Strobe frequency in the rod and frame effect.

This experiment dealt with the role of strobe frequency on the rod and frame effect in frame-dependent and frame-independent subjects in light of the destabilizing effect of strobe lighting on body posture. Analysis showed that the frame effect was resistant to strobe illumination and was significantly stronger at 2 Hz than at 9 Hz. Since the stroboscopic effect was not related to the extent of the frame effect observed in normal light, there was no over-all dependence on the different components of the visual field (static and kinetic). Moreover, analysis of eye movements during stroboscopic exposure confirmed previous observation of a visual scanning style related to orienting activity.

Respective contribution of orientation contrast and illusion of self-tilt to the rod-and-frame effect.

The visual angle subtended by the frame seems to be an important determinant of the contribution of orientation contrast and illusion of self-tilt (ie vection) to the rod-and-frame effect. Indeed, the visuovestibular factor (which produces vection) seems to be predominant in large displays and the contrast effect in small displays. To determine how these two phenomena are combined to account for the rod-and-frame effect, independent estimates of the magnitude of each component in relation to the angular size subtended by the display were examined. Thirty-five observers were exposed to three sets of experimental situations: body-adjustment test (illusion of self-tilt only), the tilt illusion (contrast only) and the rod-and-frame test, each display subtending 7, 12, 28, and 45 deg of visual angle. Results showed that errors recorded in the three situations increased linearly with the angular size. Whatever the size of the frame, both mechanisms, contrast effect (tilt illusion) and illusory effect on self-orientation (body-adjustment test), are always present. However, rod-and-frame errors became greater at a faster rate than the other two effects as the size of teh stimuli became larger. Neither one nor the other independent phenomenen, nor the combined effect could fully account for the rod-and-frame effect whatever the angular size of the apparatus.