TETON (Vanlehn, K. , Ball, W.)
A unified theory of cognition should be developed piecewise,
by modeling specific empirically verifyiable
behaviors and specifying
how they relate to each other to form a cognitive
architecture.
This methodology leads to a development of models of higher level
cognitive functions first, such as goal reconstruction, which are
then mapped explicitly onto lower level functions such as memory, a
attention, and motor control.
TETON's architecture is composed of three parts, a long term
memory, a working or short term memory , and an
execution cycle. The
long term memory stores procedures (operators) and selection rules. The
working memory is itself comprised of four parts, but its main use is
as storage for recent results of the execution cycle, which are goals.
The execution cycle uses selection rules in the long term memory to
choose an operator. This operator is then interpreted and it's result
placed in working memory. The architecture will automatically subgoal
if an impass is reached (no operator is preferred
, and there are multiple methods for resolving such impasses.
The long term memory is a knowledge base which is used to
store operators, selection rules, and general facts. An operator has some
selection critera, and a set of preconditions. If the preconditions are met,
then the body is interpreted, and can effect working memory elemetns or
may delibertly create subgoals. Operators also contain 'shortcuts' which
inform the interpreter that his goal has already been achieved. This situation
occurrs during
'goal - reconstruction', which happens when one or more
goals are forgotton. When it does occurr, it allows TETON to simply
mark the goal as completed without executing it. The other type of
knowledge in the long term memory are selection rules. Selection rules are
of three forms, Consideration, Rejection, and Preference. They operate, as
thier names suggest, by forming a set of rules to consider, rejecting some,
and creating a partial order of the rest. This is very similar to the way Soar
(J.Laird))
choses operations.
The main working memory contains goals produced by recent computations
Goals in TETON are slot based structures which can represent past, present,
and future goals. They also contain the state to be achieved, and a link
to the state they were created from. Other information may also be present -
dependant on the needs of the procedures in long term memory. There are three
other divisons of working memory besided main, including a situation memory which
is a representation of the external environment, a limited capacity buffer which
stores recent important events, and a scratchpad which is used for lookahead search
in planning actions. Theor
The execution cycle first decides what to do by interpreting
selection rules, then preforms the operation by interpreting body of the
operator selected. The execution cycle will automaticlly create an impass
if no operator is uniquely selected. Impasses are resolved by multiple methods,
including psychologicaly plausable ones such as simply skipping it or randomly
choosing an operator. One important property of this selection method is that
it is NON-LIFO, in other words any current goal can be selected for achievement
, not simply the last goal created. The execution cycle also checks to see if
the current goal has a shortcut condition. The presence of a shortcut condition
is a flag that the goal has already been achieved, and that what the executor is
now working on is what is called 'goal-reconstruction', because one or more
previously accomplished goals have been forgotton. In this case the goals do not
need to be executed and can simply be marked as completed.
TETON's knowledge is stored mainly as
procedural knowledge
in
it's long term memory component. The long term memory component is
impenertrable to the rest of the system, and the representation language
used is not very usefull for representing declaritive knowledge, but
nothing prohibits such knowledge from being incorporated in the future.
The long term memory component also contains
strategic knowlege which is
used to impose a partial ordering on the set of procedures selected
during the
execution cycle in an attempt to determine the sole best
procedure to execute in the current situation.
TETON is basically a
problem solving and planning
architecture.
Learning is not incorporated currently, but nothing inherent to the
architecture prohibits it's inclusion. One of the authors main points
was the incorporation of 'goal reconstruction' into TETONS problem solving
abilities. Goal reconstruction is the process of
rebuilding lost goal
structures through the incorporation of some external notation which
acts as a reminder of previously achieved goals. The process is included
for
phychological validity, in terms of limited short term memory and
opportunistic problem solving. TETON's short term memory is limited, and
therefore 'forgets' some subgoals if distracted for some time. The system
then has the ability to reconstruct these forgotton goals given a
top level goal and utilize the shortcut operator slot to avoid re - executing
these goals. The same reconstruction method applies to problems which are
given already partially solved, the system will attempt to resolve the
entire problem, but those goals which are already completed in the
environmnent
are 'shortcut'.
The problems TETON has been exposed to are simple, static
subtraction problems. The problems are presented visually, and the
goal of the system is to solve the problem using methods similar to those
used by humans. This includes utilizing the environment as a means of
representing aspects of the problem it does not have the ability to
represent internally. Currently, in TETON, the external representation is
limited to information regarding the partial completeness of a problem.
TETON assumes a perfect sensing of the environment as presented, and
is not concerned with speed of response. The architecture however seems to
impose no restrictions on Scalability in either of these areas.
One of the main limitation of the TETON architecture currently is
its focus on Procedural knowledge. This effects its
scalability because
for Procedures to be effective in diverse environments, the must have access
to larger and larger amounts of declaritive knowlege, or facts. TETON
however is meant as more of a
Psychologically plausable computational model
of human cognition, in particular, the phycological ramifications of
goal reconstructuction are explored and empirically tested.
The environment, along with our limited capacity to represent
information determines the algorithm we use to solve mathematical problems.
Other Architectures
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