Fig. 3.

Priority of lowest FM1 frequencies for delay perception. (A) Experiment to test whether the lowest FM1 frequencies of 25 to 35 kHz are both necessary and sufficient for echo delay discrimination. The bat on the Y-shaped platform is trained to broadcast sonar sounds into microphones (m, left and right) which lead to return of electronic “virtual reality” echoes from loudspeakers (s, also left and right). The correct response is to move forward toward the loudspeaker that delivers S+ echoes for a mealworm reward (rewarded S+, blue, 3,160 and 3,460 μs delays simulating distances of 54 and 60 cm and having easily perceived Δt = 300 μs glint separation versus unrewarded S−, purple, 3,660 μs delay, simulating 63 cm distance using a normally easily discriminated 500 μs longer delay than S+). Left–right positions of S+ and S− were randomized in experiments. Each stimulus condition (high-pass filtering, red list, or low-pass filtering, blue list) was tested for 150 trials per bat; performance was assessed as the percentage of correct responses (ranging from 0% perfect; to 50% chance). S+ echoes are subjected to sharp low-pass cutoff frequency (blue numbers) or high-pass cutoff frequency (red numbers) filtering (115 dB/octave cutoff frequencies in small steps from 99 down to 20 kHz or from 20 up to 68 kHz). (B) Two-choice results obtained for different high-pass and low-pass truncations of echo spectra. Top shows a spectrogram of a typical FM bat broadcast (frequency is horizontal, time is vertical; harmonics are FM1, FM2). Bottom shows the mean performance (±1 SD) of four big brown bats in two-choice tests with different high-pass (red) and low-pass (blue) cutoff frequencies. An individual bat’s performance is shown in light gray. Error percentages show that the presence of frequencies around 29 to 32 kHz at the tail end of the FM1 sweep (orange vertical arrow) is essential; absent these frequencies, performance is near chance for all of the remaining frequencies, even if 70 to 80% of the other frequencies are still present (high-pass conditions). Additional confirmation that these frequencies are special comes from the results of previous jamming avoidance experiments (green triangles marking individual preferred frequencies and detection performance for three big brown bats; ref. 35). These bats defend a narrow span of frequencies (24 to 32 kHz) at the tail end of the FM1 sweep by shifting FM1 up or down, away from single-frequency jamming sounds at these frequencies. They do not react to jamming at other frequencies (35).