Picture this: You’re sitting in a noisy classroom with 30 students. Your professor is just about to start class when you get the urge to use the bathroom. Quickly, you raise your hand and ask for permission to leave. Among the noise, your professor responds with a single syllable answer. Your friend is confident that the professor said “No” while you interpreted the answer as “Go”. What is responsible for this difference?
Auditory processing is the ability of the brain to interpret sound waves that are received by the ear (White-Schwoch et al., 2016). It has been noted from previous studies that this impressive processing ability tends to become weaker in older adults and in children with language disabilities; two populations who struggle in adverse listening environments. One hypothesis is that neurons found in the inferior colliculus (a small structure in the midbrain) become out of sync when stimulated by sound. Instead of firing a message at the same time, they will fire randomly. This variation results in an increased difficulty of interpreting speech. Unfortunately, studies on internal brain structures in humans are difficult to complete. Therefore, White-Schwoch et al. aimed to see if localized (internal) recording of neurons in an animal brain react similar to scalp (external) neuron recordings in humans in response to sound. If this is the case, then animal models could be used to study localized neuron responses and relate this to human auditory processing.
To complete this study, brain responses to a sound stimulus were recorded for 50 children and 10 guinea pigs (guinea pigs have the same hearing spectrum as human speech). The stimulus used in this experiment was a single consonant-vowel syllable, in this case [da]. This sound was presented in a quiet condition and a noise condition (continuous background chatter). For the animal recordings, neuron activity was traced internally (and externally) across regions in the brain. For the human recordings, brain response from children ages 3-5 years were traced externally by placing stickers with electrodes across the scalp. By using young children, researchers were able to record a more accurate auditory response due to less exposure time to harsh noise environments compared to adults.
Upon the completion of 84 trials on guinea pigs and 50 trials on children, researchers had started to discover an exciting trend. Overall, it was found that guinea pigs and children had identical, consistent brain reactions to the stimulus. While examining the results, reactions to the stimulus were separated into three components: the onset (beginning of the [d] sound), transition between consonant and vowel (d-a), and the vowel sound itself [a]. Neural responses to the initial [d] sound had the most variability (difficult speech processing), while responses to the vowel were the most consistent (easier speech processing). This trend was also seen when the effect of background noise was added, but sound processing was easier in the quiet environment.
Results from this experiment were successful to prove that humans and guinea pigs have the same auditory processing ability, but an additional question was established: What happens chemically in the brain? By using further animal models, research on rats had shown that inhibitory neurotransmitter receptors (?-Aminobutyric acid [GABA] and glycine receptors) decline as we age, which results in less synchronization of neuron excitement, providing a link to difficult speech perception in older adults (White-Schwoch et al., 2016). As well, N-methyl-D -asparate receptors (NMDA-Rs) generally bind with glutamate (an excitatory neurotransmitter) and allow for synchronized neuron response to sound, resulting in accurate speech perception. However, developmental delays in children disrupt this binding process in the brain, resulting in unsynchronized responses to sound (difficult speech perception), leading to language disabilities.
Overall, thanks to experimental success of this study, these findings can be used to reach a better understanding of auditory processing differences in people, perhaps leading to advances in improving the lives of children and older adults living with language difficulties.
Source: White-Schwoch, T., Nicol, T., Warrier, C.M., Abrams, D.A., Kraus, N. (2016). Individual Differences in Human Auditory Processing: Insights From Single-Trial Auditory Midbrain Activity in an Animal Model. Oxford University Press. 1-21.
