The role of motor command feedback in electrosensory processing.

Sensory motor-coordination and the descending modulation of sensory perception can be particularly well studied in the mormyrid electrosensory lateral line lobe (ELL). In this first order sensory processing network, electroreceptive primary afferent input is integrated with a corollary discharge signal which modulates neuron excitability immediately after the generation of an electric organ discharge. Corollary discharge feedback to the electrosensory lobe allows the brain to distinguish between reafferent sensory input, generated by autostimulation of cutaneous electroreceptors (resulting from the fish's own electric discharge) and exafferent sensory input, evoked by stimulation of the same cutaneous electroreceptors by an external electric source. Mechanisms of this type define the context of incoming sensory information and are the first step in the dynamic regulation of perception. The corollary discharge pathway originates from a collateral branch of the electromotor command neuron axons. It is relayed via bulbar and mesencephalic command-associated nuclei and reaches the ELL by way of projections from the cerebellar posterior granular eminence to the superficial layer of the ELL, and from juxtalobar and juxtalemniscal nuclei to the deeper layers of ELL. ELL is a geometrically organized laminar structure containing a variety of cell types. A number of them combine a spiny dendritic tree in the superficial molecular layer with non-spiny basal dendrites in plexiform or deeper layers. Sensory input may reach the basal dendrites of these neurons either directly or indirectly, via granule cells in the deeper layers of ELL, on which the primary afferent fibers terminate. All neurons recorded intracellularly in the ELL show strong interaction between electrosensory and corollary discharge input. Corollary discharge gating of sensory processing is plastic and depends on dynamic sensory-motor association.