Measuring evoked responses to speech

Our auditory brain is very good at detecting speech, which we are able to understand when listening in considerable background noise. However measuring the evoked response of the brain to speech using surface electrodes is challenging. A number of groups worldwide are carrying out research in this area, but the limitations and clinical applications of such methods are not yet fully understood.Artificial stimuli such as clicks and tone pips evoke robust responses such as the Auditory Brainstem Response that, whilst only fractions of a μV in amplitude, can be recorded reliably with appropriate averaging and filtering. Evoked responses to such stimuli are widely used in neonatal hearing assessment and have had great clinical impact as they allow hearing aids to be fitted to infants from a few weeks of age, minimising effects of auditory deprivation. When we fit hearing aids to infants the signal that we are generally most interested in making audible is speech. However measuring the response to clicks and tone pips does not allow us to directly demonstrate that the brain is responding to speech. Also the response of hearing aids to speech on a typical user setting may be different from that produced using artificial stimuli such as clicks or tone pips, so using artificial non-speech sounds may have limitations for evaluating hearing aid benefit.By measuring evoked responses to speech we may be able to demonstrate that hearing aids give access to speech and/or adjust hearing aids to maximise benefit for the user, but key questions for clinical use are the reliability and duration of measurements. Various approaches have been used in the literature to explore the measurement of evoked responses to speech, using stimuli ranging from simple repeating speech tokens such as ‘da’ through words and sentences to running speech. Also evoked responses to speech have been measured from the brainstem level through to the cortex. In this talk I will summarise key approaches that have been used to measuring evoked responses to speech and discuss their potential application to evaluate or adjust hearing aids. I will also give an update on findings from the EPSRC funded project EP/M026728/1 ‘Personalized fitting and evaluation of hearing aids with EEG responses’. In this project we are exploring the limits of measuring evoked responses to speech with a view to using the most promising methods to evaluate or adjust hearing aids for the end user.