THE ROLE OF MOTOR COMMAND FEEDBACK IN ELECTROSENSORY PROCESSING

Sensory motor-coordination and the descending modulation of sensory pe rception can be particularly well studied in the mormyrid electrosenso ry lateral line lobe (ELL). In this first order sensory processing net work, electroreceptive primary afferent input is integrated with a cor ollary discharge signal which modulates neuron excitability immediatel y after the generation of an electric organ discharge. Corollary disch arge feedback to the electrosensory lobe allows the brain to distingui sh between reafferent sensory input, generated by autostimulation of c utaneous electroreceptors (resulting from the fish's own electric disc harge) 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 a re the first step in the dynamic regulation of perception. The corolla ry discharge pathway originates from a collateral branch of the electr omotor command neuron axons. It is relayed via bulbar and mesencephali c command-associated nuclei and reaches the ELL by way of projections from the cerebellar posterior granular eminence to the superficial lay er of the ELL, and from juxtalobar and juxtalemniscal nuclei to the de eper 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 gr anule cells in the deeper layers of ELL, on which the primary afferent fibers terminate. All neurons recorded intracellularly in the ELL sho w strong interaction between electrosensory and corollary discharge in put. Corollary discharge gating of sensory processing is plastic and d epends on dynamic sensory-motor association.