Gender and Ear Influences on the Speech-Evoked Middle Latency Response (MLR)

Abstract

Auditory evoked potentials (AEP) are used to evaluate auditory system function from the level of the auditory nerve to the auditory cortex and association areas. For auditory evoked potentials to reach their full power as an assay of hearing and brain function, it is important to understand stimulus- and subject-related variables. The middle latency response (MLR) is one type of auditory evoked potential that reflects the activity of the auditory nervous system at levels including the auditory thalamus and primary auditory cortex. Whereas gender and laterality-related differences have been found at the level of the inner ear and brainstem, limited studies have investigated gender differences at the level of the auditory thalamus and primary auditory cortex. Additionally, the use of complex stimuli, such as a consonant-vowel token, and presentation of stimuli in noise has been investigated for other evoked potentials, but few studies have used this type of stimulus to elicit the MLR. Therefore, the current study was undertaken to evaluate the effect of gender and laterality (ear) differences (subject-related parameters), and stimulus complexity and masking (stimulus-related parameters), on the MLR. Gender differences were found in the current study, revealing shorter MLR component latencies and larger amplitudes in females compared to males. No ear-related differences were evident, however. The speech token /da/ was effective in evoking an MLR that displayed latency and amplitude characteristics like those found in studies that used click or tone-burst stimuli. Contralateral masking noise resulted in reduction of the MLR amplitude, which is the classical definition of suppression with respect to this specific AEP. This study clearly establishes gender as a significant subject-related parameter, and the use of complex stimulus paradigms that can be applied to clinical applications of MLR.