Ultra-precise quantal timing: Evidence from simultaneity thresholds in long-range apparent movement

Conditions for the disappearance of long-range apparent movement were inves tigated. In an experiment on beta motion, critical interstimulus intervals (ISIs) of downward simultaneity thresholds for stimuli presented in continu ous alternation were determined for exposure durations (EDs) varying from 3 to 160 msec. Each subject performed each test twice. Data were collected i n three sessions, each for one of three angular separations (3 degrees, 6 d egrees, and 12 degrees) and the full set of EDs. The distribution of critic al ISIs collapsed across subjects, EDs, and angular separations shows sharp maxima at regular distances within a range of 0-110 msec ISI. Significant or near-significant peaks were found at ISIs of 5, 9, 22, 27, 43, 55, and 1 07 msec. Although mean critical ISIs shifted with spatial separation, no es sential shift of the main maxima occurred. Evidence of a periodic modulatio n with a period duration of 4.5 msec was obtained from the distributions of differences between critical ISIs of the first tests and their replication s, which exhibit extremely low standard deviations (<10 msec). These result s agree well with previous analyses (Geissler, 1987, 1992) that led to a ta xonomic model of quantal timing, briefly summarized in this paper. Further consequences are discussed and related to earlier developments (Geissler, 1 991, 1992, 1997).