The slow pathway in the electrosensory lobe of Gymnotus omarorum: Field potentials and unitary activity

• Self-activation pattern of the electrosensory lobe of G. omarorum is described.
• Current source density analysis shows a typical sequence of sinks and sources.
• Six main types of neurons were identified according to their activation pattern.
• Sinks and spikes indicate three main volleys of activity across the lobe.
• This suggests different electrosensory processing in pulse and wave fish.

This is a first communication on the self-activation pattern of the electrosensory lobe in the pulse weakly electric fish Gymnotus omarorum. Field potentials in response to the fish’s own electric organ discharge (EOD) were recorded along vertical tracks (50 μm step) and on a transversal lattice array across the electrosensory lobe (resolution 50 μm × 100 μm). The unitary activity of 82 neurons was recorded in the same experiments. Field potential analysis indicates that the slow electrosensory path shows a characteristic post-EOD pattern of activity marked by three main events: (i) a small and early component at about 7 ms, (ii) an intermediate peak about 13 ms and (iii) a late broad component peaking after 20 ms. Unit firing rate showed a wide range of latencies between 3 and 30 ms and a variable number of spikes (median 0.28 units/EOD). Conditional probability analysis showed monomodal and multimodal post-EOD histograms, with the peaks of unit activity histograms often matching the timing of the main components of the field potentials. Monomodal responses were sub-classified as phase locked monomodal (variance smaller than 1 ms), early monomodal (intermediate variance, often firing in doublets, peaking range 10–17 ms) and late monomodal (large variance, often firing two spikes separated about 10 ms, peaking beyond 17 ms). The responses of multimodal units showed that their firing probability was either enhanced, or depressed just after the EOD. In this last (depressed) subtype of unit the probability stepped down just after the EOD. Early inhibition and the presence of early phase locked units suggest that the observed pattern may be influenced by a fast feed forward inhibition. We conclude that the ELL in pulse gymnotiformes is activated in a complex sequence of events that reflects the ELL network connectivity.