Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 06 table of contents.
Just as the eye has a delay system to cling onto sensory information, so does the ear. The auditory information store, dubbed echoic memory by Neisser (1967), lasts one or two seconds. Echoic memory can also be called the auditory store or auditory sensory register.
What is echoic memory and how long does it last? How did Guttman and Julesz test the duration of echoic memory and what did they determine?
One creative experiment designed to measure echoic memory was carried out by Guttman and Julesz (1963). They used a computer to generate repeating segments of white noise. White noise is composed of all frequencies randomly mixed together. It sounds like "shhhh" and cannot be described or memorized. The computer made it possible to put together a repeating pattern of white noise with no gap between repetitions. The subjects had no clue that a sound was being repeated. Guttman and Julesz instructed subjects to put on headphones, listen to the noise, and report what they heard.
If the repeating segment of white noise lasted longer than a few seconds, the subjects never realized it was repeating. They heard a continuous whooshing sound with no pattern. If the segment of white noise was less than two seconds long, the subjects suddenly realized they heard a repeated sound. They still could not describe the sound (other than saying "shhhh") but they knew it was being repeated.
To detect a repeating pattern of random frequencies, subjects must use a memory system capable of preserving an exact copy of the noise from one repetition to the next. This is what echoic memory does: it preserves the exact pattern of sound for one or two seconds.
How does the "Why did you say?" phenomenon illustrate echoic memory?
A less scientific demonstration of echoic memory is the "What did you say?" phenomenon, which goes like this:
Person #1: "What time is it?"
Person #2: "What did you say? Oh, 2:30."
The second person hears the question after asking, "What did you say?" This is due to echoic memory, which holds the sound of the question for a second or two. Even if you were not paying attention to the words when they were uttered, you can "hear" them when you turn your attention to them. This can be annoying to the person who starts repeating the question only to be interrupted by an answer.
Development of brain scanning technology made it possible to observe echoic memory in the brain. Using MEG (magnetoencephalography),Lu, Williamson, and Kaufman (1992) were able to show activity in a portion of the auditory cortex (part of the cerebral cortex which responds to sound) lasting two to five seconds after a sound stimulus.
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