Psych 215L:
Language Acquisition
Lecture 2
The Mechanism of Acquisition
and
Some Child Language Research Methods
Levels of Representation
Marr (1982)
Describing vs. Explaining
“…it gradually became clear that something important was
missing that was not present in either of the disciplines of
neurophysiology or psychophysics.
The key observation is
that neurophysiology and psychophysics have as their
business to
describe
the behavior of cells or of subjects but
not to
explain
such behavior….What are the problems in
doing it that need explaining, and what level of description
should such explanations be sought?”  Marr (1982)
On Explaining (Marr 1982)
“…[need] a clear understanding of
what
is to be computed,
how
it is to be done, the
physical assumptions
on which the
method is based, and some kind of
analysis of the algorithms
that are capable of carrying it out.”
“This was what was missing  the analysis of the problem as
an informationprocessing task
.
Such analysis does not
usurp an understanding at the other levels  of neurons or of
computer programs  but it is a necessary complement to
them, since without it there can be no real understanding of
the function of all those neurons.”
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On Explaining (Marr 1982)
“But the important point is that if the notion of different types
of understanding is taken very seriously, it allows the study of
the
informationprocessing basis of perception
to be made
rigorous
.
It becomes possible, by separating explanations
into different levels, to make explicit statements about what is
being computed and why and to construct theories stating
that what is being computed is optimal in some sense or is
guaranteed to function correctly.
The ad hoc element is
removed…”
On Explaining (Marr 1982)
“But the important point is that if the notion of different types
of understanding is taken very seriously, it allows the study of
the
informationprocessing basis of perception
to be made
rigorous
.
It becomes possible, by separating explanations
into different levels, to make explicit statements about what is
being computed and why and to construct theories stating
that what is being computed is optimal in some sense or is
guaranteed to function correctly.
The ad hoc element is
removed…”
Our goal: Substitute “language acquisition” for
“perception”.
The three levels
Computational
What is the goal of the computation?
What is the
logic of the strategy by which it can be carried out?
Algorithmic
How can this computational theory be implemented
in a procedure?
What is the representation for the
input and output, and what is the algorithm for the
transformation?
Implementational
How can the representation and algorithm be realized
physically?
The three levels:
An example with the cash register
Computational
What does this device do?
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 Fall '11
 pearl
 Machine Learning, Bayesian inference, inductive bias, Language Acquisition Research

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