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Designing Sociable Robots 47

bias

inputs gains node output

threshold, T

0 Amax
Activation level, A
A= (Σ inputs * gains) + bias

Figure 4.2
Aschematicofabasiccomputationalprocess.Theprocessisactivewhentheactivationlevel A exceedsthreshold T .

Networks of units Units are connected to form networks of interacting processes that
allow for more complex computation. This involves connecting the output(s) of one unit to
the input(s) of other unit(s). When a unit is active, besides passing messages to the units
connected to it, it can also pass some of its activation energy. This is called spreading activa-
tion and is a mechanism by which units can influence the activation or suppression of other
units (Maes, 1991). This mechanism was originally conceptualized by Lorenz (1973) in his
hydraulic model. Minsky (1988) uses a similar scheme in his ideas of memory formation
using K-lines.
Subsystems of networks Groups of connected networks form subsystems. Within each
subsystem the active nodes perform special computations to carry out tasks for that subsys-
tem. To do this, the messages that are passed among and within these networks must share
a common currency. Thus, the information contained in the messages can be processed
and combined in a principled manner (McFarland & Bosser, 1993). Furthermore, as the
subsystem becomes more complex, it is possible that some agents may conflict with others
(such as when competing for shared resources). In this case, the agents must have some
means for competing for expression.
Common currency This raises an important issue with respect to communication within
and between different subsystems. Observable behavior is a product of many interacting
processes. Ethology, comparative psychology, and neuroscience have shown that observable
behavior is influenced by internal factors (motivations, past experience, etc.) as well as by
external factors (perception). This demands that the subsystems be able to communicate and
influence each other despite their different functions and modes of computation. This has led
ethologists such as McFarland and Bosser (1993) and Lorenz (1973) to propose that there