Cortical responses to speech stimuli in hearing impaired infants measured by fMRI and auditory evoked potentials

The overall aim of the present research was to perform an exploratory analysis of the relationship between the P1 auditory evoked potential response and residual auditory cortical function as shown by fMRI activation maps in moderate or severe to profoundly hearing impaired subjects.;Electrophysiological testing included evaluation of P1 cortical auditory evoked response with a hearing aid on using a Klatt generated /ba/ stimulus in the soundfield at 75 dBSPL. Subjects unable to wear an amplification device during the testing were stimulated using an Eartone 3A insert earphone at a minimum of 10 dB sensation level based on aided audiometric results. An fMRI paradigm consisting of Narrow Band Noise (NBN) and stories was administered under sedation at the end of a clinical scan in a 3 Tesla system using sound presentation levels of 10 dB sensation level based on audiometric results. Stimuli were interleaved with silence in a block-periodic counterbalanced fMRI design with 30-second on-off intervals. Results were subjected to a correlation analysis to search for a relationship between P1 characteristics and the number of activated fMRI pixels detected within specified regions of interest in the auditory cortex. In addition, a multiple regression analysis was conducted to assess the prediction of the P1 latency in our sample based on duration of hearing aid use and age at the time of the fitting.;The importance of early intervention for congenital hearing loss has been well established using a variety of measures, including speech and language outcome, cognitive and educational assessment techniques, and electrophysiologic measures. The P1 auditory evoked potential has been acknowledged and characterized as a biological indicator of auditory neurodevelopment in normal hearing children and in children with cochlear implants. (Sharma, et al., 2005). In this sample of infants with hearing aids, multiple regression analysis results suggest that the age of hearing aid fitting and duration of hearing aid use are significant predictors of shifts in P1 latency even during very early stages of development (F (2,8) = 10.266, p= .006). Combining both predictors in the model explained 72% of the overall variance (R2 = .720). These findings provide additional support for the essential influence of early sensory exposure on auditory neuromaturation during the first year of life using a non-invasive physiological measure.;Activity related cortical signals reflecting higher auditory cortical function could be more clearly understood by interrelating electrophysiologic findings with non-invasive mapping techniques. fMRI evaluation of the central auditory system prior to cochlear implantation may offer a possible means of objective assessment of auditory nervous system pathways. Auditory cortex activation observed in this sample of hearing impaired toddlers was highly variable between subjects. Group and individual maps of BOLD activation were also markedly dissimilar from what has been observed in normal hearing subjects within the same age range (Smith, et al., 2008). In all subjects, common areas of BOLD activity outside of auditory regions included positive BOLD within the medial frontal gyrus and anterior cingulate cortex, and negative BOLD responses in the inferior frontal gyrus. Areas that were shown to possess a positive correlation with shifts in P1 latency included the left middle frontal gyrus and inferior frontal gyrus, the anterior cingulate, and bilaterally in insular and occipital areas.;Results indicated that although fMRI activation patterns do not exhibit a predictive relationship with P1 latency (r(10) = .038, p = .456), the strength of the auditory BOLD response was inversely correlated with P1 amplitude (rS = -.85, p = .001), suggesting more localized regions of cortical responsivity in subjects with robust P1 waveforms. This finding was difficult to interpret in light of fMRI sedation effects, and may be a reflection of normal cortical maturational processes that are known to influence the anatomical generators of the P1 response. In order to verify and further characterize a possible inverse connection between neural and vascular activity in thalamocortical and primary auditory cortical regions, more information is needed regarding developmental effects auditory deprivation on P1 amplitude. (Abstract shortened by UMI.)...