Figure 4.

(A) Superimposed snapshots of a reach-to-grasp action directed at an Efron block. Red double-headed arrows indicate “grip aperture”, the Euclidean distance between the tracked markers placed on the tips of the thumb and index-finger (B) sample trajectories of the thumb and index-finger (blue circles) during a precision pincer grasp as the hand reaches out toward the object. The grip aperture is indicated in red. The light blue line reflects the peak grip aperture, which is achieved well-before the fingers contact the object. (C) Grip aperture plotted as a function of time (e.g., percent movement time). The peak grip aperture is again indicated in light blue. (D) Peak grip aperture shows a positive linear relationship to the target size of the object, and so it is thought to reflect the visuomotor system’s anticipatory estimate of the target’s width. The slopes can be used as indicators of “grip scaling.” (E) The slopes for grasping and manual estimation for both the controls (open circles) and DF (X’s) across studies in which Efron blocks were used, the visual conditions were “ecological” (i.e., online visual feedback was available), and the controls were gender-matched and age-appropriate for DF. Although DF scales her grasp to the width of the Efron blocks, her slopes are significantly shallower than those of the controls, using either independent or paired-samples variants of the t-test (p_max < 0.04). The slopes of DF’s manual estimations are essentially zero and clearly different from those of the controls (p_max < 6 × 10⁻³). Critically, the difference in slopes between the grasping and manual estimation tasks falls well-outside of the range of the controls (p_max < 5 × 10⁻³). In other words, across a number of comparable studies of DF’s grasping and perceptual estimation ability, her performance when grasping Efron blocks is sharply dissociated from her performance when perceptually estimating their width.