The hypothesis introduced by Smeets and Brenner concerning the perpendicular approach of the thumb and index finger during grasping has heuristic value, but it also has limitations. Among the limitations are the following: (a) the approach parameter is not directly testable and it is unclear how the values of deceleration at contact and movement time are set theoretically; (b) it is questionable that motion of the thumb and index finger are independent; (c) reliance on the minimum-jerk account ignores critiques of that account; and (d) the model begs the question of how the effectors proximal to the index finger and thumb are controlled. We briefly review an alternative model that can handle these challenges.
D.A. Rosenbaum and C. Elsinger are with the Dept of Psychology, Pennsylvania State University, University Park, PA 16802. R.J.G. Muelenbroek is with Nijmegen Institute for Cognition and Information, University of Nijmegen, PO Box 9104,6500 HE, Nijmegen, The Netherlands. J. Vaughan is with the Dept. of Psychology, Hamilton College, Clinton, NY 13323.