Imagine that your hearing sensitivity for pure tones is exquisite: not affected by the kind of damage that occurs through frequent exposure to loud music or other noises. Now imagine that, despite this, you have great problems in understanding speech, even in a quiet environment. This is what occurs if you have a temporal processing disorder, potentially diagnosed as auditory neuropathy, dyslexia, specific language impairment or auditory processing disorder. Your ears works fine, yet, for some reason, your brain fails to acknowledge this.
Sound is characterized by its frequency content (low-to-high tones) and its temporal dynamics (rapid fluctuations in loudness), which together determine its pitch and timbre. The temporal aspect becomes important when the sound is complex, i.e., contains many harmonics, such as a kitten vocalization (Figure), a musical note, or a spoken sentence. The scale of the dynamics varies from the periodicity in a high tone (rapid fluctuations inversely proportional to the frequency), that in a vowel reflecting the vocal cord vibration (Figure), and in the prosody of a spoken sentence. A harmonic sound, such as a musical note, can have both a periodicity pitch and a frequency pitch.
Sound is a fast and tiny fluctuation in the ambient air pressure that causes movement of the eardrum, transmitted via tiny bones in the middle ear to fluid in the inner ear, which causes a vibration mimicking the sound pressure fluctuations in the basilar membrane. Siting on top of that membrane are the hair cells that function as microphones and produce electric fluctuations that cause the release of a chemical, causing the auditory nerve fibers to fire action potentials (electric pulses) in synchrony with the sound pressure fluctuations. The auditory nerve comprises about 30,000 fibers that normally produce action potentials in mutual synchrony.
Problems with conveying these high periodicities can occur in the interface (synapse) between hair cell and auditory nerve fiber; either through a genetic problem or by long exposure to occupational or recreational noise. This problem is called ‘auditory synaptopathy’. Problems can also occur due to dysfunction of the insulating layer around each auditory nerve fiber, also caused by gene expression problems; this is called ‘auditory neuropathy’. The phenotype of both disorders is good frequency sensitivity –near normal audiogram– but severely disrupted speech understanding. In the case of synaptopathy where the auditory nerve fibers are functioning properly, cochlear implants provide excellent rehabilitation. In the case of neuropathy, however, the successful restoration of speech perception with cochlear implants is much less. Cochlear implants consist of electrode arrays that are surgically implanted into the cochlea and receive electrical input from a sound processor, as a treatment for severe to profound sensorineural hearing loss.
Quite often dysfunctions in speech understanding occur in dyslexia, specific language impairment, and auditory processing disorders. But in these cases it is harder to pinpoint specific structural problems in the central nervous system. The underlying communality may be developmental delays in auditory temporal processing, ultimately resulting in attention overload or other cognitive problems. There is no agreement on the underlying causes here, but again processing of simple tones appears intact whereas processing of complex sounds is affected.
General neurological disorders can also involve problems with auditory temporal processing, but typically more in the longer time frames such as rhythm, intonation and prosody, and word order discrimination. So for instance, in patients with schizophrenia, autism and epilepsy, we often find abnormalities in brain rhythms and in the connections between brain structures. There are also suggestive genetic links between schizophrenia and epilepsy, between schizophrenia and dyslexia, and between autism and specific language impairment.
In these disorders the auditory system is not the only one affected, and often the integration of the auditory and other sensory modalities is affected as well. The amount of complex information in need of processing by the brain poses a dilemma, one which I recently phrased as such: “We live in a multi-sensory world and one of the challenges the brain is faced with is deciding what information belongs together. Our ability to make assumptions about the relatedness of multi-sensory stimuli is partly based on their temporal and spatial relationships. Stimuli that are proximal in time and space are likely to be bound together by the brain and ascribed to a common external event…. Audio-visual temporal processing deficits are found in dyslexia, in specific language impairment and in auditory processing disorder. In autism, the temporal integration window for audio-visual tasks is lengthened and this relates to speech processing deficits. In autism, schizophrenia, and epilepsy all temporal processing disorders were multi-modal.”
Understanding, diagnosing and rehabilitating the various auditory temporal processing disorders requires a good understanding of the basic temporal processing carried out in the inner ear and auditory nerve, as well as in the central auditory nervous system. Basic measures of temporal processing are all based on, and can largely be understood on, the basis of faithful transmission of the periodicities in sound, via the electrical pulses generated in the auditory nerve. These are the bottom-up mechanisms, but they are not sufficient for understanding the clinical problems. This requires incorporating attention and general cognitive mechanisms of communication, and integrating these top-down processes with the bottom-up ones.
Featured image: (Nature of time) 1903 by Poul la Cour by Morten Bisgaard. Public domain via Wikimedia Commons.