For those with pure word deafness, actions always speak louder than words

The phone rings — you hear it. The caller ID displays — you read it. You pick up the phone — you say hello. But no matter how hard you listen, you can’t understand a single word that’s said either by you or the caller.

No, you haven't just crossed over into the Twilight Zone; you have a rare syndrome called pure word deafness (PWD). Individuals with PWD cannot understand any speech, even if they can identify other sounds and read written words with no difficulty whatsoever¹.

We know that the primary culprits responsible for PWD are accidents or illnesses that damage the brain and its blood vessels, including tumors, seizures, dementia and strokes². What’s less well known is where in the brain these traumatic events wreak havoc. To find out, let's first trace the journey of sound through the air and into the brain.

Back to that voice on the phone. Before your brain can even hope to interpret its meaning, the sound waves must first travel down the ear canal where they bounce off the eardrum before rattling a group of small bones called ossicles. The movement of the ossicles causes fluid inside the cochlea (a snail-shaped part of the inner ear) to vibrate.

Fig 1. How sound waves travel through the ear. 

The rippling motion of this fluid allows ions to flood into a special type of neurons called hair cells. The hair cells then convert sound wave information into an electrical signal that is transmitted to the brain. Given that you can still identify your ringtone, detecting sound vibrations and sending them to your brain isn’t the problem. The PWD must lay waste upon your auditory processing system further along the pathway.

We move onto the final frontier for the auditory information to traverse: the brain. After traveling through a series of nerves, the voice — encoded in neural signals — reaches the primary auditory cortex, a part of the brain that interprets sounds. Because you are still able to tell the difference between the sound of a phone rining and the sound of a voice, this initial auditory processing must be intact. We’re nearing the final destination of the voice on the phone, but we’re not there yet.

Next up, the neural signals from the voice arrive at Wernicke’s area, a brain region that processes language both when it arrives from outside sources (the voice on the phone), and when it’s produced and ready to exit into the world (your voice). At long last, we’ve arrived at the sticky spot that the sound can’t get through in patients with PWD³.

We know Wernicke’s area is involved with PWD because studies have shown that the majority of individuals who developed PWD suffered damage in or near this region⁴. Furthermore, a 2013 report describes a woman who developed PWD following damage to Wernicke's area, and, amazingly, after the damage healed, she regained her ability to understand spoken language⁵.

It’s unclear exactly how language is processed in this area, but scientists think that understanding language requires high speed auditory processing that normal sounds do not (consonants occur in 5 to 15 millisecond bursts). Supporting this hypothesis, research has shown that if spoken to at an exceptionally slow speed, PWD patients can comprehend words much better⁶.

Efforts to re-teach PWD patients to hear language have proven challenging. Phoneme discrimination therapy has yielded some results, but patients struggle to distinguish between the sounds “ba”, “da” and “la”, because the consonants “b”, “d”, and “l” differ only in a few milliseconds of information. Furthermore, even while patients could begin to differentiate between phonemes,they still could not understand the language they heard. The most promising approach for PWD patients is to couple the presentation of verbal sounds with visual lip-reading information.

PWD is a rare disorder, but it leaves us with a universal message — your brain must know how to talk to itself before others can talk to you. And like an elaborate game of Telephone, it only takes one misunderstanding in the grapevine of auditory processing to shut down any meaningful conversation.

Sources:

[1]  Watson, N. V., & Breedlove, S. M. (2012). The mind's machine. Sunderland, MA: Sinauer Associates, Inc.

[2] Krival, K. (2011). Pure word deafness. In J. S. Kreutzer, J. DeLuca & B. Caplan (Eds.), Encyclopedia of clinical neuropsychology (pp. 2088-2091). New York: Springer New York. doi:10.1007/978-0-387-79948-3_918

[3]  Shoumaker RD, Ajax ET, Schenkenberg T. (1977). Pure word deafness. (auditory verbal agnosia). Dis Nerv Syst, 38(4), 293.

[4]  Guilbeau, C. (2013). Auditory verbal agnosia.

[5]  Poppick, L. (2013). Pure word deafness means woman can hear sounds but not words. Huffington Post.

[6]  Poeppel, D. (2001). Pure word deafness and the bilateral processing of the speech code. Cognitive Science, 25, 679-693.