Sensory Systems

Annelid worms are simultaneously an interesting and difficult model system for understanding the evolution of animal vision. On the one hand, a wide variety of photoreceptor cells and eye morphologies are exhibited within a single phylum; on the other, annelid phylogenetics has been substantially re-envisioned within the last decade, suggesting the possibility of considerable convergent evolution. This article reviews the comparative anatomy of annelid visual systems within the context of the specific behaviors exhibited by these animals. Each of the major classes of annelid visual systems is examined, including both simple photoreceptor cells (including leech body eyes) and photoreceptive cells with pigment (trochophore larval eyes, ocellar tubes, complex eyes); meanwhile, behaviors examined include differential mobility and feeding strategies, similarities (or differences) in larval versus adult visual behaviors within a species, visual signaling, and depth sensing. Based on our review, several major trends in the comparative morphology and ethology of annelid vision are highlighted: (1) eye complexity tends to increase with mobility and higher-order predatory behavior; (2) although they have simple sensors these can relay complex information through large numbers or multimodality; (3) polychaete larval and adult eye morphology can differ strongly in many mobile species, but not in many sedentary species; and (4) annelids exhibiting visual signaling possess even more complex visual systems than expected, suggesting the possibility that complex eyes can be simultaneously well adapted to multiple visual tasks.

Information processing in the auditory system shows considerable adaptive plasticity across different timescales. This ranges from very rapid changes in neuronal response properties—on the order of hundreds of milliseconds when the statistics of sounds vary or seconds to minutes when their behavioral relevance is altered—to more gradual changes that are shaped by experience and learning. Many aspects of auditory processing and perception are sculpted by sensory experience during sensitive or critical periods of development. This developmental plasticity underpins the acquisition of language and musical skills, matches neural representations in the brain to the statistics of the acoustic environment, and enables the neural circuits underlying the ability to localize sound to be calibrated by the acoustic consequences of growth-related changes in the anatomy of the body. Although the length of these critical periods depends on the aspect of auditory processing under consideration, varies across species and brain level, and may be extended by experience and other factors, it is generally accepted that the potential for plasticity declines with age. Nevertheless, a substantial degree of plasticity is exhibited in adulthood. This is important for the acquisition of new perceptual skills; facilitates improvements in the detection or discrimination of fine differences in sound properties; and enables the brain to compensate for changes in inputs, including those resulting from hearing loss. In contrast to the plasticity that shapes the developing brain, perceptual learning normally requires the sound attribute in question to be behaviorally relevant and is driven by practice or training on specific tasks. Progress has recently been made in identifying the brain circuits involved and the role of neuromodulators in controlling plasticity, and an understanding of plasticity in the central auditory system is playing an increasingly important role in the treatment of hearing disorders.