Activities of rhombencephalic units in mormyrid fish.

Abstract

1. The activity of the lateral lobe and cerebellar units involved in electroreceptive mechanisms were studied in the weakly electric fish, Gnathonemus petersii (Mormyridae). 2. The lateral lobe units were of two types, those which responded with a certain number of impulses to each electric organ discharge (EOD) and those which responded similarly but only in the presence of an object. The cerebellar units showed a regular, spontaneous activity which was depressed by each EOD. 3. The functional characteristics of the lateral lobe units were similar to those of primary neurons. They could, however, be distinguished from each other by the following criteria: in the lateral lobe units, both the number and the latency of impulses were greater, and the impulse frequency lower than in primary neurons. Under steady stimulus conditions the lateral lobe impulse frequency fluctuated significantly more than those of primary neurons. 4. The number and latency of the lateral lobe impulses could be modified by positioning a plastic (non conductive) or a metal (conductive) plate close to the fish. The latency of the depressive period in the cerebellar units could be changed by the field strength. 5. The units responded to the presence as well as to the movement of an object. Some of these units responded differently for headwards or tailwards movements along the body axis. Best directional sensitivity was found for units which responded to a moving object only beyond a certain speed and for either direction at different positions. 6. Metal and plastic objects in general provoked opposite effects yielding information about the quality and the relative position of the object. 7. In contrast to high frequency fish, the lateral lobe units studied in Gnathonemus appear not necessarily to integrate over more than one EOD.