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From: toddo@uhmanoa.UUCP (Todd Ogasawara)
Newsgroups: net.ai,net.philosophy,net.cog-eng
Subject: Re: Gibson's theory of perception
Message-ID: <153@uhmanoa.UUCP>
Date: Wed, 23-Jul-86 02:16:15 EDT
Article-I.D.: uhmanoa.153
Posted: Wed Jul 23 02:16:15 1986
Date-Received: Thu, 24-Jul-86 02:52:07 EDT
References: <1782@mtuxo.UUCP> <3483@sdcc3.ucsd.EDU>
Reply-To: toddo@uhmanoa.UUCP (Todd Ogasawara)
Organization: University of Hawaii at Manoa
Lines: 64
Xref: mnetor net.ai:1061 net.philosophy:2193 net.cog-eng:237

In article <3483@sdcc3.ucsd.EDU> ln63szf@sdcc3.ucsd.EDU (Rick Frey) writes:
>> In this view, the
>> natural light sources present in the environment, the reflective
>> properties of the surfaces of objects, and the optical characteristics
>> of the atmosphere are as much a part of the human perceptual system as
>> the eyes and the brain. Thus, the retinal stimulation pattern is not the
>> input to perception, but rather an internal stage in the process. The
>> input to the perceptual process is the object itself; 
>
>Gibson's major mistake.  This simply isn't true.  Obviously it has to be
>the light that makes it to the organism.  If no light reaches the
>organism, then there's no perception.  Period.  Unless the organism
>*perceives* the object, it doesn't exist to the perceiver.  Perception
>starts once there is something to perceive, and that only can begin when
>perceptual information about the object makes its way to the perceiver.
>
[before I begin, let me say that I agree with most of what Rick has to say]

Visual perception and interpretation is of course based on light striking
the retina.  However, reflected light from from an object is not really
the only way to perceive objects.

The best example of this is the filling in process that takes place to
prevent our "perceiving" the blind spot of each eye.  The optic nerves
collect at a point of the retina where there is no receptors.  Yet we
do not perceive a black hole in our visual field.  You might say well
this is because the corresponding input from the other eye covers up.
Simple to test and disprove this: close one eye.  Still no black hole
in visual space.  The black hole is simply assumed to be there by the
brain and is "filled-in".

Another example is my original dissertation topic: subjective contours.
Artists have long known that you can "suggest" contours and edges by
manipulating the placement of black areas.  There are ways to place
lines and black areas to force the visual system to perceive whole
and sometimes complex objects (curved, etc.)  For some good illustrations,
and an easy to find reference, check out:

	Kanizsa, Gaetano (April 1976).  Subjective Contours.
	Scientific American, 4: 48-52

Another example of perceiving objects without the need of the object's
reflected light falling on the retina is visual hallucinations.  High
fever, sensory deprivation, psychoactive drugs, etc. all can cause this
experience.

...todd


-- 
Todd Ogasawara, University of Hawaii
Dept. of Psychology & U. of Hawaii Computing Center

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