R Kompass | PubMed Health

PubMed2000

Alternative perceptual states 'apparent motion' and 'perceived simultaneity' lead to differences of induced EEG rhythms.

An investigation of the cortical response (EEG) to periodically presented stimuli producing an ambiguity between long-range apparent motion and flicker is reported. ERPs to stimulus onsets differed slightly between the two percepts, in accordance with the results of Manning et al. (1988), Selmes et al. (1997). Induced rhythms exhibited a strong increase in induced beta and gamma powers at electrode positions T7 and T8 during the perception of apparent motion in two out of 10 participants. In addition, a small overall increase in alpha power at 12-13 Hz and a decrease in delta power below 3.5 Hz during perceived motion were found. The results indicate that a variety of different neural rhythms are involved in the perception of long-range apparent motion.

Adult↗

PubMed1999

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

Conditions for the disappearance of long-range apparent movement were investigated. In an experiment on beta motion, critical interstimulus intervals (ISIs) of downward simultaneity thresholds for stimuli presented in continuous alternation were determined for exposure durations (EDs) varying from 3 to 160 msec. Each subject performed each test twice. Data were collected in three sessions, each for one of three angular separations (3 degrees, 6 degrees, and 12 degrees) and the full set of EDs. The distribution of critical 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 107 msec. Although mean critical ISIs shifted with spatial separation, no essential shift of the main maxima occurred. Evidence of a periodic modulation with a period duration of 4.5 msec was obtained from the distributions of differences between critical ISIs of the first tests and their replications, which exhibit extremely low standard deviations (< 10 msec). These results agree well with previous analyses (Geissler, 1987, 1992) that led to a taxonomic model of quantal timing, briefly summarized in this paper. Further consequences are discussed and related to earlier developments (Geissler, 1991, 1992, 1997).

Adult↗