[ACT-R-users] Spreading Activation in ACT-R

[Jerry.Ball at mesa.afmc.af.mil]

In ACT-R, the activation of a DM chunk is a combination of the base level
activation and the source activation spread from buffers in the focus of
attention. The activation of all DM chunks (consistent with the retrieval
template) is computed (in parallel) at the time of a retrieval and the most
highly activated chunk is retrieved (subject to noise). It has recently been
brought to my attention that there is no carryover of source activation from
DM retrieval to DM retrieval. The source activation is recomputed on each
retrieval given the contents of the buffers at the time of the retrieval. On
the assumption that base level activations reach asymptote for frequently
used DM chunks (following the power law of learning), it is unclear how base
level activation alone can account for recency effects of DM chunks which
have reached asymptote, since changes in base level will be "in the noise".
Something like carryover source activation would appear to be needed.

 

As a practical example of where carryover source activation would be useful,
considering the recognition of idiomatic expressions like "kicked the
bucket". At the processing of the word "bucket", recognition of the idiom
"kicked the bucket" instead of just the word "bucket" requires activation
from both "kicked" and "bucket". With carryover source activation, the
activation of "kicked the bucket" by "kicked" would carryover to the
activation of "kicked the bucket" by "bucket". Without carryover source
activation, it will be necessary for "kicked" to remain in the focus of
attention along with "bucket" in order to spread source activation to
"kicked the bucket". 

 

More generally, whenever a composite chunk is competing with noncomposite
chunks (e.g. words vs. letters, phrases vs. words, objects vs. features,
collections vs. individuals), without carryover of source activation, the
elements of the composite chunk will need to be retained in the focus of
attention to activate the composite chunk given the current implementation
of spreading activation (and assuming retrievals are not specific to the
composite type). So long as composite chunks are limited to a few slots and
values, retaining the noncomposite chunks in the focus of attention should
work to activate the composite chunk. However, carryover source activation
offers an alternative to this approach.

 

Speculating on other potential uses of carryover source activation, whereas
long-term learning would presumably be reflected in base level activations,
short term learning effects might involve an interaction of base level and
carryover source activation. This could provide an explanation for spacing
effects in learning and the discontinuity in the rate of decay that is often
found across short and longer time frames.

 

Even more speculatively, carryover source activation might be viewed as
residual neural activity akin to Grossberg's notion of resonance, whereas
base level activation results from long term changes in neural potentiation.

 

Jerry

 

  

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