Alt-BEAM Archive
Message #08486
To: beam@sgiblab.sgi.com
From: Bruce Robinson Bruce_Robinson@telus.net
Date: Mon, 20 Dec 1999 10:46:52 -0800
Subject: [alt-beam] Re: Lets Hypothesise...
Darrell Johnson wrote:
>
> Well, I'm not trying to say that we measure distances
> in units in our heads.. units are an abstraction. But
> in order to grab the cup, your brain tells your hand
> to move the distance between where your hand *was* to
> where the cup *is*.
Actually, in these types of situations three entirely different things
can be involved.
When we're reaching for something nearby that we're familiar with, our
eyes estimate the distance and direction based on the surroundings (e.g.
relative distance from the edge of the table). Our joints provide
feedback to control the motion of the hand and arm. Thus, look at a cup
of coffee on the table in front of you, close your eyes, and reach for
it. Usually you don't spill it. Try it with one eye closed -- usually
not a problem.
When we're reaching for something nearby that we aren't familiar with,
or which is potentially dangerous (e.g. a beaker of acid), our eyes are
constantly estimating the DIFFERENCE between where our hand is and where
the object is. Feedback to control the arm/hand is based on minimizing
the difference. Also possible with one eye closed, but the approach is
usually slower.
Finally, when we're approaching something that's some distance away, we
rely on triangulation from our eyes to estimate the distance. A person
blind in one eye can substitute a sense of scale for triangulation,
provided there are objects of a known size in view, but it isn't true
triangulation.
All three of these measurement methods are useful in different
situations, so it's worthwhile experimenting with all of them.
Bruce
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