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Children and adults diagnosed with learning disorders show highly variable perceptual and cognitive profiles. Many factors can contribute to the diagnosis of learning problems including basic perceptual physiology. BioMARK (Biological Marker of Auditory Processing) is a neurophysiological test used to quickly and objectively identify disordered processing of sound - the auditory brainstem response (ABR) to speech - which has been associated with learning impairments in many children.
Until now, auditory processing evaluations have consisted of behavioural measures which are subjective in nature, and may in some cases be confounded by client variables such as attention, memory, motivation, or simply a failure to understand the task at hand. As a supplementary diagnostic tool, BioMARK provides a measure that is objective and non-invasive, and directly assesses the neural processing of sound.
A BioMARK test is particularly important for individuals with speech and language disorders (reading, auditory, autism, dyslexia). Brainstem measures are related to the encoding of linguistic information and can provide a biological marker for auditory function in children with language-based learning problems. Of the 5% to 10% of children diagnosed with learning disabilities, approximately 30% can be explained by subtle auditory deficits of neural processing that contribute to delayed and abnormal reading skills. If this is the case, an abnormal BioMARK response will appear on the test, and the origin of the learning difficulty identified. Clients with neural processing difficulties are also those who will benefit the most from Auditory Training Programs, so BioMARK also acts as a screening tool for therapy recommendations.
Anyone can undertake a BioMark test providing they can remain awake and quiet for 20-30 minutes while watching a movie, for example, since the test requires no active participation from the client. Electrodes are placed on the head and an earphone in the right ear for delivery of the acoustic stimulus. A ‘Click’ evoked ABR is performed first to confirm normal standard ABR, then results from the BioMARK test using a ‘Da’ sound are compared to an age appropriate normative database to assess the latency, slope and spectral amplitude, and produce a BioMARK score.
In humans, acoustic stimulation captured by the ear is transformed by the cochlear into small but measurable electrical potentials – firing (activation) of neurons. This neuronal firing within the auditory processing pathway, creates a pattern of voltage fluctuations lasting for about half a second called an auditory evoked potential (AEP). With enough repetitions of an acoustic stimulus, they may be visualised in a time-voltage waveform. Using specific recording electrode placement on the scalp, amplification, selected filters, and a post-stimulus timeframe, it is possible to detect neural activity arising from structures spanning the auditory nerve, through the brainstem, and to the cortex.
As the latency (time after stimulus) of AEPs increases, the origin of the neural firing becomes more central. Early latency (the first 10 ms) corresponds roughly to the brainstem, middle latency (10-80 ms) to the thalamus/cortex, and late latency (80-500+ ms) to the cortex. It is the early latency upon which the ABR is based, relating to the neuronal firing between the receptor potentials of the cochlea and neurogenic responses arising from the auditory nerve and low midbrain structures.
With decreasing stimulus intensity, wave latencies increase systematically until the hearing threshold is reached, below which the response is absent. Although adult-like responses are attained by age two, there is a developmental time course, and it is possible to test hearing in newborns with age-appropriate norms. Thus, the ABR is an accurate and objective measure of hearing threshold and is possible in individuals who are unable to be tested behaviourally.
Nina Kraus is a Professor at Northwestern University, and together with the Kraus Lab, developed the BioMARK technique for the diagnosis of physiological disorders in auditory processing. Her research investigates the neural encoding of speech and music and its plasticity, how this encoding is disrupted in clinical populations, and how it reacts to differing levels of expertise. Further information and research can be found at: Northwestern University Neuroscience Laboratory website.
The Listen And Learn Centre is one of very few clinics in Australia that has BioMARK available to test biological markers of auditory function.