The study appears in the July 29 issue of Science. The distinction is enabled by the fact that the bats' sonar pulses have two distinct components, or harmonics, at different frequency levels. The higher-frequency harmonic forms a narrower beam than the widespread low harmonic, so central targets receive and reflect both harmonics in roughly equal measure. Off-target objects, on the other hand, fall outside the narrower beam of the high harmonic and thus reflect proportionally more of the low-frequency sounds.
The harmonic structure also assists in isolating insect targets from background reflections—higher-frequency sounds diminish more quickly in air, so the high harmonic returns to the bat more weakly when reflected off of distant objects. The reflected mixture of the two harmonics allows the bat to focus on what it is targeting—the echoes that come back with both harmonics intact.
Bates has contributed to Scientific American in the past. Bates and her co-authors, James Simmons of Brown and Tengiz Zorikov of Georgian Technical University's Institute of Cybernetics in Tbilisi, used an experimental setup with a live bat at the foot of a Y-shaped platform. The bats were trained to respond to a target echo from one arm of the platform and to avoid a "clutter" echo from the other arm.
Microphones near the bat and loudspeakers at the ends of the Y could be used to manipulate the echo, either by filtering out one or the other harmonic or by artificially delaying the echo of one harmonic relative to another. By so doing, the researchers could isolate the importance of the harmonic structure in the bats' separation of target from clutter. The researchers also tested how harmonics play into a facet of the bats' signal-processing machinery known as amplitude-latency trading.
Auditory neurons in the big brown bat that process echolocation signals respond more slowly to diminished echoes. Not all bats echolocate in the same way. Some bats use a CF constant frequency sonar portion, some an FM frequency modulated sweep , and some a combination of the two. Each of the two, CF and FM, server a different purpose.
CF echolocation is best suited to detection of targets read: dinner and determining doppler shift. FR echolocation is best for honing in on the details of an object and for more precisely determining its distance. The choice between CF and FR may depend on the bat's environment. When bats produce either a CF or an FM pulse or both as with the figure below , they produce not only the primary frequency, but a number of harmonics related to their primary frequency.
Of great interest is often the second harmonic, which both serves as the best measure of Doppler shift so that the bat might correct it heading for its own speed and as an identifier of the bat's call. The pattern of harmonics is slightly different and thus unique among bats, so that many bats echolocating in the same space can distinguish their calls from those of the bats around them.
When a bat begins its echolocation it usually makes short millisecond long pulses of sonar and listens for the returning echoes see above. If the echo indicates prey, it will generally fly towards the source of the echo while continually emitting pulses of increasing rapidity and shorter duration. A schematic representation of bat auditory cortex. For some time the importance of the Doppler shift has been eluded to in this summary. The Doppler shift is a phenomena that occurs with sound waves.
As a bat produces pulses of sonar they are reflected back from a target. However, the pulses are not just reflected back 'as is,' there is some compression of the sound waves which causes them to reflect back at a higher frequency. In the case of an insect, the wing beat of the insect causes its own Doppler shift to be superimposed on the existing Doppler shift. The mustached bat can detect these ripples in the Doppler shift and this leads them to their insect target. The mustached bat engages in what is known as Doppler-shift compensation, that is the bat lowers its own emitted CF so that the Doppler shifted echo returning from the insect is precisely in the range where the bat has the best chance of detecting it Thus the specialization at the level of the basilar membrane and spiral ganglion makes intuitive sense.
But the bat takes this analysis one level higher. In the tonotopic organization of the auditory cortex there is a relatively large portion of the cortex dedicated to the frequencies between This is known as the DSCF area Doppler shifted constant frequency area , illustrated in pink in the above figure.
This area's frequencies are specialized for the mustached bat based on its resting frequency of 61 KHz; however, another species of bat with a different resting frequency would have its own frequency focus. This area is often referred to as an 'acoustic fovea', drawing a comparison between the fovea in the visual system of some animals and the DSCF area in bats.
The neurons of the DSCF area have a particular frequency and amplitude that will excite them maximally. The cells are also arranged in a columnar fashion, similar to the cytoarchitecture in the visual cortex of the cat.
Following that analogy, cells in perpendicular colums in the DSCF area respond to one particular frequency and amplitude. Unlike other areas in the bat's cortex, cells in the DSCF area only respond to the frequency and amplitude of the echo of the 2nd harmonic of the CF. They do not respond to the emitted pulse. The area is hypothesized to be used in discriminating minute differences in frequency, the sort that would cause a flying insect to appear above the background noise and existing Doppler shift.
Suga has hypothesized that this is the "region responsible for the precision of the Doppler-shift compensation but not for performing the actual compensation. This is the area in the cortex where the bat calculates the actual Doppler-shift from the target.
The bat does this by comparing the frequencies between the CF pulse and its 2nd or 3rd harmonic echo. The cells are arranged according to the 1st harmonic of the CF longitudinally.
Along the width of the area, cells are arranged according to the frequency of the returning echo. It has been found that the velocities from zero to 4 meters per second are overrepresented in that map because of the bats need for precision at those speeds for landing or catching prey.
The colors in the figure correspond to the colors in the first figure in the neural substrates section. Gross connectivity from auditory input to cortex.
Although there is more to echolocation in bats than what has been mentioned in this web page, I believe it is clear how exquisite the system for the production and analysis of echolocation signals is in the bat. Many structures at all levels of the nervous system, from basilar membrane to cortex, have been dedicated to permitting the mustached bat to navigate and catch prey.
As research continues on this species and others, more information is being gained on what occurs at the level of the auditory brainstem and the thalamus that permits for such detailed processing at the cortical level. It seems impossible to approach the bat from anything but a neuroethological perspective, since it appears that each specialization is driven by the environment of the bat, and the particular frequencies that it produces.
All pictures and figures from Suga, N. Bisonar and neural computation in bats. Jim Buzbee's bat house site has every bat link imaginable The University of Michigan's collection of information on bats. An amazing archive of bat pictures! Griffin, D.
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