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The Moon Illusion

By Grant Ocean

Oculomotor micropsia and macropsia

        Oculomotor micropsia is not a new discovery, but it is only recently utilized by McCready and others to explain the Moon illusion. I have discussed the phenomenon briefly in the previous article, and now I am going to discuss it in more details. Briefly oculomotor micropsia is conventionally understood as this: if there is only one object, or a dominant object, in the field of view, our eyes try to converge and accommodate to its distance. The muscles that control convergence and accommodation send signals to our brain, which are important distance cues. But when one's eyes converge and accommodate on a distance nearer than that of the object, that object appears to subtend a smaller angle than if one's eyes were converged and accommodated upon it. Micropsia means "appearing small", and here refers to the visual angle subtended by an object. The reverse effect, macropsia, occurs when the eyes converge or accommodate at a distance greater than the object being judged. In summary, (1) as a rule, when the eyes focus and converge to a closer distance than a viewed object its angular size appears to become smaller (micropsia) than it was before the readjustment; (2) when the eyes adjust to a farther distance, the angular size of a viewed object appears to become larger (macropsia) than it was before.
        Why would the eyes converge or accommodate at distances different than the object of interest in the field of view? Several reasons have been given. A very large number of nearby objects spread over the field of view may bias the brain towards convergence at their distance. For most people, the functions of convergence (aiming the eyes to a common point) and accommodation (focus setting of the eye's lenses) are "locked together" so if one converges to nearby objects, the accommodation adjusts to that distance also, and vice-versa. Proponents of this hypothesis as explanation of the moon illusion argue that if there are a number of distant objects in the field of view, as there would be when observing the rising or setting full moon, the brain adjusts accommodation and convergence to them. But when viewing the full moon directly overhead, in a clear sky, there are no other distance cues, and the eye adjusts to its resting focus a distance of 1 or 2 meters. This makes the perceived angular size of the overhead moon seem smaller.
        On the surface there are some similarities between oculomotor micropsia and macropsia and my approach to the Moon illusion; but there are some fundamental differences between them. According to the proponents of oculomotor micro/macropsia, the fact that the horizon moon is perceived as larger is because our brain adjusts accommodation and convergence to the distant objects in the field of view; as a result, we have oculomotor macropsia. Put the situations where the distant objects are not visible at all, e.g., at sea, aside, the distant objects on the horizon are physically much closer than the horizon moon and perceived as such also. This is against the rule of oculomotor macropsia, which is that when the eyes adjust to a farther distance, the angular size of a viewed object appears to become larger (macropsia) than it was before.What the proponents have observed is not the oculomotor macropsia as described by the above rule, but is the rule of perceived size of objects. As I have stated in The Ames Room Illusion, the fundamental rule of perceived size is that when you move an observational frame of reference, whether it is a ruler, a thumbnail or any object, away from your eye and toward an object, the target object will appear larger; and when you move that reference frame away from an object and toward your eye, the object will appear smaller. In addition, the observational frame of reference is associated with the converging point which in turn is associated with the visual field volume. Hence, the larger appearance of the horizon moon is determined by the place of the converging point and the reduced visual field volume, not by oculomotor macropsia.
        The micropsia and macropsia of our size perception are believed to be caused by the muscles that control convergence and accommodation sending signals to our brain, and it is why they are called oculomotor micropsia and macropsia. As such, the size perception induced by the oculomotor information sent to our brain which in turn interprets as further away and smaller, or as nearer and larger. Accordingly, it is the subjective interpretation based on the oculomotor information that determines our size perception, not the objective information and rules. If accommodation and related oculomotor system were involved in the moon illusion, the elderly people who have lost nearly all accommodation would not perceive the illusion. But they do perceive the Moon illusion. And persons with eye lens implants have no accommodation, and they do perceive the Moon illusion as well. Covering one eye removes convergence from consideration, but that does not eliminate the Moon illusion. Also, the convergence and accommodation will not work for objects further than 10 meters, let alone very far objects such as the objects on the horizon. My proposal is that micropsia and macropsia do happen; but they are not caused by our oculomotor system as believed by many perception researchers, but by the converging point which can be moved closer to the object when we focus on it and accompanying visual field volume changes.
        The understanding of oculomotor micropsia and macropsia is quite confusing, and misleading sometimes. For instance, according to New World Encyclopedia, oculomotor macropsia causes objects to appear larger when they appear far away based on distance cues such as buildings or trees, and oculomotor micropsia makes objects appear smaller when we perceive them to be closer. Oculomotor micorpsia exists in order to make it easier and faster to turn our heads the right amount to perceive a nearby object that might pose a threat. Oculomotor macropsia exists because it is a necessary byproduct of using oculomotor micropsia, and there is no harm done if we misperceive an object that is far away. These explanations based on the evolutionary theory are contradictory and quite confusing. On the contrary, the closer objects that appear larger would be easier to get our attention and therefore be more adaptive for our survival and reproduction. And the faraway predators do not pose an immediate threat; so their smaller appearance is not very harmful for our survival and therefore is not unadaptive.


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References


     Nanavati, S. (2009). A history and experimental analysis of the moon illusion. The New School Psychology Bulletin, Vol. 6, No. 1.
     Trehub, A. (1991). The Cognitive Brain. MIT Press.     

Related Information on the Web:

http://facstaff.uww.edu/mccreadd/index.html
http://www.lhup.edu/~dsimanek/3d/moonillu.htm
http://en.wikipedia.org/wiki/Moon_illusion
http://retina.anatomy.upenn.edu/~bart/scriptie.html
http://www.pnas.org/content/97/1/500.full
http://nspb.net/index.php/nspb/article/view/6/3

Appendix A


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