Echo SPL, training experience, and experimental procedure influence the ranging performance in the big brown bat, Eptesicus fuscus.

Four Eptesicus fuscus were trained in a range discrimination experiment to choose the closer of two phantom targets. Echo attenuation was roving between trials returning echoes ranging from -10 dB to -50 dB SPL (sound pressure level) relative to emission SPL. Discrimination thresholds were determined. After sufficient training, ranging performance was stable and about the same in the range between -20 dB and -50 dB with range difference thresholds around 300 microseconds. At -10 dB, performance was poor even after long training. After additional training at a constant relative echo SPL of -30 dB and a delay difference of 300 microseconds the performance measured with roving echo SPL improved at all relative echo SPL between -20 dB and -50 dB but not at -10 dB. The new experimental procedure improved the performance by additional learning, and the bats generalized over a wide range of relative echo SPL. Threshold improved to 100 microseconds when measured at a constant relative echo SPL of -30 dB, again indicating the influence of the experimental procedure. In correspondence to neurophysiological data the ranging performance deteriorates if the echo SPL is close to the emission SPL. Signal duration and emission SPL were variable during range discrimination.