Kerry M. B. Bennett

Reach to grasp: the response to a simultaneous perturbation of object position and size.

Abstract This study assessed the reach to grasp movement and its adaptive response to a simultaneous perturbation of object location and size. The aim was to clarify the means by which integration between the neural pathways modulating transport and manipulation is achieved. Participants (n = 11) were required to reach 30 cm to grasp a central illuminated cylinder of either small (0.7 cm) or large (8 cm) diameter. For a small percentage of trials (20/100) a visual perturbation was introduced unexpectedly at the onset of the reaching action. This consisted of a shift of illumination from the central cylinder to a cylinder of differing diameter (large in session A; small in session B) that was positioned 20° to the left (n = 10 trials) or to the right (n = 10) of the central cylinder. The subject was required to grasp the newly illuminated cylinder. Movement duration for these “double” (position and size) perturbed trials was much longer than those of control trials to the central cylinder (session A: by an average of 250 ms; session B: 180 ms), and the increased values were much greater than those reported previously in “single” perturbation studies where either size or location of the object was perturbed. Initial signs of a response to the “double” perturbation were seen almost simultaneously in the transport parameter of peak arm deceleration and in the manipulation parameter of maximum grip aperture, but these changes were not evident until more than 400 ms after movement onset, a response onset much later than that found in “single” perturbation studies. It is proposed that the visual change resultant from the double perturbation activates integration centres that at first gate the flow of information to the parallel channels of transport and manipulation. Following processing of this information, these centres act to instigate a synchronised and coordinated response in both components. These results add support to the existence of neural centres dedicated to the integration of parallel neural pathways, and which exercise flexibility in the degree to which these components are “coupled” functionally.

Interference from Distractors in Reach-to-grasp Movements

Descriptions of interference effects from non-relevant stimuli are extensive in visual target detection and identification paradigms. To explore the influence of features of non-relevant objects on reach-to-grasp movements, we instructed healthy normal controls to reach for and pick up a cylinder (target) placed midsagittally 30 cm from the starting position of the hand. In Experiment 1, the target was presented alone, or accompanied by a narrower, wider, or same-size distractor positioned to the left or right of the target. In Experiment 2, the target was presented alone or accompanied by a distractor, which was slanted at a different orientation to the target. Reflective markers were placed on the wrist, thumb, and index finger of the right hand, and infra-red light-detecting cameras recorded their displacement through a calibrated 3-dimensional working space. Kinematic parameters were derived and analysed. Consistent changes in the expression of peak velocity, acceleration, and deceleration were evident when the distractor was narrower or wider than the target. The impact of the orientation of the distractor, conversely, was not marked. We discuss the results in the context of physiological findings and models of selective attention.

The reach-to-grasp movement in Parkinson's disease: response to a simultaneous perturbation of object position and object size.

Abstract This study assessed the adaptive response of the reach-to-grasp movement of 12 Parkinson’s disease (PD) and 12 control subjects to a simultaneous perturbation of target object location and size. The main aim was to test further the reported dysfunction of PD subjects in the simultaneous activation of movement components. Participants were required to reach 30 cm to grasp a central illuminated cylinder of either small (0.7 cm) or large (8 cm) diameter. For a small percentage of trials (20/100) a visual perturbation was introduced unexpectedly at the onset of the reaching action. This consisted of a shift of illumination from the central cylinder to a cylinder of differing diameter, which was positioned 20° to the left (n=10) or to the right (n=10). The subject was required to grasp the newly illuminated cylinder. For the Parkinson’s disease subject group, the earliest response to this ’double’ perturbation was in the parameter of peak reaching acceleration, which was on average 50 ms earlier for ’double’ perturbed than for non-perturbed trials. The grasp component response followed more than 500 ms after the earliest transport response. For the control subjects initial signs of a response to the ’double’ perturbation were seen almost simultaneously in the transport parameter of peak arm deceleration, and in the manipulation parameter of maximum grip aperture, but these changes were not evident until more than 400 ms after movement onset. These results indicate that the basal ganglia can be identified as part of a circuit which is involved in the integration of parallel neutral pathways, and which exercise flexibility in the degree to which these components are ’coupled’ functionally. With basal ganglia dysfunction the activation of integration centres that at first gate the flow of information to the parallel channels of reach and grasp seems inefficient.

Human inferior parietal cortex 'programs' the action class of grasping

If one writes with a pen grasped between the toes, or a pencil held in the mouth, the handwriting style may be of poor quality but can be identified as belonging to a particular individual. Like other actions, such as grasping or pointing, different body parts can be used to produce the movement. These findings, of reasonably consistent spatial and temporal productions by different effectors, have been used to argue for the concept of motor equivalence and the existence of motor programs abstracted from particular effectors. In this study subjects were required to perform an action (grasping a sweet) with different effectors (the mouth or the hand) while the brain was scanned. Activation of the inferior parietal lobe during real and imagined mouth grasping, and during real hand grasping actions was demonstrated. Primate neurophysiological research has implicated this region in a movement-planning role. Our results confirm the importance of the inferior parietal lobe in integrating converging multimodal sensory information for coding of general action patterns in humans.

How perceived object dimension influences prehension

A kinematic study assessed the effects of the perceived dimensions of an object upon the patterning of a prehension movement involving that object. If an apple was perceived as two-dimensional, subjects utilized a large precision grip between the index finger and thumb. If the apple was perceived as three-dimensional, whole hand prehension involving all the digits was utilized. A visual perturbation from perceived two-dimensional to three-dimensional at movement onset resulted in a transition from the 2D precision grip pattern to the 3D whole hand prehension. These results suggest that visual mechanisms for interpreting the dimensions of an object directly influence motor selection pathways, and do not necessarily access a three-dimensional central nervous system representation of the object.

Upper limb movement differentiation according to taxonomic semantic category.

THE size or shape of an object, its perceptual features, determine the patterning of an arm and hand action involving that object. Little is known about the role played by the taxonomic semantic attributes of an object for perceptuomotor processing. In this study we investigated whether the semantic relationship between two target stimuli influences the kinematics of a bilateral reach-to-grasp action. The results showed that reach-to-grasp movements preceding the action of putting living-thing pairs together were faster, and showed earlier settings of reach and grasp temporal parameters, than movements involving pairs of non-living things. It is hypothesized that this reflects the recruitment of different categorical perceptuomotor pathways.

Prehension movements and perceived object depth structure

This study asked the question, “Will the motor pattern to a perceived two-dimensional (2-D) object differ from that same object when it is perceived as three dimensional (3-D)?” Subjects were required to reach and grasp an apple that could appear to be 2-D or 3-D. Two experimental sessions were conducted. In Condition A, the apple was initially perceived to be 2-D, but, for 20% of trials, it suddenly shifted to a 3-D apple at movement onset. In Condition B, the apple was initially perceived to be 3-D, but, for 20% of trials, it suddenly shifted to a 2-D silhouette of the same apple. For control trials, subjects grasped the perceived 2-D apple as if it were a disc (82%), and they grasped the 3-D apple, as they would a normal apple, with a whole-hand grasp (86%). For Condition A perturbed trials, there was a rapid change from a 2-D precision grip to a 3-D whole-hand prehension, whereas the converse was true for the opposite perturbation. Peak acceleration was anticipated for Condition A perturbed trials but not for Condition B perturbed trials. These results indicate that the motor patterns we use in interacting with an object are strongly influenced by the way we perceive the object in real time, and that object affordances, such as dimension, can override the influence exerted by existing representations.