ACT-R Parallelism

[Mike Byrne]

> I seem to be a little confused about the parallel aspect of ACT-R
> P/M.  I am working with visual objects and the documentation for
> ACT-R P/M says that, "the vison module checks the icon to see if
> there is a feature in the icon that matches the chunk requested
> by ACT-R.  If one existes, a chunk representing that visual
> location is created and returned to ACT-R in zero time."  I guess
> I also assumed that all chunks returned by ACT-R P/M in zero time
> would have the same activation levels, but as far as I can tell,
> they don't.  

It is correct that not all chunks of type VISUAL-LOCATION will have
the same activation.  This will be a function of what the model is
currently attending to, because the slots of the currently-attended
visual object are activation sources.  Thus the VISUAL-LOCATION
chunk associated with that object will have a higher activation
than other VISUAL-LOCATION chunks, unless they happen to be stuffed
into the goal.

> So... if they don't have the same activation levels, then they
> really are not returned in parallel, because some are more active
> than others.  

I'm sorry, but I don't understand your logic here.  Differential
activation of VISUAL-LOCATION chunks doesn't affect the behavior of
the Vision Module one iota.  That is, the activation level of a
VISUAL-LOCATION doesn't affect how rapidly it will be returned with
a TIME NOW test.  That time is always zero, regardless of the
chunk's activation.  

For example, consider the following production LHS, and assume
there are two objects on the screen, one with an x-coordinate of
500 and one with an x-coordinate of 150:

=goal>
   isa          example
=loc1>
   isa          visual-location
   time         now
   screen-x     150
=loc2>
   isa          visual-location
   time         now
   screen-x     500
   
This production would find (or create) two VISUAL-LOCATION chunks
and the total matching time for this production would be zero. 
This qualifies as parallel in my book, and will happen irrespective
of the activations of those chunks, which could be different
depending on what's in the goal and what VISUAL-OBJECT is currently
attended.

Now, if you are retrieving a VISUAL-LOCATION chunk from memory
(i.e. doing it *without* a TIME NOW specifier), then you are
correct, they will be retrieved serially and will be affected by
having differential activation levels.  If you omit the TIME NOW
specifier, then RPM never gets called; you are simply requesting a
pair of retrievals from declarative memory, which works the way it
always has.

> If they were returned in parallel, they all should have the same
> activations.  

This is simply not true; see my explanation above.

> So.. do I have to do a match on all the portions of the screen
> that I want returned as chunks with the same activation? 
> Couldn't I do that without ACT-R P/M?

Again, I apologize, but I don't understand what you want here. 
Could you clarify or re-ask this question?  I suspect some code
would *really* help here.

-Mike

===========================================================
Mike Byrne, Ph.D.                             byrne at acm.org
Assistant Professor, Psychology Department
Rice University, MS-25          http://chil.rice.edu/byrne/
6100 Main Street                      +1 713-348-3770 voice
Houston, TX  77005-1892                 +1 713-348-5221 fax