anges in loudness compared to two groups of untrained rats, and found that the auditory cortex in the expert rats contained groups of neurons that had become selective for specific volume levels -- they fired only at those levels and were quiet otherwise. This physiological change in the brain, called "plasticity," has been widely observed in humans and animals who have learned new skills.

Then came the breakthrough discovery: the expert rats were processing volume changes in a new and different way. In the brains of the untrained rats, the overall neural response rate increased as the sound got louder and louder, as the classical model would predict. In the expert rats, however, the overall response rate of the selective neurons increased until the sound reached a loudness threshold of 40 decibels -- and then leveled off while the loudness increased 100-fold, from 40 to 80 decibels. "At first glance, this was not good," observes Polley: If their neurons were not increasing their firing rate, how were the expert rats registering the increase in volume? David T. Blake, PhD, UCSF assistant research physiologist and a co-author of the study, cracked the puzzle. Instead of looking for a simple increase in firing rate, Blake measured the rate at which the firing changed, either up or down. This rate turned out to be in exact proportion to the increase in volume -- and at the same ratio as the firing rate increase. Tests confirmed that the untrained rats' brains were not registering volume increases in this new way; it had been learned by the expert rats as they became better at discriminating changes in volume.

Polley concludes, "There is still proportionality between response strength in the brain and the stimulus. But now neurons are much more selective, and can represent sound intensity with decreasing firing rates as well as increasing firing rates." This system is "optimal" for representing subtle changes in loudness, reasons Polley, because "it give
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Contact: Jennifer O'Brien
jobrien@pubaff.ucsf.edu
415-476-2557
University of California - San Francisco
10-Dec-2004

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