Credit: Daniel Robert

Humans and other "higher" animals aren't so special when it comes to making life-or-death decisions in an instant, a Cornell University study of insect hearing has found. Even the lowly cricket employs a sophisticated capability, called categorical perception, when its life (or love life ) is at stake.

"Crickets -- and probably many other types of animals -- have found a simple way to build a system that responds quickly," said Robert A. Wyttenbach, the Cornell postgraduate associate of neurobiology and behavior who gave crickets a perception test originally developed for human infants. "Crickets have to make a yes-or-no decision in a hurry, and ones that waffle become bat bait."

For a cricket flying through the night air, life is a sound spectrum-filling cacophony, Wyttenbach said, but only two sound sources really matter: other crickets, calling at 4 to 5 kHz (4-5 kilohertz, the familiar chirping that is just above the highest note on the piano), and insect-eating bats, emitting ultrasound that helps echolocate their prey at 25 to 80 kHz (humans with good hearing can detect sounds up to 20 kHz).

Steering toward the low-frequency cricket sound might help the flier find a mate, or at least the company of other crickets. However, failing to steer away from bats' ultrasound could cost the cricket not only its life but its place in the gene pool, explained Ronald R. Hoy, the Cornell professor of neurobiology and behavior whose laboratory conducted the experiments. Researchers led by Hoy reported in 1978 that crickets can detect bats' ultrasound and take evasive action, but those flight studies never answered the question: What is the cut-off line between "good" sounds and "bad", and how does the insect decide?

The study is reported in the journal Science,(Sept. 13, 1996), in a report by Wyttenbach, Hoy and Michael L. May, a former Cornell graduate student, "Categorical Perception of Sound Frequency by Crickets."

The Cornell biologists set up a cricket flight chamber, wired for sound, computerized data-gathering and a built-in breeze from a fan, to make the cricket think it is flying while tethered in midair. "We weren't looking for the so-called continuous perception," Wyttenbach said, explaining that continuous perception allows human color vision to discriminate hundreds of variations between "blue" and "green," even though humans cannot precisely label each bluish-sort-of-green color.

"A more useful ability in some situations is our ability to tell the difference between sounds like 'ba' and 'pa,' " Wyttenbach noted. "Even though there is a continuum of variation between 'ba' and 'pa' based on voice onset time or VOT, which instruments can detect, our ears don't discriminate. We hear either 'ba' or 'pa' and that's how we label it. That is categorical perception, and it looked like crickets might categorically perceive sounds as either 'other crickets' or 'bats.' "

Categorical perception previously had been shown at other laboratories in monkeys, chinchillas and some species of birds, Wyttenbach knew, but getting crickets to explain how they make up their minds would be tricky. His flight chamber experiments were showing that playback sounds mimicking cricket calls made the tethered insects "fly" toward the speaker. And sounds in the bat's ultrasound range made the crickets veer away from the speaker. Still, he couldn't very well ask the crickets: "Do 20 kHz and 40 kHz sound the same or different?"

So the neurobiologists turned to a test suggested by Cornell Professor of Psychology Elizabeth Spelke for another category of notoriously uncooperative research subjects, human babies. Spelke's habituation-dishabituation test determines precisely what stimulus level is "different enough" to get a subject's attention when it has grown accustomed to (or habituated to ) repeated stimuli. Babies respond less and less when they hear "pa" repeated over and over; they pay attention again when the experimenter throws in a dishabituating "ba."

Repeated stimulation at 20kHz habituated Wyttenbach's crickets, and each time they steered less and less vigorously from the speaker. (In the real world, that habituated response would have pleased the bats.) Next, the neurobiologist tried to get the crickets' attention and dishabituate them back to normal behavior by playing a variety of sounds. Sounds between 5 and 16 kHz were different enough (from 20 kHz ) to change the insects' behavior, he demonstrated, and 16kHz was the dividing line. He showed that crickets label sounds above 16kHz as "foe" and those below as "friend," and that they are capable of categorical perception.

Does this mean that crickets, with just a couple hundred thousand neurons in their brains, are as intelligent as humans, with billions of neurons? "No, of course not," Hoy said. "It does say that all of us animals have to solve many of the same problems. Some with many fewer brain cells have come up with a comparable, workable solution. We may now expect to find categorical perception in many different systems, between insects and humans."