The spectrogram correlation and transformation (SCAT) model of the sonar receiver in the big brown bat (Eptesicus fuscus) consists of a cochlear component for encoding the bat's frequency modulated (FM) sonar transmissions and multiple FM echoes in a spectrogram format, followed by two parallel pathways for processing temporal and spectral information in sonar echoes to reconstruct the absolute range and fine range structure of multiple targets from echo spectrograms. The outputs of computations taking place along these parallel pathways converge to be displayed along a computed image dimension of echo delay or target range. The resulting image depicts the location of various reflecting sources in different targets along the range axis. This series of transforms is equivalent to simultaneous, parallel forward and inverse transforms on sonar echoes, yielding the impulse responses of targets by deconvolution of the spectrograms. The performance of the model accurately reproduces the images perceived by Eptesicus in a variety of behavioral experiments on two-glint resolution in range, echo phase sensitivity, amplitude-latency trading of range estimates, dissociation of time- and frequency-domain image components, and ranging accuracy in noise.
1. Flights of three big brown bats (Eptesicus fuscus) landing on a hand and catching a suspended mealworm were video analysed. 2. Results were consistent with the bats using the same basic control procedure in the quite different approach tasks--namely keeping tau (r) = kr and tau (a)/tau (r) = k alpha r. Here r is the current distance to the destination; alpha is the angle between the current direction of the destination and the goal direction of final approach (beta min); tau (r) = r/r, tau (alpha) = alpha/alpha; and kr, k alpha r are constants. 3. The bats were each quite consistent on a particular task (hand or mealworm) in the values they used for the control parameters kr, k alpha r and beta min. However, different values were used in the two tasks, which reflected the different behaviour required at the destination. Flights to hand required twisting and landing upside down and approach angle beta min was closer to vertical and kr was smaller and corresponded to decelerating nearly to a stop. In contrast, the mealworms were caught in mid flight and approach angle beta min was shallower and speed of approach was about constant. 4. tau (r) might be registered acoustically by tau (echo-delay) or by tau (echo-intensity). tau (alpha) might be registered by the bat's directional hearing and gravity sense. 5. The bat's learned the tasks easily, suggesting that the control procedure they used in the experiments was part and parcel of the natural skills they had developed in the wild.
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