Neural basis of communication in the high frequency electric fish,Eigenmannia virescens (Jamming Avoidance Response)

Abstract

1. Higher order P and T units were analyzed in the first relay of the electrosensory system, the posterior lobe of the medulla, and in the second relay, the torus semicircularis of the midbrain. Among P and T units subtypes were seen similar to those in the periphery: units with and without displacement of firing patterns dependent on+or −ΔF (Figs. 1, 8, 9). 2. Higher order P units showed precise firing within restricted parts of the beat cycle (100 ms range), so that the temporal displacement of maximum activity for+and −ΔF was more obvious. In T units the precision of firing within the EOD cycle (μs range) and therefore a discrimination of the +and −ΔF modulation patterns was vastly improved (Figs. 1, 8, 9). 3. P units in the posterior lobe which were specialized for the detection of moving objects showed distorted responses when foreign fields of smallΔF were presented simultaneously with moving objects. The resolution for the object response was improved, when the fish shifted the EOD away from the foreign frequency, i.e. when theΔF (beat frequency) increased (Fig. 3). 4. The torus semicircularis inEigenmannia has more than 11 layers among which a mere superficial one containes T units. More superficial and deeper layers showed P unit activity of different degree of complexity or units responsive to other sensory modalities. T and P units as in the posterior lobe formed a congruent topographical map of the fish's electroreceptive surface (Figs. 4, 5). 5. In the torus besides simple higher order P and T units complex units were found. Complex units in their response patterns showed signs of convergency of P and T unit subtypes (Figs. 11, 12). 6. Besides complex P and T units there were special units capable of signalling without any need of temporal references the sign of theΔF and other dynamic properties of the beating electric fields. “ΔFp units” responded with regular bursts of activity during+ΔF and with little activity. during −ΔF. The –ΔF activity was below the background firing level when no foreign field interfered. The units also showed tuning to an optimalΔF of 3–5 Hz (Fig. 13). 7. These “ΔFp units” which are in line with the P unit system have the necessary properties to interfere with the pacemaker in the medulla during the JAR. If the neurons would act upon the pacemaker with an inhibitory synapse, an inhibition and thus frequency decrease of the pacemaker would occur during+ΔF. During −ΔF the pacemaker would be relieved from the background inhibition and subsequently speed up (voltage controlled oscillator) (Fig. 14). 8. “ΔFt neurons”, obviously in the line of the T unit system, have been found in the reticular formation close to the pacemaker. These neurons showed a different average phase of the spike in the EOD cycle dependent on+or −ΔF. These neurons could speed up or slow down the pacemaker by raising the membrane potential to threshold with a different slope depending on the time of the PSP in the regenerative cycle of the pacemaker (phase controlled oscillator) (Fig. 15). 9. Torus involvement in the control of the pacemaker was shown by electrical stimulation which yielded three types of frequency shifts (Fig. 16).