The impact of frequency compression on cortical evoked potentials and perception

Abstract

provided invaluable technical assistance without which this work could not have been completed.Many thanks are owed to my family and friends. v ABSTRACT Nonlinear frequency compression is a signal processing technique used to increase the audibility of high frequency speech sounds for hearing aid users with sloping, high frequency hearing loss. However, excessive compression ratios may reduce spectral contrast between sounds and negatively impact speech perception. This is of particular concern in infants and young children, who may not be able to provide feedback about frequency compression settings. This study explores use of an objective cortical auditory evoked potential that is sensitive to changes in spectral contrast, the auditory change complex (ACC), in the verification of frequency compression parameters.We recorded ACC responses in adult listeners to a spectral ripple contrast stimulus processed with a range of frequency compression ratios (1:1 to 4:1). Vowel identification, consonant identification, speech recognition in noise (QuickSIN), and behavioral ripple discrimination thresholds were also measured under identical frequency compression conditions. In Experiment 1, these measures were completed in ten adult normal hearing individuals to determine the effects of this type of signal processing in individuals with optimal hearing. In Experiment 2, these same measures were repeated in ten adults with sloping, high frequency hearing loss, which is the clinical population for whom this signal processing technique was intended.Increasing the compression ratio did not affect vowel identification for the NH group but did cause a significant decrease in vowel identification for the hearing impaired listeners. Changes in compression ratio were associated with significant changes in ACC amplitude, consonant identification, ripple discrimination threshold, and speech perception in noise scores.These results indicate that the ACC response, like speech and non-speech perceptual measures, is sensitive to frequency compression ratio. However, it was observed that the amplitude of ACC responses elicited by our single ripple contrast vi stimulus was not strongly correlated with behavioral ripple discrimination or measures of speech perception in quiet or in noise. It may be the case that using a different ripple density in the evoking stimulus (e.g. 1 RPO, 2 RPO), or determination of ACC threshold by using multiple ripple densities, would result in stronger correlation to behavioral measures across frequency compression conditions. vii