Plasticity in the mormyrid ELL

electrosensory lateral line lobe (ELL) of mormyrid electric fish. The mormyrid ELL is one member of a group of cerebellum-like sensory structures found in fish, amphibians and mammals (Fig. 1). These structures possess a molecular layer, an underlying sensory input layer and an intervening layer of large cells. The large cells have apical dendrites that receive input from parallel fibers, and basilar dendrites that receive input from the periphery, either directly from primary afferent fibers or indirectly via interneurons. The large cell layer contains the output neurons that convey sensory information to higher stages of the system. The parallel fibers of the molecular layer arise from an external granule cell mass that receives a rich variety of information, including corollary discharge signals associated with motor commands, descending signals from higher stages of the same sensory modality and signals from other sensory modalities such as proprioception (Montgomery et al., 1995; Fig. 2). Several of these structures in fish have been shown to be adaptive sensory processors and to generate negative images of predictable features in the sensory inflow. Addition of such negative images to the actual or concurrent sensory input minimizes the predictable features and allows novel or unexpected inputs to stand out more clearly (Bell, 1982; Montgomery and Bodznick, 1994; Bodznick, 1993; Bastian, 1995; for a review, see Bell et al., 1997a). The negative images are generated by a process of association between various types of predictive signals and particular patterns of sensory input. The predictive signals that are effective are the same as those that are conveyed by parallel fibers, i.e. corollary discharge signals, descending signals from higher levels of the same sensory system and proprioceptive signals conveying information about body or fin movements. Recent work with mormyrids (Bell et al., 1997c), gymnotids (Bastian, 1996b; Wang and Maler, 1997) and elasmobranchs (Bodznick et al., 1996) indicates that the generation of negative images in these fish is largely due to plasticity at the synapses made by parallel fibers and other types of descending input to the molecular layer. This paper is concerned with mormyrid fish. The following sections present work on the plasticity as studied with natural sensory and motor signals in these fish, on the functional circuitry of the mormyrid ELL and on the phenomenology and mechanism of plasticity at the cellular level.