Event related auditory evoked potentials (AEP) can be elicited without the participant in an experiment needing to do anything. One such AEP is generated in the brain stem in response to simple repetitive acoustic stimuli (e.g., a sharp click). Such brain stem responses are often used to evaluate the integrity of the auditory pathway. When using more complex acoustic stimuli, e.g., a spoken syllable, the brain stem response to can be divided into transient (onset response) and sustained parts (frequency-following response, FFR). Brainstem responses appear to provide information about how the sound structure of a speech syllable is encoded by the auditory system.
In the very explicitly named paper “Context-Dependent Encoding in the Human Auditory Brainstem Relates to Hearing Speech in Noise: Implications for Developmental Dyslexia”, Chandrasekaran, Hornickel, Skoe, Nicol and Kraus from Northwestern University’s Auditory Neuroscience Laboratory recorded electrical activity at Cz (with responses bandpass ﬁltered from 70 to 2000, the low-pass cutoff of 70 Hz was chosen to reduce any cortical contribution) to investigate how responses to auditory stimuli are modulated by the context of speech (context dependent coding).
What they did was to play a synthesized speech syllable to children who had no neurological abnormalities or learning disabilities as they viewed a video. The context manipulation consisted of presenting the syllable either in a repetitive (predictable) or in a variable (unpredictable) fashion. As the title suggests, the authors examined the brain responses of good and poor readers. What did they find? Here is the summary:
We examined context-dependent encoding of speech in children with and without developmental dyslexia by measuring auditory brainstem responses to a speech syllable presented in a repetitive or variable context. Typically developing children showed enhanced brainstem representation of features related to voice pitch in the repetitive context, relative to the variable context. In contrast, children with developmental dyslexia exhibited impairment in their ability to modify representation in predictable contexts. From a functional perspective, we found that the extent of context-dependent encoding in the auditory brainstem correlated positively with behavioral indices of speech perception in noise. The ability to sharpen representation of repeating elements is crucial to speech perception in noise, since it allows superior “tagging” of voice pitch, an important cue for segregating sound streams in background noise. The disruption of this mechanism contributes to a critical deficit in noise-exclusion, a hallmark symptom in developmental dyslexia.
Chandrasekaran, Hornickel, Skoe, Nicol and Kraus (2009). Context-Dependent Encoding in the Human Auditory Brainstem Relates to Hearing Speech in Noise: Implications for Developmental Dyslexia, Neuron, 64,311–319.
See also Russo et al (2004). Brainstem responses to speech syllable. Clinical Neurophysiology, 115, 2021–2030.