From: brucec@phoebus.labs.tek.com (Bruce Cohen)
Subject: Re: half silvered lenses (was Re: Direct Neural Input (Was Re:
Date: 9 Nov 91 20:42:58 GMT
Organization: Computer Research Lab, Tektronix Inc.



In article <1991Nov9.033143.7224@milton.u.washington.edu> craig@utcs.utoronto.
ca (Craig Hubley) writes:

> In article <1991Nov5.033024.17522@watserv1.waterloo.edu> dstamp@watserv1.
> waterloo.edu (Dave Stampe-Psy+Eng) writes:
> 
>>Yes, exactly.  What I was proposing involved being able to interact with
>>virtual *objects*, while letting the user see his hands, the computer
>>keyboard, etc.  This solves a LOT of the psychophysical and kinesthetic
>>problems.  But the time delay problem kills it, for now at least. (sigh).
> 
> I don't understand why.  Seems to me that the answer is practice, practice,
> practice, which is absolutely justifiable if you are going to provide such
> a useful device.   I don't find it all that easy to walk around in VR
> anyway...

There is a bound on how long that time delay can be before coordinated
physical activity is nearly impossible.  That's because for
non-ballistic physical activity such as picking up objects, hand-eye
coordination is necessary.  That's a negative-feedback control system;
like all such systems it goes unstable (typically starts to oscillate)
when there's too much delay in the feedback loop.  Practice doesn't seem
to help much, the only way it could help really is to allow the user to
develop habits of activity using pre-planned ballistic motions, but they
have really limited use in new situations.

As an example, suppose that you're wearing a glove and goggles, and you
decide to pick up the bottle labelled "Drink Me" on the table in front
of you.  As you move your hand towards the bottle, your brain tracks the
relative position of hand and bottle visually against the kinesthetic
information from your arm that tells how the arm and its joints are
oriented.  As you make a change in the motion of some muscle, to correct
for an error in the approach tracjectory, the brain assumes some maximum
amount of time between the instigation of the movement and detection of
the effect.  If the display takes substantially longer than this maximum
(which is on  the order of 100-200 milliseconds) to update the display,
the brain's response is to assume that the correction didn't occur, and
to increase the correction.  Out on the arm, which really did correct,
this will cause overshoot and oscillation.  If you manage to get close
enough, you'll probably knock the bottle off the table.

------------------------------------------------------------------------
Speaker-to-managers, aka
Bruce Cohen, Computer Research Lab        email: brucec@crl.labs.tek.com
Tektronix Laboratories, Tektronix, Inc.                phone: (503)627-5241
M/S 50-662, P.O. Box 500, Beaverton, OR  97077