Learning & Plasticity

In order to make sense of sound, our auditory system needs to be able to learn from experience, and to adapt to changing environments and circumstances. These adaptive processes in the auditory system occur over different time scales, some taking a few seconds, others lasting a lifetime. Chapter 7 of "Auditory Neuroscience" charts the development of our auditory perceptual abilities and places this in the context of the maturation of the ear and the central auditory pathways. We consider the role of sensory experience in shaping the developing auditory pathway and its impact on the emergence of auditory functions ranging from sound localization to speech perception. We also look at the neural mechanisms that allow the auditory system to remain "plastic" in later life, and therefore able to learn and adapt to changing inputs. The following web pages provide further information on this subject. 

Sounds heard by unborn babies in utero

This extract of a You Tube video entitled "Life in the Womb" describes the types of sound that the human fetus is likely to be exposed to in utero. Human fetuses are known to become sensitive to sound well before birth, but to think that they hear just as grown ups do would be very naive. While experience of the sounds that reach the ears of the fetus, which can originate from inside as well as outside the mother's body, may well influence the development of the auditory pathways, the brain of unborn babies, and even neonates, is very immature in many respects. Consequently, developmental changes are observed in most auditory perceptual abilities, with adult function sometimes taking several years to be attained.

 

Prism adaptation of spatial hearing in barn owls

Under construction

 

Videos kindly provided by Dr William DeBello from the Center for Neuroscience and Department of Neurobiology, Physiology, and Behavior, University of California, Davis ()

Pena, J. L., & DeBello, W. M. (2010). Auditory processing, plasticity, and learning in the barn owl. Institute for Laboratory Animal Research Journal 51, 338-352

Knudsen, E. I., & Knudsen, P. F. (1989). Vision calibrates sound localization in developing barn owls. Journal of Neuroscience 9, 3306-3313.

 

1) Barn owls are able to accurately localize a target, such as a mouse on the ground, using either visual or auditory cues. This is illustrated in the following movie. The first owl to appear flies straight to its target and catches the mouse. The second owl has recently been fitted with prism glasses: its behavior looks fairly normal, but it mislocalizes the mouse and misses out.

2) This movie shows the same prism-mounted owl unsuccessfully trying to catch a mouse (it consistently aims to the right of its intended target). After several attempts, however, this animal does succeed in catching the mouse, indicating that visuo-motor adaptation is taking place to compensate for the presence of the prisms.

3) Eventually, the prism-mounted owl's auditory spatial mapping also adapts, allowing it to catch a mouse in complete darkness. Watch out for the owl perched at the back, and the mouse running along the far right edge of the arena. (This third video was taken with infrared light which is invisible to the owl). 

Song learning in birds

Examples of the subsong, plastic song and adult song, which are produced at different ages by zebra finches can be found at: http://www.sciencemag.org/cgi/content/full/320/5876/630/DC1

"Subsong" - thought to be equivalent to a human baby's babbling - produced by a normal 38 day old juvenile bird. 


This early vocal behavior does not rely on area HVC (the "high vocal center"), which contains the neural circuit for adult song production. Thus, bilateral HVC deactivation does not alter subsong produced by the same zebra finch in the above example. 


The next phase of song production, called "plastic song" is characterized by the presence of variable syllables. The normal plastic song produced by a 50 day old juvenile bird is shown below.


Song produced by the same 50 day old bird after bilateral lesions of HVC. Although the bird still vocalizes, the song has lost its structure and resembles the more primitive subsong.


Normal undirected singing produced by an adult bird. 


Singing produced by the same adult bird after bilateral lesions of HVC. The distinct syllables that characterize normal adult songs are lost.