Auditory Scene Analysis by Echolocation in Bats:

Insect capture Studies & Aim of the Bat's Sonar Beam

Hanging bats

    Auditory perception involves the organization of sound, or the analysis of auditory scenes. The concept of auditory scene analysis applies broadly to all hearing vertebrates, but is perhaps most clearly illustrated in the echolocating bat, a mammal that probes the environment with sonar vocalizations and extracts three-dimensional information from the features of returning echoes. Exploiting the bat's specialized acoustic imaging system, our current behavioral studies focus on the processing of dynamic auditory signals for the perception of complex auditory scenes.

Photo courtesy, Elisabeth Kalko

    One set of perceptual experiments evaluates whether the bat can integrate information over a series of echoes to develop a three-dimensional representation of a complex, moving target. This represents an important step in determining the extent to which the mammalian auditory system can build up information from dynamic acoustic signals over time.

    Illustration of a bat pursuing a prey item in the vicinity of trees. Each panel represents a new slice in time, each separated by a fixed interval. In each panel, the bat's position relative to the trees and insect changes, as the bat pursues its prey. Below the drawing is a schematic that displays the relative timing of sonar pulses (solid bars) and target echoes (individual trees and insect echoes are represented by filled triangles, circles, and x's). Bottom shows separate plots of the time delay between each sonar pulse and returning echo depends on the distance between the bat and the reflecting object, plotted separately for each of the objects.

    Other experiments examine the phenomenon of auditory stream segregation in bats for the perceptual organization of space. In all of these experiments, we monitor the features of the bat's echolocation sounds as they discriminate sonar targets to study the animal's active control over echo information under different task conditions. The aims of this work are to develop a broad understanding of complex signal processing in biological systems and to establish an empirical foundation for integrative models of spatial information processing and the perceptual organization of sound.

    Schematic of setup for video and sound recordingsof tethered insect captures byecholocating bats.
    Two video cameras were mounted in the roomto permit 3D reconstruction of the bat's flight path. Video recordings were synchronized with audio recordings taken with ultrasonic microphones delivering signals to either a digital acquisition system (IoTech Wavebook) or a reel-to-reel high-speed recorder (Racal).

    The perceptual organization of acoustic space permits complex spatially-guided behaviors. Biosonar, for example, incorporates a sensorimotor feedback system in which the animal uses information contained in echoes to guide appropriate orienting responses. Echolocating bats in flight experience dynamic auditory spatial information, and we are conducting adaptive motor studies to examine head and pinna movements as well as sonar vocalization patterns in animals that are tracking and intercepting moving targets. A fundamental question underlying this research is whether the echolocating bat integrates sonar information between successive echoes to predict the future position of a moving target.

    These experiments involve behavioral analyses of the bat capturing insects while in flight. The bat is trained to intercept mealworms suspended by monofilament fishing line from the ceiling of the large flight room, and high-speed video and sound recordings are used to monitor the animal's motor behaviors. Smooth, erratic, and discontinuous motion of the tethered mealworm are produced by a track and cart attached to the ceiling of the flight room (Animation). In these studies, we are measuring the response latencies of adjustments in the bat's head-aim and flight path as well as distance-appropriate changes in vocal production under conditions that challenge the bat's sensorimotor system. A central objective of this work is to generate new data on sensorimotor feedback control in biological systems. This work also provides us with a valuable behavioral assay to anchor our neurophysiological studies of sensorimotor integration.

    Bat taking tethered insect           Sonar signals produced by Eptesicus Fuscus (FM)
                                                 Photo courtesy Steven Dear

    The sonar beam of the bat is not omni-directional. As a first approximation one may think of it as consisting of a major lobe aligned along the bat's head and aimed slightly downwards. The half power width of this lobe for the species of bat that we study, Eptesicus fuscus, is around 40 degrees of azimuth. Thus the beam may be said to have a direction-the axis of its main lobe. The genesis of this project was the hypothesis that the sonar beam direction in echolocating bats would be something of an analog to gaze in visual animals. The aims of this project are:

    • To find out just how qualified the hypothesis linking gaze in visual animals to the direction of the sonar beam is
    • To investigate questions related to selective attention and tracking in echolocating bats, again in loose analogy to visual animals
    • To study the spatial structure of the beam itself. (Especially a mysterious structure which we suscpect to be a large ventral lobe)
    The beam aim is a good indexof target selection in multi-target environments.
    The direction of the bat's sonar beam does not always follow the direction ofits flight path.

    Currently we have set up a giant (well, WE think so) planar array of 16 microphones arranged in a U shape round our flight room. A simple bandpass filter followed by a envelope detector circuit attached to each microphone returns the envelope of the signals (Animation). These signal envelopes are analysed offline and are used to reconstruct the overall beampattern.

    Behavioral experiments are designed to reveal the orientation of the bat's sonar beam as it searches for, selects and tracks a tethered insect target udner various conditions.