Sara A. Winges

The role of vision on hand preshaping during reach to grasp

During reaching to grasp objects with different shapes hand posture is molded gradually to the objects contours. The present study examined the extent to which the temporal evolution of hand posture depends on continuous visual feedback. We asked subjects to reach and grasp objects with different shapes under five vision conditions (VCs). Subjects wore liquid crystal spectacles that occluded vision at four different latencies from onset of the reach. As a control, full-vision trials (VC5) were interspersed among the blocked vision trials. Object shapes and all VCs were presented to the subjects in random order. Hand posture was measured by 15 sensors embedded in a glove. Linear regression analysis, discriminant analysis, and information theory were used to assess the effect of removing vision on the temporal evolution of hand shape. We found that reach duration increased when vision was occluded early in the reach. This was caused primarily by a slower approach of the hand toward the object near the end of the reach. However, vision condition did not have a significant effect on the covariation patterns of joint rotations, indicating that the gradual evolution of hand posture occurs in a similar fashion regardless of vision. Discriminant analysis further supported this interpretation, as the extent to which hand posture resembled object shape and the rate at which hand posture discrimination occurred throughout the movement were similar across vision conditions. These results extend previous observations on memory-guided reaches by showing that continuous visual feedback of the hand and/or object is not necessary to allow the hand to gradually conform to object contours.

Common Input to Motor Units of Intrinsic and Extrinsic Hand Muscles During Two-Digit Object Hold

Anatomical and physiological evidence suggests that common input to motor neurons of hand muscles is an important neural mechanism for hand control. To gain insight into the synaptic input underlying the coordination of hand muscles, significant effort has been devoted to describing the distribution of common input across motor units of extrinsic muscles. Much less is known, however, about the distribution of common input to motor units belonging to different intrinsic muscles and to intrinsic-extrinsic muscle pairs. To address this void in the literature, we quantified the incidence and strength of near-simultaneous discharges of motor units residing in either the same or different intrinsic hand muscles (m. first dorsal, FDI, and m. first palmar interosseus, FPI) during two-digit object hold. To extend the characterization of common input to pairs of extrinsic muscles (previous work) and pairs of intrinsic muscles (present work), we also recorded electromyographic (EMG) activity from an extrinsic thumb muscle (m. flexor pollicis longus, FPL). Motor-unit synchrony across FDI and FPI was weak (common input strength, CIS, mean +/- SE: 0.17 +/- 0.02). Similarly, motor units from extrinsic-intrinsic muscle pairs were characterized by weak synchrony (FPL-FDI: 0.25 +/- 0.02; FPL-FPI: 0.29 +/- 0.03) although stronger than FDI-FPI. Last, CIS from within FDI and FPI was more than three times stronger (0.70 +/- 0.06 and 0.66 +/- 0.06, respectively) than across these muscles. We discuss present and previous findings within the framework of muscle-pair specific distribution of common input to hand muscles based on their functional role in grasping.

Effects of Object Compliance on Three-Digit Grasping

Compared with rigid objects, grasping and lifting compliant objects presents additional uncertainties. For any static grasp, forces at the fingertips depend on factors including the locations of the contact points and the contact forces must be coordinated to maintain equilibrium. For compliant objects, the locations and orientations of the contact surfaces change in a force-dependent manner, thus changing the force requirements. Furthermore, every force adjustment then results in additional changes in object shape. This study characterized force and muscle activation patterns in this situation. Fingertip forces were measured as subjects grasped and lifted a 200-g object using their thumb, index, and ring fingers. A spring was sometimes placed under the index and/or ring finger contact surface. Surface electromyographic activity was recorded from ten hand muscles and one proximal arm muscle. The patterns of grip (normal) force and muscle activity were similar across conditions during the load and lift phases, but their amplitude depended on whether the contact surface was compliant. Specifically, the grip force increased smoothly during the load phase of the task under all conditions. To the contrary, the tangential contact (load) force did not increase monotonically when one or more of the contact surfaces were compliant, resulting in a decoupling of the grip and load forces.

Patterns of muscle activity for digital coarticulation

Although piano playing is a highly skilled task, basic features of motor pattern generation may be shared across tasks involving fine movements, such as handling coins, fingering food, or using a touch screen. The scripted and sequential nature of piano playing offered the opportunity to quantify the neuromuscular basis of coarticulation, i.e., the manner in which the muscle activation for one sequential element is altered to facilitate production of the preceding and subsequent elements. Ten pianists were asked to play selected pieces with the right hand at a uniform tempo. Key-press times were recorded along with the electromyographic (EMG) activity from seven channels: thumb flexor and abductor muscles, a flexor for each finger, and the four-finger extensor muscle. For the thumb and index finger, principal components of EMG waveforms revealed highly consistent variations in the shape of the flexor bursts, depending on the type of sequence in which a particular central key press was embedded. For all digits, the duration of the central EMG burst scaled, along with slight variations across subjects in the duration of the interkeystroke intervals. Even within a narrow time frame (about 100 ms) centered on the central EMG burst, the exact balance of EMG amplitudes across multiple muscles depended on the nature of the preceding and subsequent key presses. This fails to support the idea of fixed burst patterns executed in sequential phases and instead provides evidence for neuromuscular coarticulation throughout the time course of a hand movement sequence.

Neural Control of Hand Muscles During Prehension

In the past two decades a large number of studies have successfully characterized important features of the kinetics and kinematics of object grasping and manipulation, providing significant insight into how the Central Nervous System (CNS) controls the hand, one of the most complex motor systems, in a variety of behaviors. In this chapter we briefly review studies of hand kinematics and kinetics and highlight their major findings and open questions. The major focus of this chapter is on the neural control of the hand, an objective that has been pursued by studies on electromyography (EMG) of hand muscles. Here we review what has been learned through different yet complementary methodological approaches. In particular, the study of single motor unit activity has revealed how the distribution of common neural input within and across hand muscles might reflect a muscle-pair specific organization. Studies of motor unit population have revealed important synergistic patterns of muscle activity while also revealing muscle-pair specific patterns of neural coupling. We conclude the chapter with the results of recent simulation studies aiming at combining advantages of single and multi-unit recordings to maximize the amount of information that can be extracted from EMG signal analysis.

Multi-Digit Control of Contact Forces During Rotation of a Hand-Held Object

Rotation of an object held with three fingers is produced by modulation of force amplitude and direction at one or more contact points. Changes in the moment arm through which these forces act can also contribute to the modulation of the rotational moment. Therefore force amplitude and direction as well as the center of pressure on each contact surface must be carefully coordinated to produce a rotation. Because there is not a single solution, this study sought to describe consistent strategies for simple position-to-position rotations in the pitch, roll, and yaw axes. Force amplitude and direction, and center of pressure on the contact surfaces (and thus the moment arm), were measured as human subjects rotated a 420 g force-transducer instrumented object, grasped with the thumb, index and ring fingers (average movement time: 500 ms). Electromyographic (EMG) activity was recorded from five intrinsic and three extrinsic hand muscles and two wrist muscles. Principal components analysis of force and EMG revealed just two main temporal patterns: the main one followed rotational position and the secondary one had a time course that resembled that of rotational velocity. Although the task could have been accomplished by dynamic modulation of the activity of wrist muscles alone, these two main dynamic EMG patterns were seen in intrinsic hand muscles as well. In contrast to previous reports of shifting in time of the phasic activity bursts of various muscles, in this task, all EMG records were well described by just two temporal patterns, resembling the position and velocity traces.

From Single Motor Unit Activity to Multiple Grip Forces: Mini-review of Multi-digit Grasping

Abstract This paper is a mini review of kinetic and kinematic evidence on the control of the hand with emphasis on grasping. It is not meant to be an exhaustive review, rather it summarizes current research examining the mechanisms through which specific patterns of coordination are elicited and observed during reach to grasp movements and static grasping. These coordination patterns include the spatial and temporal covariation of the rotation at multiple joints during reach to grasp movements. A basic coordination between grip forces produced by multiple digits also occurs during whole hand grasping such that normal forces tend to be produced in a synchronous fashion across pairs of digits. Finally, we address current research that suggests that motor unit synchrony across hand muscles and muscle compartments might be one of the neural mechanisms underlying the control of grasping.

Distinct digit kinematics by professional and amateur pianists

Many everyday tasks such as typing, grasping, and object manipulation require coordination of dynamic movement across multiple joints and digits. Playing a musical instrument is also one such task where the precise movement of multiple digits is transformed into specific sounds defined by the instrument. Through extensive practice musicians are able to produce precisely controlled movements to interact with the instrument and produce specific sequences of sounds. The present study aimed to determine what aspects of these dynamic movement patterns differ between pianists who have achieved professional status compared to amateur pianists that have also trained extensively. Common patterns of movement for each digit strike were observed for both professional and amateur pianists that were sequence specific, i.e. influenced by the digit performing the preceding strike. However, group differences were found in multi-digit movement patterns for sequences involving the ring or little finger. In some sequences, amateur subjects tended to work against the innate connectivity between digits while professionals allowed slight movement at non-striking digits (covariation) which was a more economical strategy. In other sequences professionals used more individuated finger movements for performance. Thus the present study provided evidence in favor of enhancement of both movement covariation and individuation across fingers in more skilled musicians, depending on fingering and movement sequence.

Spatial and temporal aspects of cognitive influences on smooth pursuit

It is well known that prediction is used to overcome processing delays within the motor system and ocular control is no exception. Motion extrapolation is one mechanism that can be used to overcome the visual processing delay. Expectations based on previous experience or cognitive cues are also capable of overcoming this delay. The present experiment was designed to examine how smooth pursuit is altered by cognitive information about the time and/or direction of an upcoming change in target direction. Subjects visually tracked a cursor as it moved at a constant velocity on a computer screen. The target initially moved from left to right and then abruptly reversed horizontal direction and traveled along one of seven possible oblique paths. In half of the trials, a cue was present throughout the trial to signal the position (as well as the time), and/or the direction of the upcoming change. Whenever a position cue (which will be referred to as a timing cue throughout the paper) was present, there were clear anticipatory adjustments to the horizontal velocity component of smooth pursuit. In the presence of a timing cue, a directional cue also led to anticipatory adjustments in the vertical velocity, and hence the direction of smooth pursuit. However, without the timing cue, a directional cue alone produced no anticipation. Thus, in this task, a cognitive spatial cue about the new direction could not be used unless it was made explicit in the time domain.

Does temporal asynchrony affect multimodal curvature detection

Multiple sensory modalities gather information about our surroundings to plan appropriate movements based on the properties of the environment and the objects within it. This study was designed to examine the sensitivity of visual and haptic information alone and together for detecting curvature. When both visual and haptic information were present, temporal delays in signal onset were used to determine the effect of asynchronous sensory information on the interference of vision on the haptic estimate of curvature. Even under the largest temporal delays where visual and haptic information were clearly disparate, the presentation of visual information influenced the haptic perception of curvature. The uncertainty associated with the unimodal vision condition was smaller than that in the unimodal haptic condition, regardless of whether the haptic information was procured actively or under robot assistance for curvature detection. When both visual and haptic information were available, the uncertainty was not reduced; it was equal to that of the unimodal haptic condition. The weighting of the visual and haptic information was highly variable across subjects with some subjects making judgments based largely on haptic information, while others tended to rely on visual information equally or to a larger extent than the haptic information.