The simulated slug in essay 14 is always motivated to search for food and to eat while it’s moving over food. Adding satiety is a natural step to explore motivation. Before modifying, the slug is spontaneously motivated to move in a semi-random direction and to pause when it’s over food, but when the slug is full, it doesn’t need to search for food and can slow to save energy. As with the original slug, the spontaneous motivation is still to explore and eat, but satiety inhibits that motivation.
Modifying the exploration motivation system resembles [Brooks 1990] subsumption system, where higher layers modulate lower systems. In contrast, a hunger system would be more of a stimulus-response system than a spontaneous motivation system. As an engineering problem, the intrinsic motivation is more robust because the system recovers if the satiety/hunger system is faulty, while a hunger-stimulus system would leave the animal to starve because it’s not motivated to search for food.
Exploring vs eating
The satiety signal could suppress feeding instead of suppressing searching for food. Because a slug can eat while moving, the two systems are essentially independent. In behavior, this distinction between foraging and eating is the distinction between approach behavior and consummatory behavior, which are handled in different brain areas.
In vertebrates, the brainstem controls consummation, specifically the parabrachial nucleus (B.pb) and B.nts. [Grill 2002], while the hypothalamus controls approach toward potential food. Besides its role in eating, B.pb is an extremely important area for motivation and learning because it responds to unconditioned stimuli (US), like taste, pain, irritation, etc. Unconditioned stimuli is the raw sensation for punishment and reward learning. So, this essay simulates hypothalamic modulation instead of B.pb, but simulating either at this level of detail is trivial.
Simulation
When the slug is over food, satiety increases as if it were eating. Satiety decays over time. When the satiety level is above a threshold, the slug slows.
In the alternate (B.pb) behavior, the slug could suppress its pausing behavior over food. In the above screenshot, that pause would suppress the last yellow arrest spikes, and the slug would move normally despite crossing over food.
fn satiety_speed_update(mut body: ResMut<Body>) {
if body.get_satiety() >= 0.75 && ! body.is_sensor_food() {
body.speed(0.5);
}
}Actually, this code looks wrong, because the satiety shouldn’t depend on the food sensor. I’m leaving it because it’s wrong in an interesting way. The correct behavior suppresses locomotion, which is the intent of the speed 0.5. If the sensing food overrides the locomotion suppression, that conflict-resolution needs its own node. Satiety needs to suppress locomotion, and sensing food needs to unsuppress locomotion. In this code, satiety is unsuppressing itself. The behavior is correct, but the control is wrong.
In mammals, the satiety system is complicated. For example, in B.pb the taste of spoiled food is spontaneous and needs to be suppressed to eat [Grill 2002]. Satiety allows the sensation of rotten food to proceed, which causes the animal to stop eating. Hunger suppresses the sensation, allowing the animal to eat.
Which all sounds needlessly complicated, but might make sense as an evolutionary system. Since a slug or a filter-feeder eats continuously, unless its food is rotten or toxic, evolution would need to create a “stop eating toxic food” system as a higher priority than satiety. Once the stop eating system exists, it can hijack that system for satiety; the animal stops eating because it feels nearly sick. Evolution doesn’t care if the logic doesn’t make sense, as long as the system works.
References
Brooks, R.A. (1991), ‘Intelligence without Representation’, Artificial Intelligence 47, pp. 139-159.
Grill, Harvey J., and Joel M. Kaplan. “The neuroanatomical axis for control of energy balance.” Frontiers in neuroendocrinology 23.1 (2002): 2-40.