Sound learning may hinge on cue contrasts

A person walking down the street can quickly use acoustic cues to locate, say, the

position of a car approaching from behind or of a radio blaring from an open

window. The blast of the car’s horn reaches the ear nearest the car first, and the

deejay’s booming voice sounds slightly louder in the ear nearest the radio.

With training, people can improve on such sound perceptions, but some basic

acoustic skills respond far more than others do, a new study suggests. Volunteers

given extensive practice became progressively better at perceiving slight

differences in the loudness of sounds delivered simultaneously to their right and

left ears, say Beverly A. Wright and Matthew B. Fitzgerald, both neuroscientists

at Northwestern University in Evanston, Ill.

Yet given just as much practice, other trainees exhibited only modest gains in

their perception of subtle alterations in the timing of equally loud sounds

entering each ear, Wright and Fitzgerald report in the Oct. 9 Proceedings of the National Academy of Sciences. Most learning on this task occurred in early

practice sessions and then leveled off, the researchers say.

“There are at least two different acoustic-learning mechanisms involved in sound-source location, and one is much more trainable than the other,” Wright says. This

line of research may lead to more-effective treatments for speech and language

disorders related to hearing problems, in her view. It may also improve training

for jobs that require sharp hearing and the rapid identification of sound sources.

However, the reasons for greater improvement in one distinction but not the other

“remains a puzzle,” remarks psychologist Merav Ahissar of Hebrew University in

Jerusalem in a comment published with the new report. It’s not inherently more

difficult to perceive the arrival time of sounds or to undergo training in this

skill, says Ahissar, who studies acoustic perception.

Earlier research had indicated that separate groups of nerve cells sort out these

two lines of information before they converge elsewhere in the brain’s acoustic

cortex.

Wright and Fitzgerald studied 32 adults, ages 18 to 44. None had any hearing

problem. While wearing headphones, volunteers completed two initial trials. In

one, they tried to discern loudness changes in pairs of tones presented

simultaneously to each ear. In the other, they attempted to note changes in the

timing of comparably loud tones presented to each ear.

Half the participants then received training on one or the other of the two

acoustic tasks. Over 9 or 10 days, each worked at the task daily for 1 hour.

In the trained group, performance on each task rose noticeably over the first

couple of hours of practice, the scientists say. After that, volunteers exhibited

few gains in their perception of changes in sound timing. In contrast, enhanced

discrimination of differing sound levels continued to improve throughout training.

When tested 2 weeks after the initial trials, people who had received training on

loudness differences performed substantially better on that task than their

untrained peers. Training in telling apart the timing of sounds yielded only a

small advantage over no training.

“This is the first step toward being able to pick and choose tactics for rapidly

training people to make specific types of auditory discriminations,” Wright says.

Further research needs to examine whether the same acoustic learning patterns

occur in real-life situations, she adds.

The discovery of sharp contrasts in the ability to learn specific types of

acoustic discriminations coincides with research on visual training, comments

psychologist Robert L. Goldstone of Indiana University in Bloomington. “There’s a

lot of plasticity in the ability to learn to discriminate some types of visual

cues, whereas there’s almost no learning for others,” he says.