Amanda Fujiki and Dr. David McPherson, Department of Communication Disorders

The term auditory processing disorder (APD) refers to a diverse collection of problems in the processing of auditory information. An individual with an APD may have hearing acuity within normal limits; that is, the middle ear (conductive) and inner ear (sensory) systems may function typically. The individual may demonstrate a breakdown occurring within the central nervous system, however, and this breakdown may limit the individual’s ability to understand, interpret, and use auditory information. Auditory processing disorders are thought to interfere with language development and literacy learning (Geffner, 2007, MacFarland & Cacace, 2009). The effects of APD may be highly variable ranging from mild language/learning disorders to severe language and communicative impairment. In addition, APD may be associated with difficulties in social communication and social skill.

Disorders of Auditory processing are difficult to detect. They cannot be diagnosed with routine speech audiometry because the breakdown or auditory processing occurs in the central auditory nervous systems as opposed to the sensory auditory system. Individuals with disorders of auditory processing are able to hear sounds, but they have difficulty interpreting the meaning of those sounds. Masking is widely used during audiological testing to evaluate the effects of noise on auditory threshold and function. As such, it is a critical component of audiological evaluation. Masking level difference (MLD) provides an indication of the improvement in an individual’s ability to detect a sound signal presented to both ears under different masking conditions (signal and masking noise in the same phase vs. signal and masking noise in a different phase).

One of the main goals of auditory processing testing has been to identify areas of the CNS that may be damaged or dysfunctional in the auditory processing pathways and surrounding structures. Binaural interaction tests primarily assess brainstem function; however, most behavioral binaural interaction tests are only sensitive to severe brainstem dysfunction and, as such, have limited clinical usefulness (Olson et al, 1976). As an exception to this, the MLD has been found to be sensitive to less extreme dysfunctions (Noffsinger et al, 1984), making the MLD one of the few binaural interaction tests that are appropriate for clinical auditory processing assessment.

The purpose of the current study is to examine the neurophysiological bases of masking level differences (MLD) in healthy young adults using brain mapping. We employ a software program developed by Professor David McPherson to examine an adaptive procedure while recording the scalp distribution of the brainwave for the purposes of source localization. We present a noise (masker) and a pure tone (signal) binaurally. The MLD test determines thresholds for two conditions: MoSo and MoSπ. The target stimulus for each condition is a 500 ms presentation of noise with the signal embedded in the center of the noise. The non-target stimulus is the noise without the signal. The participants are asked to press a button only when they heard the target stimulus (i.e, 500 Hz tone).

This project is well on its way, we are currently gathering data from 20 participants (ten male and ten female) between the ages of 18 and 26. All participants are right handed, typical hearing individuals with no history of head injury. We expect that our results will show that the scalp distribution of the MLD is different from the scalp distribution of the masker or tone alone. Data provided by this study will lead to more sensitive and reliable measures of auditory processing disorders.

I would like to express my gratitude for this ORCA grant. My participation in Professor McPherson’s lab has been the highlight of my undergraduate career. I have gained skills and knowledge that otherwise would have been unavailable to me and Dr. McPherson’s mentorship has been invaluable to me. I realize it is an incredible opportunity for an undergraduate student to receive an award like this. Thank you so much!

References

• Geffner, D. (2007). Central auditory processing disorders: Definition, description, and behaviors. In D. Geffner, & D. Ross-Swain, (Ed.), Auditory processing disorders: Assessment,management, and treatment. San Diego: Plural Pub.
• McFarland, D. J., & Cacace, A. T. (2009). Models of central auditory processing abilities and
  disorders. In A. T. Cacace, & D. J. McFarland (Eds.), Controversies in central auditory
  processing disorder (pp. 93-107). San Diego: Plural Pub.
• Noffsinger, D., Schaefer, A.B., & Martinez, C.D. (1984). Behavioral and objective estimates of auditory brainstem integrity. Sem Hear, 5, 337-349.
• Olsen, W.O., Noffsinger, D., & Carhart, R. (1976). Masking level differences encountered in clinical populations. Audiol, 14(4), 287-301.