Several flickering tasks run simultaneously

[David McFarlane]

Ah, checked my own E-Prime FAQ, and here is the 
link of where I have addressed this kind of 
question 
before: 
http://groups.google.com/group/e-prime/browse_thread/thread/fda0b9bbe3a149c3

-- David McFarlane, Professional Faultfinder
(Now next time someone asks this, I will try to 
direct them to the thread from today :))


At 7/7/2011 03:03 PM Thursday, David McFarlane wrote:
>Mich beat me to the punch, and with the answer 
>that I would have given at first.  And Mich adds 
>a crucial concern that I would have overlooked, 
>namely that your chosen flicker times only work 
>if they are each commensurate with your display 
>refresh rate, which they probably are not (your 
>chosen flicker frequencies translate to periods 
>of 83.3, 100, 116.3, and 113.3 ms, respectively, 
>and you would have to find a common divisor to 
>all of those in the range of roughly 8.3 to 16, 
>good luck!).  But I can't resist weighing in further...
>
>Setting aside your incommensurate flicker rates, 
>can E-Prime do this?  Well (as Mich already 
>said), yes, in principle; but in practice, for 
>this particular task you might be far better off 
>using something like Presentation, or MATLAB 
>with the Psychophysics Toolkit (my own first 
>choice for your task).  You would like a toolset 
>that includes pre-made objects that can flicker 
>themselves at specified rates, and then set 
>those running.  AFAIK, EP has no such facility, 
>but maybe Presentation or MATLAB or Psychopy do, and then Bob's your uncle!
>
>Next, you could try Mich's fine suggestion, but 
>once again I fear that incommensurability will 
>raise its ugly head.  Now you not only need a 
>common divisor, but you need a List that 
>contains the pattern of on-off images for each 
>time point until the entire temporal pattern 
>repeats, and that will not happen until you 
>reach the least-common-multiple of the 
>individual period times, and in your case that 
>will be greater than, say, 83.3 * 113.3 = 9.5 s, 
>and perhaps as great as 83.3 * 100 * 116.3 * 
>113.3 = 30.7 h.  Yikes!  Of course, if you are 
>not wedded to these period times, then you might 
>adust them to more workable values (there's a nice academic exercise for you).
>
>But I said E-Prime could do this, and here's 
>how.  It would require some intricate code, the 
>same way that you could do it in any 
>full-featured programming language, such as C or 
>C++ or even JavaScript, and I have done things 
>like this myself in C.  You would need to 
>contruct your own "event loop" (do a search of 
>the Group or the Forum on that phrase to see 
>where I have discussed this before).  In short, 
>you create an array that holds the upcoming 
>transition time for each object (in this case, 
>your four flickering targets).  Each time 
>through the loop it just checks each of these 
>times against the current clock time, and when 
>any one reaches its next transition time then 
>the loop updates that target and adjuts its 
>transition time for the next one.  Your loop 
>just does that for the duration of the 
>display.  Here is some pseudo-code for that 
>(using my own odd mixture of C-like and other notation):
>
>while( presentation_ongoing )
>     for( i = 1 to nTargets )
>         if( Clock.Read >= target[i].tNext )
>             target[i].toggle
>             target[i].tNext = target[i].tNext + target[i].tPeriod
>
>Of course, you also have to initialize .tPeriod 
>and .tNext for each target, and I left out the 
>mechanics of how to implement anything like a 
>.toggle method to redraw a target between its 
>two states, but I leave those as exercises.
>
>-- David McFarlane, Professional Faultfinder
>
>
>(P.s.  For those who know more C-like notation, 
>the pseudo-code above could be more nicely written as
>
>while( presentation_ongoing )
>     for( i = 1 to nTargets )
>         if( Clock.Read >= target[i].tNext )
>             target[i].toggle
>             target[i].tNext += target[i].tPeriod
>
>Isnt' that nice?)
>
>
>At 7/6/2011 05:12 AM Wednesday, Michiel Spape wrote:
>>Oh, wait, simultaneous tasks, you say?
>>I think there are a few ways of handling this...
>>How about you make one slide with your four 
>>targets. These are four text "sub objects". 
>>Each target has a colour: white (as in, off, 
>>since the background is white), or black. See 
>>first how you can make them black or white 
>>(it's the background colour thing). Then, set 
>>each to [Target1Colour], [Target2Colour], 
>>[Target3Colour] and [Target4Colour] instead. 
>>The slide has a duration of slightly below your 
>>maximum refresh rate (i.e. 15 ms if your 
>>monitor runs at 60 Hz), and onset sync is on.
>>Make a list, sequential selection, 1 procedure, 
>>and set Exit after to 1 sec (i.e., it repeats 
>>for one second). Drag the slide you just made to the one procedure.
>>Add 4 procedures to the list: Target1Colour, 
>>Target2Colour, Target3Colour, Target4Colour. 
>>Think of the list as one second, and add levels 
>>accordingly: if your monitor runs at 60 Hz, 
>>there should be at least 60 levels. Find out, 
>>at each level, which of the targets is black. 
>>For instance, your second target, flickering at 
>>10 Hz, should be black at the first level (time 
>>= 0 ms), 7th level (time = 100 ms), 13th level 
>>(time = 200 ms) and so on, but white at level 2-6, and so on.
>>
>>Well, voila, there you go, it should now 
>>flicker. Of course, nothing is randomised, but 
>>it should work, and I think you might be able 
>>to take it from there. Also, no code yet.
>>Best,
>>Michiel
>>
>>Michiel Spapé
>>Research Fellow
>>Perception & Action group
>>University of Nottingham
>>School of Psychology
>>www.cognitology.eu
>>
>>
>>-----Original Message-----
>>From: e-prime at googlegroups.com 
>>[mailto:e-prime at googlegroups.com] On Behalf Of pati-confidence
>>Sent: 06 July 2011 06:17
>>To: E-Prime
>>Subject: Several flickering tasks run simultaneously
>>
>>I want to build a BCI system, which presents four targets on LCD
>>screen, with flickering frequencies of 12Hz, 10Hz, 8.6Hz and 7.5Hz.
>>respectively.
>>
>>And the four targets are flickering simultaneously.
>>
>>Can E-prime achieve the goal?
>>
>>Any tips will be appreciate.  Thank you.
>
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