A Riemannian geometry theory of human movement: The geodesic synergy hypothesis

Mass-inertia loads on muscles change with posture and with changing mechanical interactions between the body and the environment. The nervous system must anticipate changing mass-inertia loads, especially during fast multi-joint coordinated movements. Riemannian geometry provides a mathematical framework for movement planning that takes these inertial interactions into account. To demonstrate this we introduce the controlled (vs. biomechanical) degrees of freedom of the body as the coordinate system for a configuration space with movements represented as trajectories. This space is not Euclidean. It is endowed at each point with a metric equal to the mass-inertia matrix of the body in that configuration. This warps the space to become Riemannian with curvature at each point determined by the differentials of the mass-inertia at that point. This curvature takes nonlinear mass-inertia interactions into account with lengths, velocities, accelerations and directions of movement trajectories all differing from those in Euclidean space. For newcomers to Riemannian geometry we develop the intuitive groundwork for a Riemannian field theory of human movement encompassing the entire body moving in gravity and in mechanical interaction with the environment. In particular we present a geodesic synergy hypothesis concerning planning of multi-joint coordinated movements to achieve goals with minimal muscular effort.     

A note on accepting the null hypothesis: problems with respect to the mass-spring and pulse-step models of movement control

Models are sometimes developed on the premise that there are no differences between two or more conditions regardless of experimental manipulation. This article argues that unless a very sensitive experiment (or series of experiments) was conducted, such conclusions are often premature. Design and statistical factors such as alpha level, variability, sample size, and treatment effect are all determinants of the sensitivity of the experiment. It is sometimes the case that an experiment produces differences between means and yet the null hypothesis is not rejected because of high variability within experimental conditions or because too few subjects were used. Unless the probability of detecting a difference between groups is known, it would seem unwise to conclude that no difference exists. Judgment on the model should be suspended. Examples are taken from the mass-spring and pulse-step models fro voluntary movement control to illustrate the problem.     

Empirical evaluation of the uncanny valley hypothesis fails to confirm the predicted effect of motion

The uncanny valley hypothesis states that the acceptability of an artificial character will not increase linearly in relation to its likeness to human form. Instead, after an initial rise in acceptability there will be a pronounced decrease when the character is similar, but not identical to human form (Mori, 1970/2012). Moreover, it has been claimed but never directly tested that movement would accentuate this dip and make moving characters less acceptable. We used a number of full-body animated computer characters along with a parametrically defined motion set to examine the effect of motion quality on the uncanny valley. We found that improving the motion quality systematically improved the acceptability of the characters. In particular, the character classified in the deepest location of the uncanny valley became more acceptable when it was animated. Our results showed that although an uncanny valley was found for static characters, the deepening of the valley with motion, originally predicted by Mori (1970/2012), was not obtained.     

Varied intensities of training, predicted maximum oxygen uptake and the minimum threshold hypothesis

In this study, 24 sedentary first-year male university students aged 17 to 24 yr. were assigned to one of four groups. Three of the groups underwent a 6-wk. training program consisting of a 30-min. bicycle ergometer ride, three times per week. A fourth group acted as a control. The experimental groups trained at workload intensities corresponding to predetermined heart rates of 125, 140, and 155 bpm, respectively. Analysis of scores indicated that changes in aerobic capacity occurred only if the intensity of training exceeded a minimal level. Essentially, these results replicated data from other studies.     

The one-target advantage: a test of the movement integration hypothesis.

Two experiments were conducted to compare the temporal structure of single aiming movements to two-component movements involving either a reversal in direction or an extension. For reversal movements, there was no cost associated with the movement time for the first segment of the movement. However, regardless of movement direction, initiation instructions, handedness or effector, two-component extension movements were always associated with a longer movement time for the first movement segment. This disadvantage for extension movements, but not reversal movements, is consistent with the notion that there is interference between the execution of the first movement and implementation of the second movement. By contrast, because the muscular force used to break the first movement is also used to propel the second movement, reversal movements are organised as an integrated unit.     

Categorization by movement direction: Retrieval-induced forgetting of motor sequences grouped by motion features

We investigated the organized storage of motor sequences in memory by assuming that processes related to interference at retrieval are indicative of memory organization. Effects resulting from these processes, thus, would allow inferences on how motor sequences are represented and organized. Participants learned motor sequences that were categorized by the direction of the initial movement. The subsequent selective retrieval of a subset of sequences of one category resulted in retrieval-induced forgetting (RIF) for the non-retrieved sequences of the same category. RIF occurred in an explicit recall test (Experiment 1), as well in an implicit test assessing memory with novel cues (Experiment 2). The results suggest that RIF affected motor programmes and that other cues as the used effectors (here movement direction) can be used for the organization of procedural memory. Basic retrieval dynamics apparently operate within the declarative and procedural systems in a similar way.     

Effects of movement frequency and joint kinetics on the joint coordination underlying bimanual circle drawing

Ten healthy participants performed bimanual symmetric and asymmetric circle drawing at 4 frequencies. The authors partitioned the variance of the joint configuration across repetitions into 1 component representing equivalent joint configurations with respect to achieving stability of the mean hand path (i.e., goal-equivalent variance, GEV) and 1 component leading to a variable hand path (non-goal-equivalent variance, NGEV) across cycles. Higher frequencies led to increased NGEV related to control of the nondominant hand and to the relative position and orientation between the hands during asymmetric drawing. The results were related to differences in muscle and interaction moments between the arms, and they suggest a possible relationship between the ability to use intersegmental forces and the stability of interlimb synergy.     

Temporal constraints in the control of prehensile movement

Three experiments were conducted to investigate the control of the manipulation (i.e., finger-thumb aperture) and transportation (i.e., wrist velocity) components in prehensile movement (Jeannerod, 1981, 1984). In all experiments, subjects were seated and instructed to grasp a dowel mounted on a joystick following a discrete movement over a set distance. Thus, the amount of dowel movement following the grasp could be determined. In Experiment 1, the tolerance (i.e., the amount of allowable dowel movement) was manipulated using a computer-generated boundary around the dowel. The results indicated that the transportation component changed dependent on the tolerance condition, and there were trends that maximum aperture was also affected. Experiment 2 manipulated both tolerance and dowel size (i.e., diameter) factorially in a within-subject design. Dowel size affected only the manipulation component, supporting Jeannerod's (1981) earlier work, but tolerance clearly influenced both components. Experiment 3 investigated Wing, Turton, and Fraser's (1986) proposition that speed of movement influences aperture size. Distance and movement time were combined factorially to produce conditions with different average velocities. Maximum aperture was dependent on the movement time rather than the speed of movement. The relation between the control of the components was examined by using a new method of calculating within-trial correlations between aperture size and wrist velocity in Experiments 2 and 3. The correlations were related to the temporal aspects of the movement with higher correlations in the rapid movement time conditions. Also, the temporal occurrence of maximum aperture remained invariant across the different movement conditions. In general, the results suggest a strong functional linkage between the two components, which may be dependent on the temporal characteristics of the movement.     

Saccade size control in reading: Evidence for the linguistic control hypothesis

In Experiment 1, it is shown that during reading the eye makes larger saccades near long words than near short words. The effects are reduced when the subject’s peripheral vision is diminished by the use of a moving “window” centered on the subject’s fixation point, outside of which letters are replaced by Xs. In Experiment 2, it is shown that even if linguistic predictions are kept constant, the eye tends to make longer jumps when approaching THE than when approaching a three-letter verb. This “THE-skipping” effect is weaker if THE is compared with an auxiliary (HAD, WAS, or ARE) than if it is compared with a less frequently occurring verb (ATE, RAN, MET). It follows that knowledge of the lexicon can combine with information from peripheral vision fast enough to influence saccade size from moment to moment.     

Pupil diameter tracks changes in control state predicted by the adaptive gain theory of locus coeruleus function

An important dimension of cognitive control is the adaptive regulation of the balance between exploitation (pursuing known sources of reward) and exploration (seeking new ones) in response to changes in task utility. Recent studies have suggested that the locus coeruleus-norepinephrine system may play an important role in this function and that pupil diameter can be used to index locus coeruleus activity. On the basis of this, we reasoned that pupil diameter may correlate closely with control state and associated changes in behavior. Specifically, we predicted that increases in baseline pupil diameter would be associated with decreases in task utility and disengagement from the task (exploration), whereas reduced baseline diameter (but increases in task-evoked dilations) would be associated with task engagement (exploitation). Findings in three experiments were consistent with these predictions, suggesting that pupillometry may be useful as an index of both control state and, indirectly, locus coeruleus function.     

The role of proprioception in the perception and control of human movement: Toward a theoretical reassessment

The theoretical role of proprioception in the perception and control of human movement is elusive because of the obvious inability to manipulate experimentally the various receptive systems. Individuals who have had the metacarpophalangeal joint and joint capsule removed and replaced with silastic inserts afford a unique opportunity to evaluate a principal source of proprioception, namely, slowly adapting joint afferents. In a set of experiments, we show that such individuals show no deficits in finger localization following joint replacement. We take this and other complementary findings as a preliminary basis for proposing a dynamic rather than kinematic account of movement production. In addition, we provide a reconceptualization of the function of proprioceptive information in the CNS. Our arguments focus on proprioceptive inputs as tuning or modulating interneuronal pools rather than providing dimension-pecific information to the brain, as is commonly assumed.     

Hierarchical control of rapid movement sequences

Are movement sequences executed in a hierarchically controlled fashion? We first state explicitly what such control would entail, and we observe that if a movement sequence is planned hierarchically, that does not imply that its execution is hierarchical. To find evidence for hierarchically controlled execution, we require subjects to perform memorized sequences of finger responses like those used in playing the piano. The error data we obtain are consistent with a hierarchical planning as well as execution model, but the interresponse-time data provide strong support for a hierarchical execution model. We consider three alternatives to the hierarchical execution model and reject them. We also consider the implications of our results for the role of timing in motor programs, the characteristics of motor buffers, and the relations between memory for symbolic and motor information.     

In defense of the advance specification hypothesis for motor control

Summary Choice reaction times often depend on the relations between the responses that are possible. Rosenbaum, Inhoff, and Gordon (1984) accounted for these effects with an advance specification (AS) hypothesis, according to which subjects prepare for a choice by creating a single motor program with the features shared by the response alternatives; after the choice signal is presented, subjects are assumed to fill in the features that were missing. In a recent article, Heuer (1987) argued that the AS hypothesis cannot account for a number of results that he has obtained. He suggested hat a better hypothesis relies on the idea of programming interactions (PI), according to which programs for alternative responses have mutual facilitative or inhibitory influences, depending on their similarity. Heuer (1987) reported an experiment that was partly consistent with the PI hypothesis and, as implied by him, inconsistent with the AS hypothesis. We show that the results of this experiment, as well as the results of Heuer's earlier experiments, are in fact consistent with the AS hypothesis and inconsistent with the PI hypothesis. We show in addition that there are a number of logical problems with the PI hypothesis which make it a less attractive model of choice than the AS hypothesis.Supported in part by National Science Foundation grant BNS-8710933 and a Research Career Development Award from the National Institutes of Health to the first author.     

Quadrupedal movement training improves markers of cognition and joint repositioning

IntroductionExercise, and in particular balance and coordination related activities such as dance, appear to have positive effects on cognitive function, as well as neurodegenerative conditions such as dementia and Parkinson’s disease. Quadrupedal gait training is a movement system requiring coordination of all four limbs that has previously been associated with cognitive development in children. There is currently little research into the effect of complex QDP movements on cognitive function in adults.PurposeTo determine the effects of a novel four-week quadrupedal gait training programme on markers of cognitive function and joint reposition sense in healthy adults.MethodsTwenty-two physically active sports science students (15 male and 7 female) were divided into two groups: a training group (TG) and a control group (CG). All participants completed the Wisconsin Card Sorting Task (WCST) and were tested for joint reposition sense before and after a four-week intervention, during which time the TG completed a series of progressive and challenging quadrupedal movement training sessions.ResultsParticipants in the TG showed significant improvements in the WCST, with improvements in perseverative errors, non-perseverative errors, and conceptual level response. This improvement was not found in the CG. Joint reposition sense also improved for the TG, but only at 20 degrees of shoulder flexion.ConclusionsPerformance of a novel, progressive, and challenging task, requiring the coordination of all 4 limbs, has a beneficial impact on cognitive flexibility, and in joint reposition sense, although only at the specific joint angle directly targeted by the training. The findings are consistent with other studies showing improvements in executive function and joint reposition sense following physical activity.     

Thoughts regarding the qualitative analysis of human movement

The qualitative analysis of human movement is composed of many parts. Trying to assess the contribution of each of these components to the whole process of qualitative analysis is difficult. Evaluating a qualitative approach to human movement analysis using quantitative procedures can lead to misinterpretations. Qualitative analysis of movement has components important to its success which cannot be gauged by traditional biomechanical standards.