Infant Hearing Development: Translating Research Findings into Clinical Practice Lori J. Leibold Department of Allied Health Sciences The University of North Carolina at Chapel Hill Auditory Development All aspects of sound processing develop. Different skills develop at different rates Hearing requires a brain. Infants are not little adults. Overview What is developing? How do we study hearing during infancy? Maturation of the sensory representation of sound Maturation of perceptual processing» Development of the ability to listen in noise 1
Overview What is developing? How do we study hearing during infancy? Maturation of the sensory representation of sound Maturation of perceptual processing» Development of the ability to listen in noise What is Developing? Primary or Sensory Central or Perceptual Hearing (Thanksgiving Edition) 2
Overview What is developing? How do we study hearing during infancy? Maturation of the sensory representation of sound Maturation of perceptual processing» Development of the ability to listen in noise To understand how hearing develops We need to understand the anatomy We need to understand the physiology We need to understand the behavior Observer-Based Psychoacoustic Procedure Allows us to obtain a threshold! Werner Olsho, 1987 3
Translation to Clinical Practice: The Psychometric Function Upper Asymptote From Gescheider, 1985 Infants Upper Asymptote Typically around 0.85 correct for all stimuli» e.g., Trehub et al., 1980; Olsho et al., 1988; Leibold and Werner, 2007 1.0 Upper asymptote ~ 0.85 Proportion Correct Infants will not respond on every stimulus presentation (even for sounds that are easily audible). Importance of the use of catch trials. Stimulus Intensity Translation to Clinical Practice: Yield (Success) Rate Not all VRA sessions will be a success. From Gravel, 2001 4
Overview What is developing? How do we study hearing during infancy? Maturation of the sensory representation of sound Maturation of perceptual processing» Development of the ability to listen in noise Maturation of the Peripheral Auditory System Cochlea Mature at birth Conductive Apparatus Not fully mature until 11 years Largest improvements occur during infancy Brainstem Neural Transmission Immature early in infancy Substantial improvements by 6 months Sensory Representation: What is the Quality of the Input? Are re frequency, intensity, and temporal characteristics of sound represented as they are in the mature system? The sensory representation of sound appears to be fairly precise by about 6 months of age. 5
Translation to Clinical Practice: Establishing Normative Data 6-month olds likely have a fairly precise (perhaps noisy ) sensory representation of sound Studies of typical development have provided the foundation for identifying hearing loss during infancy» We know a lot about typical development for absolute thresholds, OAEs, ABRs, and middle ear measurements Implications for Studying the Development of Complex Auditory Perception We are now in a position to start examining how infants perform on more complex auditory tasks Infant-adult differences in performance on complex tasks are not likely due to an immature sensory representation» At least for children who are typically developing Overview What is developing? How do we study hearing during infancy? Maturation of the sensory representation of sound Maturation of perceptual processing» Development of the ability to listen in noise 6
Examples of Perceptual Processes Sound source segregation» How are concurrent acoustic components assigned to the appropriate source? Selective auditory attention» Attention to relevant features within a sound» Attention to one sound source among several Pattern perception Music perception Sound Source Segregation How are concurrent acoustic components assigned to the appropriate source? Potential Cues for Sound Segregation (at least for adults) Spectral separation Spectral profile Harmonicity Spatial separation Temporal separation Temporal onsets and offsets Temporal modulations 7
Sound Source Segregation Frequency Time Frequency Time Sound Source Segregation in Infants: Methodological Challenges I hear four streams, but I m only paying attention to the man s voice. Too bad I m not talking yet Selective Auditory Attention: Attention to Sound Features 8
Selective Auditory Attention: Attention to Sound Sources Linus Edgar Allen Poe Translation to Clinical Practice: Listening in Noise Infants and young children have difficulty hearing in the presence of competing sounds» Detecting tones» Recognizing speech Children with hearing loss appear to have even greater difficulty An Infant s s Environment is Often Noisy There are often multiple sound sources Sounds sources differ across environments Acoustic uncertainty can be high 9
A Child s s Classroom is Usually Noisy Standards exist to minimize noise and reverberation (ANSI, 2002) Most classrooms do not meet these standards (e.g., Knecht et al., 2002) Average classroom acoustics offer a greater challenge for children than for adults (e.g., Jamieson et al., 2004) What is noise? Any unwanted sound. Studying the Effects of Noise in the Lab: Auditory Masking Auditory masking is the reduction in audibility of a sound in the presence of another sound. Signal Level (db SPL) Signal Absolute Threshold 10 db SPL Amount of Masking Signal Level (db SPL) =Masked Threshold-Absolute Threshold =10 db Masked Threshold 20 db SPL Signal Masker 10
Commonly Used Maskers White Noise Broadband Noise Narrowband Noise Speech-shaped Noise Multi-talker babble 2-Talker Speech Multi-tonal Complexes Mechanisms of Masking: Energy Based Masking can represent a failure of frequency selectivity. Auditory filter model» Ear operates as bank of filters» Only energy in the auditory filter around the signal frequency masks the signal Energy-based Masking Intensity (db SPL) Intensity (db SPL) Interval 1 Interval 2 11
Detecting a Narrow-Band Signal in Noise But is this energy-based masking? Schneider et al. 1989 Mechanisms of Masking: Perceptual Speech Spectrogram Perceptual (central) factors can also produce masking Masking despite an adequate sensory representation Sounds in natural environments have:» Multiple spectral components» Time-varying spectra» Stimulus uncertainty The Masking Formerly Known As Distraction Masking Adults Infants Amplitude (db SPL) Amplitude (db SPL) Werner and Bargones (1991) 12
Distraction Masking: 4-104 Year-Olds Leibold et al., in progress Consequences of Broadband Listening Immature listening strategies appear to make children susceptible to interference from background sounds (maskers) May also make infants and children sensitive to features adults don t hear Broadband Noise (Energetic) Masker 3000 1000 300 Time (ms) Leibold and Werner, 2006 13
Random 2-tone 2 (Informational) Masker 3000 1000 300 Time (ms) Leibold and Werner, 2006 Masked Thresholds for Infants and Adults Leibold and Werner, 2006 Developmental Effects Still Extend into the School-Aged Years Leibold and Neff, 2007 14
Recognizing Speech in Noise Children need a greater signal-to-noise ratio compared to adults (e.g., Elliot et al., 1979) Child-adult differences increase as masker complexity increases (e.g., Hall et al., 2005) Age-related changes across childhood» Infants and younger children often have more difficulties in noise than older children Word Recognition in Speech-Shaped Shaped Noise or 2-Talkers2 3% 17% 21% 10% Bonino et al., in preparation Can We Improve Infants Performance? Fixing the Masker Spectra 3000 1000 300 Time (ms) Leibold and Werner, 2006 15
Fixed Spectra: Infants Leibold and Werner, 2004 Fixed Spectra: School-Aged Children Leibold and Neff, 2007 Benefit of Carrier Phrase Say the Word PBK Words in a 2-Talker 2 Masker Bonino et al., in preparation 16
Acknowledgements Human Auditory Development Lab» Angela Yarnell Bonino, Laura Fleenor, Caitlin Rawn, Ashley Halbach, Jack Hitchens, Chas Phillip, Alex O Dell Collaborators» Lynne Werner, Emily Buss, Donna Neff, Walt Jesteadt, NIDCD (R03 DC008389)» Pat Stelmachowicz, Joseph Hall, III 17