Re: Effect of Eye Position on Perception

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 10 Apr 1997 11:31:40 -0500
>From: P3D Ronald W Doerfler <doerfler@xxxxxxxxxxxxxxxxxxxx>
>Subject: Effect of Eye Position on Perception

>I just saw this news item and thought it was interesting, particularly
>the last line:

>=============

>        SAN FRANCISCO, April 9 (UPI) -- Italian and U.S. researchers say  
>conventional wisdom on how you see things is wrong. 
>        It has long been thought that the perceived location of objects in  
>relation to the observer depends only on the image reflected in the 
>eye's retina -- but the latest evidence shows that's just not so. 
>        It also depends on the direction of the gaze. To ensure that the  
>visual world remains stable as we look around us, in addition to the 
>retinal image, the brain must also take into account the movements of 
>the eyes and head. 
>        In two reports in the British journal Nature Wednesday, researchers  
>from the Istituto di Neurofisiologia del CNR in Pisa and the Universita 
>di Roma ``La Sapienza'' in Rome and the University of California, Los 
>Angeles, say there's a lot more to seeing things than meets the eye. 
>        David Burr of the Pisa center says, ``Conventional wisdom has it that  
>the perceived spatial relationship between visual stimuli depends on 
>retinal information alone -- and does not require any extra information 
>about eye position or the direction of the gaze. Conventional wisdom is 
>wrong.'' ...

I think something's seriously wrong here - in the wording of the article that
Ron quoted, or in the author's understanding of vision research, or in the
last few centuries of vision research as described by the article. What it
sounds like the article is claiming is that researchers have always believed
that eye position has nothing to do with perception of spatial relations.
But depth perception is one kind of perception of spatial relations, and
convergence is one feature of eye position, which implies that the author
is claiming that vision researchers have always believed that eye convergence
plays no role in depth perception!

Such a claim would both defy common sense (if I hold my finger a foot in front
of my face and converge my eyes to look at my finger, then the image of the
finger is at the same relative position on both retinas, and yet I do not
perceive my finger as being infinitely far away), and contradict several
references I have handy:
 -"Seeing" - edited by Carterette and Friedman, Academic Press, 1975:
     - pp 17-18 shows that Descartes believed depth perception to be a
       combination of convergence and image position on the retinas
       ("Tractus de Homine", Amsterdam, 1686)
 -"Eye and Brain" by Richard L. Gregory, Princeton University Press, 1990:
     - pp 72ff describe the roles of "convergence depth perception"
       and "disparity depth perception".

There are also problems with the non-3D aspects. When I shift my eyes or head
to the side, I don't think the whole universe has suddenly moved. The human
visual system works largely through a scanning process that causes a visual
model of the world to be built up - it's hard to imagine any visual researchers
failing to perceive that there must be some sort of motion/position 
compensation mechanism, so it's hard to believe that this could be a recent
discovery.

I'm inclined to suspect that this particular case is due to misinformed or
ambiguous writing (or possibly something in the text that was not quoted),
but there have actually been cases in which fundamental misunderstanding of
simple phenomena has persisted among the scientific community until recently.
A few years ago there was an article in Scientific American indicating that
there had recently been a correction to a huge and fundamental error in the
mathematical model of how a person swings on a swing. Apparently among those
physicists who thought it was worthwhile to model the operation of a swing,
none of them had remembered their own experiences, or thought to watch their
kids or try it for themselves before creating the model!

I'm working on an article that begins "Scientists have long believed that
a human can determine the temperature of an object only by eating it, but
a recent discovery shows that it's sometimes possible to judge the temperature
just by touching it with your fingers." ;-)

John R


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