Decades of research on how bats use
echolocation to keep a focus on their targets not only lends support
to a long debated neuroscience hypothesis about vision but also could
lead to smarter sonar and radar technologies.
Amid a neuroscience debate about how
people and animals focus on distinct objects within cluttered scenes,
some of the newest and best evidence comes from the way bats “see”
with their ears, according to a new paper in the Journal of
Experimental Biology. In fact, the perception process in question
could improve sonar and radar technology.
Bats demonstrate remarkable skill in
tracking targets such as bugs through the trees in the dark of night.
James Simmons, professor of neuroscience at Brown University, the
review paper’s author, has long sought to explain how they do that.
It turns out that experiments in
Simmons’ lab point to the “temporal binding hypothesis” as an
explanation. The hypothesis proposes that people and animals focus on
objects versus the background when a set of neurons in the brain
attuned to features of an object all respond in synchrony, as if
shouting in unison, “Yes, look at that!” When the neurons do not
respond together to an object, the hypothesis predicts, an object is
relegated to the perceptual background.
Because bats have an especially acute
need to track prey through crowded scenes, albeit with echolocation
rather than vision, they have evolved to become an ideal testbed for
the hypothesis.
“Sometimes the most critical
questions about systems in biology that relate to humans are best
approached by using an animal species whose lifestyle requires that
the system in question be exaggerated in some functional sense so its
qualities are more obvious,” said Simmons, who plans to discuss the
research at the 2014 Cold Spring Harbor Asia Conference the week of
September 15 in Suzhou, China.
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Finding with frequencies: Bats send out
harmonic pairs of frequencies to sense where things are. The strength
differences in the high and low frequencies in the pair (minimal in
red, greater in blue) help the bat focus on the target front and
center. Image: James Simmons/Brown University
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A focus of frequencies
Here’s how he’s determined over the
years that temporal binding works in a bat. As the bat flies it emits
two spectra of sound frequencies — one high and one low — into a
wide cone of space ahead of it. Within the spectra are harmonic pairs
of high and low frequencies, for example 33 kilohertz and 66
kilohertz. These harmonic pairs reflect off of objects and back to
the bat’s ears, triggering a response from neurons in its brain.
Objects that reflect these harmonic pairs in perfect synchrony are
the ones that stand out clearly for the bat.
Of course it’s more complicated than
just that. Many things could reflect the same frequency pairs back at
the same time. The real question is how a target object would stand
out. The answer, Simmons writes, comes from the physics of the
echolocation sound waves and how bat brains have evolved to process
their signal. Those factors conspire to ensure that whatever the bat
keeps front-and-center in its echolocation cone will stand out from
surrounding interference.
The higher frequency sounds in the
bat’s spectrum weaken in transit through the air more than lower
frequency sounds. The bat also sends out the lower frequencies to a
wider span of angles than the high frequencies. So for any given
harmonic pair, the farther away or more peripheral a reflecting
object is, the weaker the higher frequency reflection in the harmonic
pair will be. In the brain, Simmons writes, the bat converts this
difference in signal strength into a delay in time (about 15
microseconds per decibel) so that harmonic pairs with wide
differences in signal strength end up being perceived as way out of
synchrony in time. The temporal binding hypothesis predicts that the
distant or peripheral objects with these out-of-synch signals will be
perceived as the background while front-and-center objects that
reflect back both harmonics with equal strength will rise above their
desynchronized competitors.
With support from sources including the
U.S. Navy, Simmons’s research group has experimentally verified
this. In key experiments (some dating back 40 years) they have sat
big brown bats at the base of a Y-shaped platform with a pair of
objects – one a target with a food reward and the other a
distractor – on the tines of the Y. When the objects are at
different distances, the bat can tell them apart and accurately crawl
to the target. When the objects are equidistant, the bat becomes
confused. Crucially, when the experimenters artificially weaken the
high-pitched harmonic from the distracting object, even when it
remains equidistant, the bat’s acumen to find the target is
restored.
In further experiments in 2010 and
2011, Simmons’ team showed that if they shifted the distractor
object’s weakened high-frequency signal by the right amount of time
(15 microseconds per decibel) they could restore the distractor’s
ability to interfere with the target object by restoring the
synchrony of the distractor’s harmonics. In other words, they used
the specific predictions of the hypothesis and their understanding of
how it works in bats to jam the bat’s echolocation ability.
If targeting and jamming sound like
words associated with radar and sonar, that’s no coincidence.
Simmons works with the U.S. Navy on applications of bat echolocation
to navigation technology. He recently began a new research grant from
the Office of Naval Research that involves bat sonar work in
collaboration with researcher Jason Gaudette at the Naval Undersea
Warfare Center in Newport, R.I.
Simmons said he believes the evidence
he has gathered about the neuroscience of bats not only supports the
temporal binding hypothesis, but also can inspire new technology.
“This is a better way to design a
radar or sonar system if you need it to perform well in real-time for
a small vehicle in complicated tasks,” he said.
In addition to the Office of Naval
Research (grants: N00014-04-1-0415, N00014-09-1-0691), support for
Simmons’ research comes from the National Science Foundation
(IOS-0843522), the National Institutes of Health (R01-MH069633), the
NASA/RI Space grant, and the Brown Institute for Brain Science.
Contact
David Orenstein
401-863-1862
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