Electrolocation

Summary

• 1. Two categories of electroreceptive neurons of the posterior lateral line lobe (PLLL), termed E and I cells, were studied in the weakly electric fishApteronotus albifrons. The E cells respond with short latency, modal latency of 3.6 ms, to increases in electric organ discharge (EOD) amplitude with increased action potential frequency. The I cells responded after a longer time, modal latency of 6.8 ms, to the same stimulus with a decrease in spike frequency (Figs. 1 and 2). Mean spontaneous activity of the E cells was less than that of the I cells, 44 spikes/s compared to 59 spikes/s, and the E cells usually had a preferred phase of firing within the EOD cycle whereas the I cells did not (Fig. 2).

• 2. The mean responses of the E and I cells to various sized 1 Hz modulations of EOD amplitude were fit by power functions having exponents of 0.42 and 0.36, respectively (Fig. 10). The responses of E cells to AMs of different frequencies were similar to those of electroreceptors in that the most effective AM frequency was approximately 64 Hz. The I cells were most sensitive to lower AM frequencies, from 2 to 16 Hz, and the best AM frequency was dependent upon the stimulus intensity (Fig. 4). The absolute value of the responses, change in firing frequency, of either the E or the I cells was much lower than receptor responses to the same stimulus. However, evaluation of the differences in the sizes of the receptors' and the PLLL cells' responses along with estimates of the variances of measures of their spike frequency show that the E and I cells are approximately 16 times as sensitive as the receptors.

• 3. The E cells respond to moving metal objects with a strong increase in frequency followed by a rebound decrease and the I cells respond to plastic objects in the same manner. These responses decrease with increasing object distance lateral to the fish and the rate of this response decay is very close to that predicted on the basis of the cells responses to electronically produced AMs that have no movement component (Figs. 5 and 10).

• 4. The receptive fields of the E and I cells mapped with small spherical metal and plastic objects consist of an area of increased spike frequency followed by a decrease and these areas reflect the cells initial response to the object followed by a rebound response of opposite sign (Figs. 6 and 7). Additional regions of reduced activity boardering or preceding the main response area and differences in the amplitude of various components of the response dependent upon movement direction may be due to more complex receptive field properties.

• 5. The areas of the excitatory regions of these cells' receptive fields containing responses of at least 40% of the peak response averaged 85 mm² for the E cells and 96 mm² for the I cells and these areas are significantly larger than the same measures made on the receptors' fields. Approximately 6 to 15 electroreceptors are estimated to converge on a single E cell or on an I cell, via an inhibitory interneuron. This estimate is based on the differences in receptive field sizes and the density of receptors on the skin (Fig. 11) and this estimate agrees well with convergence estimates based on neuroanatomical methods.