Grasping is a fascinating action of the hand just as the hand is an organized structure with exquisite precision and flexibility. The hand has many functions whereby we reach for objects, grasp and lift them or manipulate or use them to act on other things (Castiello, 2005, p. 726). Behavioral consequences from the anatomical variations of the hand due to adaptation have been studied. Modern studies focus on the relationship between brain function and the hand.
History tells us that the researcher Napier did some good work on “precision and power grips” (Napier, 1980). The biomechanical and neurophysical constraints were explained using his model. The rationality by which he explained each of the movements of the hand with regard to variations like aspects of movement such as force, posture, duration and, speed was amazing. The intended activity decided what type of grip was necessary. The techniques in behavioral neuroscience, neuroimaging, and electrophysiology help to reveal where exactly in the brain these processes have started. The sensorimotor transformation relates to the older views.
Anthropologists have agreed that the evolution of Primates is closely related to a life in the trees (Lemelin, 2007, p.329). The forward-facing eyes and grasping extremities have been interpreted as adaptations for foraging for insects on branches and leaves. They have prehensile hands, long limbs, mobile joints, the diagonal sequence in quadruped walking, protracted arm limb position, and forelimb compliance.
The grasping varies in humans and macaques depending on the object grasped (Castiello, 2005, p. 726). Grasping has been described with reference to the grip aperture which happens to be the separation between the thumb and index finger and the size of the maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped (Jeannerod, 1984 as cited in Safstrom, 2008, p. 265). Discrepancies between the visual and haptic information are adjusted by adapting the MGA (Weigelt and Bock, 2007). The aperture becomes larger when the haptic information is larger than the visual appearance. This automatic process is believed to be independent of whether the subject is aware of the difference in size (Sa¨fstro¨m and Edin, 2004). Contact forces are built up after about 30msec of fingertip contact with the object due to momentum exchange (Biegstraaten et al. 2005). Cutaneous afferents encode the amplitude and direction of force used (Johansson and Birznieks, 2004). Information is conveyed by these afferents with very high acuity. The receptors in joints and muscles also allow contact information to be passed. Contact between the fingertips and objects is predicted dynamically by the CNS and timing prediction errors (Sa¨fstro¨m, 2008, p. 265).
The components of the grasp
Many sequential processes are involved in the ability to grasp objects. Spatial localization of the object, movement of the hand towards the object and the enclosing of the fingers around the o ect are the actions involved (Sa¨fstro¨m, 2008, p. 276). What triggers the mechanism is still unclear. It could be by sensory feedback or feedforward prediction of what could happen. These mechanisms are separately controlled and independent of each other. The reaching-for-grasping movement aims to position the fingers on the object at appropriate places and the hand is opened and closed so that the fingers move towards the contact surface through orthogonal fashion (Sa¨fstro¨m, 2008, p. 276). The central nervous system controls the timing of contact. The movement of the thumb into the grasping position and the thumb-finger opposition should not be underestimated in the action of grasping (Arimoto, 2008, p. 156). In opposition, the pulp surface of the thumb is placed squarely in contact with another digit.
Another study has described the reaching-out-to-grasp action as having two motor components, hand transport and grip formation (Grosskopf, 2005, p. 230). The reaching or the transport component involves the contractions of the proximal muscles acting on the shoulder and elbow joints. Pre-shaping-of-the-fingers configuration adapts to the size and shape of the
object which the hand is approaching. The dominant hand is the right in 90% of people and is involved in precision tasks like writing and dextrous manipulation (Grosskopf, 2005, p. 230).
The nondominant hand has a stabilizing function. Interlimb differences have been noted (Sainburg 2002). Other studies have said that no significant differences in grip aperture were noted between the dominant and nondominant hands. One difference noted was that pre-shaping was faster in the dominant hand (Grosskpof, 2005, p. 238).
Neuroscience
A whole cortical pathway is dedicated to the vision-based functioning of reaching and grasping. Relationships exist between the action-oriented dorsal pathway and the categorization- oriented ventral pathway (Chinellato, 2005, p. 366). Lesions of the primary motor cortex or corticospinal fibers have been found to have disrupted the grasping in humans. The independent finger movement may be lost initially to be regained later but the power grip remains intact all through. Studies have not been done for lesions of ventral premotor cortex in humans. Lesions of the superior parietal lobe produce optic ataxia which leads to a disorder of visuomotor transformation (Glover, 2003). When a patient with optic ataxia reached out, the finger grip aperture was abnormally large and had no correlation with the object size. The grip aperture profile may have many peaks for the action rather than a normal single one. Extensive damage including areas neighboring to the superior parietal lobe produces an exaggerated anticipatory opening of the fingers and causes awkward grasps. Cognitive cues and previous experience of the size could help to determine the size of the object without the grasp. Quantitative measures of grasping have not been done so a real comparison cannot be made between the macaque grasp and human grasp (Castiello, 2005, p. 730). Earlier studies used PET or positron emission tomography. Now more studies are using fMRI or functional Magnetic Resonance Imaging which provides superior spatial resolution.
Robotics
Reproducing artificial beings with relevant skills characterizing the intelligence of animals and humans is robotics. The grasping and manipulative skills are especially required in robotics. Robots closer to the biological reality and thereby more intelligent are being produced as part of the technological advances in neuroscience. The performances of prehension still have a large interval between those of primates and robots. A better vision-based grasping is the requirement. The FARS (Fagg-Arbib-Rizzolatti-Sakata) model is the latest attempt at manufacturing a robot modeling on the sensorimotor mechanism of visual –based grasping (Chinellato, 2005, p.369). Engineers have to develop repeatable and robust algorithms to deliver the geometry and mechanics, data processing and noisy measurements. Recent models consider the reference input for control as “the difference vector between the hand and the target expressed in eye-centered reference frame” (Soueres, 2007, p. 54). This idea goes well with the current models which suggest a multi-sensory spatial resolution. The innovative visual servoing framework in robotics may be considered using a deported camera. The task Jacobian is a function “of each joint of the eye-to-hand kinematic chain. Mathematical arguments can be derived to show that the motor control not only depends on the angular joint parameters of the arm but also on the position of the head and the gaze direction (Soueres, 2007, p. 54).
A long time may yet be taken to produce a robot that emulates human grasping. The current knowledge on the neurophysiology of vision-based grasping must be analysed to determine the basis of the robot (Chinellato, 2005, p. 374)
References
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- Biegstraaten M, Smeets JB, Brenner E (2005) The relation between force and movement when grasping an object with a precision grip. Exp Brain Res 171:347–357
- Castiello, U. (2005). The Neuroscience of grasping .Nature, Vol.6, 2005
- Chinellato, E. & Pobil, A.P. del (2005). Vision and Grasping: Humans vs. Robots, in Mechanisms, Symbols, and ModelsUnderlying Cognition. (Eds.) José Mira & José R. Álvarez Springer-Verlag, Berlin, Heidelberg
- Glover, S. Optic ataxia as a deficit specific to the on-line control of actions. Neurosci. Biobehav. Rev. 27, 447–456 (2003)
- Johansson RS, Birznieks I (2004) First spikes in ensembles of human tactile afferents code complex spatial fingertip events. Nat Neurosci 7:170–177
- Lemelin, P. and Schmitt, D. (2007). Origins of Grasping and Locomotor Adaptations in Primates: Comparative and Experimental Approaches Using an Opossum Model
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- Soueres, P. et al (2007). Robotics Insights for the Modeling of Visually Guided Hand Movements in Primates. In Biology and Control Theory: Current Challenges Quienec et al, Springer-Verlag, Berlin Heidelberg
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