source activation

[Christian J Lebiere]

Wolfgang,

Excerpts from mail: 17-Dec-97 source activation by Wolfgang Schoppek at uni-ba 
> In my attempts to construct my own ACT-R (4.0) models I realized that I
> don't understand the relation between "being the focus" and "source
> activation". Hitherto I believed that only the chunk on top of the
> goal-stack is a source of activation. But following the traces of a very
> simple model I realized that pushing a chunk on the stack with
> (Focus-on-Goal) does not raise the activation-level of the chunk.
> Setting a source value with (sdp  :Source) causes such a raise without
> the chunk being the focus. I couldn't find an answer to this question
> neither in the literature nor in the manual. So can anybody help me with
> my fundamental problem?

Unlike earlier ACT-R versions (i.e. ACT-R 2.0), in ACT-R 4.0 setting a
chunk as the focus does not make it a source of activation, but instead
makes its components, i.e. the chunks which appear as values in its
slots, as new sources of activations.  The total goal activation (aka W)
is divided evenly among those sources.  Therefore, the more chunks in
the goal, the lower their individual level of source activation.  This
has been of central interest to many recent models, including those of
working memory, list memory and individual differences:

Anderson, J. R., Reder, L. M. & Lebiere, C. (1996) Working Memory:
Activation limitations on retrieval. Cognitive Psychology, 30, 221-256.

Anderson, J. R., Bothell, D., Lebiere, C. & Matessa, M. (in press).  An
integrated theory of list memory.  Journal of Memory and Language.

Lovett, M., Reder, L. M., Lebiere, C. Modeling Individual Differences in
a Digit Working Memory Task. Proceedings of the Annual Meeting of the
Cognitive Science Society. Stanford, CA.  August 1997.

Christian