Re: Vision

From: Harnad, Stevan (
Date: Wed Apr 23 1997 - 21:26:17 BST

> From: Hurden, Jackie <>
> during my revision of chapter 4 I found that I could not
> adequately describe the differences between viewpoint based
> theories and object based theories.
> could I please have a kid-sib definition of these two
> fundamental principles?! - if poss before Friday?

Hi Jackie, Don't forget the Skywriting Archive. Below are excerpts from
my Skywriting summary of Chapter 4. If you don't know where it
is and how to access it on the Web, ask someone who
knows. Cheers, Stevan

When children are very young, they do not know how an object looks from
a viewpoint other than their own. If you show them an object that
is green on one side and red on the other, and then put it down in front
of them with the green side facing them and ask them what it looks
like to a person looking at the other side, they say it's green.
Studies of the effect of brain damage have shown that there are two
kinds of spatial perception: VIEWPOINT-BASED (or egocentric)
and OBJECT-BASED (allocentric) spatial perception. These systems
are separate in the brain because one can be damaged and the other
Children first see objects egocentrically (view-based): they do not
know how an object looks from a viewpoint other than their own.
How do we see 3-D shapes as invariant despite the many changes
in the shadows they cast on our retina?
According to a theorem in solid geometry, if you mark three points on
the surface of a 3-D object, and look at the shadow it casts in 2D,
then the whole shape of the object can be recovered from just 2
different 2-D views. This is a view-based invariance. It means that your
visual system (or a machine's) could tell what the shape of a 3-D object
was from just two views of it.

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