Reaching out for an object is often described as consisting of two components that are based on different visual information. Information about the object's position and orientation guides the hand to the object, while information about the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object (on the basis of its shape, surface roughness, and so on) and then moving one's thumb and fingers more or less independently to these positions. We modelled this description using a minimum jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variables such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behaviour (Smeets & Brenner, 1999a). When we use a dynamic model, based on the digits being attracted to the goal positions and repelled by other parts of the objects (and each other), we could model an ever wider range of behaviour.
Our alternative view assumes that the opening of the hand emerges from the trajectories of the digits. We therefore studied the movements of the digits in grasping (Smeets & Brenner, 2001). Our predictions are: that the well-known relation between object size and grip aperture holds for each digit; that the same relation holds if the object is grasped with two hands instead of with the thumb and finger of one hand; that maximum deviation, variability and duration of the digit movements are related; and that variations in the timing of the maximum deviation of one digit are independent of those in the other digit. In accordance with our predictions, we found that the maximum deviation of both digits increased with 0.75 times the object radius, independent of the hand(s) used. The movements of the thumb were more variable than those of the index finger, which was reflected by a larger deviation earlier in the movement. The timing of the maximum deviation of the two digits was independent. These results on the digits' movements are consistent with our view that grasping can be understood as the largely independent movements of the digits. The results are not in conflict with the hypothesis that the grip is controlled during grasping, but can only be explained by extending that hypothesis post hoc.
Everyone has their own idiosyncratic way of making movements, for instance how finger and thumb have subtle differences in their trajectories when moving to a target position. We found that the idiosyncratic differences between the digits in a single-digit pushing and tapping movement corresponded to the idiosyncratic differences when moving together in grasping. When making such tapping movements while wearing prism-spectacles, we found that it is possible to adapt the two digits in opposite directions. Most interestingly, when subsequently performing a grasping movement, we observed a clear after-effect in grip aperture. This last finding is probably the most direct indication that grip aperture in grasping is based on moving digits to positions, rather than scaling grip aperture to object size.
Publications on prehension
- Bozzacchi C, Brenner E, Smeets JB, Volcic R & Domini F. (2018). How removing visual information affects grasping movements. Experimental Brain Research. on-line (reprint, DOI)
- Schot, W. D., Brenner, E., Smeets, J. B. J. (2017). Unusual prism adaptation reveals how grasping is controlled. eLife, 6, e21440. (reprint, DOI)
- Voudouris, D., Smeets, J.B.J., Brenner, E. (2016) Fixation biases towards the index finger in almost-natural grasping PLoS ONE, 11:e0146864. (reprint, DOI)
- Roche, K., Verheij, R., Voudouris, D., Chainay, H., Smeets, J.B.J. (2015) Grasping an object comfortably: orientation information is held in memory Experimental Brain Research, 233:2663-2672. (preprint, DOI)
- Borchers, S., Verheij, R, Smeets, J.B.J., Himmelbach, M. (2014) The influence of object height on maximum grip aperture in empirical and modelled
data. Journal of Experimental Psychology: Human Perception and Performance, 40:889-896 (reprint, DOI)
- Paulun, V.C., Kleinholdermann, U., Gegenfurtner, K.R., Smeets, J.B.J., Brenner, E. (2014) Center or side: biases in selecting grasp points on small bars. Experimental Brain Research, 232:2061-2072 (reprint, DOI)
- Verheij, R., Brenner, E., Smeets, J.B.J. (2014) Why does an obstacle just below the digits' paths not influence a grasping movement while an obstacle to the side of their paths does? Experimental Brain Research, 232:103-112 (reprint, DOI)
- Verheij, R., Brenner, E., Smeets, J.B.J. (2014) The influence of target object shape on maximum grip aperture in human grasping movements. Experimental Brain Research, 232:3569-3578 (reprint, DOI)
- Verheij R, Brenner E, Smeets JBJ (2013) Why are the digits' paths curved vertically in human grasping movements? Experimental Brain Research, 224:59-68 (reprint, DOI)
- Verheij, R., Brenner, E., Smeets, J.B.J. (2013) Gravity affects the vertical curvature in human grasping movements Journal of Motor Behavior, 45:325-332 (reprint, DOI)
- Voudouris, D., Smeets, J.B.J., Brenner, E. (2013) Ultra-fast selection of grasping points Journal of Neurophysiology,110:1484-1489 (reprint, DOI)
- Verheij R, Brenner E, Smeets JBJ (2012) Grasping Kinematics from the Perspective of the Individual Digits: a Modelling Study. PLoS One, 7(3): e33150 (reprint, DOI)
- Voudouris D, Smeets JBJ, Brenner E (2012) Do Humans Prefer to See Their Grasping Points? Journal of Motor Behavior, 30:475-494 (reprint, DOI)
- Voudouris D, Smeets JBJ, Brenner E (2012) Do obstacles affect the selection of grasping points? Human Movement Science, 31:1090-1102 (reprint, DOI)
- Schot WD, Brenner E, Smeets JBJ (2011) Grasping and hitting moving objects. Experimental Brain Research 212:487-496 (reprint, DOI)
- Smeets JBJ, Martin JN, Brenner E (2010) Similarities between digits' movements in grasping, touching and pushing. Experimental Brain Research 203:339-346. (reprint, DOI)
- Schot WD, Brenner E, Smeets JBJ (2010) Posture of the arm when grasping spheres to place them elsewhere. Experimental Brain Research 204:163-171 (reprint, DOI)
- Voudouris D, Brenner E, Schot WD, Smeets JBJ (2010) Does planning a different trajectory influence the choice of grasping points? Experimental Brain Research 206:15-24 ( reprint, DOI)
- Smeets JBJ, Brenner E, and Martin J (2009). Grasping Occam's Razor. In: Progress in Motor Control V: A Multidisciplinary perspective. (Berlin: Springer Verlag), pp. 499-522 (reprint)
- Hesse C, de Grave DDJ, Franz VH, Brenner E, Smeets JBJ (2008) Planning movements well in advance. Cognitive Neuropsychology 25:985-995 (reprint, DOI)
- Cuijpers RH, Brenner E, Smeets JBJ (2008) Consistent haptic feedback is required but it is not enough for natural reaching to virtual cylinders. Human Movement Science 27:857-872 (reprint, DOI)
- Smeets JBJ, Brenner E (2008) Grasping Weber's Law. Current Biology 18:R1089-R1090 (reprint, DOI)
- Biegstraaten M, de Grave DDJ, Smeets JBJ, Brenner E. (2007) Grasping the Müller-Lyer illusion: not a change in length. Experimental Brain Research 176:497-503 (reprint).
- Kleinholdermann U, Brenner E, Franz VH Smeets JBJ (2007) Grasping trapezoidal objects. Experimental Brain Research 180:415-420 (reprint, DOI).
- Biegstraaten M, Smeets JBJ, Brenner E (2006) The relation between force and movement when grasping an object with a precision grip. Experimental Brain Research 171:347-357 ( reprint, DOI).
- Cuijpers RH, Brenner E, Smeets JBJ (2006) Grasping reveals visual misjudgements of shape. Experimental Brain Research 175:32-44 (reprint,DOI).
- de Grave DDJ, Biegstraaten M, Smeets JBJ, Brenner E. (2005) Effects of the Ebbinghaus figure on grasping are not due to misjudged size Experimental Brain Research 163:58-64 (reprint).
- Cuijpers RH, Smeets JBJ, Brenner E (2004) On the relation between object shape and grasping kinematics. Journal of Neurophysiology 91:2598-2606 (reprint).
- Biegstraaten M, Smeets JBJ, Brenner E (2003) The influence of obstacles on the speed of grasping. Experimental Brain Research, 149:530-534 (reprint, DOI).
- Smeets JBJ, Glover S, Brenner E (2003) Modeling the time-dependent effect of the Ebbinghaus illusion on grasping. Spatial Vision, 16:311-324 (reprint).
- Smeets JBJ, Brenner E (2002) Does a complex model help to understand grasping? Experimental Brain Research, 140:132-135 (reprint).
- Smeets JBJ, Brenner E, Biegstraaten M (2002) Independent control of the digits predicts an apparent hierarchy of visuomotor channels in grasping. Behavioural Brain Research, 136:427-432 (reprint).
- Smeets JBJ, Brenner E, de Grave DDJ, Cuijpers RH (2002) Illusions in action: consequences of inconsistent processing of spatial attributes. Experimental Brain Research, 147:135-144 (reprint).
- Smeets JBJ, Brenner E (2001) Independent movements of the digits in grasping. Experimental Brain Research, 139:92-100 (reprint).
- Smeets JBJ, Brenner E (2000) Grasping neurones. Motor Control 4:121-123 (reprint).
- Smeets JBJ, Brenner E (1999a) A new view on grasping. Motor Control 3:237-271 (reprint).
- Smeets JBJ, Brenner E (1999b) Grip formation as an emergent property. Response to commentaries on "A new view on grasping". Motor Control 3:316-325. (reprint)
- Brenner E, Smeets JBJ (1996) Size illusion influences how we lift but not how we grasp an object. Experimental Brain Research, 111:473-476 (reprint).
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