Effects of end-goal on hand shaping

The aim of the present study was to determine whether hand shaping was affected by planning of an action subsequent to object contact. Ten subjects (5 females and 5 males, ages 19-33) were requested to reach toward and grasp a convex object between the thumb and the four fingers of the right hand and to perform one of the following actions: 1) lift up the object; 2) insert the object into a niche of a similar shape and size as the object, or 3) insert the object into a rectangular niche much larger than the object. Flexion/extension at the metacarpal-phalangeal and proximal interphalangeal joints of all digits were measured using resistive sensors embedded in a glove. Although all experimental conditions required grasping the same object, we found different covariation patterns among finger joint angles across conditions. Gradual preshaping of the hand occurred only when planning object lift or when the end-goal required object placement into the tight niche. In contrast, for the larger niche, gradual preshaping was not evident for the ring and the little finger. Further, reaching movements were faster for movements ending with the larger niche than for the other movement conditions. The present results suggest that hand shaping takes into account end-goal in addition to object geometry. We discuss these findings in the context of forward internal models that allow the prediction of the sensorimotor consequences of motor commands in advance to their execution.     

Higher-order action planning for individual and joint object manipulations

Many actions involve multiple action steps, which raises the question how far ahead people plan when they perform such actions. Here, we examined higher-order planning for action sequences and whether people planned similarly or differently when acting individually or together with an action partner. For individual performances, participants picked up an object with one hand and passed it to their other hand before placing it onto a target location. For joint performances, they picked up the object and handed it to their action partner, who placed it onto the target location. Each object could be grasped at only two possible grasping positions, implying that the first selected grasp on the object determined the postures for the rest of the action sequence. By varying the height of the target shelf, we tested whether people planned ahead and modulated their grasp choices to avoid uncomfortable end postures. Our results indicated that participants engaged in higher-order planning, but needed task experience before demonstrating such planning during both individual and joint performances. The rate of learning was similar in the two conditions, and participants transferred experience from individual to joint performance. Our results indicate similarity in mechanisms underlying individual and joint action sequence planning.     

Multifocal intraparietal activation during discrimination of action intention in observed tool grasping

The way we grasp an object varies depending on how we want to use that object, and this knowledge can be used to predict the object-related behavior of others. In this study, we assessed the neural correlates that determine the action intention of another person based on observed prehensile movements. Fourteen right-handed volunteers watched video clips of a person performing right-handed transitive grasping gestures that were either aimed at displacing or using a tool-object. Clips showing the grasping and displacement of neutral shapes served as a control condition. By discrimination of the actor's intention, three roughly symmetrical foci were activated in the anterior, middle, and caudal segments of the intraparietal sulci, and in the fusiform gyri and parts of the lateral occipital complex. Anterior intraparietal activation has been associated with the representation of object goals (object specific), and the present findings extend its involvement to functional goals (use-specific). Activation in the middle intraparietal region during intention discrimination was very similar to the activation elicited in a saccadic localizer task, suggesting a relation with spatial attention and eye movements. The caudal intraparietal region has been related with visuospatial guidance of reaching, and its activation during action intention discrimination indicates that the visuospatial properties of the observed reaching movement contribute to understanding of actions. As these parietal regions are strongly linked with motor behavior, our results appear to support the motor simulation hypothesis for action understanding with the preferential recruitment of the mirror-neuron system. This could at least be the case when no contextual information other than the visual properties of the movement is provided to discriminate the intention of an observed hand action.     

What does the brain do when you fake it? An FMRI study of pantomimed and real grasping

Given that studying neural bases of actions is very challenging with fMRI, numerous experiments have used pantomimed actions as a proxy to studying neural circuits of real actions. However, the underlying assumption that the same neural mechanisms mediate real and pantomimed actions has never been directly tested. Moreover, the assumption is called into question by neuropsychological evidence suggesting that real actions depend on the dorsal stream of visual processing whereas pretend actions also recruit the ventral stream. Here, we directly tested these ideas in neurologically intact subjects. Ten right-handed participants performed four tasks: 1) grasping real three-dimensional objects, 2) reaching toward the objects and touching them with the knuckle without hand preshaping, 3) pantomimed grasping in an adjacent location where no object was present, and 4) pantomimed reaching toward an adjacent location. As expected, in the anterior intraparietal area, there was significantly higher activation during real grasping than that during real reaching. However, the activation difference between pantomimed grasping and pantomimed reaching did not reach statistical significance. There was also no effect of pantomimed grasping within the ventral stream, including an object-selective area in the lateral occipital cortex. Instead, we found that pantomimed grasping was mediated by right-hemisphere activation, particularly the right parietal cortex. These results suggest that areas typically invoked by real actions may not necessarily be driven by "fake" actions. Moreover, pantomimed grasping may not tap object-related areas within the ventral stream, but rather may rely on mechanisms within the right hemisphere that are recruited by artificial and less practiced actions.     

Activity of human motor system during action observation is modulated by object presence

Neurons in the monkey mirror neuron system (MNS) become active when actions are observed or executed. Increases in activity are greater when objects are engaged than when the actions are mimed. This modulation occurs even when object manipulation is hidden from view. We examined whether human motor systems are similarly modulated during action observation because such observation-related modulations are potentially mediated by a putative human MNS. Transcranial magnetic stimulation (TMS) was used to elicit motor-evoked potentials (MEPs) of a grasping muscle while participants observed actual or pantomimed grasping movements whose endpoints were sometimes hidden from view. MEP amplitudes were found to be modulated by object presence. Critically, the object-based modulation was found when the participant directly observed object manipulation and when the object manipulation had to be inferred because it was hidden. These findings parallel studies of MNS activity in monkeys and support the hypothesis that the MNS influences motor system activity during action observation. Although the object-based modulation of MEP amplitudes was consistent with the hypotheses, the direction of the modulation was not—MEP amplitudes decreased during action observation in contrast to the increase that has previously been observed. We suggest that the decrease in MEP amplitude on object-present trials resulted from inhibitory mechanisms that were activated to suppress the observation-evoked response codes from generating overt muscle activity.     

Gaze behavior when reaching to remembered targets

People naturally direct their gaze to visible hand movement goals. Doing so improves reach accuracy through use of signals related to gaze position and visual feedback of the hand. Here, we studied where people naturally look when acting on remembered target locations. Four targets were presented on a screen, in peripheral vision, while participants fixed a central cross (encoding phase). Four seconds later, participants used a pen to mark the remembered locations while free to look wherever they wished (recall phase). Visual references, including the screen and the cross, were present throughout. During recall, participants neither looked at the marked locations nor prevented eye movements. Instead, gaze behavior was erratic and was comprised of gaze shifts loosely coupled in time and space with hand movements. To examine whether eye and hand movements during encoding affected gaze behavior during recall, in additional encoding conditions, participants marked the visible targets with either free gaze or with central cross fixation or just looked at the targets. All encoding conditions yielded similar erratic gaze behavior during recall. Furthermore, encoding mode did not influence recall performance, suggesting that participants, during recall, did not exploit sensorimotor memories related to hand and gaze movements during encoding. Finally, we recorded a similar lose coupling between hand and eye movements during an object manipulation task performed in darkness after participants had viewed the task environment. We conclude that acting on remembered versus visible targets can engage fundamentally different control strategies, with gaze largely decoupled from movement goals during memory-guided actions.     

Discharge of primate magnocellular red nucleus neurons during reaching to grasp in different spatial locations

Reaching to grasp is of fundamental importance to primate motor behavior. One descending motor pathway that contributes to the control of this behavior is the rubrospinal tract. An important source of origin of the rubrospinal tract is the magnocellular red nucleus (RNm). Forelimb RNm neurons discharge vigorously during reach-to-grasp movements. RNm discharge is important for hand use, as coordinated whole-limb movements without hand use are not associated with strong discharge. Because RNm is functionally linked to muscles of the entire forelimb, RNm discharge may also contribute to use of the proximal limb that accompanies hand use. If RNm contributes to proximal limb use, we predict discharge to differ for reaches that differ in proximal limb involvement but require the same grasp. We tested this prediction by measuring discharge of individual RNm neurons while monkeys reached to grasp objects in four spatial locations in front of them. The animals reached from the waist to locations to the left, right, above, and below the shoulder of the "reaching" limb. RNm neurons of our sample were activated strongly during reach-to-grasp, and discharge of a third of the neurons tested depended on the spatial location of the object grasped. Discharge of RNm neurons and EMG activity of many of the distal and proximal forelimb muscles we tested were larger for reaching to grasp in the upper and/or right than lower and left target locations. Based on comparisons of each individual neuron's discharge patterns during reaches with and without preshaping the hand, we conclude that target location-dependent modulations in discharge rate of the majority of RNm neurons whose discharge differed for reaching to grasp in the four target locations contributed to aspects of hand preshaping that covaried with reach direction.     

Planning an action

 The motor control of a sequence of two motor acts forming an action was studied in the present experiment. The two analysed motor acts were reaching-grasping an object (first target) and placing it on a second target of the same shape and size (experiment 1). The aim was to determine whether extrinsic properties of the second target (i.e. target distance) could selectively influence the kinematics of reaching and grasping. Distance, position and size of both targets were randomly varied across the experimental session. The kinematics of the initial phase of the first motor act, that is, velocity of reaching and hand shaping of grasping, were influenced by distance of the second target. No kinematic difference was found between movements executed with and without visual control of both hand and targets. These results could be due to computation of the general program of an action that takes into account extrinsic properties of the final target. Conversely, they could depend on a visual interference effect produced by the near second target on the control of the first motor act. In order to dissociate the effects due to second target distance from those due to visual interference, two control experiments were carried out. In the first control experiment (experiment 2) subjects executed movements directed towards spatial locations at different distances from the first target, as in experiment 1. However, the near second target was not presented and subjects were required to place the object on an arbitrary near position. Distance of the second (either real or arbitrary) target affected the reaching component of the first motor act, as in experiment 1, but not the grasp component. In the second control experiment (experiment 3), the pure visual interference effect was tested. Subjects were required to reach and grasp the object and to lift it in either presence or absence of a second near stimulus. No effect on the initial phase of the first motor act was observed. The results of the this study suggest a dissociation in the control of reaching and grasping, concerning not only visual analysis of extrinsic properties of the immediate target but also visual analysis of the final target of the action. In other words, the notion of modularity for the motor control can be extended to the construction of an entire action. Received: 26 July 1996 / Accepted: 19 December 1996     

On the other hand: overflow movements of infants' hands and legs during unimanual object exploration

Motor overflow is extraneous movement in a limb not involved in a motor action. Typically, overflow is observed in people with neurological impairments and in healthy children and adults during strenuous and attention-demanding tasks. In the current study, we found that young infants produce vast amounts of motor overflow, corroborating claims of symmetry being the default state of the motor system. While manipulating an object with one hand, all 27 of the typically developing 4.5- to 7.5-month-old infants who we observed displayed overflow movements of the free hand (on 4/5 of unimanual actions). Mirror-image movements of the hands occurred on 1/8 of unimanual actions, and the hands and legs moved in synchrony on 1/3 of unimanual acts. Motor overflow was less frequent when infants were in a sitting posture and when infants watched their acting hand, suggesting that upright posture and visual examination may help to alleviate overflow and break obligatory symmetry in healthy infants.     

Trajectory of human movement during sit to stand: a new modeling approach based on movement decomposition and multi-phase cost function

The ability to predict accurately the weights of objects is essential for skilled and dexterous manipulation. A potentially important source of information about object weight is through the observation of other people lifting objects. Here, we tested the hypothesis that when watching an actor lift an object, people naturally learn the object’s weight and use this information to scale forces when they subsequently lift the object themselves. Participants repeatedly lifted an object in turn with an actor. Object weight unpredictably changed between 2 and 7 N every 5th to 9th of the actor’s lifts, and the weight lifted by the participant always matched that previously lifted by the actor. Even though the participants were uninformed about the structure of the experiment, they appropriately adapted their lifting force in the first trial after a weight change. Thus, participants updated their internal representation about the object’s weight, for use in action, when watching a single lift performed by the actor. This ability presumably involves the comparison of predicted and actual sensory information related to actor’s actions, a comparison process that is also fundamental in action.     

Kinematics fingerprints of leader and follower role-taking during cooperative joint actions

Performing online complementary motor adjustments is quintessential to joint actions since it allows interacting people to coordinate efficiently and achieve a common goal. We sought to determine whether, during dyadic interactions, signaling strategies and simulative processes are differentially implemented on the basis of the interactional role played by each partner. To this aim, we recorded the kinematics of the right hand of pairs of individuals who were asked to grasp as synchronously as possible a bottle-shaped object according to an imitative or complementary action schedule. Task requirements implied an asymmetric role assignment so that participants performed the task acting either as (1) Leader (i.e., receiving auditory information regarding the goal of the task with indications about where to grasp the object) or (2) Follower (i.e., receiving instructions to coordinate their movements with their partner’s by performing imitative or complementary actions). Results showed that, when acting as Leader, participants used signaling strategies to enhance the predictability of their movements. In particular, they selectively emphasized kinematic parameters and reduced movement variability to provide the partner with implicit cues regarding the action to be jointly performed. Thus, Leaders make their movements more “communicative” even when not explicitly instructed to do so. Moreover, only when acting in the role of Follower did participants tend to imitate the Leader, even in complementary actions where imitation is detrimental to joint performance. Our results show that mimicking and signaling are implemented in joint actions according to the interactional role of the agent, which in turn is reflected in the kinematics of each partner.     

Functional magnetic resonance adaptation reveals the involvement of the dorsomedial stream in hand orientation for grasping

Reach-to-grasp actions require coordination of different segments of the upper limbs. Previous studies have examined the neural substrates of arm transport and hand grip components of such actions; however, a third component has been largely neglected: the orientation of the wrist and hand appropriately for the object. Here we used functional magnetic resonance imaging adaptation (fMRA) to investigate human brain areas involved in processing hand orientation during grasping movements. Participants used the dominant right hand to grasp a rod with the four fingers opposing the thumb or to reach and touch the rod with the knuckles without visual feedback. In a control condition, participants passively viewed the rod. Trials in a slow event-related design consisted of two sequential stimuli in which the rod orientation changed (requiring a change in wrist posture while grasping but not reaching or looking) or remained the same. We found reduced activation, that is, adaptation, in superior parieto-occipital cortex (SPOC) when the object was repeatedly grasped with the same orientation. In contrast, there was no adaptation when reaching or looking at an object in the same orientation, suggesting that hand orientation, rather than object orientation, was the critical factor. These results agree with recent neurophysiological research showing that a parieto-occipital area of macaque (V6A) is modulated by hand orientation during reach-to-grasp movements. We suggest that the human dorsomedial stream, like that in the macaque, plays a key role in processing hand orientation in reach-to-grasp movements.     

An investigation into manual asymmetries in grasp behavior and kinematics during an object manipulation task

Manual asymmetries in the control of movements have been investigated in a variety of experimental paradigms. Initial studies demonstrated that the dominant right hand has advantages over the non-dominant left hand in many aspects of motor control. However, more recent studies have shown that the presence and extent of these asymmetries depends on the task context and accuracy demands. Typically, manual asymmetries on a motor planning and motor execution level are examined separately. However, given that recent research has demonstrated that specific task constraints do not influence both levels equally, the purpose of the present experiment was to investigate manual asymmetries in motor planning and execution. To this end, initial grasp behavior (motor planning) and kinematics (motor execution) were examined in thirteen right-handed participants during a unimanual grasping and placing task. We specifically manipulated grasping hand, target location, object end orientation, and object grasp time at the start location. There were three main findings. First, motor planning or movement execution was similar regardless of grasping hand. Second, prospectively planned actions were influenced by target location and the required end orientation of the object. Third, the amount of time spent in an initial posture did not influence initial grasp postures. However, it did alter the movement kinematics during the grasping (approach phase) and placing (transport phase) portion of the task. We posit that grasping and placing movements are comprised of an initial grasp and a transport component, which are differentially influenced by task constraints.     

Knowing your nose better than your thumb: measures of over-grasp reveal that face-parts are special for grasping

Typically, when a grasping response is made, the hand opens wider than the target object. We show that this over-grasp response is reduced when we reach to parts of our own face, relative to when we reach to other body parts or to neutral objects. This is not due to reaching to different parts of body space, as over-grasp responses are indifferent to whether or not other body parts or neutral objects are placed close to the face. It is also not due to differences in perceptual knowledge of the size of the target object. We conclude instead that the familiarity of face parts influences the grasping response directly. Subsequent experiments demonstrate that the movement representation determining any effect is not based on a torso-centred frame, and not abstracted from the specific hand used for grasping. We discuss the implications of the results for understanding and measuring motor representations for familiar actions.     

Semantics affect the planning but not control of grasping

The semantic meaning of a word label printed on an object can have significant effects on the kinematics of reaching and grasping movements directed towards that object. Here, we examined how the semantics of word labels might differentially affect the planning and control stages of grasping. Subjects were presented with objects on which were printed either the word "LARGE" or "SMALL." When the grip aperture in the two conditions was compared, an effect of the words was found early in the reach, but this effect declined continuously as the hand approached the target. This continuously decreasing effect is consistent with a planning/control model of action, in which cognitive and perceptual variables affect how actions are planned but not how they are monitored and controlled on-line. The functional and neurological bases of semantic effects on planning and control are discussed.     

Attention modulates motor system activation during action observation: evidence for inhibitory rebound

Perceiving another individual’s actions activates the human motor system. We investigated whether this effect is stronger when the observed action is relevant to the observer’s task. The mu rhythm (oscillatory activity in the 8- to 13-Hz band over sensorimotor cortex) was measured while participants watched videos of grasping movements. In one of two conditions, the participants had to later report how many times they had seen a certain kind of grasp. In the other condition, they viewed the identical videos but had to later report how many times they had seen a certain colour change. The colour change and the grasp always occurred simultaneously. Results show mu rhythm attenuation when watching the videos relative to baseline. This attenuation was stronger when participants later reported the grasp rather than the colour, suggesting that the motor system is more strongly activated when the observed grasping actions were relevant to the observer’s task. Moreover, when the graspable object disappeared after the offset of the video, there was subsequent mu rhythm enhancement, reflecting a post-stimulus inhibitory rebound. This enhancement was again stronger when making judgments about the grasp than the colour, suggesting that the stronger activation is followed by a stronger inhibitory rebound.     

The effects of delay on the kinematics of grasping

 We examined the effect on manual prehension of introducing a 5-s delay between viewing a target object and initiating a grasping movement. Subjects were tested in four conditions: three involved grasping the object and a fourth involved estimating its size. In the main experimental condition (Open Loop Delay), subjects viewed a target object for 300 ms, but did not initiate a grasping movement until an auditory signal was presented 5 s later. In this condition, subjects had to rely on stored visual information for guiding their grasp after the delay. In another condition (Open Loop), subjects initiated their grasping movement as soon as the target appeared. In both of these open-loop conditions, subjects reached out and grasped the object without seeing their hand. In the third grasping condition (Closed Loop), the target object and the hand were visible throughout the reach. In the three grasping conditions, subjects were instructed to pick up the object across its width using their index finger and thumb. In a final condition (Perceptual Estimation), subjects gave a manual estimate of the object’s width with their index finger and thumb after viewing the object for 300 ms. In all four conditions, subjects were presented with a target object in which the height, length and width were independently varied from trial to trial. The results of the experiment indicated that reaching and grasping movements made in the Open-Loop and Closed-Loop conditions did not differ in any kinematic measures. In contrast, when subjects performed in the Open-Loop Delay condition, their reaches took significantly longer and achieved peak velocity proportionately earlier. As well, their maximum grip aperture was significantly larger. In addition, reaching movements in all three grasping conditions were affected by both the object’s width (the ’relevant’ dimension) and height. The manual estimates in the Perceptual-Estimation condition, however, reflected only the object’s width. These results, together with evidence from other studies, suggest that motor actions performed after a delay use different transformations than those used for ’real-time’ grasping. We argue that the stored visual information used to drive delayed actions arises from a perceptual rather than a visuomotor analysis of the target object. Received: 10 July 1998 / Accepted: 4 December 1998     

Planning movements well in advance

It has been suggested that the metrics of grasping movements directed to visible objects are controlled in real time and are therefore unaffected by previous experience. We tested whether the properties of a visually presented distractor object influence the kinematics of a subsequent grasping movement performed under full vision. After viewing an elliptical distractor object in one of two different orientations participants grasped a target object, which was either the same object with the same orientation or a circular object without obvious orientation. When grasping the circular target, grip orientation was influenced by the orientation of the distractor. Moreover, as in classical visuomotor priming, grasping movements were initiated faster when distractor and target were identical. Results provide evidence that planning of visually guided grasping movements is influenced by prior perceptual experience, challenging the notion that metric aspects of grasping are controlled exclusively on the basis of real-time information.     

Mirror neurons and intention understanding: Dissociating the contribution of object type and intention to mirror responses using electromyography

Since their discovery in the monkey and human brain, mirror neurons have been claimed to play a key role in understanding others' intentions. For example, “action‐constrained” mirror neurons in inferior parietal lobule fire when the monkey observes a grasping movement that is followed by an eating action, but not when it is followed by a placing action. It is claimed these responses enable the monkey to predict the intentions of the actor. These findings have been replicated in human observers by recording electromyography responses of the mouth‐opening mylohyoid muscle during action observation. Mylohyoid muscle activity was greater during the observation of actions performed with the intention to eat than of actions performed with the intention to place, again suggesting an ability to predict the actor's intentions. However, in previous studies, intention was confounded with object type (food for eating actions, nonfood for placing actions). We therefore used electromyography to measure mylohyoid activity in participants observing eating and placing actions. Unlike previous studies, we used a design in which each object (food, nonfood) could be both eaten and placed, and thus participants could not predict the actor's intention at the onset of the action. Greater mylohyoid activity was found for the observation of actions performed on food objects, irrespective of intention, indicating that the object type, not the actor's intention, drives the mirror response. This result suggests that observers' motor responses during action observation reflect the presence of a particular object, rather than the actor's underlying intentions.