response and the simple response were extensively trained beforehand. During the testing session, the trained complex response pattern was performed in all trials. The latency of the untrained and trained complex responses decreased from the short to the medium and long cue-target intervals. The latency of the complex response was longer than that of the simple response, except in the case of the trained responses and the long cue-target interval. These results suggest that the preparation of untrained complex responses cannot be completed in advance, this being possible, however, for trained complex responses when enough time is available. The duration of the 1st submovement, 1st pause and 2nd submovement of the untrained and the trained complex responses increased from the short to the long cue-target interval, suggesting that there is an increase of online programming of the response possibly related to the degree of certainty about the moment of target appearance.
PMCID: PMC3854281  PMID: 22473319
Cueing; Cue-target interval; Response complexity; Response practice; Reaction time; Movement time
16.  Action selection and refinement in subcortical loops through basal ganglia and cerebellum 
Subcortical loops through the basal ganglia and the cerebellum form computationally powerful distributed processing modules (DPMs). This paper relates the computational features of a DPM's loop through the basal ganglia to experimental results for two kinds of natural action selection. First, functional imaging during a serial order recall task was used to study human brain activity during the selection of sequential actions from working memory. Second, microelectrode recordings from monkeys trained in a step-tracking task were used to study the natural selection of corrective submovements. Our DPM-based model assisted in the interpretation of puzzling data from both of these experiments. We come to posit that the many loops through the basal ganglia each regulate the embodiment of pattern formation in a given area of cerebral cortex. This operation serves to instantiate different kinds of action (or thought) mediated by different areas of cerebral cortex. We then use our findings to formulate a model of the aetiology of schizophrenia.
PMCID: PMC2440782  PMID: 17428771
modularity; serial order; pattern classification; error correction; schizophrenia; presynaptic inhibition
17.  Handwriting Movement Kinematics for Quantifying EPS in Patients Treated with Atypical Antipsychotics 
Psychiatry research  2010;177(1-2):77-83.
Ongoing monitoring of neuroleptic-induced extrapyramidal side effects (EPS) is important to maximize treatment outcome, improve medication adherence and reduce re-hospitalization. Traditional approaches for assessing EPS such as parkinsonism, tardive akathisia, or dyskinesia rely upon clinical ratings. However, these observer-based EPS severity ratings can be unreliable and are subject to examiner bias. In contrast, quantitative instrumental methods are less subject to bias. Most instrumental methods have only limited clinical utility because of their complexity and costs. This paper describes an easy-to-use instrumental approach based on handwriting movements for quantifying EPS. Here, we present findings from psychiatric patients treated with atypical (second generation) antipsychotics. The handwriting task consisted of a sentence written several times within a 2 cm vertical boundary at a comfortable speed using an inkless pen and digitizing tablet. Kinematic variables including movement duration, peak vertical velocity and the number of acceleration peaks, and average normalized jerk (a measure of smoothness) for each up or down stroke and their submovements were analyzed. Results from 59 psychosis patients and 46 healthy comparison subjects revealed significant slowing and dysfluency in patients compared to controls. We observed differences across medications and daily dose. These findings support the ecological validity of handwriting movement analysis as an objective behavioral biomarker for quantifying the effects of antipsychotic medication and dose on the motor system.
PMCID: PMC2859992  PMID: 20381875
18.  Relative performance of the two hands in simple and choice reaction time tasks 
There is evidence that the left hemisphere is more competent for motor control than the right hemisphere. This study investigated whether this hemispheric asymmetry is expressed in the latency/duration of sequential responses performed by the left and/or right hands. Thirty-two right-handed young adults (16 males, 16 females; 18-25 years old) were tested in a simple or choice reaction time task. They responded to a left and/or right visual target by moving their left and/or right middle fingers between two keys on each side of the midline. Right hand reaction time did not differ from left hand reaction time. Submovement times were longer for the right hand than the left hand when the response was bilateral. Pause times were shorter for the right hand than the left hand, both when the responses were unilateral or bilateral. Reaction time results indicate that the putatively more efficient response preparation by the left hemisphere motor mechanisms is not expressed behaviorally. Submovement time and pause time results indicate that the putatively more efficient response execution by the left hemisphere motor mechanisms is expressed behaviorally. In the case of the submovements, the less efficient motor control of the left hand would be compensated by a more intense attention to this hand.
PMCID: PMC3932976  PMID: 24345871
Motor lateral asymmetry; Reaction time; Movement time; Response complexity; Unilateral response; Bilateral response
19.  A neural mechanism of speed-accuracy tradeoff in macaque area LIP 
eLife  2014;3:e02260.
Decision making often involves a tradeoff between speed and accuracy. Previous studies indicate that neural activity in the lateral intraparietal area (LIP) represents the gradual accumulation of evidence toward a threshold level, or evidence bound, which terminates the decision process. The level of this bound is hypothesized to mediate the speed-accuracy tradeoff. To test this, we recorded from LIP while monkeys performed a motion discrimination task in two speed-accuracy regimes. Surprisingly, the terminating threshold levels of neural activity were similar in both regimes. However, neurons recorded in the faster regime exhibited stronger evidence-independent activation from the beginning of decision formation, effectively reducing the evidence-dependent neural modulation needed for choice commitment. Our results suggest that control of speed vs accuracy may be exerted through changes in decision-related neural activity itself rather than through changes in the threshold applied to such neural activity to terminate a decision.
eLife digest
Many actions involve a trade-off between speed and accuracy, with typing being a good example: the faster you try to type a sentence, the more mistakes you are likely to make. Mathematical models have successfully reproduced the speed-accuracy trade-off, but it is not clear how the brain represents and weighs up these two factors. Now, Hanks et al. have shown how single neurons in a region of the brain called the lateral intraparietal cortex vary their firing rate to optimize the balance between speed and accuracy.
Two macaque monkeys were trained to fixate on a single dot on a screen and then move their eyes in one of two directions in response to movies of random dots on a video screen. Initially, the monkeys received a reward immediately after every correct response, whereas incorrect responses were punished with a very short time-out. Under these conditions, the optimal strategy is to respond quickly at the expense of accuracy. In a separate block of trials, the monkeys were again rewarded for correct responses, but this time their reward was delayed if they responded too quickly. The most effective strategy now is to respond accurately, but more slowly.
In both the ‘high speed’ and ‘high accuracy’ conditions, the firing of neurons in lateral intraparietal cortex increased while the dots were on the screen. As soon as the firing rate reached a threshold—representing the point at which the monkey had accumulated enough evidence to make a decision about the direction of movement—the monkey moved its eyes. Previous theories had suggested that when speed was the priority, the level of activity required to trigger a decision would be lower than when accuracy was emphasized. Surprisingly, however, the threshold did not differ between the ‘high speed’ and ‘high accuracy’ conditions. Instead, neurons displayed a higher initial firing rate whenever speed was prioritized, enabling the monkey to make a decision on the basis of less evidence.
This finding is consistent with human brain imaging studies that have shown increased baseline activity in decision-making circuitry when speed is prioritized over accuracy. Studying these mechanisms could help to reveal why some individuals are more impulsive decision-makers than others.
PMCID: PMC4054775  PMID: 24867216
decision making; parietal cortex; speed-accuracy tradeoff; rhesus macaque monkey; other
20.  Advance information and movement sequencing in Gilles de la Tourette's syndrome. 
Tourette's syndrome is a chronic neurological disorder manifested by involuntary motor tics and vocalisations. Because the basal ganglia have been implicated in the pathology underlying Tourette's syndrome, the present two procedures, both involving sequential movements, sought to determine the extent to which patients with Tourette's syndrome were reliant on, and could utilise different levels of advance information. Patients with Tourette's syndrome were found to be more reliant than controls on external visual cues to execute rather than to initiate a motor programme. When there was a high level of reduction in advance information--that is, a visual pathway to be followed was extinguished well in advance of each successive movement--executions progressively slowed as the sequence was traversed. Similarly, if no advance information was provided before each move, movement execution was slower than that of controls. The movement initiation times of patients with Tourette's syndrome were, however, similar to those of controls, as were their movement execution times when advance visual information was available. It seems that patients with Tourette's syndrome, like parkinsonian patients who are known to have a basal ganglia disorder, require external sensory cues to sequence a motor programme effectively. The present study found evidence consistent with the hypothesis that patients with Tourette's syndrome, like patients with Parkinson's disease, may be dysfunctional in internal switching mechanisms. Alternatively, with limited visual guidance, patients with Tourette's syndrome, regardless of medication or depression state, may require more time to plan and programme each next submovement, and under such conditions may require external visual cues to direct attention effectively to given targets. Although the underlying pathogenesis is still speculative, it is concluded that there is much to support the notion that Tourette's syndrome may stem from abnormalities of the major pathways between the basal ganglia and the frontal lobes.
PMCID: PMC1073315  PMID: 7876849
21.  Feasibility of Stereo-Infrared Tracking to Monitor Patient Motion During Cardiac SPECT Imaging 
IEEE transactions on nuclear science  2004;51(5 II):2693-2698.
Patient motion during cardiac SPECT imaging can cause diagnostic imaging artifacts. We investigated the feasibility of monitoring patient motion using the Polaris motion-tracking system. This system uses passive infrared reflection from small spheres to provide real-time position data with vendor stated 0.35 mm accuracy and 0.2 mm repeatability. In our configuration, the Polaris system views through the SPECT gantry toward the patient's head. List-mode event data was temporally synchronized with motion-tracking data utilizing a modified LabVIEW virtual instrument that we have employed in previous optical motion-tracking investigations. Calibration of SPECT to Polaris coordinates was achieved by determining the transformation matrix necessary to align the position of four reflecting spheres as seen by Polaris, with the location of Tc-99m activity placed inside the sphere mounts as determined in SPECT reconstructions. We have successfully tracked targets placed on volunteers in simulated imaging positions on the table of our SPECT system. We obtained excellent correlation (R2 > 0.998) between the change in location of the targets as measured by our SPECT system and the Polaris. We have also obtained excellent agreement between the recordings of the respiratory motion of four targets attached to an elastic band wrapped around the abdomen of volunteers and from a pneumatic bellows. We used the axial motion of point sources as determined by the Polaris to correct the motion in SPECT image acquisitions yielding virtually identical point source FWHM and FWTM values, and profiled maximum heart wall counts of cardiac phantom images, compared to the reconstructions with no motion.
PMCID: PMC2600515  PMID: 19081781
22.  Relationship Between Extraretinal Component of Firing Rate and Eye Speed in Area MST of Macaque Monkeys 
Journal of neurophysiology  2005;94(4):2416-2426.
We have isolated extraretinal and retinal components of firing during smooth pursuit eye movements in the medial-superior-temporal area (MST) in the extrastriate visual cortex. Awake macaque monkeys tracked spots in total darkness to eliminate image motion inputs from the background. For 300 ms during sustained tracking at different speeds, the target was stabilized on the moving eye, practically eliminating image motion inputs from the tracking target. The extraretinal component of firing rate during image stabilization was direction selective and related to eye speed but sometimes showed a different preferred speed from the retinal component of the same neuron's responses. The highly variable firing rate of individual MST neurons allowed an ideal observer to predict target speed correctly on 25% of trials. Pooling the data from 71 MST neurons improved the correct response rate to 50%. Behavioral experiments imposed brief perturbations of target velocity to assess the gain of visual-motor transmission for pursuit. The average response to perturbations increased as a function of target speed. However, the size of the responses to individual perturbations allowed an ideal observer to predict target speed correctly on only 35% of the trials. The imprecision of MST responses argues that the output of MST may be a poor candidate to drive eye velocity and so may instead regulate another component of pursuit. The good agreement between the eye velocity precision of the behavioral responses to perturbations of target motion and the firing of MST neurons raises regulation of the visual-motor gain of pursuit as one candidate component.
PMCID: PMC2582193  PMID: 15944236
23.  3D active workspace of human hand anatomical model 
If the model of the human hand is created with accuracy by respecting the type of motion provided by each articulation and the dimensions of articulated bones, it can function as the real organ providing the same motions. Unfortunately, the human hand is hard to model due to its kinematical chains submitted to motion constraints. On the other hand, if an application does not impose a fine manipulation it is not necessary to create a model as complex as the human hand is. But always the hand model has to perform a certain space of motions in imposed workspace architecture no matter what the practical application does.
Based on Denavit-Hartenberg convention, we conceived the kinematical model of the human hand, having in mind the structure and the behavior of the natural model. We obtained the kinematical equations describing the motion of every fingertip with respect to the general coordinate system, placed on the wrist. For every joint variable, a range of motion was established. Dividing these joint variables to an appropriate number of intervals and connecting them, the complex surface bordering the active hand model workspace was obtained.
Using MATLAB 7.0, the complex surface described by fingertips, when hand articulations are all simultaneously moving, was obtained. It can be seen that any point on surface has its own coordinates smaller than the maximum length of the middle finger in static position. Therefore, a sphere having the centre in the origin of the general coordinate system and the radius which equals this length covers the represented complex surface.
We propose a human hand model that represents a new solution compared to the existing ones. This model is capable to make special movements like power grip and dexterous manipulations. During them, the fingertips do not exceed the active workspace encapsulated by determined surfaces. The proposed kinematical model can help to choose which model joints could be eliminated in order to preserve only the motions important for a certain application. The study shows that all models, simplified or not, exhibit a pronounced similitude with the real hand motion, validated by the fingertips' computed trajectories. The results were used to design an artificial hand capable to make some of the hand's functions with a reduced set of degrees of freedom.
PMCID: PMC1876463  PMID: 17472756
24.  Estimation of the Rigid-Body Motion from Three-Dimensional Images Using a Generalized Center-of-Mass Points Approach 
IEEE transactions on nuclear science  2006;53(5):2712-2718.
We present an analytical method for the estimation of rigid-body motion in sets of three-dimensional SPECT and PET slices. This method utilizes mathematically defined generalized center-of-mass points in images, requiring no segmentation. It can be applied to compensation of the rigid-body motion in both SPECT and PET, once a series of 3D tomographic images are available. We generalized the formula for the center-of-mass to obtain a family of points co-moving with the object's rigid-body motion. From the family of possible points we chose the best three points which resulted in the minimum root-mean-square difference between images as the generalized center-of-mass points for use in estimating motion. The estimated motion was used to sum the sets of tomographic images, or incorporated in the iterative reconstruction to correct for motion during reconstruction of the combined projection data. For comparison, the principle-axes method was also applied to estimate the rigid-body motion from the same tomographic images. To evaluate our method for different noise levels, we performed simulations with the MCAT phantom. We observed that though noise degraded the motion-detection accuracy, our method helped in reducing the motion artifact both visually and quantitatively. We also acquired four sets of the emission and transmission data of the Data Spectrum Anthropomorphic Phantom positioned at four different locations and/or orientations. From these we generated a composite acquisition simulating periodic phantom movements during acquisition. The simulated motion was calculated from the generalized center-of-mass points calculated from the tomographic images reconstructed from individual acquisitions. We determined that motion-compensation greatly reduced the motion artifact. Finally, in a simulation with the gated MCAT phantom, an exaggerated rigid-body motion was applied to the end-systolic frame. The motion was estimated from the end-diastolic and end-systolic images, and used to sum them into a summed image without obvious artifact. Compared to the principle-axes method, in two of the three comparisons with anthropomorphic phantom data our method estimated the motion in closer agreement to than of the Polaris system than the principal-axes method, while the principle-axes method gave a more accurate estimation of motion in most cases for the MCAT simulations. As an image-driven approach, our method assumes angularly complete data sets for each state of motion. We expect this method to be applied in correction of respiratory motion in respiratory gated SPECT, and respiratory or other rigid-body motion in PET.
PMCID: PMC2600504  PMID: 19081775
25.  A neural tracking and motor control approach to improve rehabilitation of upper limb movements 
Restoration of upper limb movements in subjects recovering from stroke is an essential keystone in rehabilitative practices. Rehabilitation of arm movements, in fact, is usually a far more difficult one as compared to that of lower extremities. For these reasons, researchers are developing new methods and technologies so that the rehabilitative process could be more accurate, rapid and easily accepted by the patient. This paper introduces the proof of concept for a new non-invasive FES-assisted rehabilitation system for the upper limb, called smartFES (sFES), where the electrical stimulation is controlled by a biologically inspired neural inverse dynamics model, fed by the kinematic information associated with the execution of a planar goal-oriented movement. More specifically, this work details two steps of the proposed system: an ad hoc markerless motion analysis algorithm for the estimation of kinematics, and a neural controller that drives a synthetic arm. The vision of the entire system is to acquire kinematics from the analysis of video sequences during planar arm movements and to use it together with a neural inverse dynamics model able to provide the patient with the electrical stimulation patterns needed to perform the movement with the assisted limb.
The markerless motion tracking system aims at localizing and monitoring the arm movement by tracking its silhouette. It uses a specifically designed motion estimation method, that we named Neural Snakes, which predicts the arm contour deformation as a first step for a silhouette extraction algorithm. The starting and ending points of the arm movement feed an Artificial Neural Controller, enclosing the muscular Hill's model, which solves the inverse dynamics to obtain the FES patterns needed to move a simulated arm from the starting point to the desired point. Both position error with respect to the requested arm trajectory and comparison between curvature factors have been calculated in order to determine the accuracy of the system.
The proposed method has been tested on real data acquired during the execution of planar goal-oriented arm movements. Main results concern the capability of the system to accurately recreate the movement task by providing a synthetic arm model with the stimulation patterns estimated by the inverse dynamics model. In the simulation of movements with a length of ± 20 cm, the model has shown an unbiased angular error, and a mean (absolute) position error of about 1.5 cm, thus confirming the ability of the system to reliably drive the model to the desired targets. Moreover, the curvature factors of the factual human movements and of the reconstructed ones are similar, thus encouraging future developments of the system in terms of reproducibility of the desired movements.
A novel FES-assisted rehabilitation system for the upper limb is presented and two parts of it have been designed and tested. The system includes a markerless motion estimation algorithm, and a biologically inspired neural controller that drives a biomechanical arm model and provides the stimulation patterns that, in a future development, could be used to drive a smart Functional Electrical Stimulation system (sFES). The system is envisioned to help in the rehabilitation of post stroke hemiparetic patients, by assisting the movement of the paretic upper limb, once trained with a set of movements performed by the therapist or in virtual reality. Future work will include the application and testing of the stimulation patterns in real conditions.
PMCID: PMC2259362  PMID: 18251996

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