Functional origins of the vertebrate cerebellum from a sensory processing antecedent

The structure of the cerebellar cortex is remarkably similar across vertebrate phylogeny. It is well developed in basaljawed fishes, such as sharks and rays with many of the same cell types and organizational features found in other vertebrategroups, including mammals. In particular, the lattice-like organization of cerebellar cortex (with a molecular layer of parallel fibres,interneurons, spiny Purkinje cell dendrites, and climbing fires) is a common defining characteristic. In addition to the cerebellarcortex, fishes and aquatic amphibians have a variety of cerebellum-like structures in the dorso-lateral wall of the hindbrain.These structures are adjacent to, and in part, contiguous with, the cerebellum. They derive their cerebellum-like name from thepresence of a molecular layer of parallel fibers and inhibitory interneurons, which has striking organizational similarities to themolecular layer of the cerebellar cortex. However, these structures also have characteristics which differ from the cerebellum. Forexample, cerebellum-like structures do not have climbing fibres, and they are clearly sensory. They receive direct afferent inputfrom peripheral sensory receptors and relay their outputs to midbrain sensory areas. As a consequence of this close sensory association,and the ability to characterise their signal processing in a behaviourally relevant context, good progress has been made indetermining the fundamental processing algorithm in cerebellar-like structures. In particular, we have come to understand thecontribution to signal processing made by the molecular layer, which provides an adaptive filter to cancel self-generated noise inelectrosensory and lateral line systems. Given the fundamental similarities of the molecular layer across these structures, coupledwith evidence that cerebellum-like structures may have been the evolutionary antecedent of the cerebellum, we address the question:do both share the same functional algorithm? [Current Zoology 56 (3): 277-284, 2010].

Behavior and physiology of mechanoreception: separating signal and noise

The mechanosensory lateral line is found in all aquatic fish and amphibians.It provides a highly sensitive and versatile hydrodynamic sense that is used in a wide range of behavior.Hydrodynamic stimuli of biological interest originate from both abiotic and biotic sources,and include water currents,turbulence and the water disturbances caused by other animals,such as prey,predators and conspecifics.However,the detection of biologically important stimuli often has to occur against a background of noise generated by water movement,or movement of the fish itself.As such,separating signal and noise is“of the essence”in understanding the behavior and physiology of mechanoreception.Here we discuss general issues of signal and noise in the lateral-line system and the behavioral and physiological strategies that are used by fish to enhance signal detection in a noisy environment.In order for signal and noise to be separated,they need to differ,and we will consider those differences under the headings of:frequency and temporal pattern;intensity discrimination;spatial separation;and mechanisms for the reduction of self-generated noise.We systematically cover the issues of signal and noise in lateral-line systems,but emphasize recent work on self-generated noise,and signal and noise issues related to prey search strategies and collision avoidance.