Amy Ragsdale and Dr. David McPherson, Department of Communication Disorders
Source localization is a tool used to identify a precise visual location within the brain which allows for interpretation of the source of the brain activity. In this research, source localization is used to provide a more accurate identification and evaluation of the neurophysiological sources that contribute to the brain’s responses to speech-like and non-speech stimuli.
The interest of this research study was to perform accurate localization of event-related potentials (ERP). ERPs are reliably recorded using EEG data and measure the brain’s electrophysiological response that is directly the result of perception. We examined two ERPs associated with auditory processing, the Mismatch Negativity (MMN) and P300. Previous research has shown that speech is perceived differently from other acoustic input. Our hypothesis was that by use of source localization we could confirm that there are significant differences in the cortical processing of speech sounds vs. non-speech sounds.
ERP data of 20 test subjects from a previous study of ours were used in this project. The ERP results were separated into two categories, speech and non-speech. Within the two categories, there were 6 individual data sets. Using Curry multi-modal neuroimaging software, we combined our ERP data with MRI data to generate three-dimensional models of the brain. Because we did not have the equipment to acquire MRIs of our test subjects, we used a standard MRI provided by Curry.
In this study, we used dipoles to examine the location of the MMN and P300 responses on our 3D models. A dipole presupposes that there is a single source of activity and radiating from that is a field of positivity and a field of negativity, much like a magnet. These dipoles represent a single source that explains the electrophysiological activity.
Our results showed that the dipoles generated within each individual data set remained in the same location for both the MMN and P300. This suggests that parallel processing takes place at the level of the MMN and P300 generators. However, the location of the dipoles were significantly different when comparing the categories of speech and non-speech.
Figure 1 shows the source localization of the MMN and P300 for a non-speech stimuli. The location of the dipole is associated with Brodmann Area 9, an area of the brain associated with attention and working memory. Figure 2 shows the source localization for a speech stimuli. The dipole is located in Brodmann area 22, or Wernicke’s area, and is associated with understanding speech. Similar results were found for other speech and non-speech stimulus. These results confirm that there are areas where significant differences are found in the cortical processing of speech and non-speech sounds.
The information obtained in this study will help to further the understanding of language processing, which will be beneficial in treating speech and hearing disorders. Clinically, the normative patterns involved in speech vs. non-speech processing found in this study could be compared with those of individuals with known neural processing disorders. Deviations in the subject’s ERPs could help clinicians create a more complete differential diagnosis.
Overall, this was an invaluable learning experience that increased my passion for medicine. Not only have I learned about the inner workings of the auditory system, this project has taught me the value of research in discovering new ways to help people.