The optic tectum of the gymnotiform electric fish, Eigenmannia: labeling of physiologically identified cells.

A total of 47 tectal neurons of the weakly electric fish, Eigenmannia, were studied physiologically and labelled by intracellular injection of Lucifer Yellow. With the exception of two cell types, all cells could be classified in accordance with the Golgi studies of Sas and Maler. The dominant stimulus modality of neurons was correlated with their laminar location. Neurons of the stratum opticum only responded to visual stimuli, such as modulations of the light level or the motion of an object. They showed, however, no directional preferences for motion. Neurons of the stratum griseum centrale were predominantly driven by electrosensory stimuli, most often those associated with the movement of an object, and generally were very sensitive to the direction of motion. Integration of different sensory modalities was found in neurons with dendrites invading laminae with different sensory inputs. In addition, small axons of interneurons appear to relay information across laminae. Large multipolar neurons in the deep tectum responded to the motion of objects, often preferring a particular direction of motion. Some of these large multipolar neurons of the deep tectum also discriminated the sign of the frequency difference between a mimic of a neighbor's sinusoidal electric organ discharge and the animal's own signal. These neurons are potential candidates for the control of the jamming avoidance response. These neurons were morphologically indistinguishable from large multipolar neurons of the deep tectum that either responded to moving objects or to acoustical stimuli. Individual large cells of the deep tectum project to various targets (Fig. 1) and probably contribute to the control of different behavioral responses. This suggests that the nature of such responses would then depend upon the constitution of sets of neurons recruited by a given stimulus situation, and the role of individual tectal neurons would neither be particularly specific nor very significant.