is an increased and stereotyped response to an external stimulus (Lorenz, 1981; Manning, 1979;
McFarland, 1981). This response comprises an elaborated temporal sequence of component
actions. Unlike reflex behaviour, the intensity and duration of a fixed-action pattern is not
controlled by the presence of a given stimulus. In other words, the execution of the fixed-action
pattern could continue even if the stimulus is removed. The escape response in animals is an
example of fixed-action patterns. This type of reactive behaviour involves a sequence of evasive
actions, and requires a persistence of the environmental signals.
The reactive behaviours are modelled in our BPS by incorporating a new layer over the
first one. The creation of this second layer includes the definition of the following components:
the Perceptual Persistents blackboard level, and two new internal behaviours: perceptual
persistence and external behaviour selector. The inclusion of these components in BeCA allows
us to model reactive behaviours by taking into account two new elements not present in the first
layer: the persistence of external signals and a process of behaviour selection among different
reactive behaviours.
The Perceptual Persistents level models a type of short term memory. At this level, the
strongest external signals, initially projected onto the External Perceptions level, persist for
more time. The signals at the Perceptual Persistents level are created or modified by the
perceptual persistence internal behaviour. The condition of an elementary behaviour of this type
is satisfied when at least one of the following facts has taken place: at the External Perceptions
level there has been created an external signal specified in the condition of an elementary
behaviour, and/or at the Perceptual Persistents level there has been created a signal specified
in the condition of an elementary behaviour, or the intensity of this signal has been modified.
The elementary behaviours that have satisfied their condition enter into a competition process.
The activation level of each elementary behaviour is specified by expressions (1) and (2). The
new OT signal created on the Perceptual Persistents level by the perceptual persistence internal
behaviour will be the activation level AT in expression (2), if it is greater than a threshold 2T,
and zero otherwise. The time during which this signal will be active in the Perceptual
Persistents level will depend on the value of parameter 6, which is a decay factor, in expression
(1):
(DψT-(ι-r)qr + ¾⅛s + Z,,.+ (1)
where O T is the strength of the previous signal on the Perceptual Persistents level, Fa S is the
coupling strength related to the signal O S on the External Perceptions level, and Fa T is the
negative coupling strength with which the signal O T laterally inhibits the signal O T. The final
activation level A T is calculated by hyperbolically converging the temporary activation level
AtmpiT to a value MaxiT using expression2:
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