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From: jmm@ski.UUCP (Joel M. Miller)
Newsgroups: net.med,net.info-terms
Subject: Re: VDT's and eyes.
Message-ID: <214@ski.UUCP>
Date: Fri, 13-Dec-85 00:15:17 EST
Article-I.D.: ski.214
Posted: Fri Dec 13 00:15:17 1985
Date-Received: Tue, 17-Dec-85 04:57:31 EST
References: <>
Reply-To: jmm@unix.UUCP (Joel M. Miller)
Organization: Smith-Kettlewell Institute, S.F., CA.
Lines: 51
Xref: watmath net.med:2985 net.info-terms:750

In article <> mmm@weitek.UUCP (Mark Thorson) writes:
>Here's an interesting phenomenon.  Run your finger along the line of dots
>from left to right.  Do it slowly, so that it takes about two seconds for your
>finger to cross the screen.  Track your fingertip with your eyes.  You should
>be able to see a "race lights" effect, where you see the dots moving.
>
>.............................................................................
>
>After a few times, you will be able to track the moving dots without the aid
>of your finger.  The interesting part is the difficulty in making the dots
>go from right to left.  I can see the dots move that direction when I use my
>finger, but not otherwise.  I suppose this is due to developing left-to-right
>eye muscle coordination for reading.
>
>Mark Thorson  (...!cae780!weitek!mmm)
>

A fascinating visual phenomenon!  Its explanation, I think, involves
the nature of "smooth pursuit" eye movements and flickering VDT
displays.

When you look around a stationary scene, you eye moves in little
flicks, called "saccades".  Most people cannot move their eyes smoothly
("smooth pursuit") unless there is a smoothly-moving target to track.
This moving target can be either an actually moving target or a
stationary target that is only "apparently" moving. There are many
kinds of "apparent motion" but, for now it is sufficient to know that
if something seems to move, it can be a stimulus for smooth pursuit.

The moving finger provides a target that enables the eyes to move
smoothly.  The speed of movement at which this phenomenon occurs passes
each point in about 0.025 sec, which (I think) is about the interval
between screen refreshes.  Thus, as your gaze moves, each point flashes
on in the same retinal position (the retina is the light-sensitive
surface lining the inside of the eye) as did the previous point, so
that the retinal situation is unchanging.  However, your brain knows
your eye is moving (it made it move) and figures that the only way an
object's image could remain stationary on a moving retina is for the
object to be moving along with the eye.  That's what you see.

Unless you are one of those people that can execute smooth pursuit eye
movements in the absence of a moving target (I'm not), you'll have to
get each half-cycle going with your finger, but then the finger can be
removed, and the apparent movement of the dots will drive smooth
pursuit and sustain the phenomenon to the end of the line.

I don't find any any directional assymetry, myself.
-- 
Joel M Miller; Smith-Kettlewell Institute of Visual Sciences
2232 Webster St; San Francisco CA 94115; 415/561-1703
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