Acoustic imaging in the bat brain
At Brown University, researchers in the Department of Neuroscience investigate how bats construct a detailed picture of their world using sound. This process, known as echolocation, involves emitting high-frequency sounds and interpreting the returning echoes. The work, pioneered by figures like Professor James A. Simmons, focuses on the big brown bat, Eptesicus fuscus. This species emits ultrasonic, frequency-modulated (FM) calls that sweep from around 100 kHz down to 20 kHz. By analyzing the delay and spectral pattern of the echoes, the bat perceives the distance, size and texture of objects.
The precision of this system is extraordinary. Behavioral experiments show that bats can resolve two separate reflecting points that are just 0.3 millimeters apart. This is accomplished by detecting time differences in returning echoes as small as 2 microseconds. To form these acoustic images, the bat's brain processes information about the time delay between its vocalization and the returning echo. This "echo-delay" is the primary cue for determining an object's distance. Each meter of distance adds approximately 6 milliseconds to an echo's travel time. The bat's auditory system contains specialized neurons that are tuned to specific echo delays, typically from 2 to 40 milliseconds, which corresponds to target distances of about 34 to 590 centimeters.