Andrew Dimitrijevic, Ph.D.
Meet the Researcher
Andrew Dimitrijevic, Ph.D. received his Ph.D. at the University of Toronto working on auditory steady-state responses with Dr. Terrence Picton.
He went on to do two postdoctoral fellowships, at University of British Columbia (with Dr. David Stapells) and at University of California Irvine (with Dr. Arnold Starr). He is currently at the Cincinnati Children’s Hospital and studies the electrophysiology of human hearing and development in children with auditory processing disorders, cochlear implants and single-sided deafness.
The Research
Cincinnati Children's Hospital Medical Center
Sensory and Cognitive Processing in Children with Auditory Processing Disorders: Behavior and Electrophysiology
Central Auditory Processing Disorder (CAPD) can be defined as having a listening difficulty despite having normal hearing. One theory of CAPD is that this bottom-up processing isn’t working properly, a bit like listening to a de-tuned radio or TV. However, when the sound code reaches the cortex, it is mixed with a variety of signals from other systems, including vision, memory and attention. A second theory of CAPD is that the problem occurs at this level of mixing. In this ‘top-down’ theory, inappropriate control signals from high-level thinking systems, especially memory and attention, are thought to lead to misunderstanding of the code produced in the auditory system. Unfortunately, these two theories are difficult to tease apart. For example, a typical statement by a parent of a child with CAPD is that (s)he seems unaware when being spoken to. This could indicate poor listening due to inattention, or due to an inability to process speech sounds in the auditory system. Understanding which theory is correct may be important for treatment of CAPD. This research aims to tease apart these two theories by examining how the brain processes sound. One aspect of this research will examine how the brain encodes pitch and level fluctuations in sound. Both of these sound qualities are the “building blocks” of speech. If there are deficits at this level of neural processing then perhaps a “bottom up” or sound encoding problem exists. Another aspect of this research will examine a more cognitive approach and examine how the brain deals with speech in noise. This will be indexed by use of brain oscillations which are thought to reflect neural networks across different parts of the brain. Therefore by approaching CAPD from these two directions, it may be possible to show whether their listening difficulties are due to bottom-up or top-down processing problems.
Research Area: Electroencephalography, behavior, psychoacoustics, sensory processing, cognitive processing
Long-term goal: This research addresses whether there are subtypes of Central Auditory Processing Disorder (CAPD), arising from deficits of bottom-up or top-down processing. Bottom-up processing refers to how the sound signal is encoded up to the level of the brain (i.e., ear to auditory nerve through the brainstem and up to the brain). Top-down processing is what the brain does with that information and includes cognition and attention. Understanding the mechanism of the CAPD will help direct clinicians as to what intervention may be most appropriate. For example, bottom-up problems may be best dealt with using a ‘communication device’. These resemble a cell phone ear piece and they are activated wirelessly by a small microphone worn by a teacher or parent. Top-down problems, on the other hand, may be better remedied by auditory training. Developing a test to tease apart these two scenarios and use this information to guide intervention is the long term goal of this project.
