Processing of ampullary input in the brain: comparison of sensitivity and evoked responses among elasmobranch and siluriform fishes

1. Averaged evoked potentials (AEP) in response to pulses and sine waves of electric current, either as a homogeneous field or a large dipole field around the fish, have been recorded from the brain in 5 genera of elasmobranchs and 10 genera of siluriform fishes. 2. In the elasmobranchs, AEP's and spike-hash bursts are described from medulla, cerebellum, mesencephalon and telencephalon. A complex sequence of early and late waves is found in each level. Two sets of factors influence the latency and amplitude of the component waves. One is stimulus parameters including intensity, stimulus orientation, sign of the change of current, position of dipole, serial number in the first few cycles of a train as well as interval or frequency of a regular series. Separate component peaks of the complex AEP appear to have different dependencies upon these factors. To sinusoidal stimuli the best response by a certain criterion is at 20--30 Hz in a shark (Carcharhinus), 10--15 Hz in a ray (Potamotrygon); these are much higher than the values based on behavior from previous authors. The lowest threshold, with moderate averaging, was 0.015 muV/cm (= 0.8 nA/cm2) in marine species, less than 50 muV/cm (= 0.7 nA/cm2) in the freshwater ray. 3. The other set of factors determining AEP is brain parameters, including the recording locus, the type of electrode and electrode advance and the state of the brain (anesthesia, etc.). There is evidence of topographic segregation of some stimulus parameters, but mapping was not undertaken. Responsive regions are described in the several brain levels. 4. The best loci for electric AEP are distinct from those for acoustic and photic AEPs. No interaction between these was found. 5. All of the siluriforms tested show electroreception by the criterion of high sensitivity AEP. No obvious specialization was noted among the diverse species, but most were represented by a single experiment. Taken together with the previously known species, the sample of nine families of this large order suggests that this is a general characteristic of the order. 6. The best responses were found in the region of the torus semicircularis and in the lateral lobe. The lowest threshold by the criterion used was less than 0.15 mV/cm (= 2 nA/cm2), but some were nearly 2 mV/cm. Compared to much lower values in Ictalurus, this suggests species differences although part of the difference may be due to failure to record from the optimal locus. The AEP may be useful as a method of suveying for other electroreceptive groups. It is also potentially useful to reveal discriminable parameters of importance, such as dipole position, orientation, polarity and frequency.