Binaural hearing Prof. Dan Tollin on the Hearing Throne, Oldenburg Hearing Garden
Outline of the lecture Cues for sound localization Duplex theory Spectral cues do demo Behavioral demonstrations of pinna cues SOC circuits for encoding ITD/ILDs How do we show that the role of the SOC is to encode binaural localization cues? Behavior Effects of lesions on behavior Or even better, effects of stimulation on behavior Anatomical evidence Response properties of neurons: how do they respond to manipulations of that parameter? ITDs: characteristic delay
Visual vs auditory localization Visual world is mapped onto the retina. Visual localization requires preserving anatomical retinotopy. Spatial information is not represented in the cochlea. Auditory system has to compute space
What are the acoustic cues for localization? Binaural cues for horizontal localization Interaural time differences (ITDs) Interaural level differences (ILDs) Spectral cues for vertical localization Counter-intuitively, the spectrum of the sound helps us to localize it in the vertical dimension
Duplex theory of sound localization Low frequencies Requires phase locking (< 3 khz) Very high resolution (jnd = 5-10 sec) High frequencies Short (reflects from head, ears)
Effect of wavelength and size of object constant, size varies Size constant, varies
Head-related transfer functions (HRTFs)
Spectral cues are important for vertical localization
Demo of pinna cues
Pinna cues can be distorted chronically in human subjects Hofman et al., 1998
Distorted HRTFs affect vertical sound localization Human subjects can adapt to distorted HRTFs Curiously, there is no aftereffect Hofman et al., 1998
Pinna movements aid bats in wire avoidance during flight Mogdans et al., 1988
How do we encode these pinna cues? The type IV cells in the DCN have properties that suggest sensitivity to the notches in pinna cues. Lesions of DCN also suggest a role of DCN in localization in elevation.
Reiss and Young, 2005 Type IV responses to notched noise
Young et al., 1992 Type IV units are sensitive to notch noise stimuli, like HRTFs
Type IV cells show strong inhibition with a notch at CF
Effects of DAS lesion on head orientation Control DAS lesion May, 2000
Virtual acoustic space In addition to providing information on source elevation, the HRTFs of the sound also externalize the sound when heard over headphones. Since the HRTF measurements are not from your own ears, these effects vary considerably from subject to subject.
Saccade task
Adapted from Geisler, 1998, courtesy W. Welker What do the ears do?
Pinna movements during localization
With the head restrained, cats undershoot acoustic targets Populin and Yin, 1998
Pinna movements to visual and acoustic targets Populin and Yin, 1998
Pinna movements are largely on the ipsilateral side Populin and Yin, 1998
Pinna movements have very short latencies
Auditory localization improves markedly with the head unrestrained
With the head free, cats localize sounds about as well as barn owls and people
What are the ears doing? Vestibulo-auricular reflex (VAR): The left pinna moves reflexly to compensate for a leftward head movement VAR
The vestibular-auricular reflex (VAR)
Frens and van Opstal, 1995 Localization in elevation requires wide band spectral sounds
Cat 28 A 40 Noise 500 Hz 1000 Hz 2500 Hz 6000 Hz 20 0 o o o o o -20 Elevation (deg) B -40 40 20 0-20 -40 Target Response Target Response -40-20 0 20 40 B Azimuth (deg) Gaze Shift Elevation (deg) 40 20 0-20 -40 = 4.42 Gain =.86 n = 549 = 7.73 Gain = 0.01 n = 257 = 7.94 Gain =.11 n = 340 = 7.74 Gain =.07 n = 315 = 7.13 Gain =.07 n = 318 40 Gaze Shift Azimuth (deg) 20 0-20 -40 = 4.14 Gain =.78 n = 549 = 19.43 Gain =.85 n = 257 = 18.29 Gain = 1.00 n = 340 = 12.00 Gain =.85 n = 315 = 15.66 Gain =.69 n = 318-40 -20 0 20 40 Motor Error (deg)
Binaural beats (or rotating tones) f f + f b
Characteristic delay Rose, Gross, Geisler and Hind, 1966 Jerzy Rose
Shig Kuwada, U Conn Kuwada and Yin, 1983
Yin and Kuwada, 1983
Other important or interesting binaural phenomena Binaural masking level differences Huggins pitch
Binaural masking level differences (BMLDs)