What is an Auditory Processing Disorder?

Binaural. The fact that you have two ears is important. Each ear hears tones and loudness. Your brain takes the perceptions that arrive at the inner ear and sends them through very complex pathways that start deep in the brain, but ultimately arrive at the cortex, the top layer of the brain. We call the network specialized for receiving sound in the brain the "Auditory Pathways" or the "Central Auditory Pathways" of the brain.

Spatial hearing. These pathways are crucial to your ability to hear in space. Spatial hearing allows you to judge whether someone talking to you is either near or far away. Hearing in space requires amazing coordination within your brain of the auditory inputs from the two ears. The brain is constantly comparing the two input streams in terms of time, frequency and intensity, and this allows us to differentiate sounds and different speakers. Remember that we don't hear just one frequency at a time. In everyday life we have multiple sound sources producing multiple waves of sound pressure that enter the ear simultaneously . For instance, if you stand alone in your kitchen, you hear the hum of the refrigerator and the ticking of the clock and the dripping of water from the faucet simultaneously. Your brain takes input from each of your ears, compares the input and comes to a conclusion about what the sound represents. In a room filled with talking people, think of how much more sound is entering your ears over the course of a few minutes.

Many simultaneous inputs. Typical listening involves hearing multiple sound sources all the time, and each input is an array of tones that need to be sorted out and processed. When you are able to understand speech in a noisy setting or able to understand a distorted signal, it is because your auditory pathways have been very efficient in synchronizing the two inputs from your two ears.

Breakdown. What happens when the auditory pathways of the two ears do not work together efficiently? It appears that the result are Auditory Processing Disorders: ability to hear and understand auditory signals in ideal environments, but not in noisy ones; difficulty with fine auditory distinctions that may signal the differences between two different words; difficulty separating out what signal is important in a noisy situation and often getting side-tracked and listening to the not-important signal; difficulty interpreting fine intonation patterns that we use to differentiate between statements and sarcasm, or between true questions and rhetorical questions.

Metaphor. Let's look at the processing through a metaphor. The ear has four parts: the outer ear, the middle ear, the inner ear and the central auditory pathways. Think of these four parts like a baseball diamond. To understand clearly and correctly you have to hit a "home run" when it comes to sound and that means touching all the bases. If a sound doesn't even get past the outer ear, that is like a batter who was called "out" before reaching first base. Even if the sound reaches third base, that still isn't good enough for the team to score. The ears work in synchrony only when all four bases have been reached. So a problem with the central auditory pathways is like a batter who slams the ball to the outermost part of the stadium and runs to first, second and third bases, but then seems to forget what to do next and perhaps runs a lap around left field instead of heading for home. That won't help the team's score.

Comparing. Auditory Pathways from each of the two ears must constantly compare multiple incoming sounds in order to make fine timing and loudness judgments. When your brain does this successfully you are performing what scientists call "Auditory Stream Analysis" - separating out and making meaning of multiple simultaneous inputs. This skill is crucial for efficient auditory processing.

Developmental skill. Auditory processing improves with maturation . Babies are not very good at it, but as they grow, their ability to hear in noise and understand distorted signals improves. By the time a child reaches seven years of age, auditory processing skills should be developed enough to attend to a main signal in a complex sound environment - like classrooms!

Testing. Some people have a delay in maturation of the Auditory Pathways, but catch up after a few years. Others have Pathways that aren't working efficiently as a permanent characteristic. To understand how the Central Auditory Pathways are working, audiologists have special tests that measure the coordination and efficiency of the Auditory Pathways. (Those are the tests offered by Portland APD).

Audiogram normal. The first part of a typical hearing test (an "audiogram ") evaluates the hearing of each ear separately. The second part of a hearing test evaluates how the two ears work under ideal conditions: when it is quiet and there is no background noise. Most people with Auditory Processing Disorders have a perfectly normal audiogram. Their problem comes when the two ears have to work together in not-ideal environments: in noise or understanding distorted signals. An audiogram is a prerequisite to an evaluation for Auditory Processing Disorders.

Getting home. To take our analogy a little further: if a batter is called out at first or second or third base, everyone understands that the hit will not be counted as a home run. But if the batter does get through the first three bases, we want to find out why the batter isn't reaching home base. Analyzing that last step is what Portland APD is about.