I want to thank everyone who responded both publicly and privately
to my request for help in demonstrating opponent-process after-images.
I found the suggestions very helpful, and I appreciated receiving the
PowerPoint slides some sent along. I probably have enough American
flags for an invasion of Canada now except, of course, you can only
see them after you've seen them, if you know what I mean.
I made some informal observations at my computer and with my class to
see if I could figure out what was going on. For my class, I asked
them to write down the colour names for each stimulus. I restricted
them to red, green, yellow and blue names only, to make it easier to
categorize their responses.
Here's what I think (disclaimer: unsystematic observations, limited
validity), First, the advice to use a fixation point was helpful, and
enhanced the vividness of the after-images. We saw after-images with
both PowerPoint and with the overhead projector, so intensity of the
stimulus isn't a critical factor.
The main problem seems to be the colour of the after-images. This is
important because I emphasize that the linking of red with green, and
blue with yellow was the critical observation leading to the
development of opponent-process colour theory. Yet my PowerPoint red
square elicited consistent responses of blue rather than green.
Fiddling with the characteristics of the colour at my computer (hue,
saturation, luminosity, adding other wavelengths) seemed to have
little effect on the after-image as long as the test colour still
seemed red. As another test, I tried one of the PowerPoint
slides I received from A TIPSter (on my computer, not in class), just
in case it was my red that was at fault. But it still produced blue,
not green. It's hard to maintain in class that red is linked with
green when they just saw that it was linked with blue.
I didn't try exposing a green square in class. But on my computer, it
produces a kind of purple which I'd probably call red in a
forced-choice. So the primary difficulty is that red doesn't produce
green on PowerPoint.
When I switched to a square coloured red with a marking pen
on a transparency and shown with an overhead projector, it produced
near-unanimous responses of the hoped-for green. Similarly, a
marking-pen yellow projected with an overhead produced reports of
blue.
The PowerPoint-presented flag as well as a version I copied onto a
transparency both produced the expected results; that is, the yellow
elicited responses of blue, and the green elicited responses of red.
But the flag stimulus doesn't test the red stimulus, which is where
the problem lies.
Also, as an intro demo, the flag is entertaining and impressive.
But it makes it hard to concentrate on individual colours if the
interest is in exactly what colour is linked to what. Also, I would
expect that prior familiarity with the flag might exert a bit of a
bias on the results, even for Canadians (who are probably more
familiar with the American flag than their own, a sad commentary).
So my overall conclusion is that for unknown reasons, a simple red
square on PowerPoint elicits an after-image of blue rather than green,
at least in my hands. It must have something to do with the way in
which PowerPoint generates the colour. I'm going to stick to overhead
projector displays instead, and so avoid embarrassing hedging about
what the class really should be seeing.
-Stephen
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Stephen Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
Bishop's University e-mail: [EMAIL PROTECTED]
Lennoxville, QC
J1M 1Z7
Canada Department web page at http://www.ubishops.ca/ccc/div/soc/psy
Check out TIPS listserv for teachers of psychology at:
http://www.frostburg.edu/dept/psyc/southerly/tips/
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