Thanks to all who previously chipped in on this topic. I have been
doing a bit of research, using the following script:
+++++++++++++++++++++
on mouseUp
-- scrollbars to fiddle with the various times
-- duration of the image of the person
get the thumbpos of scrollbar "milliseconds"
-- duration of the gap between picture and mask
put the thumbpos of scrollbar "gap" into tgap
-- this is the duration of the mask
put the thumbpos of scrollbar "show" into tshow
-- present a submarine style cross to focus attention
show group "targit"
wait 1000 milliseconds
hide group "targit"
put the milliseconds into tstart
show image "adf03"
wait it milliseconds
hide image "adf03"
put the milliseconds - tstart & return after field "actualtime"
wait tgap milliseconds
show grc "rectangle"
wait tshow milliseconds
hide grc "rectangle"
end mouseUp.
++++++++++++++++++++++
What happens is that a crosshair style target appears for 1 sec. Then
a picture of a person appears for 'it' milliseconds, then there is a
gap of tgap milliseconds, then a mask appears for tshow milliseconds.
The mask is a rect filled with a pattern. Its purpose is a technical
one, called backward masking (no, not like on heavy metal records). If
an image appears briefly followed by the mask, the amount of conscious
psychological processing permitted by the person viewing it can be
truncated. Images which would be recognisable at a given display
duration are rendered invisible but still processed psychologically.
Don't ask how, it just works. (If your really want to know, take a
look here --> http://www.ac-psych.org/?id=3 )
You can above see that field 'actualtime' accumulates the duration of
the display of the picture of the person (plus the time taken to do the
timing) over successive runs. With the duration of the person image
set at 30 ms, (gap = 40 ms and mask = 160 ms), I shouldn't be able to
see the image of the person, at least not conciously, but I can.
Now I expected to get variable effects in appearance, because I am
testing on a MacBook, so my guess is that the LCD just won't keep up
with these rapid display changes. What I was planning to do was shift
the test stack to a CRT box, and set the refresh rate to 100Hz (in fact
I think it goes to 138Hz). In the literature, I can see that images
can be displayed for a single cycle at 60Hz, and the effect can work).
What surprised me on the MacBook is the recorded variability of the
durations, irrespective of the fact that I can see the person when I
shouldn't be able to. The mean measured display time (set as above) is
38 ms, min 32 and max 50, Standard Deviation = 4.46 (over 30 trials).
I can slide the scrollbar to such a short duration when I can't see the
image of the person, but of course I can't know whether this is because
the backward masking is working, or because the image really isn't
appearing!
One of the things which occurred to me is that I could adapt the test
script above to do a kind of calibration routine, so that the
milliseconds is set to fall in the middle of the distribution of actual
durations, so that some will be a little shorter than 30ms, and others
a little more.
I would welcome any thoughts or comments on what I am doing, and
suggestions for doing it better.
Best Wishes,
David Glasgow
Carlton Glasgow Partnership
http://www.i-psych.co.uk
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