Monday, May 08, 2006

Development of the Moon Illusion?

Everybody knows the moon illusion: The moon looks bigger when it's near the horizon than when it's high overhead, despite the fact that it subtends the same visual angle in each case. This is part of what makes certain famous Ansel Adams photographs look weird. The illusion disappears if you view the moon through a paper towel roll, blocking out visual information about the horizon.

Various explanations of the moon illusion have been offered; I take no stand on them, but merely offer the following observation.

I was driving with my four-year-old son, Davy, along a stretch of road beside some hills. The moon was full. Because of the hills, the visual horizon from our perspective was wavy. As we drove, the horizon would rise up closer to the moon, as a hill rose to greet it, as it were, then drop away from the moon when there was a gap in the hills. The moon was very striking that night and I'd already noticed it, but had said nothing to Davy.

From the back seat, Davy said, "The moon is getting bigger and smaller!" I said, "It is?" and looked toward the moon again. It seemed to stay a constant size to me.

Now, one might expect Davy's experience to be the normal one, since the moon was sometimes nearer, sometimes farther from the visual horizon from our perspective. Perhaps that is the original experience of children, but adults learn to compensate somehow?


Brendan Ritchie said...

Hey Eric,

Perhaps what changes with age is the sort of illusion one thinks the moon illusion is.
My own understanding of the illusion is that it has to do with the proximity to the horizon that causes us to think it is bigget than it appears (I owe this understanding to Tim Schoeder. So if it is wrong, I blame him). That is, the main is still an image of the same size in our visual field, but the conscious thoughts/judgments we make about its size change. So, a distinction: we can have illusions of perception (like if, in fact, the moon DID take up more area in our visual field for some reason) and illusion of thought (like the above account of the moon illusion suggests). So, when young, our experience is the same as when we are older, but we think it is a illusion of perception, and it is not till we have experience of the phenomena that we realize that it looks the same, and the illusion is one of thought.
In defense of the illusion of thought account of the moon phenomena, I know that it is comon to experience the same illuision with other things (like shorelines, and cranes).

Brendan Ritchie said...


So many spelling and grammar errors...even Tim's last name...ugh

Eric Schwitzgebel said...

An interesting thought -- but I probably wouldn't draw as sharp a line between "perception" and "thought" as your proposal seems to require, brendan. At least, it seems that our "thoughts" can influence our "perceptions" at a very basic, phenomenological level.

That our perceptual experience of the (perspectival) size of an object is determined by the size of its retinal projection is a common assumption that is perhaps implicit in what you said -- and it may be right -- but I raise some concerns for that view in "Do Things Look Flat?" (one of my forthcoming essays, available on my homepage).

Brendan Ritchie said...

Thanks for the reply Eric,

I think you are right that I am drawing a sharp distinction between perception and thought, perhaps too sharp. And I would also concede that our thoughts can influence our perceptual experiences. However, I think there are really need cases that we can have illusions of thought (as I defined them) as well.

A case that always sticks out in my mind involved a woman with anterograde amnesia who initially did not confabulate (form false beliefs, memories, etc). After the onset of amnesia she suffered from a severe head injury, which caused her to confabulate: namely, she formed the belief that her husband was her father! Even though her father was dead and her husband looked nothing like her father (both facts, as I recall, she was aware of) she could not help forming the belief that her husband was her father, when she looked at him. I think it is faily obvious in this case, if my report of it is correct, that what is not involved is a error in perception, but in thought. If this is right, then perhaps there is a case to be made for the sharp(ish) distinction I am making.

As for your second point, I will be at the SPP (poster presentation) so I look forward to seeing you present the paper

Eric Schwitzgebel said...

Cool case. Some of the work on delusions might be relevant there (e.g., Tim Bayne).

I'll try to catch your poster at the SPP!