Effects of gravitational and optical stimulation on the perception of target elevation.

To examine the combined effects of gravitational and optical stimulation on perceived target elevation, we independently altered gravitational-inertial force and both the orientation and the structure of a background visual array. While being exposed to 1.0, 1.5, or 2.0 Gz in the human centrifuge at NASA Ames Research Center, observers attempted to set a target to the apparent horizon. The target was viewed against the far wall of a box that was pitched at various angles. The box was brightly illuminated, had only its interior edges dimly illuminated, or was kept dark. Observers lowered their target settings as Gz was increased; this effect was weakened when the box was illuminated. Also, when the box was visible, settings were displaced in the same direction as that in which the box was pitched. We attribute our results to the combined influence of otolith-oculomotor mechanisms that underlie the elevator illusion and visual-oculomotor mechanisms (optostatic responses) that underlie the perceptual effects of viewing pitched visual arrays.

Effect of structured visual environments on apparent eye level.

Each of 12 subjects set a binocularly viewed target to apparent eye level; the target was projected on the rear wall of an open box, the floor of which was horizontal or pitched up and down at angles of 7.5 degrees and 15 degrees. Settings of the target were systematically biased by 60% of the pitch angle when the interior of the box was illuminated, but by only 5% when the interior of the box was darkened. Within-subjects variability of the settings was less under illuminated viewing conditions than in the dark, but was independent of box pitch angle. In a second experiment, 11 subjects were tested with an illuminated pitched box, yielding biases of 53% and 49% for binocular and monocular viewing conditions, respectively. The results are discussed in terms of individual and interactive effects of optical, gravitational, and extraretinal eye-position information in determining judgements of eye level.

Relation of split apparent motion to metacontrast.

The apparent suppression of the target in metacontrast is often accompanied by "split" apparent motion. In Experiments 1, 4, 5, and 6 "neighboring stimuli" (similar to and flanking the mask stimuli) were added to the display, and subjects rated both metacontrast and split motion. Under some conditions, both split motion and metacontrast were completely eliminated (Experiment 1), supporting the assumption that apparent motion is necessary for metacontrast. However, under other conditions, neighboring stimuli caused a much stronger depression of metacontrast than of split motion (Experiments 4 and 5), sometimes even enhancing the latter (Experiment 6), supporting the assumption that the mechanisms underlying the two phenomena are essentially independent. Further, peripheral presentation and close spacing of target and mask with no neighboring stimuli (Experiments 2 and 3) gave strong metacontrast while completely eliminating split motion, showing clearly that apparent motion is not necessary for metacontrast. Results are interpreted in terms of a "fusion" process underlying metacontrast and a "direction-sensitive unit" underlying apparent motion. Interactions between these two processes that might account for the common co-occurrence of motion and metacontrast are proposed.