Entry 041: Implausible Speech
Last entry we discussed a patient who could consciously perceive most every sound except a human voice. Here I describe two people who had nearly the opposite problem: they could identify spoken language without error. What they couldn’t identify is anything else.
The first person was a 52-year-old stroke patient from Mexico. He could speak without impediment, and, importantly, could understand other people’s speech. But he was unable to identify most other sounds, ranging from babies crying to people coughing. His deficit was remarkably specific.
The second patient hailed from Japan. His speech was similarly preserved, yet he displayed the same difficulties as his 52-year-old colleague. He had severe difficulties identifying environmental noises, such as car horns, and ringing telephones.
Both patients suffered from auditory agnosia. People with this condition have impairments in perceiving and/or identifying sounds. The deficit can be global or, as the symptoms of these two men show, almost implausibly precise. There appears to be a bright dividing line between our ability to recognize human voices and virtually every other sound in our perceptual soundscape.
Understanding that implausibility invites us to continue our discussion about auditory processing. We left off at the inner ears, whose shape I likened, like almost everyone else who teaches this material, to a snail. The end goal of this snail-like inner ear is to convert mechanical energy from vibrations into electrical signals the brain can understand. Deficits like auditory agnosia show us when that understanding goes awry.
The inner ear sends its electrically encoded information into our skulls via the auditory nerve. After the inner ear converts the vibrations into electricity, the signal is funneled through that nerve, which then introduces its information to the brain.
BRAIN PROCESSING
And introduce it does, though a bit circuitously.
The first stop is the thalamus, an egg-shaped structure in the middle of the brain. We discussed the thalamus in Entry 012. Its name is derived from a Greek word meaning “bedroom” (or “inner chamber) which, puzzlingly, has nothing to do with eggs.
The thalamus acts not so much like a bedroom as it does an air traffic control tower. Its job is to relentlessly route high-flying sensory signals to specific brain regions for further processing. That includes hearing. The thalamus directs signals from the auditory nerve into the primary auditory cortex, specifically to a region called area A1. These sound processing tissues – you’ve got both left and right ones - lie just above your ears.
This left side/right-side distinction is important for many reasons, including processing assignments. The left side deals with most aspects of speech. The right side processes rhythmic information and music and something called the prosody of speech, (more on that later). Both are important for comprehensive sound perception.
We’re not done with our story, of course. At some point we’ll need to explain auditory agnosia. Here’s a hint: Because the job assignments are uneven in those cortices, you can lose the function of one region while leaving the other functions perfectly intact.
It is to those differences that we turn next.
REFERENCES
Polster, M.R., and S.B. Rose. "Disorders of Auditory Processing: Evidence for Modularity in Audition" Cortex 34, no. 1 (1998): 47-65.
Spreen, O. et al. "Auditory Agnosia without Aphasia." Arch Neurology 13 (1965): 84-92.
Fujii, T. et al. "Auditory Sound Agnosia without Aphasia Following a Right Temporal Lobe Lesion." Cortex 26 (1990): 263-68.