Abstract
Purpose: An afterimage (AI) is perceived to move during voluntary saccades in the dark, presumably because of extraretinal eye-movement signals. Perceived AI displacement decreases with the frequency of equal-amplitude saccades, suggesting that extraretinal signals for saccades are a temporally low-pass filtered version of the actual eye movements (Gruesser et al., Vision Res., 1987). As head movements also produce retinal image motion, the goal of this study was to assess extraretinal signals for voluntary head roll and compare their temporal-frequency dependence to the extraretinal signals for saccades. Methods: Six normal observers synchronized voluntary horizontal saccades or head roll in the dark to a metronome at frequencies from 0.5 to 1.7 Hz. The observer represented the perceived change in AI direction during saccades or AI orientation during head roll using a hand-held rod. Eye movements were recorded by a ViewPoint Eye Tracker. Head movements and settings of perceived AI direction or orientation change were recorded on video tape. Results: To estimate the fraction of saccades and head roll that are accounted for by extraretinal signals, the perceived change of AI direction or orientation was normalized to the amplitude of eye or head motion (mean values = 17 and 30 deg, respectively). As the frequency of saccades increased from 0.5 to 1.7 Hz, the normalized change in perceived AI direction decreased from ca. 1.0 to 0.4. The normalized change in perceived AI orientation decreased comparably, from ca. 0.8 to 0.4. The normalized change in perceived AI orientation still fell similarly with temporal frequency, from ca. 1.2 to 0.5, assuming partial compensation for observers' torsional eye movements during head roll (mean amplitude = 15 deg). Conclusion: The estimated extraretinal signals for saccades and head roll fall off similarly with temporal frequency, which is consistent with low-pass temporal filtering of both signals at a common neural site.
R01-EY05068, P30-EY07551 & 003652-0185-2001 (Texas ARP)