EVOCATION AND CHARACTERIZATION OF PERCEPTS OF APPARENT MOTION ON THE FACE

The percepts evoked by sequential stimulation of sites in close spatia l proximity (less than or equal to 2.5 cm) on the face were studied. B oth method-of-limits and magnitude-estimation procedures were used to identify and characterize alterations in the percepts produced by syst ematic changes in the temporal and spatial parameters of the sequence. Each site was stimulated by a vertically oriented row of miniature vi brating probes. Apparent motion was consistently perceived when the de lay between the onsets of sequentially activated rows (interstimulus o nset interval, or ISOI) fell within a relatively narrow range of value s, the lower limit of which approximated 5 msec. Both the upper limit and the perceived smoothness and continuity of the motion percepts (go odness of motion) increased with the duration for which each row stimu lated the skin over the range evaluated, 15-185 msec. For the successi ve 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, ma ximum goodness of motion decreased with increases in the apparent velo city of motion. When the number of sequentially activated rows was inc reased from two to four or more, the quality of the motion percepts im proved. For the successive activation of multiple closely spaced rows, values of ISOI at which numerical estimates of goodness of motion wer e highest approximated integral fractions of the duration for which ea ch row stimulated the skin. In this situation, the probes rose and fel l in a regular, step-locked rhythm to simulate an edge-like or rectang ular object moving across the skin. The goodness of motion so attained was relatively independent of the apparent velocity of motion.