Exp Brain Res Research Articles

O hand, where art thou? Mapping hand location across the visual field during common activities

Humans employ visually-guided actions during a myriad of daily activities. These ubiquitous but precise manual actions rely on synergistic work between eye and hand movements. During this close cooperation between hands and eyes, the hands persist in sight in a way which is unevenly distributed across our visual field. One common assertion is that most hand actions occur in the lower visual field (LVF) because the arms are anatomically lower than the head, and objects typically rest on waist-high table surfaces. While experimental work has shown that humans are more efficient at reaching for and grasping targets located below their visual midline (Goodale and Danckert, Exp Brain Res 137:303–308, 2001), there is almost no empirical data detailing where the hands lie in the visual fields during natural hand actions. To build a comprehensive picture of hand location during natural visually guided manual actions, we analyzed data from a large-scale open-access dataset containing 100 h of non-scripted manual object interactions during domestic kitchen tasks filmed from a head-mounted camera. We found a clear vertical visual asymmetry with hands located in the lower visual scene (LVS) in more than 70% of image frames, particularly in ipsilateral space. These findings provide the first direct evidence for the established assumption that hands spend more time in the lower than in the upper visual field (UVF). Further work is required to determine whether this LVF asymmetry differs across the lifespan, in different professions, and in clinical populations.

P 84 Altered sensorimotor integration of afferent inhibition in patients with complex regional pain syndrome

Introduction: The complex regional pain syndrome (CRPS) is a chronic pain condition which frequently results in sensorimotor dysfunction of the affected limb. With respect to pathophysiology an impaired interaction in sensorimotor cortex has been hypothesized 1 . Sensorimotor interactions can be assessed through transcranial magnetic stimulation (TMS) that is preceded by an electrical stimulation of peripheral motor or sensory nerves (short/ long afferent inhibition; SAI/LAI). Previous studies revealed only minor alterations of LAI using peripheral motor stimulation in CRPS 2 . This study uses a more comprehensive assessment of SAI and LAI, induced by motor and sensory peripheral stimulation, to assess interaction of the sensorimotor system in CRPS. Methods : 13 unilateral upper limb affected patients with CRPS (9 females, 54 ± 17.8 years) and 11 controls (HCs; 8 females, 52± 18.3 years) received TMS on either hemisphere. SAI and LAI were induced by a motor (m_) or sensory (s_) median nerve stimulation preceded by TMS with 20ms (m_SAI) and 200ms (m_LAI) or 23ms (s_SAI) and 203ms (s_LAI), respectively 3 . Twenty paired and single TMS stimuli were randomly applied for each condition. Results: Pain levels were 5.1±1.5/10 at rest and 7.4±2.1/10 during movements. ANOVA revealed a significant group effect for patients vs. HCs on the affected side (F=2.2, p=0.048). Post hoc tests were corrected for multiple comparisons and showed reduced inhibition in patients after sensory stimulation (s_SAI: t=-2.51, p=0.04; s_LAI: t=-2.53 p=0.038) and motor stimulation with ISI 200ms (m_LAI: t=-2.62, p=0.032). Afferent sensorimotor inhibition was lower in patients with longer disease duration (r=-0.66, p=0.014). Discussion: We demonstrate altered sensorimotor integration of afferent inhibition in CRPS. Since afferent inhibition after sensory stimulation is thought to be mediated via interconnections especially between Brodmann area 3b of somatosensory and the motor cortex, our results complement existing literature about maladaptive cortical plasticity of these regions in CRPS 4,5 . Impairment in LAI, that is thought to involve higher-order somatosensory cortical areas and the posterior parietal cortex is in line with existing literature on chronic pain 2,4 . Future longitudinal investigation should evaluate intra- and intercortical sensorimotor integration systematically in CRPS to better understand their impact in development of sensorimotor symptoms. Literature: 1 Harris AJ. Cortical origins of pathological pain. Lancet 1999. 2 Morgante et al. Normal sensorimotor plasticity in complex regional pain syndrome with fixed posture of the hand. Mov Disord. 2017. 3 Sailer A, et al. Brain. 2003. 4 Chen et al. Modulation of motor cortex excitability by median nerve and digit stimulation. Exp Brain Res. 1999. 5 Pfannmöller et al. Investigations on maladaptive plasticity in the sensorimotor cortex of unilateral upper limb CRPS I patients. RNN. 2019.

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents.

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Bimanual coordination associated with left- and right-hand dominance: testing the limb assignment and limb dominance hypothesis

In an experiment conducted by Kennedy et al. (Exp Brain Res 233:181–195, 2016), dominant right-handed individuals were required to produce a rhythm of isometric forces in a 2:1 or 1:2 bimanual coordination pattern. In the 2:1 pattern, the left limb performed the faster rhythm, while in the 1:2 pattern, the right limb produced the faster pattern. In the 1:2 pattern, interference occurred in the limb which had to produce the slower rhythm of forces. However, in the 2:1 condition, interference occurred in both limbs. The conclusion was that interference was not only influenced by movement frequency, but also influenced by limb dominance. The present experiment was designed to replicate these findings in dynamic bimanual 1:2 and 2:1 tasks where performers had to move one wrist faster than the other, and to determine the influence of limb dominance. Dominant left-handed (N = 10; LQ = − 89.81) and dominant right-handed (N = 14; LQ = 91.25) participants were required to perform a 2:1 and a 1:2 coordination pattern using Lissajous feedback. The harmonicity value was calculated to quantify the interference in the trial-time series. The analysis demonstrated that regardless of limb dominance, harmonicity was always lower in the slower moving limb than in the faster moving limb. The present results indicated that for dominant left- and dominant right-handers the faster moving limb influenced the slower moving limb. This is in accordance with the assumption that movement frequency has a higher impact on limb control in bimanual 2:1 and 1:2 coordination tasks than handedness.

Immediate action effects motivate actions based on the stimulus\u2013response relationship

The theory of event coding, an influential framework for action planning, suggests that humans first integrate stimulus, response, and action effect into representation (an event file) via their contingencies, and then, the activation of expected action effects drives the associated response. While previous studies have typically examined such functions of action effects after, rather than before or during, the acquirement of the representation, Eitam et al. (Exp Brain Res 229:475–484, 2013a) demonstrated that the presence of immediate feedback to action (i.e., action effects) can instantly elicit faster responses than delayed feedback. However, the underlying mechanism of this faciliatory effect remains unclear. Specifically, while the response–effect relationship has been highlighted, the role of stimuli has not been investigated. To address this issue, the present study conducted four experiments. We first reproduced the faciliatory effects of immediate action effects with between- and within-participants design (Experiments 1 and 2, respectively). Then, we assessed whether immediate action effects facilitate response speed, when stimuli (Experiment 3) and a combination of stimuli and responses (Experiment 4) determined the delay of action effects. The identical response was executed faster when driven by stimuli associated with immediate effects than by those associated with lagged effects. This result indicates that immediate action effects do not reinforce the execution of specific motor actions itself, but facilitate actions depending on the stimulus–response relationship. We discuss the potential mechanism of the facilitation effect.

Perceptual information supports transfer of learning in coordinated rhythmic movement

In this paper, we trained people to produce 90° mean relative phase using task-appropriate feedback and investigated whether and how that learning transfers to other coordinations. Past work has failed to find transfer of learning to other relative phases, only to symmetry partners (identical coordinations with reversed lead–lag relationships) and to other effector combinations. However, that research has all trained people using transformed visual feedback (visual metronomes, Lissajous feedback) which removes the relative motion information typically used to produce various coordinations (relative direction, relative position; Wilson and Bingham, in Percept Psychophys 70(3):465–476, 2008). Coordination feedback (Wilson et al., in J Exp Psychol Hum Percept Perform 36(6):1508, 2010) preserves that information and we have recently shown that relative position supports transfer of learning between unimanual and bimanual performance of 90° (Snapp-Childs et al., in Exp Brain Res 233(7), 2225–2238, 2015). Here, we ask whether that information can support the production of other relative phases. We found large, asymmetric transfer of learning bimanual 90° to bimanual 60° and 120°, supported by perceptual learning of relative position information at 90°. For learning to transfer, the two tasks must overlap in some critical way; this is additional evidence that this overlap must be informational. We discuss the results in the context of an ecological, task dynamical approach to understanding the nature of perception–action tasks.

The mesencephalic-hypoglossal nuclei loop as a possible central pattern generator for rhythmical whisking in rats.

It has been previously demonstrated that the Me5 nucleus is involved in the genesis of reflex activities at whisker pad level. Specific Me5 neurons, which provide sensory innervation of the macrovibrissae, are monosynaptically connected with small hypoglossal neurons innervating the extrinsic muscles that control macrovibrissal movements. Artificial whisking, induced by the electrical stimulation of the peripheral stump of the facial nerve and the electrical stimulation of the XII nucleus or the infraorbital nerve, induced evoked responses in the whisker pad extrinsic motor units, along with a significant increase in the electromyographic activity of the extrinsic pad muscles (Mameli et al. in Acta Oto-Laryngol 126:1334-1338, 2006; in Pfűgers Arch Eur J Physiol 456:1189-1198, 2008; in Brain Res 1283:34-40, 2009; in Exp Brain Res 234:753-761, 2016). In anaesthetized rats, we evaluated the possible involvement of this Me5-XII loop in the genesis of rhythmical whisking. The anatomical findings showed that in addition to the ipsilateral, even the contralateral Me5 nucleus could be retrogradely labeled by the Dil tracer injected into the whisker pad of one side, they, furthermore, showed labeled axons extending across the midline between the two nuclei. The electrophysiological findings agreed with the neuroanatomical results, since the mechanical or artificially induced deflection of the whiskers of one side, evoked in the Me5 contralateral nucleus different patterns of responses. The hypothesis that the Me5-XII loops, along with their cross-linked relationship, could act as a "central generator" responsible for the stereotyped symmetrical pattern of macrovibrissal movements such as rhythmical whisking has been discussed.

The influence of obstacles on grasp planning.

When reaching to grasp for an object in the absence of obstacles, the choice of contact points is highly consistent within and between healthy humans, suggesting a preplanning of grasping movements (Gilster et al. in Exp Brain Res 217:137-151, 2012). In real life, objects may obstruct the favored contact points at a target object, requiring adjustments to avoid collision. In the present study, we investigated how an obstacle that directly obstructs the favored contact points for two-digit grasping changes the planning and execution of reach-to-grasp movements. Furthermore, we elucidated to what extent an obstacle placed at various angular positions around the target object (thereby not directly obstructing the favored contact points) still influences trajectories, contact points, and time-related parameters. When obstacles directly obstructed favored contact points participants either chose a completely new contact point or grasped the object only slightly away from the favored contact point. Obstacles located near the favored contact points but not directly obstructing them still resulted in a repulsive effect, meaning that contact points were shifted away from the obstacle to ensure sufficient distance to the obstacle. We found that the position of an obstacle even influences the direction in which the fingers set off. This leads to a deviation of the trajectory very early in the time course, yielding longer movement times if the main contact points are obstructed. Taken together, the early significant influence of obstacles on the grasping movement supports the assumption that grasping movements are preplanned.

T50. Freezing of gait does not modulate neural oscillations in motor cortices

Introduction Walking abilities are severely affected in patients with freezing of gait (FOG), a common phenomenon in advanced stages of Parkinson’s disease (PD). Despite the high occurrence of FOG, little is known about the underlying mechanisms. Interestingly, it was reported that patients with FOG are still able to ride a bike (Snijders AH et al., Bicycling breaks the ice for freezers of gait, Mov Disord 2011). While similar brain networks are involved in walking and bicycling (Fukuyama H et al., Brain functional activity during gait in normal subjects: A SPECT study, Neurosci Lett 1997; Christensen LOD et al., Cerebral activation during bicycle movements in man, Exp Brain Res 2000), it is not completely understood why both types of movement are differently affected in PD patients with FOG. A recent study from our group demonstrated that the oscillatory activity within the subthalamic nucleus (STN) differs between patients with and without FOG (Storzer L et al., Bicycling suppresses abnormal beta synchrony in the Parkinsonian basal ganglia, Ann Neurol 2017). In particular, it was shown that patients with FOG present a characteristic increase of oscillatory activity at ∼18 Hz. The aim of the study was to examine movement-associated cortical oscillatory activity in PD patients with and without FOG. Methods We analyzed EEG data of 19 PD patients with FOG (65.5 ± 7.2 years), 13 PD patients without FOG (61.6 ± 8.1 years), and 16 healthy controls (63.3 ± 7.3 years). Patients were recorded OFF dopaminergic medication using a portable EEG system (Porti, TMSi, Enschede, The Netherlands). Data were recorded during unconstrained walking and bicycling on a stationary bicycle. Measurements consisted of a baseline rest period, continuous movement, and an alternating sequence of rest and movement, i.e. bicycling or walking, for 10 s each. Furthermore, measurements included 180° turns around the body axis during walking condition. Turns were rated for the occurrence of FOG episodes by two different raters. Analyses were focused on oscillatory activity in the beta band and the Cz electrode overlying the leg area of both motor cortices. Statistical testing was performed by fitting linear mixed models using package lme 1.1–13 for R. Results Bicycling and walking were associated with beta power suppression (13–35 Hz). This suppression was stronger for bicycling than for walking ( β = −0.37, p β = −0.11, p = 0.72). Furthermore, within the group of Freezers we did not find cortical differences between turns with FOG and turns without FOG either( β = 0.66, p = 0.4). Conclusion We did not observe cortical beta power changes specifically related to freezing. This finding does not support the notion that the primary motor cortex is crucially involved in generating freezing.

Using proprioception to control ongoing actions: dominance of vision or altered proprioceptive weighing?

Visual feedback is crucial for movement accuracy (e.g., Keele and Posner, J Exp Psychol 77:155-158, 1968). As well, proprioception has been argued to be important for the control of voluntary movements (e.g., Bagesteiro et al., Exp Brain Res 171:358-370, 2006; Rossetti et al., J Neurophysiol 74:457-463, 1995). While tendon vibration (TVib) has been used to perturb proprioceptive information during limb matching tasks (Goodwin et al., Science 175:1382-1384, 1972), the current study employed between-trial dual-muscle TVib, coupled with vision occlusion, to assess the use of vision and proprioception for the online control of voluntary movements. Participants (n = 17) performed an upper-limb reaching task (30cm). TVib influenced both accuracy and precision of movement endpoint. Critically, variability analyses showed that participant's performance was most affected by TVib at 75% of the movement duration, even in the presence of vision. These findings demonstrate that between-trial dual-muscle tendon vibration can perturb proprioceptive feedback, and further, suggest that proprioception can be important for the online control of reaches, even when vision is available.

Normal temporal binding window but no sound-induced flash illusion in people with one eye.

Integrating vision and hearing is an important way in which we process our rich sensory environment. Partial deprivation of the visual system from the loss of one eye early in life results in adaptive changes in the remaining senses (e.g., Hoover et al. in Exp Brain Res 216:565-74, 2012). The current study investigates whether losing one eye early in life impacts the temporal window in which audiovisual events are integrated and whether there is vulnerability to the sound-induced flash illusion. In Experiment 1, we measured the temporal binding window with a simultaneity judgement task where low-level auditory and visual stimuli were presented at different stimulus onset asynchronies. People with one eye did not differ in the width of their temporal binding window, but they took longer to make judgements compared to binocular viewing controls. In Experiment 2, we measured how many light flashes were perceived when a single flash was paired with multiple auditory beeps in close succession (sound induced flash illusion). Unlike controls, who perceived multiple light flashes with two, three or four beeps, people with one eye were not susceptible to the sound-induced flash illusion. In addition, they took no longer to respond compared to both binocular and monocular (eye-patched) viewing controls. Taken together, these results suggest that the lack of susceptibility to the sound-induced flash illusion in people with one eye cannot be accounted for by the width of the temporal binding window. These results provide evidence for adaptations in audiovisual integration due to the reduction of visual input from the loss of one eye early in life.

Multisensory temporal processing in own-body contexts: plausibility of hand ownership does not improve visuo-tactile asynchrony detection.

Tracking one's own body is essential for environmental interaction, and involves integrating multisensory cues with stored information about the body's typical features. Exactly how multisensory information is integrated in own-body perception is still unclear. For example, Ide and Hidaka (Exp Brain Res 228:43-50, 2013) found that participants made less precise visuo-tactile temporal order judgments (TOJ) when viewing hands in a plausible orientation (upright; typical for one's own hand) compared to an implausible orientation (rotated 180°). This suggests that viewing one's own body relaxes the precision for perceived visuo-tactile synchrony. In contrast, visuo-proprioceptive research shows improvements for multisensory temporal perception near one's own body in asynchrony detection tasks, implying an increase in precision. Hence, it is unclear whether viewed hand orientation generally modulates the ability to detect small asynchronies between vision and touch, or if this effect is specific to TOJ tasks. We investigated whether viewed hand orientation affects detection of visuo-tactile asynchrony. In two experiments, participants viewed model hands in anatomically plausible or implausible orientations. In one experiment, we stroked the hands to induce the rubber hand illusion. Participants were asked to detect short delays (40-280ms) between vision (an LED flash on the model hand) and touch (a tap to fingertip of the participant's hidden hand) in a two-interval forced-choice task. Bayesian analyses show that our data provide strong evidence that viewed hand orientation does not affect visuo-tactile asynchrony detection. This study suggests the mechanisms for fine-grained time perception differ between visuo-tactile and visuo-proprioceptive contexts.

The power law as behavioral illusion: reappraising the reappraisals.

Marken and Shaffer (Exp Brain Res 235:1835-1842, 2017) have argued that the power law of movement, which is generally thought to reflect the mechanisms that produce movement, is actually an example of what Powers (Psychol Rev 85:417-435, 1978) dubbed a behavioral illusion, where an observed relationship between variables is seen as revealing something about the mechanisms that produce a behavior when, in fact, it does not. Zago et al. (Exp Brain Res. https://doi.org/10.1007/s0022-017-5108-z , 2017) and Taylor (Exp Brain Res, https://doi.org/10.1007/s00221-018-5192-8 , 2018) have "reappraised" this argument, claiming that it is based on logical, mathematical, statistical and theoretical errors. In the present paper we answer these claims and show that the power law of movement is, indeed, an example of a behavioral illusion. However, we also explain how this apparently negative finding can point the study of movement in a new and more productive direction, with research aimed at understanding movement in terms of its purposes rather than its causes.

Effect of visual field motion on vestibulo-myogenic response during upright stance: A pilot study

Effect of visual field motion on vestibulo-myogenic response during upright stance: A pilot study

The role of Dopamine in Preparatory Inhibition: What can we learn from Parkinson’s disease?

The role of Dopamine in Preparatory Inhibition: What can we learn from Parkinson’s disease?

Mechanical perturbations can elicit triggered reactions in the absence of a startle response.

Perturbations delivered to the upper limbs elicit reflexive responses in stretched muscle at short- (M1: 25-50ms) and long- (M2: 50-100ms) latencies. When presented in a simple reaction time (RT) task, the perturbation can also elicit a preprogrammed voluntary response at a latency (< 100ms) that overlaps the M2 response. This early appearance of the voluntary response following a proprioceptive stimulus causing muscle stretch is called a triggered reaction. Recent work has demonstrated that a perturbation also elicits activity in sternocleidomastoid (SCM) over a time-course consistent with the startle response and it was, therefore, proposed that the StartReact effect underlies triggered reactions (Ravichandran et al., Exp Brain Res 230:59-69, 2013). The present work investigated whether perturbation-evoked SCM activity results from startle or postural control and whether triggered reactions can also occur in the absence of startle. In Experiment 1, participants "compensated" against a wrist extension perturbation. A prepulse inhibition (PPI) stimulus (known to attenuate startle) was randomly presented before the perturbation. Rather than attenuating SCM activity, the responses in SCM were advanced by the PPI stimulus. In Experiment 2, participants "assisted" a wrist extension perturbation. The perturbation did not reliably elicit startle but despite this, two-thirds of trials had RTs of less than 100ms and the earliest responses began at ~ 70ms. These findings suggest that SCM activity following a perturbation is the result of postural control and is not related to startle. Moreover, an overt startle response is not a prerequisite for the elicitation of a triggered reaction.

Failures in adaptive locomotion: trial-and-error exploration to determine adequate foot elevation over obstacles.

Lifting the limb sufficiently to clear an obstacle seems like a straightforward task, yet trips are a common cause of falls across all ages. Examination of obstacle contacts in the lab revealed a progressive decrease in foot elevation with repeated exposures, ultimately resulting in failure (Heijnen et al. Exp Brain Res 23:219-231, 2012). The purpose of this study was to determine if the progressive decrease in foot elevation continued when knowledge of obstacle contact was removed. Twenty-one young adults (mean 20.0±1.0years; 8 males) crossed a 20cm obstacle in a 12m walkway for 150 trials. The obstacle was covertly lowered between the lead and trail limb crossing of the obstacle, which eliminated obstacle contact with the trail limb if the limb was too low. The average failure rate was 8%, substantially higher than the 1-2% observed for stationary, visible obstacles. Therefore, tactile information from obstacle contact was instrumental for guiding the trail limb; visual information and joint angle information were insufficient for most participants. Foot elevation change over successive trials varied across participants, and was categorized as (1) asymptotic decrease (N=11, 52%), with foot elevation converging to obstacle height, (2) linear decrease (N=7, 33%), and (3) stable (N=3, 14%). The asymptotic and stable groups appeared to have reasonable knowledge of obstacle height; the linear group did not. The asymptotic behavior is consistent with participants exploring the region above the obstacle through trial-and-error to determine appropriate foot elevation.

Vertically arrayed stimuli and responses: transfer of incompatible spatial mapping to Simon task occurs regardless of response-device orientation.

Conde et al. (Exp Brain Res 233:3313-3321, 2015) found that the Simon effect for vertically arrayed stimuli and responses was reduced after 100 prior practice trials with an incompatible mapping of the stimulus locations and responses. This finding was contrary to Vu's (Mem Cognit 35:1463-1471, 2007) finding of no transfer effect with 72 trials of prior practice. Conde et al. proposed that the different results were due to their responses being coded as top and bottom in the frontal plane, whereas Vu's were coded as far and near in the transverse plane. We conducted four experiments to test this possibility in which participants responded with keypresses using their thumbs on a numeric keypad held vertically (upright in the frontal plane) or horizontally (flat in the transverse plane). Experiment 1 showed that, without any prior practice, a similar sized Simon effect was obtained when the response device was oriented in the transverse plane as when it was oriented in the frontal plane. In Experiments 2 and 3 participants performed with the same device orientation in the incompatible practice and Simon transfer tasks, with orientation manipulated between-subjects in the former and within-subjects in the latter. The Simon effect was reduced in both cases, with no significant difference in transfer effect for transverse and frontal planes. In Experiment 4, the device orientation differed between the incompatible practice and Simon transfer tasks, and the Simon effect was reduced similarly across both response-device orientations. Thus, the differences between Conde et al.'s and Vu's findings cannot be attributed to the response-device orientation. Our results are consistent with the view that people code response locations in the transverse plane as top and bottom, rather than far and near, in agreement with the terminology of "top row" and "bottom row" for computer keyboards.

The speed-curvature power law of movements: a reappraisal.

Several types of curvilinear movements obey approximately the so called 2/3 power law, according to which the angular speed varies proportionally to the 2/3 power of the curvature. The origin of the law is debated but it is generally thought to depend on physiological mechanisms. However, a recent paper (Marken and Shaffer, Exp Brain Res 88:685-690, 2017) claims that this power law is simply a statistical artifact, being a mathematical consequence of the way speed and curvature are calculated. Here we reject this hypothesis by showing that the speed-curvature power law of biological movements is non-trivial. First, we confirm that the power exponent varies with the shape of human drawing movements and with environmental factors. Second, we report experimental data from Drosophila larvae demonstrating that the power law does not depend on how curvature is calculated. Third, we prove that the law can be violated by means of several mathematical and physical examples. Finally, we discuss biological constraints that may underlie speed-curvature power laws discovered in empirical studies.

When an object appears unexpectedly: foot placement during obstacle circumvention in children and adults with Developmental Coordination Disorder.

Adjustments to locomotion to avoid an obstacle require a change to the usual pattern of foot placement, i.e. changes to step length and/or step width. Previous studies have demonstrated a difficulty in individuals with Developmental Coordination Disorder (DCD) in controlling stability while both stepping over and while circumventing an obstacle. In a previous study, we have considered the way in which individuals with DCD prepare for the possibility of an obstacle appearing (Wilmut and Barnett in Exp Brain Res 235:1531-1340, 2017). Using a parallel data set from this same task on the same individuals, the aim of the current study was to investigate the exact nature of changes in foot placement during obstacle avoidance, as this was not clear from previous work. Children and adults aged from 7 to 34years of age took part in the study. Forty-four met the criteria for a diagnosis of DCD and there were 44 typically developing (TD) age and gender-matched controls. Participants walked at a comfortable pace down an 11m walkway; on 6 out of 36 trials a 'gate' closed across their pathway which required circumvention. These 6 'gate close' trials were analysed for this study. The number and magnitude of step length and step width adjustments were similar across the DCD and TD groups, however, the younger children (7-11years) made a greater number of early adjustments compared to the older children and adults (12-34years of age). In contrast the adults made a greater number of adjustments later in the movement compared to the children. In terms of foot placement adjustments a clear preference was seen across all participants to use adjustments which resulted in reducing step length, stepping away from the obstacle and a combination of these. Apart from subtle differences, the individuals with DCD make step placements to circumvent an obstacle in line with their peers. It is suggested that the choice of foot placement strategy in individuals with DCD, although in line with their peers, may not be optimal for their level of motor ability.