The effects of luminance, size, and duration of a visual line on apparent vertical while the head is being inclined...

We determined orientation of a line that is seen to be vertical (i.e., apparent vertical) while the head is inclined with the trunk upright. In this condition, it has been documented that apparent vertical is independent of head orientation (i.e., orientation constancy) or is in a direction opposite to the head inclination (i.e., the Müller effect). In this study, we have focused not only on the effect of head inclination but also on visual parameters of the line that was used to indicate apparent vertical. As the visual parameters, size (5.5° and 22° in visual angle), duration (0.1 s, 3 s, and no time limit), and luminance (0.026, 0.003, and 0.001 cd/m(2) against total darkness) were varied with the head being inclined within ±30°. The main findings were: 1) the Müller effect was at best 2°, but the head inclination was judged to be much larger than it was; 2) the correlation between apparent vertical and the judgmental error of head inclination was significant but was not very high (r = -0.20); 3) the line of short duration or of low luminance facilitated the Müller effect; and 4) the magnitude of the Müller effect was large when the head was inclined to the right rather than to the left. These findings were compared with the predictions from the theory of allowing for apparent head position, the theory of ocular countertorsion, and the sensory-tonic field theory. Many aspects of the results were consistent with the predictions from the sensory-tonic field theory.