Shihab A Shamma, PhD - sas@eng.umd.edu
Mounya Elhilali, PhD
Institute for Systems Research
Electrical and Computer Engineering
University of Maryland
College Park, MD 20742
Popular version of paper 3aAB1
Presented Thursday morning, November 30, 2006
4th Joint ASA/ASJ Meeting, Honolulu, HI
Anyone who has walked into a crowded reverberant hall, with music blaring in the background, will recall the initial impression of the sound as a loud and undifferentiated noise. In short order, however, different sound streams begin to emerge as one attends to a few speakers, listen to the melody from the band, or even to one instrument in it. Humans perform this remarkable feat effortlessly, but so do many animals too.
Natural environments are often extremely cluttered and disorienting, and hence animals have developed abilities to navigate their complex auditory scenes in order to mate, feed their newborns, and avoid predators. For instance, penguin parents locate their nest by the cries of a chick in the midst of a millions-strong colony of screaming birds often situated on a flat terrain without any physical landmarks. Frogs also locate singing mates in the midst of a cacophony of calls. To accomplish these feats, it is likely that similar mechanisms have evolved in m any animals which include a mix of "bottom-up" automatic processes with complex "top-down" behaviors involving attention, expectation, learning, and memory.
Our research attempts to sort out these parallel processes and explore the strength and dynamics of their interactions. For example, in the "bottom-up" category of mechanisms, animal brains are largely hard-wired to respond and perceive sound so as to highlight the features important for the survival of the species. These might include sensitivity to specific frequency ranges or repetition rates, or an inherited ability to readily learn to recognize con-specific sounds early in life, such as speech for human infants, songs for many birds, and mating calls in frogs. In mammals, the key machinery to perform this sound analysis reside in the auditory area of the cortex, where many neurons have been found to respond selectively to specific spectral and dynamic features, a selectivity which is often referred to as the receptive field of the neurons. It has been known for sometime that different regions in the auditory cortex exhibit an ordered diversity of receptive fields that emerges automatically during development. It has also been confirmed that this organization is not fixed forever in the adult, but rather is plastic and can be modified in response to injury or following long-term training which can enhance response sensitivity of some neurons and hence re-allocate the proportion of neurons responding to different aspects of the sound.
What is surprising is our recent finding that plasticity in the adult brain is fairly rapid, and may in fact be occurring while we and other animals engage in different behaviors. More accurately, we believe that receptive fields of auditory cortical neurons change their selectivity to sound according to the specific demands of an ongoing task in a manner that promotes its successful execution. For example, when a penguin is searching for its chick, its auditory neurons may become re-tuned to the particular combination of acoustic features the make up the chick's call, pre-disposing the penguin to sense and respond to this particular call and not any other. Similarly, humans navigating a cocktail party scene as described earlier might be tuning their neurons to specific voices (and hence tuning out others!), or aligning their sensitivity to the acoustic features of one instrument in the band, facilitating its perceptual segregation from all others.
We still do not know for certain the mechanisms that give rise to this rapid plasticity, nor do we understand sufficiently the processes taking place to be able to encapsulate them in computer programs or build effective prosthetic devices. One day soon, however, we shall. Then, we will be able to mimic these abilities and build more effective hearing aids and cochlear implants, as well as automatic speech recognition systems that are robust to moderate levels of noise and clutter. In fact, it is likely that the basic processes that enable animals to overcome acoustic clutter are analogous to those employed in visual and other sensory clutter, and hence the implications of this research go beyond the purely auditory realm.
So, next time you interact with your environment, be it a heated discussion, listening to music, or viewing an interesting image, you should realize that in the process, you are changing your mind and altering your perception of the world forever.