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.
A map of the world made of time
The bat's brain does not register these delays; it organizes them spatially. In the auditory cortex of many bat species, these delay-tuned neurons are arranged topographically, forming a "chronotopic" map. This is a calibrated neural representation of distance. Neurons that respond to short echo delays (nearby objects) are clustered in one area, while neurons that respond to long delays (distant objects) are in another. As a bat flies toward an object, the echo delays shorten, and this creates a wave of neural activity that moves across this cortical map.
This organization is analogous to the retinotopic maps found in the visual cortex of sighted animals, where adjacent neurons process information from adjacent points in the visual field. In the bat, the auditory cortex creates a map of the world where one axis is organized by time delay, representing distance. This neural architecture allows the bat to process a stream of echoes from a complex environment—an "acoustic scene"—and segregate different objects from the background clutter. The research at Brown University explores how this neuronal processing gives rise to what the bat actually perceives, using the timing of neural responses to construct a dynamic image of its surroundings.