Fig. 1.

Testing system schematic. A, Top view. The hand rested palm-down on an upper table, with the index finger positioned over a tunnel (not shown) through which the stimulus surfaces rose. Plastic barriers surrounding the fingertip prevented lateral and forward finger movements. Upward, downward, and backward movements were detected by a force sensor on the fingernail (not shown) and were announced by an error tone. A rotatable disk (dashed circle) just below the table housed the stimulus surfaces (partial expanded view on left). B, Side view of the lower table, concealed during testing. Rotation of the stepper motor positioned the appropriate stimulus surface under the finger, after which the linear actuator moved with velocity V₁, freeing the rod supporting mass M to pivot around pointP. The stimulus surface contacted the finger with forceF= Mg and velocityV₂ = (V₁)(d₂/d₁). After a 1 sec contact period, the actuator retracted to return the rod and stimulus surface to their resting positions. M= 10 or 50 gm; V₂ = 4 cm/sec. For clarity, only 19 of the 40 stimulus surfaces are shown.