Who Needs Emotions? The Brain Meets the Robot

356 conclusions
The Motivated Toad
Earlier, we examined the basic, overlapping circuitry for fleeing and feeding
in the toad but did not show how this circuitry could be modulated by
motivational systems. Toads can be conditioned to treat objects that normally
elicit escape behavior as prey. Toads which are allowed to eat mealworms
out of an experimenter’s hand can be conditioned to respond to the
moving hand alone (Brzoska & Schneider, 1978). Heatwole and Heatwole
(1968) showed that the upper size threshold for acceptable prey increases
with long-term food deprivation while the lower size threshold remains
constant. In spring, prey-catching behavior decreases or fails to occur. Indeed,
prey recognition is “exchanged” for female recognition, introducing
mating behavior. Moving females release orienting, approaching, and clasping
behaviors in the male (Heusser, 1960; Kondrashev, 1976). In many species,
males will attempt to clasp practically any moving object including other
males during mating season (Wells, 1977).
Betts (1989) carried forward the modeling of tectal–pretectal interactions
(reviewed in Arbib, 1987) to include the effects of motivation. The basic idea
is that perceptual schemas can be modulated by broadcast signals for drive
levels such as those for feeding or mating, thus shifting the balance of behavior.
For example, standard models of prey catching address the finding that
ablation of the pretectum results in disinhibition of prey catching, with animals
snapping at objects much larger than normal prey (Ewert, 1984); modulating
the level of pretectal inhibition can thus shift the balance between feeding
and fleeing. Betts (1989) further suggested parallels between the effects of
pretectal ablation and the conditioning results and changes that occur during
the mating season. T5 neurons in the tectum have a variety of responses, including
those which we classify as prey recognition. Betts modeled T5 neuron
function by distinguishing the T5 base of T5 cells within the tectum from the
pretectal inhibition which modulates it. A T5 neuron then has the potential
to be, for example, either a prey or mate feature detector depending on this
inhibition. Betts suggests that the subclasses of T5 neurons with different
detector properties should be regarded as more or less stable states of a modulated
system capable of adaptability and changeability.
In summary, the frog has basic systems for feeding, fleeing, and mating;
but we can distinguish circuitry that provides basic “subroutines” from circuitry
that uses motivational factors to bias their deployment. This separates
the motivation from the behavior. From my point of view, fear is not a behavior,
such as freezing, but rather a process that biases the system to be
more likely to emit such a behavior. Freezing is one of the many possible
behaviors that express fear. Which one is chosen depends on learning, species,
and all kinds of other bias. Part of the task of a model of emotion and