milliseconds timing & subliminal stimuli

[David Glasgow]

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