Bat echolocation

Neuroethology - Biology 419/580

Bowling Green State University, Spring 2004

Bat Echolocation - Hardware and Mechanisms

Auditory processing for Bat Echolocation

A. The ear
- 1. Bat adaptations
B. The cochlea
- 1. Place coding
C. Bat auditory sensitivity

Brain systems: Separate neural circuits in distinct brain centers and cortical regions examine in parallel the signal for estimates of target distance (FM area) and velocity (CF area). Frequency modulated calls (broadband sweeps) are specifically useful for assessing distance (i.e., the time delay of the echo) as the call at each frequency represents a discrete and brief moment in time when the call was emitted. In contrast calls that remain for some time at a constant frequency (CF calls) are much more suitable to Doppler-shift analysis (i.e., variations in the frequency of the returning echo indicate movement).

Inferior colliculus

Neurons produce highly phasic responses to pulse and echo. They reliably exhibit a brief pulse of activation to the onset of the call and its echo across a range of intensities and frequencies.

Auditory Cortex:

FM-Area: Inactivation of the FM - FM area using local anesthetics prevents bats from judging distances accurately in psychophysical experiments. Neurons are highly selective in the auditory signals they respond to: they do not respond to a call or an echo alone and they do not respond to CF emissions. They only respond to the pairing of call and echo separated by a particular delay (i.e., delay selective neuron). When a call is emitted it includes the fundamental (carrier) frequency (FM1) and its various harmonics (FM2, FM3, FM4 ...). The auditory cortex compares the timing of the signal for a call (using the fundamental frequency transmitted through the bones of the skull and not the middle ear) with its echo (the auditory signal for its harmonics).
CF Area: Processes information from CF pulses with a specific pairing of call and echo. Neurons are designed as frequency-difference detectors (frequency shifts). These neurons only respond to the combination of pulse and echo where the precise time separating them is less important. Neurons respond best to a combination of the CF1 pulse, and a specific frequency of CF2 or CF3 echo. if the object to be imaged is moving then Doppler-shift will alter the precise frequency of the returning echo. A specific change in frequency between FM1 for the call and FM2 (or any of the higher harmonics) for the echo indicates a specific velocity of the object causing the echo. Somatotopic maps represent CF1 frequencies on one axis and CF2/CF3 frequencies on the other where each point on the map represents relative velocity. the DSCF (Doppler-shifted CF) area is a specialized area that occupies the largest proportion of bat CF cortex (30%). It responds only to a limited frequency range around the CF2 of the call (e.g., 60.6 - 62.2 kHz for a bat with a 61 kHz CF2). Neurons code only for the CF2 echo and not for call at any harmonic or for the echo of any harmonic other than CF2. Frequency and amplitude for CF2 are mapped systematically with the best frequency radially and echo amplitude circularly

Discussion:

How can you find your way in a cave where millions of other bats use their calls to image the surroundings. How do you distinguish echos to your signals from everyone else's?

last modified: 2/14/04