BoundedRationality_TheAdaptiveToolbox.pdf

Fast andFrugalHeuristics forEnvironmentally BoundedMinds 65
visual processing. By only being able to focus on very close objects for their first
few months of life, infants can learn about relative object size without the added
complication of size constancy. Once this environmental knowledge has been
acquired, the gradually increasing depth of visual field allows the problem of
size constancy to be learned in an independent, and therefore easier, manner.
Stronger arguments have been made for language learning. It has been put
forth that the cognitive limitations of the developing brain are a necessary hindrance
for a first language to be learned fluently. Newport (1990) has proposed
another type of "less is more" phenomenon in this domain: Lesser ability to remember
and process morphological and semantic components into form—meaning
mappings in young children allows them to learn more rapidly and
accurately those mappings that they do acquire, and then to build further upon
these as language learning proceeds. Late language learners, in contrast, may
falter when attempting to learn the full range of semantic mappings with their
mature mental capacities all at once.
This situation has been studied concretely by Elman (1993) in a neural network
model of language acquisition. When he tried to get a large, recurrent neural
network with an extensive memory to learn the grammatical relationships in
a set of several thousand sentences of varying length and complexity, the network
faltered. It was unable to pick up such concepts as noun-verb agreement in
embedded clauses, something that requires sufficient memory to keep embed
ded and nonembedded clauses disentangled. Instead of taking the obvious step
of adding more memory to the model to attempt to solve this problem, though,
Elman counterintuitively restricted its memory, making the network forget everything
after every three or four words. He hoped in this way to mimic the
memory restrictions of young children first learning language. This restricted-memory
network could not possibly make sense of the long
clause-filled sentences it encountered. Its limitations forced it to focus on the
short, simple sentences in its environment, which it did learn correctly, mastering
the small set of grammatical relationships inherent in this subset of its input.
Elman then increased the network's effective memory by forcing it to forget everything
after five or six words. It was now able to learn a greater proportion of
the sentences to which it was exposed, building on the grammatical relationships
it had already acquired. Further gradual enhancements of the network's
memory allowed it ultimately to learn the entire corpus of sentences that the full
network alone — without the benefit of starting small — had been unable to
fathom.
Elman sees the restrictions of the developing mind as enabling accurate early
learning about a small portion of the environment, which then provides a scaffold
to guide learning and hypothesizing about the rest of the environment in
fruitful, adaptive directions. Cognitive "constraints" are no longer a negative
limitation of our (or our children's) ability to behave adaptively in our environment.
Rather,