The human visual system perceives three-dimensional structures by using all kinds of means, such as perspective, binocular parallax, motion parallax, texture gradient, etc[2]. Among these, binocular stereo viewing based on binocular parallax is one of the most important means in perceiving the three-dimensional structures of objects.

In recent years, in the field of computer graphics, the three-dimensional presentation method based on binocular stereo viewing is widely used. In research on the elucidation of the human visual mechanism, studies with the use of three--dimensional presentation methods based on binocular stereo viewing as applied to computer graphics are being , carried out actively. These studies have led to new discoveries which were never anticipated in the past, and to new knowledge concerning the human visual system.

For three-dimensional presentation based on binocular viewing, various methods are used; the anaglyph method is one of them (Fig.1). According to the anaglyph method, the images of the left eye and the right eye are depicted in different colors and are observed after they are separated by means of filters corresponding to these colors. For this reason, the anaglyph method is an extremely effective one for conducting a simple experiment on binocular stereo viewing[1]. However, in the observation of moving objects which the present authors have recently conducted, a strange phenomenon was observed that a triangle rotating on a plane with a uniform depth is perceived as if it were moving in a slanted way. When this phenomenon was examined, it was confirmed that it was due to the Pulfrich effect based on the difference in brightness between red and blue colors[4]. This fact indicates that it is necessary to pay sufficient attention to the existence of this phenomenon as long as the anaglyph method is used for the presentation and observation of moving objects. The present report is intended to introduce several of phenomena observed in the course of the study on the cause of this phenomenon.