P. kuhlii above a spectrogram of its echolocation sequence.
Source: Eran Amichai, used with permission.
Many bats navigate using echolocation — emitting high frequency sound pulses and analyzing the returning echoes. The ability to accurately estimate distances is a hallmark of bat echolocation. Bats assess their distance from an object by the time it takes for their sounds to bounce off the object and return to them. They are extremely good at this task; some experiments have shown bats can discriminate distances of 3mm or smaller.
Bats achieve this accuracy with the help of delay-tuned neurons in their brains. These are cells that are activated by specific call-echo time delays, hypothesized to encode target distance. However, it is unclear whether translating a signal-to-echo delay into distance is an innate or learned behavior. When a baby bat takes off for the first time, does its brain correctly translate time delays into distance?
Speeding Up Sound
Yossi Yovel, of Tel Aviv University, and Eran Amichai, now at Dartmouth University, addressed this question by inducing an auditory illusion in echolocating bats. Since translating time into distance relies on a reference of the speed of sound, they shifted the speed of sound to see how it affected bats’ sensory behavior. Yovel and Amichai report the results today in the Proceedings of the National Academy of Sciences.
The researchers reared two groups of Pipistrellus kuhlii bat pups from birth to the age of independent flight. One group was reared in a flight chamber with normal air, while the other group was raised in a chamber in which the nitrogen in the air was replaced with helium. Since sound propagates faster in helium-enriched air than in normal air, these pups grew up in an environment where the speed of sound was 15% faster than normal.
“This means the bat calls, its sound travels quickly, hits the target, and travels back to it quickly as well,” says Amichai. “In this way, we trick the bat’s brain into thinking the target is closer.”
Next, Amichai and Yovel trained the bats to fly to a vertical target positioned 1.3m away in order to eat. Since bats adjust their echolocation parameters (such as duration and interval of their sounds) before and during flight to a target, these features can reveal where they think the target is before they take off and as they fly to it.
An adult bat flying from its perch to the target.
Source: Eran Amichai, used with permission.
After testing pups in the environments in which they were raised, the researchers took them to the other flight chamber to see if they would adjust to a new speed of sound. They also ran similar experiments on adult bats to see if adults could adjust to an increase in the speed of sound.
Out of Focus
Amichai and Yovel report that pups reared in helium-enriched air underestimated target distances, as did pups reared in the normal environment when they were exposed to helium-enriched air. Using a visual metaphor, one could say the bats “focused” their echolocation too close.
The same was true for the adult bats, who showed no flexibility in updating their speed of sound reference to the environment. All adult bats underestimated target distances and accuracy did not improve over time.
Amichai says that people’s experience diving underwater with a mask provides a useful metaphor for this study. For first-time divers, everything seems larger and closer and they cannot correctly assess sizes and distances. Experienced divers have much smaller errors, though they do not compensate consciously — their brains do the “conversion” automatically, based on experience. But when a similar error is induced in bats’ echolocation, they are unable to do the conversion.
As both pups and adults were unable to adjust to the shift in the speed of sound, the researchers suggest that it is innately encoded in the bat brain.
“Our experiments let the bats gather information about their errors,” says Amichai. “They could miss the target and then adjust their echolocation and flight accordingly, but they never did. Their sensory behavior was always completely dependent on the timing of the echoes.”
Understanding the Bat Brain
The researchers say this suggests that bats do not measure echo-delay and translate it into distance but react to delay only. They encode the world in terms of time and do not translate time into distance. In comparison, Amichai gives an example of a person reaching for their phone on a table. Their brain estimates the distance to be a foot and a half. The bat’s brain, on the other hand, will tell it the phone is 1.5 milliseconds away.
Adult P. kuhlii.
Source: Leonardoancillotto86, via Wikimedia Commons. Distributed under a CC BY 3.0 license.
So why do bats not exhibit sensory flexibility? Why is time perception innate in echolocating bats rather than learned or adaptable? Yovel and Amichai suggest that there is a strong selection pressure to have a functional sensory system as soon as possible. Newborn bat pups can begin flying by three weeks old and usually reach independence within ten weeks. At this time, they must be able to use echolocation to navigate and hunt, or risk not surviving the winter. This ecological constraint might have driven the innate neural basis of time-delay processing in the bat’s brain.
Amichai says that although this study gets us closer to imagining what it is like to be a bat, there are still large questions open, especially in how the environment, the senses, and the brain interact to influence behavior.
“We still have an incomplete understanding of how exactly the brain processes the information provided by the senses into relevant behavior,” he says. “Our study provides some insight into this, and into the experience of being a bat.”