PURPOSE: Validating through a physiological biomarker of discrimination of frequency differences in speech stimuli
WHAT IS THE Mismatched Response (MMR)?
Oddball paradigms are frequently used to study auditory discrimination by comparing event-related potential (ERP) responses from a standard, high probability sound to a deviant, low probability sound. Speech discrimination involves the comparison of two different sounds.
HOW IS TESTING FOR THE MMR DONE?
Continuous EEG (electroencephalogram) recordings will be recorded from the scalp vertex (Cz referenced to linked mastoids). A separate bipolar channel (lateral canthus – superior orbit) will be used to monitor eye movements. Eleven electrodes (F5, Fz, F6, C5, Cz, C6, P5, Pz, P6, M1, and M2) are placed on the scalp according to the International 10-20 system and were referenced to the nasion.
WHAT STIMULI WILL BE USED IN TESTING:
MMR (Mismatched Response) will be assessed in infants at 2 months of age in response to three separate speech contrasts; two “anchor” contrasts (/a/-/i/ and /ba/-/pa/) plus one “intermediate” contrast.
The intermediate contrast will be selected randomly (with equal probability) from a pool of four possible contrasts varying in difficulty: /u/-/i/, /ta/-/da/, /pa/-/ka/, and /ba/-/da/.
Therefore, we will obtain responses to each of the intermediate contrasts in ~25% of the test population, and with each tested contrast overlapping with at least one phoneme from another contrast. The use of anchor and intermediate contrasts will allow us to generate preliminary models of AORs and contrast difficulty across a wider range of contrasts than can be reasonably obtained in a single infant EEG session.
All speech sounds are equated for RMS amplitude and duration (500 ms) and will be presented at 70 dBA from a loudspeaker located 20 cm from the infant’s head. Stimuli are presented continuously with an Inter Stimulus Interval of 1200 ms in a mismatch paradigm such that the “standard” phoneme appears in 85% of the trials and the “deviant” phoneme appears in 15% of trials. For each contrast, the deviant stimulus will be selected randomly (virtual coin flip). Each block will consist of a minimum of 75 deviants and 425 standards (~15 minutes per block).
WHAT IS THE TESTING PROCEDURE?
Infants will be tested in a dim, sound-attenuated room during natural sleep state; state will be noted throughout the recording. Each EEG session will be flanked by three minutes of resting state EEG, and two minutes of resting state EEG between contrast blocks to assess sleep-state.
HOW WILL THE DATA BE ANALYZED?
All MMR magnitudes and probability scores will be collapsed into five EEG frequency bands: delta (1-3 Hz), theta (3-7 Hz), alpha (8-12 Hz), beta (13-30 Hz), and gamma (30-50 Hz). These scores will be treated as dependent variables in a partial least squares (PLS) regression analysis (Alm et al., 2009), with contrast pair treated as the independent variables. MOR magnitudes and probability scores will be collapsed into the same five EEG bands. Scores for each group (NH and HL) will then be treated to a permutation t-test (n-permutations=10001, corrected for multiple comparisons using the false discovery rate). Next, a correlation analysis will be conducted between MMR scores and SII scores to determine a relationship between AOR magnitude and speech intelligibility.