Evocation and characterization of percepts of apparent motion on the face

The percepts evoked by sequential stimulation of sites in close spatial proximity (< or = 2.5 cm) on the face were studied. Both method-of-limits and magnitude-estimation procedures were used to identify and characterize alterations in the percepts produced by systematic changes in the temporal and spatial parameters of the sequence. Each site was stimulated by a vertically oriented row of miniature vibrating probes. Apparent motion was consistently perceived when the delay between the onsets of sequentially activated rows (interstimulus onset interval, or ISOI) fell within a relatively narrow range of values, the lower limit of which approximated 5 msec. Both the upper limit and the perceived smoothness and continuity of the motion percepts (goodness of motion) increased with the duration for which each row stimulated the skin over the range evaluated, 15-185 msec. For the successive activation of only two rows, goodness of motion was not influenced by changes in their separation from 0.4 to 2.5 cm. The ISOI values at which magnitude estimates of goodness of motion were highest increased with the duration for which each row stimulated the skin. As such, maximum goodness of motion decreased with increases in the apparent velocity of motion. When the number of sequentially activated rows was increased from two to four or more, the quality of the motion percepts improved. For the successive activation of multiple closely spaced rows, values of ISOI at which numerical estimates of goodness of motion were highest approximated integral fractions of the duration for which each row stimulated the skin. In this situation, the probes rose and fell in a regular, step-locked rhythm to simulate an edge-like or rectangular object moving across the skin. The goodness of motion so attained was relatively independent of the apparent velocity of motion.