Echo SPL influences the ranging performance of the big brown bat, Eptesicus fuscus

Four bats of the species Eptesicus fuscus were trained in a two-alternative forced-choice procedure to discriminate between two phantom targets that differed in range. The rewarded stimulus was located at a distance of 52.7 cm, while the other unrewarded stimulus was further away. Only one target was presented at a time. In the first experiment we measured the range discrimination performance at an echo SPL of -28 dB relative to the bat's sonar transmission. A 75% correct performance level was arbitrarily defined as threshold and was obtained at a delay difference of 80 microseconds, corresponding to a range difference of 13.8 mm. In the second experiment the delay difference was fixed at 150 microseconds and the echo SPL varied between -8 and -48 dB relative to sonar emissions. The performance of the bats depended on the relative echo SPL. AT -28 dB the bats showed the best performance. It deteriorated at an increase of the relative echo SPL to -18 dB and -8 dB. The performance also deteriorated when the relative echo SPL was reduced to -38 dB and -48 dB. Only at low relative echo SPLs did the bats partially compensate for the reduction in echo SPL and increased the SPL of their emitted signals by a few dB. Our results support the hypothesis that neurons exhibiting paradoxical latency shift may be involved in encoding target range. This hypothesis predicts a decrease in performance at high echo SPLs as we found it in our experiments. The observed reduction in performance at very low echo SPLs may be due to a decrease in S/N ratio.