John Medina's Brain Rules

Entry 042: Top-down management

Heres’ a fact concerning sound perception that’s about as hard to ignore as it is hard to believe: You don’t hear what is out there. You hear what your brain thinks is out there.

Want proof?

Suppose I ask you to listen to this recorded sentence:

“The state governors met with their respective legislatures convening in the capital city.”

I might inquire: “Did you hear anything weird?” You say no. The sentence plays again, and I repeat “Did you hear anything weird?” You again say no, then inquire why I’m asking.

I respond: “One of the words was substituted with a coughing sound, and I’m wondering if you can tell me which word was replaced.”

The recording is played again. You still don’t hear the cough, even though it’s there. Your brain inexorably fills in the missing information with something that doesn’t exist.

This experiment was done years ago.

So was this one. Suppose you listen to this oddly constructed sentence: “It was found that the _eel was on the orange.” That blank exists at the start of the sixth word because no English sound resides in that position. It has been replaced with white noise. All you really hear is [white noise] eel. When I ask you to repeat what you just heard, you say “It was found that the peel was on the orange.” Not “there was static followed by the word eel.” Your brain substituted the word “peel” into the sentence.

Suppose I rerun the experiment, preserving the white noise at the same position, but changing the last word. Now it reads: “It was found that the _eel was on the axle.” Now what do you say you heard? “It was found that the wheel was on the axle,” you reply. Your brain altered what you heard yet again.

I can do this all day long. If the last word is shoe, you’ll hear the word “heel” in the sentence. If the last word is table, “meal”, is what you’ll hear.  

Experiments like these show that we hear with our brains, not our ears. They are great examples of top-down processing, something we’ve discussed before (Entry 007).

WHERE WE WERE

Top-down processing begins when auditory information is transformed into electrical signals, then enters the brain. Top-down’s signature features are annoying tendencies to a) look at prior auditory experiences, b) compare new signals to those experiences, c) mix-down the whole thing,  inserting or subtracting information in the light of previous experiences.

Top-down is why you hear “peel” if there’s an orange nearby, “wheel” if there’s an axle, “meal” if there’s a table. Though none of those words exist in the original signal, your brain cheerily supplies them free of charge.

Knowing how we typically hear things allows us to make better sense of the auditory machinery when that machinery breaks down. It begins with understanding that you don’t hear with your ears. You hear with your brain.

Hard to ignore, certainly, Perhaps hard to believe, too.

REFERENCES

Hines, T. "A Demonstration of Auditory Top-Down Processing." Behav Res Methods, Instruments & Computers 31, no. 1 (1999): 55-56.

Warren, R.W. "Perceptual Restoration of Missing Speech Sounds." Science 167, no. 392-393 (1970).