How Plastic Is Spatial Hearing?

Abstract

The location of a sound source is derived by the auditory system from spatial cues present in the signals at the two ears. These cues include interaural timing and level differences, as well as monaural spectral cues generated by the external ear. The values of these cues vary with individual differences in the shape and dimensions of the head and external ears. We have examined the neurophysiological consequences of these intersubject variations by recording the responses of neurons in ferret primary auditory cortex to virtual sound sources mimicking the animal’s own ears or those of other ferrets. For most neurons, the structure of the spatial response fields changed significantly when acoustic cues measured from another animal were presented. This is consistent with the finding that humans localize less accurately when listening to virtual sounds from other subjects. To examine the role of experience in shaping the ability to localize sound, we have studied the behavioural consequences of altering binaural cues by chronically plugging one ear. Ferrets raised and tested with one ear plugged learned to localize as accurately as control animals, which is consistent with previous findings that the representation of auditory space in the midbrain can accommodate abnormal sensory cues during development. Adaptive changes in behaviour were also observed in adults, particularly if they were provided with regular practice in the localization task. Together, these findings suggest that the neural circuits responsible for sound localization can be recalibrated throughout life.

Relearning Sound Localization with a New Ear

“…Convergence of visual and auditory inputs was also demonstrated in the brachium of the ferret IC (Doubell et al, 2000). The presence of different non-acoustic feedback signals in the IC could thus render it as a potential candidate for sound-localization plasticity also in mammals (King et al, 2001;King, 2002).…”

Spatial release from masking in normal-hearing children and children who use hearing aids

“…As these delays are highly reproducible from one device to the next (of the same make and model with the same features selected), they should not distort interaural time difference cues. Previous research has demonstrated the ability of listeners to adapt to small consistent differences in interaural time and level differences (Javer and Schwarz, 1995;King et al, 2001). As with all hearing aids, there is the likelihood of distortion of interaural time-and level-difference cues in the frequency region, typically in the range 250-500 Hz (but depending on degree of loss and size of the vent), where amplified sound and sound leaking in past the earmold or through the vent have similar magnitude (Dillon, 2001).…”

A Sensorimotor Approach to Sound Localization

“…Plasticity in the spatial properties of the auditory SC maps has been demonstrated during adulthood and facilitated by behaviors that require spatial hearing, for ferret (King et al, 2001) and barn owl (Bergan, Ro, Ro, & Knudsen, 2005). Thus a computational theory of sound localization should include mechanisms that can re-calibrate after changes in sensory experience.…”