What is APD?
What is an auditory processing disorder? Hearing is usually done with two ears: Hearing is usually done in three-dimensional space: Hearing usually involves many simultaneous inputs: Making sense of what is heard is a 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 a classroom). Breakdowns do occur: APD testing looks for those breakdowns: 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 having 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.
The fact that most people have both a fully-functioning right ear along with a fully-functioning left ear is important. Each ear hears a wide range of tones and many different levels of loudness. Your brain takes the perceptions that arrive at the two inner ears (remember, one on the right and one on the left) and starts comparing the two inputs. The incoming signals are sent through very complex analysis 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.
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. Or if sound is coming from your right side or your left side. Hearing in space requires amazing coordination within your brain of the two auditory inputs from each ear. The brain is constantly comparing the two input streams in terms of time, frequency and intensity. These comparisons allow us to differentiate sounds and different speakers.
Remember that we don't hear just one tone at a time. In everyday life we have multiple sound sources simultaneously producing multiple waves of sound pressure that enter the ear all at once. For instance, if you stand alone in your kitchen, you hear the hum of the refrigerator, the clock ticking, and the water dripping from the faucet simultaneously. The sound waves from each source enter both your right ear and your left ear. Your brain takes input from the two 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. 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.
What happens when the auditory pathways of the two ears do not work together efficiently? All of those hearing abilities you have because of your two-eared hearing might not be efficient. The ability to hear and understand auditory signals in ideal environments might be fine, but not in noisy ones; there can be difficulty with fine auditory distinctions that may signal the differences between two different words; there can be difficulty separating out what signal is important in a noisy situation (and often getting side-tracked, listening to the not-important signal); there can be difficulty interpreting fine intonation patterns that we use to differentiate between statements and sarcasm, or between true questions and rhetorical questions.
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).
Even if each ear works fine alone, two-eared coordination might be a problem:
A regular hearing test (audiogram) tests each ear separately, and finds out the softest sounds that ear can detect. Then the ability of each ear to understand speech in quiet with no other background noise is evaluated (remember, testing is done in a sound-treated booth). Most people with Auditory Processing Disorders have perfectly normal audiograms. Their problem comes when the two ears have to work together in non-ideal environments: in noise or when understanding distorted signals. So a person can have “perfect” hearing for soft tones, but still have an Auditory Processing Disorder.
Another way to think about APD:
Let's look at auditory processing through an analogy. 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.
To score, you have to reach home plate:
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. When a child or adult has fully-functioning outer, middle and inner ears, but is not understanding in poor listening environments, we need to know why that batter isn't reaching home plate. Analyzing that last step is what Portland APD is about.