Visual processing and fold-change detection by the larva of the simple chordate Ciona

Visual processing transforms the complex visual world into useful information. Ciona, a close relative of vertebrates, has one of the simplest nervous systems known, yet has a range of visuomotor behaviors. Among them are negative phototaxis and a looming-shadow response. These two behaviors are mediated by separate photoreceptor groups acting via distinct, but overlapping, neural circuits. We show here that processing circuits underlying both behaviors transform visual inputs to generate fold-change detection (FCD) outputs. In FCD, the response scales with the relative fold changes in input, but is invariant to the overall magnitude of the stimulus. Several different circuit architectures can generate FCD responses. Both the behavioral outputs and the putative circuitry for the two visuomotor behaviors point to them using different FCD circuits. Pharmacological treatment points to circuits in the posterior brain vesicle of Ciona, a region we speculate shares homology with the vertebrate midbrain, as important for FCD.