particular condition. Depending on the nature of the elementary behaviour, the components
of the vector Fa may be of a fixed or modifiable value. The existence of modifiable coupling
strengths is important because it allows the refinement of previously defined layers,
incorporating learning schemes. The vector Fa of coupling strengths of an elementary
behaviour is equivalent to a weight vector in an artificial neuron.
The blackboard acts as an internal memory, where the internal behaviours read, create,
and modify information at different blackboard levels. Each blackboard level contains
information at a different processing stage17. The actions of the internal behaviours on the
blackboard incrementally lead to the solution of a given problem. The blackboard can itself be
seen as the environment of the internal behaviours.
On the blackboard, the interface/communication mechanisms can also read and create
signals. They provide the interface between a blackboard node, and other media, such as the
external medium, internal medium (needs or goals), and other nodes. The
interface/communication mechanisms are also structured by elementary behaviours18.
The control mechanism is distributed in the functionality of the elementary and internal
behaviours.
Different types of elementary behaviours are organized forming emergent behavioural
columns, which vertically cross different blackboard levels. They emerge when the signal
created by an elementary behaviour constitutes the condition of an elementary behaviour of
a different type, and the signal created by this one is in turn the condition of another
elementary behaviour, until reaching the last blackboard level. Elementary behaviours from
different internal behaviours and communication/interface mechanisms interact with each
other through the blackboard. The result is the behavioural columns which thus emerge from
this interaction, and represent the route that signals follow through different blackboard levels.
Behavioural columns might converge or diverge.
BeCA has two defined blackboard nodes: a node that receives signals from the external
medium and determines which action should be taken upon it, and a node for processing
signals from the internal medium. Different internal behaviours, blackboard levels, and
mechanisms will be defined in these blackboard nodes as we incrementally build our BPS.
We define our BPS separated from the perceptual system, the internal medium (needs,
motivations or goals), and the motor system. This allows BeCA to be defined in a generic way,
making possible its implementation in different environments and problem domains
(perceptual and motor systems are dependent on their environment).
In the next sections, we will build our behaviours production system (BPS) following an
evolutionary bottom-up approach. We will first try to solve one problem, and once we have a
BPS that solves this problem, we will evolve the BPS as the problem evolves and becomes more
complex, but without losing the capabilities of solving the previous problem(s). In this way, we
will first build a BPS for the problem of reflex behaviours, which constitutes the initial layer.
17The names of the blackboard levels will be written starting with capitals.
18The names of interface/communication mechanisms will be also typed with italics.
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