Since essay 15 is exploring the fruit fly mushroom body, which is olfactory-focused, the simulated animal needs odor-based behavior, following attractive odors toward food. The simulation assumes an odor gradient navigation system without implementing the details. Basically, it will cheat and assume a direction vector toward the odor, because the details don’t seem to matter for the mushroom body.
In reality, the animal should use a timed gradient calculation with a single sensor (klinotaxis) or a directional navigation with multiple lateral sensors (tropotaxis). Simple animals use either navigation technique, and even bacteria can use klinotaxis with a run-and-tumble strategy.
Odor plumes in water
The gradient itself is a big oversimplification for a marine slug as used in essay 15, because odors in water don’t have a simple gradient but clump instead. [Steele et al. 2023]. Since the odor plumes, clumps and filaments drive on the waters current, following the current upstream is a more effective than computing gradients.
To following a clumped odor plume in a water current, animals move upstream, against the flow toward the source. The navigation is based on the current flow mechanosensors, not an odor gradient. The odor sensing merely enables current following, which is an interesting circuit between chemosensory and mechanosensory circuits. Odor detection provides timing and go/no-go while the mechanosensory circuit navigates, somewhat like the “what” vs “where” split in the visual cortex.
In the diagram above, the odor control of the contra-flow navigation is inhibitory, a common pattern in vertebrate brain. For example, the striatum complex (basal ganglia) tonically inhibits its output, including midbrain locomotion or optic tectum. When an action is selected, the striatum disinhibits the midbrain command neurons. Despite the complication of disinhibition – double inhibition – the system improves signal noise.
When an inhibitory neuron disables a command, the added noise doesn’t matter because the behavior is disabled, and the extra control signal noise doesn’t harm the command. When the inhibitory control is taken away, the system has clean, undisturbed sensory data. As a contrast, in an excitatory system where the odor sensor positively excited the command, the odor control signal would add noise to the mechanical sensors, reducing precision. So, despite the extra complication of a double-negative inhibitory system, it’s behaviorally superior.
Essay 15 relevance
Although this odor navigation probably won’t be part of the essay 15 simulation, I think it’s important to describe what’s left out when simplifying a model. If the simulation becomes too simplified, it can lose the essence of the behavior. The simplification is necessary to keep the model uncluttered and focused, but the dividing line is a judgement call.
References
Steele TJ, Lanz AJ, Nagel KI. “Olfactory navigation in arthropods.” J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 2023 Jul;209(4):467-488. doi: 10.1007/s00359-022-01611-9. Epub 2023 Jan 20. PMID: 36658447; PMCID: PMC10354148.