Spatio-temporal organisation of natural prehension

The History and Philosophy of Ecological Psychology

Ecological Psychology is an embodied, situated, and non-representational approach pioneered by J. J. Gibson and E. J. Gibson. This theory aims to offer a third way beyond cognitivism and behaviorism for understanding cognition. The theory started with the rejection of the premise of the poverty of the stimulus, the physicalist conception of the stimulus, and the passive character of the perceiver of mainstream theories of perception. On the contrary, the main principles of ecological psychology are the continuity of perception and action, the organism-environment system as unit of analysis, the study of affordances as the objects of perception, combined with an emphasis on perceptual learning and development. In this paper, first, we analyze the philosophical and psychological influences of ecological psychology: pragmatism, behaviorism, phenomenology, and Gestalt psychology. Second, we summarize the main concepts of the approach and their historical development following the academic biographies of the proponents. Finally, we highlight the most significant developments of this psychological tradition. We conclude that ecological psychology is one of the most innovative approaches in the psychological field, as it is reflected in its current influence in the contemporary embodied and situated cognitive sciences, where the notion of affordance and the work of E. J. Gibson and J. J. Gibson is considered as a historical antecedent.

Spatial and Temporal Patterns in Sequential Precision Reach Movements

Introduction: Sequential reach tasks are a common component of manual assembly jobs. These tasks typically involve manipulating a work object or material and reaching to successive target locations with different precision requirements. Ergonomics research on the control of hand movements has largely focused on tasks requiring discrete reaches (e.g., Bootsma & Van Wieringen, 1992; Hoff & Arbib, 1993; Jeannerod, 1984; Marteniuk et al., 1990).

The objective of this paper was to investigate spatial and temporal effects of pulley design parameters (outer diameter and groove width) on the trajectory of the threading hand in sequential reaches with different precision requirements. Additionally, we propose a scheme to segment hand trajectories into control phases based on the fingertip trajectory speed profile. Segmenting sequential reach tasks into discrete movements between two consecutive target locations will be useful towards developing models of sequential reaching movements and performance for ergonomic analysis.

Methods: Twelve right-handed adults, ages 20-26 years, participated in a laboratory experiment that required threading polyester string through a system of pulleys mounted on an acrylic work surface. Interchangeable pulleys were arranged on the perimeter of a semicircle with a radius of 46 cm at azimuths of 0°, 45°, 90°, 135°, and 180° relative to a constant origin pulley located at the center. The height of the pulleys above the floor was adjusted to place the center pulley at the participant’s standing elbow height. The thread was pulled from a spool located below the center pulley. The task involved threading the pulleys in the following sequence: origin-180°-origin-135°-origin-90°-origin-45°-origin-0°-origin. We conducted a full-factorial experiment with three pulley outer diameters (OD: 38-mm, 76-mm, and 152-mm), three groove widths (GW: 3-mm, 6-mm, and 9-mm), five pulley locations (0°, 45°, 90°, 135°, and 180°), and two threading directions (clockwise and counterclockwise), with 3 repetitions per condition. Participants were instructed to complete the task as quickly as possible while also ensuring each pulley was threaded successfully.

A motion capture marker triad on the hand dorsum tracked hand motions during the task. Hand trajectories were analyzed separately for each of the 5 origin-destination pulley location pairs. Speed profiles were analyzed to identify transition points between the transport phase, where the hand is reaching from the origin to the destination location, and the pulley interaction phase, where the hand is engaged in threading the destination pulley. The start and end points of the pulley interaction phase correspond to the first and last local speed minima that occur below a threshold set at 100-mm/s above the minimum speed when the trajectory is within the region of the destination pulley. The angle ( α) and radius ( R) of the hand position, relative to the destination pulley center, were estimated at the start ( t1) and end ( t2) points of the pulley interaction phase. Repeated measures ANOVA was used to test the effects of OD, GW, pulley location, and threading direction on the time spent in the pulley interaction phase ( TPI = t2 − t1), R1, R2, α1, α2, and the difference between α1 and α2 ( αPI = α2 - α1).

Results:

Temporal parameters: Pulley OD ( p < 0.001), GW ( p < 0.001), location ( p = 0.002), and the threading direction x pulley location interaction (p < 0.001) had a significant effect on TPI. Larger GW corresponded to less TPI (GW: Mean±SE, 3-mm: 772±34 ms, 6-mm: 473±23 ms, 9-mm: 351±18 ms). Pulley OD of 152-mm required significantly more TPI (713±35 ms) compared to the 38-mm (449±21 ms) and 76-mm (433±21 ms) OD. The CW threading direction required significantly less TPI for the 0°, 45° and, 90° pulley locations, while CCW threading direction took more TPI for the 135° and 180° pulley locations.

Spatial Parameters: The effects of OD ( p < 0.001) and pulley location ( p < 0.001) were significant for R1. Larger OD corresponded to increased R1, i.e., 38-mm OD: 76±1-mm, 76-mm OD: 87±1-mm, and 152-mm OD: 119±1-mm. Additionally, R1 increased significantly as the pulley location changed from 0°-180°. Similar trends were observed for R2 across OD and pulley location. The main effects of OD ( p < 0.001), pulley location ( p < 0.001), and threading direction ( p < 0.001) and the interaction between pulley location and threading direction ( p < 0.001) were significant for α1. Larger OD corresponded to a greater α1 (38-mm OD: 24±1°, 76-mm OD: 34±1°, 152-mm OD: 53±1°). At the 180° pulley location, α1 was significantly greater for the CCW vs. CW threading direction. At the 0°, 45° and, 90° pulley locations, α1 was greater for the CW vs. CCW threading direction. Similar trends were observed for α2 across task parameters. The main effect of pulley OD on αPI was significant ( p < 0.001) with a larger αPI for the 152-mm OD (22±1°) compared to the 38-mm OD (15±1°) and 76-mm OD (11±1°).

Discussion: These results show that pulley design parameters in a sequential reach task systematically influence the spatial properties and transition timing of hand motion trajectories between phases. Narrower GW increased the precision requirement and corresponded to slower times. Participants took more time threading the larger OD. Shorter threading times occurred when participants had a direct line of sight with the pulley groove. Pulley OD influenced the radius of the hand position at the start and end of the pulley interaction phase, whereas pulley GW had no effect. The increase in R1 and R2 for pulleys located on the contralateral side compared to the lateral side was attributed to need for line of sight with the pulley groove since the hand obstructs the view of the pulley edge on the contralateral side.

Conclusions: Analysis of sequential reaches needs to consider individual target locations and design parameters. Our findings also show the potential for modeling sequential reaches as a series of discrete reaches. A scheme to segment hand trajectories into control phases based on the fingertip trajectory speed profile was presented. Further investigation is necessary in sequential reach tasks with more realistic and complex work configurations observed in industrial settings.

Coordination of pincer grasp and transport after mechanical perturbation of the index finger

Our understanding of reach-to-grasp movements has evolved from the original formulation of the movement as two semi-independent visuomotor channels to one of interdependence. Despite a number of important contributions involving perturbations of the reach or the grasp, some of the features of the movement, such as the presence or absence of coordination between the digits during the pincer grasp and the extent of spatio-temporal interdependence between the transport and the grasp, are still unclear. In this study, we physically perturbed the index finger into extension during grasping closure on a minority of trials to test whether modifying the movement of one digit would affect the movement of the opposite digit, suggestive of an overarching coordinative process. Furthermore, we tested whether disruption of the grasp results in the modification of kinematic parameters of the transport. Our results showed that a continuous perturbation to the index finger affected wrist velocity but not lateral displacement. Moreover, we found that the typical flexion of the thumb observed in nonperturbed trials was delayed until the index finger counteracted the extension force. These results suggest that physically perturbing the grasp modifies the kinematics of the transport component, indicating a two-way interdependence of the reach and the grasp. Furthermore, a perturbation to one digit affects the kinematics of the other, supporting a model of grasping in which the digits are coordinated by a higher-level process rather than being independently controlled.NEW & NOTEWORTHY A current debate concerning the neural control of prehension centers on the question of whether the digits in a pincer grasp are controlled individually or together. Employing a novel approach that perturbs mechanically the grasp component during a natural reach-to-grasp movement, this work is the first to test a key hypothesis: whether perturbing one of the digits during the movement affects the other. Our results support the idea that the digits are not independently controlled.

A Pilot Study of the Effects of Pulley Location and Design Parameters on Hand Movements during Pulley Threading Operations

Three healthy individuals participated in a laboratory experiment that required routing a thin continuous thread through a series of pulleys mounted on a vertical work surface. Task precision demand was manipulated by altering pulley outer diameter (38 mm, 76 mm, and 152 mm) and groove width (3 mm, 6 mm, and 9 mm). The target location of each destination pulley relative to the origin at the mid-sagittal plane was also manipulated. These factors were hypothesized to influence hand motion trajectories, peak speed, and task completion time. Smaller pulley diameters and larger groove widths, representing lower precision demands, were associated with smoother trajectories and a faster task completion time. These preliminary findings suggest a systematic influence of task precision demands on movement kinematics and task performance.

Stereovision and augmented reality for closed-loop control of grasping in hand prostheses

OBJECTIVE

Technologically advanced assistive devices are nowadays available to restore grasping, but effective and effortless control integrating both feed-forward (commands) and feedback (sensory information) is still missing. The goal of this work was to develop a user friendly interface for the semi-automatic and closed-loop control of grasping and to test its feasibility.

APPROACH

We developed a controller based on stereovision to automatically select grasp type and size and augmented reality (AR) to provide artificial proprioceptive feedback. The system was experimentally tested in healthy subjects using a dexterous hand prosthesis to grasp a set of daily objects. The subjects wore AR glasses with an integrated stereo-camera pair, and triggered the system via a simple myoelectric interface.

MAIN RESULTS

The results demonstrated that the subjects got easily acquainted with the semi-autonomous control. The stereovision grasp decoder successfully estimated the grasp type and size in realistic, cluttered environments. When allowed (forced) to correct the automatic system decisions, the subjects successfully utilized the AR feedback and achieved close to ideal system performance.

SIGNIFICANCE

The new method implements a high level, low effort control of complex functions in addition to the low level closed-loop control. The latter is achieved by providing rich visual feedback, which is integrated into the real life environment. The proposed system is an effective interface applicable with small alterations for many advanced prosthetic and orthotic/therapeutic rehabilitation devices.

Quantitative model of transport-aperture coordination during reach-to-grasp movements

It has been found in our previous studies that the initiation of aperture closure during reach-to-grasp movements occurs when the hand distance to target crosses a threshold that is a function of peak aperture amplitude, hand velocity, and hand acceleration. Thus, a stable relationship between those four movement parameters is observed at the moment of aperture closure initiation. Based on the concept of optimal control of movements (Naslin 1969) and its application for reach-to-grasp movement regulation (Hoff and Arbib 1993), it was hypothesized that the mathematical equation expressing that relationship can be generalized to describe coordination between hand transport and finger aperture during the entire reach-to-grasp movement by adding aperture velocity and acceleration to the above four movement parameters. The present study examines whether this hypothesis is supported by the data obtained in experiments in which young adults performed reach-to-grasp movements in eight combinations of two reach-amplitude conditions and four movement-speed conditions. It was found that linear approximation of the mathematical model described the relationship among the six movement parameters for the entire aperture-closure phase with very high precision for each condition, thus supporting the hypothesis for that phase. Testing whether one mathematical model could approximate the data across all the experimental conditions revealed that it was possible to achieve the same high level of data-fitting precision only by including in the model two additional, condition-encoding parameters and using a nonlinear, artificial neural network-based approximator with two hidden layers comprising three and two neurons, respectively. This result indicates that transport-aperture coordination, as a specific relationship between the parameters of hand transport and finger aperture, significantly depends on the condition-encoding variables. The data from the aperture-opening phase also fit a linear model, whose coefficients were substantially different from those identified for the aperture-closure phase. This result supports the above hypothesis for the aperture-opening phase, and consequently, for the entire reach-to-grasp movement. However, the fitting precision was considerably lower than that for the aperture-closure phase, indicating significant trial-to-trial variability of transport-aperture coordination during the aperture-opening phase. Implications for understanding the neural mechanisms employed by the CNS for controlling reach-to-grasp movements and utilization of the mathematical model of transport-aperture coordination for data analysis are discussed.

Prehension is really reaching and grasping

Prehension has traditionally been seen as the act of coordinated reaching and grasping. However, recently, Smeets and Brenner (in Motor Control 3:237-271, 1999) proposed that we might just as well look at prehension as the combination of two independently moving digits. The hand aperture that has featured prominently in many studies on prehension, according to Smeets and Brenner's "double-pointing hypothesis", is really an emergent property related to the time course of the positions of the two digits moving to their respective end points. We tested this double-pointing hypothesis by perturbing the end position of one of the digits while leaving the end position of the opposing digit unchanged. To this end, we had participants reach for and grasp a metallic object of which the side surfaces could be made to slide in and out. We administered the perturbation right after movement initiation. On several occasions, after perturbing the end position of one digit, we found effects also on the kinematics of the opposing digit. These findings are in conflict with Smeets and Brenner's double-pointing hypothesis.

Effect of speed manipulation on the control of aperture closure during reach-to-grasp movements

This study investigates coordination between hand transport and grasp movement components by examining a hypothesis that the hand location, relative to the object, in which aperture closure is initiated remains relatively constant under a wide range of transport speed. Subjects made reach-to-grasp movements to a dowel under four speed conditions: slow, comfortable, fast but comfortable, and maximum (i.e., as fast as possible). The distance traveled by the wrist after aperture reached its maximum (aperture closure distance) increased with an increase of transport speed across the speed conditions. This finding rejected the hypothesis and suggests that the speed of hand transport is taken into account in aperture closure initiation. Within each speed condition, however, the closure distance exhibited relatively small variability across trials, even though the total distance traveled by the wrist during the entire transport movement varied from trial to trial. The observed stability in aperture closure distance across trials implies that the hand distance to the object plays an important role in the control law governing the initiation of aperture closure. Further analysis showed that the aperture closure distance depended on the amplitude of peak aperture as well as hand velocity and acceleration. To clarify the form of the above control law, we analyzed four different mathematical models, in which a decision to initiate grasp closure is made as soon as a specific movement parameter (wrist distance to target or transport time) crosses a threshold that is either a constant value or a function of the above-mentioned other movement-related parameters. Statistical analysis performed across all movement conditions revealed that the control law model (according to which grasp initiation is made when hand distance to target becomes less than a certain linear function of aperture amplitude, hand velocity, and hand acceleration) produced significantly smaller residual errors than the other three models. The findings support the notion that transport-grasp coordination and grasp initiation is based predominantly on spatial characteristics of the arm movement, rather than movement timing.

The developmental roots of the speed-accuracy trade-off

The speed of adult reaching movements is lawfully related to the distance of the reach and the size of the target. The authors had 7-, 9-, and 11-month-old infants reach for small and large targets to investigate a possible relation between the emergence of this speed-accuracy trade-off and the improvements in infants' ability to pick up tiny objects. By 7 months of age, infants slowed down their reaches for smaller objects. The authors concluded that it was not the ability to use a precision grip that facilitated the speed-accuracy trade-off but rather the other way around. The slowing down toward the end of the movement might set the conditions for the development of fine distal control of the hand.

Posture-based motion planning: applications to grasping

This article describes a model of motion planning instantiated for grasping. According to the model, one of the most important aspects of motion planning is establishing a constraint hierarchy--a set of prioritized requirements defining the task to be performed. For grasping, constraints include avoiding collisions with to-be-grasped objects and minimizing movement-related effort. These and other constraints are combined with instance retrieval (recall of stored postures) and instance generation (generation of new postures and movements to them) to simulate flexible prehension. Dynamic deadline setting is used to regulate termination of instance generation, and performance of more than one movement at a time with a single effector is used to permit obstacle avoidance. Old and new data are accounted for with the model.

A simple rule of thumb for elegant prehension

Reaching out to grasp an object (prehension) is a deceptively elegant and skilled behavior. The movement prior to object contact can be described as having two components, the movement of the hand to an appropriate location for gripping the object, the "transport" component, and the opening and closing of the aperture between the fingers as they prepare to grip the target, the "grasp" component. The grasp component is sensitive to the size of the object, so that a larger grasp aperture is formed for wider objects; the maximum grasp aperture (MGA) is a little wider than the width of the target object and occurs later in the movement for larger objects. We present a simple model that can account for the temporal relationship between the transport and grasp components. We report the results of an experiment providing empirical support for our "rule of thumb." The model provides a simple, but plausible, account of a neural control strategy that has been the center of debate over the last two decades.

Dissociation of covert and overt spatial attention during prehension movements: selective interference effects

In four experiments, the influence of distractor objects on the temporal evolution of the reach-to-grasp movement toward a target object (an apple) was examined. In the first experiment, the distractor was another apple, which moved laterally behind the target and occasionally changed direction toward the target, thus becoming the to-be-grasped object. In the second and third experiments, the distractor was a stationary piece of fruit, which sometimes became the to-be-grasped object because of a change in illumination. The fourth experiment was a combination of the first two experiments. In all cases, selective interference effects on the transport and manipulation components were observed only when attention to the distractor was covert rather than overt. It is proposed that covert visuospatial attention selects information about distracting but potentially important stimuli, such that a registration of significance is accomplished without the need to process all available information.

The Topology of Limb Deceleration in Prehension Tasks

Whether a subject uses a constant tau-dot strategy to decelerate a reach to grasp an object is typically determined by performing a linear regression of tau onto real time to contact. This method has the disadvantage of requiring an assumption (on the part of the experimenter) concerning the endpoint targeted by the subject. In the present contribution, an alternative, topological method is proposed that makes use of a phase-plane representation of the movement. In two experiments, experimentally obtained movement trajectories were evaluated, using this new method, and the results were found to support the constant tau-dot strategy. At the same time, however, an alternative model, associated with dynamical systems theory, was demonstrated to fit the data even better. It is argued that the topology of the phase plane suggests a softening spring description for a large part of the decelerative phase, followed by a final adjustment. Deterioration of the functionality of the adjustment made in this latter phase was observed when visual information was not available during movement execution.