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
Contact: Jennifer O'Brien
University of California - San Francisco