A bat’s ability to clearly distinguish one echo from another has long inspired those trying to develop radar and sonar systems that perform well in cluttered environments.
Scientists have been exploring the bat-inspired SCAT (spectrogram correlation and transformation receiver) for more than two decades, for example.
Now a team from Brown University, US, has taken an important step forward by getting a clearer picture of how bats do it. It’s referred to as frequency hopping, and essentially involves focussing on the echoes’ lowest frequencies.
And it has potential, neuroscientist James Simmons and colleagues suggest in a paper in the journal Proceedings of the National Academy of Sciences.
“By incorporating this feature into an existing auditory model of FM biosonar, the model can reject echoes that lack the lowest frequencies in the most recent broadcast, thus suppressing echoes of an earlier broadcast that has slightly higher low-end frequencies,” they write.
Bats locate objects by emitting ultrasonic sounds called broadcasts, which contain frequencies ranging from 25 kHz to 110 kHz, and listening to the returning echoes.
Occasionally, an initial broadcast elicits a long-delay echo reflected from a distant object, and a closely successive broadcast triggers a short-delay echo from a nearby object. They reach the bat’s ears at about the same time, but the bat deals with the potential ambiguity in matching the echo to the correct broadcast.
To investigate this further, Simmons, Chen Ming and Mary Bates trained four big brown bats (Eptesicus fuscus) to broadcast sonar sounds, which triggered loudspeakers placed on either side to deliver distinct simulated echoes consisting of various frequencies.
To receive a reward, the bats had to approach the loudspeaker that delivered an echo simulating a nearby, insect-sized virtual object. Only the lowest broadcast frequencies of 25-30 kHz were necessary and sufficient for echo perception.
The authors noted that only the second broadcast contained the lowest frequencies, and bats essentially ignored the faraway object represented by a long-delay echo elicited by the first broadcast.
“From the low-pass and high-pass results, the critical frequencies of 25 to 30 kHz are both necessary and sufficient for big brown bats to perceive differences in echo delay,” the authors write.
“As a consequence of the downward direction of the FM sweeps in the bat’s sounds, the essential 25 to 30 kHz lowest frequencies in FM1 are situated not only at the bottom of the broadcast spectrum in frequency but also at the end of the broadcast waveform in time.
“Echoes thus seem not to enter the bat’s perception until the entire echo has been received and the required lowest frequencies are available to be processed for their delay.”
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