Several investigators have claimed that the cyclopean eye is not fixed in the head, but moves along the interocular axis as a function of the stimulus situation (Erkelens, 2000; Erkelens & van de Grind, 1994; Erkelens, Muijs, & van Ee, 1996; Khan &
Crawford, 2001; Mansfield & Legge, 1996, 1997). Erkelens and van Ee (2002) assert that the concept of the cyclopean eye is inappropriate and irrelevant. In this dissertation, I will argue that these investigators confused (a) relative (exocentric) direction and absolute
(headcentric) direction and/or (b) physical descriptions and perceptual descriptions of direction (Banks, van Ee, & Backus, 1997; Khokhotva, Ono, & Mapp, 2005; Mapp &
Ono, 1999; Mapp, Ono, & Khokhotva, 2007; Ono, Lillakas, & Mapp, 2003; Ono, Mapp,
& Howard, 2002).
The cyclopean eye is not the location in the head to which perceptually aligned objects physically point. Moreover, inferences about the location of the cyclopean eye cannot be based only on observers’ reports that objects appear aligned. Inferences about the location of the cyclopean eye can be made only on reports of where objects lie with respect to the median plane of the head. The task must be a headcentric one. Despite the long history of demonstrations and experiments illustrating this point, some investigators make claims about the location of the cyclopean eye on the basis of relative direction tasks, which do not bear on the question of the location of the cyclopean eye. Indeed, all the studies cited above that claim that the location of the cyclopean eye is stimulus specific are based on only relative direction tasks. For example, Mansfield and Legge (1996, 1997) claimed that the cyclopean eye coincides with the location in the head with which their stimuli were physically aligned. Erkelens et al. (1996) claimed that since the edge of a binocularly seen near surface and the edge of a monocularly seen distant area appeared aligned when they were physically aligned to one eye, the cyclopean eye moved to that eye.
Erkelens and colleagues questioned the validity of the concept of the cyclopean eye (Erkelens & van de Grind, 1994; Erkelens et al., 1996). After conducting a series of experiments using relative direction tasks, they concluded that, “The concept of the cyclopean eye is sometimes inappropriate and always irrelevant as far as vision is
concerned.” (Erkelens & van Ee, 2002). They claimed that all experiments dealing with this issue since Ptolemy were poorly done and stated that, “Indeed we are astounded that results of many poor experiments from the literature carry so much weight.”
Figure 1.5. Stimulus used by Erkelens and colleagues in their monocular zone experiments (see text).
Figure 1.5 shows the type of stimulus used by Erkelens and colleagues. Note that, since the near surface in the figure partially occludes the distant surface, some of the stimulus elements do not physically project to a cyclopean eye located midway between the eyes. For example, the area from d to e is visible to the right eye but it is not projected to the centrally located cyclopean eye. Also, note that in the right-eye’s view, point d is physically aligned with the right edge of the near surface. From a relative direction task Erkelens et al. (1996) concluded that, “binocular space perception near monocularly
Far surface Far surface
a b c d e f
d e f g
Near surface Near surface
Left eye Right eye Left eye Right eye
A B
Cyclopean point
occluded areas is veridical and the cyclopean eye does not have a fixed position in the head, but is located between the eyes for certain visual directions and in one of the eyes for other directions.” (p. 2145).
Figure 1.6. The cyclopean illusion. When fixation changes from the near stimulus, as in (A), to the far stimulus, as in (B), the headcentric direction of the far stimulus shifts to the left. The two stimuli on the visual axis of the right eye appear on the cyclopean axis through the point of fixation (dashed lines). The location of the cyclopean axis changes with the change in the point of binocular fixation. Therefore, the concept of the cyclopean eye is needed in explaining the illusion.
Actual
Although the monocular zone experiments led Erkelens and his colleagues to conclude that the location of the cyclopean eye is stimulus specific, it was their cyclopean illusion experiment that led them to conclude that the concept of the cyclopean eye is irrelevant. The cyclopean illusion is the apparent shift in the headcentric direction of visual stimuli that occurs when the eyes change convergence, as shown in Figure 1.6.
Erkelens (2000) found that with binocular viewing all observers experienced the illusion both in dark surroundings and when the room lights were on. With monocular viewing, however, only 33% of observers experienced the illusion in darkness and none
experienced it when the room lights were on. Erkelens concluded that, “perceived direction during monocular viewing is based on signals of the viewing eye only.” (p.
2411). Erkelens and van Ee (2002) concluded that, “The concept of the cyclopean eye is
… always irrelevant as far as vision is concerned.”
Erkelens’s conclusions challenge the generally accepted view that judgments of headcentric direction are based on information from the two eyes with both monocular and binocular viewing. The primary purpose of the experiments presented in this dissertation is to examine the validity of this challenge. The general overall hypotheses are that (a) absolute (headcentric) direction judgments of monocularly viewed targets are based on retinal image position and the binocular angular position of the eyes in the head and (b) relative direction judgments only are not sufficient to specify the location of the cyclopean eye.