Amphioxus neurocircuits, enhanced arousal, and the origin of vertebrate consciousness.

Gene expression studies have recently identified the amphioxus homolog of a domain comprising the combined caudal diencephalon plus midbrain, regions implicated in locomotory control and some forms of primary consciousness in vertebrates. The results of EM-level reconstructions of the larval brain of amphioxus, reviewed here, highlight the importance of inputs to this region for light and physical contact, both of which impinge on the same synaptic zone. The neural circuitry provides a starting point for understanding the organization and evolution of locomotory control and arousal in vertebrates, and implies that one of the tasks of midbrain-based consciousness, as it first emerged in vertebrates, would have been to distinguish between light and physical contact, probably sharp pain in the latter case, by assigning different qualia to each. If so, investigating midbrain circuitry more fully could lead to a better understanding of the neural basis of some forms of sensory experience.