Untangling Speech

By HENRY FOUNTAIN (NYT) 646 words
Published: May 25, 1999


Untangling Speech
 

Back in the 1960's, fans of the Beatles spent hours playing their albums backward, destroying many a phonograph needle to decipher messages supposedly recorded in reverse concerning Paul McCartney's fate.

New research shows, however, that backward speech does not have to be unintelligible. Much as the brain can fill in missing words in a half-heard sentence at a noisy cocktail party, it can also make sense of reversed speech. If the Beatles had recorded ''Paul is dead'' in a certain way, all those needles might have been saved.

In a study reported in Nature, scientists at the California Institute of Technology and California State University at Los Angeles took a recorded sentence, broke it into equal short segments, and reversed each segment. They strung all the reversed segments together and played the sentence to listeners. When segments were about 50 milliseconds long, the sentence was intelligible. But as segments got longer, intelligibility declined, approaching zero at 200 milliseconds.

The research lends support to recent theories of how speech is perceived. These theories hold that the brain does not rely so much on analyzing the spectrum of short-term sounds as it does on lower-frequency cues like intonation and modulation. As long as the segments were short enough, the brain was still able to decipher these cues.