"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 a
Contact: Roger Segelken
Cornell University News Service