Thanks for all the suggestions! (Luck & Vogel task)

[Alex Francis]

Thanks to everyone who responded to my request for (pseudo)code for 
implementing the Luck & Vogel (1997) visual memory task.

The general consensus seems to be that it's relatively easy to do if 
there are a relatively small number of possible locations for the 
stimuli (e.g. in a more typical inhibition-of-return type of task 
with 8 circularly distributed possible stimulus locations) - just 
pick amongst the possible locations, and go from there.

To get something that looks more like the truly random distribution 
of locations I was seeking, it's also possible to make these regions 
larger than the stimuli they are to contain, and have more of them 
than the number of stimuli to be displayed, and then randomly jitter 
the location of each stimulus within the region selected for it - 
with most regions empty, and the stimuli distributed randomly with 
their respective region, the chance of getting two that line up 
exactly, either vertically or horizontally, should be quite similar 
to that in a truly random distribution. However, even this still 
gives a more spread-out, and potentially more grid-like (or 
circle-like) distribution than I would like unless the number of 
regions is extremely large.

Ultimately, it seems as if allowing for locating any stimulus 
anywhere on the screen is best done with brute force randomization 
and double-checking for overlaps, re-randomizing when necessary.  If 
the stimuli are very small (as these are) there is little chance of 
getting stuck permanently in the loop.

I have been greatly aided in this latter endeavor by Candice Morey, 
who generously shared the code she used to implement the Luck & Vogel 
(1997) task a few years ago, as described in the excellent paper 
"When visual and verbal memories compete: Evidence of cross-domain 
limits in working memory" (Morey & Cowan, 2004, Psychonomic Bulletin 
& Review, 11(2), 296-301).

Thanks,

-alex



   Alexander L. Francis         http://web.ics.purdue.edu/~francisa
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