milliseconds timing & subliminal stimuli

[Mark Smith]

David, you may well have been down this route, but the following  
little experiment I found quite interesting:

on mouseUp
   put the millisecs into t0
   show img "alex"
   unlock screen
   put the millisecs into t1
   repeat
     if the millisecs >= t1 + 30 then
       hide img "alex"
       unlock screen
       put the millisecs into t2
       exit repeat
     end if
   end repeat

   put t1 - t0 && t2 - t0 && t2 - t1
end mouseUp

The 'unlock screen' meant that I always actually saw the image (I  
believe it forces a screen refresh), and the repeat loop guarantees  
(I think) that the image is visible for at least 30 ms.

The timing measurements were (on my 1.5Ghz G4 laptop):

t1 - t0  9 to 20
t2 - t0 45 to 60
t2 - t1 reliably 36 +- 1

No idea if this is helpful, but I thought I'd throw it in...

Best,

Mark

On 27 Apr 2007, at 13:10, David Glasgow wrote:

> 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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