Like a billboard deteriorating under wind and rain, memories seem to fade over time. In zebra finches, however, memory loss appears to happen much more abruptly: According to a report in tomorrow's issue of Science, these little gray birds forget courtship songs and other sounds at distinct times, as if the brain papers right over them. "This is the first time we may have a handle on how long memories last," says neurobiologist Fernando Nottebohm, the study's head author.
Nottebohm and his colleagues at Rockefeller University in New York City played recorded finch songs and other random sounds like human speech to captive finches hooked up to electrodes. They played the same sound over and over, recording the firing of neurons in the medial neostriatum (NCM), a brain center that responds vigorously to new sounds. After about 100 repetitions, the NCM response had dropped off 60%, indicating that the birds had become familiar with a sound.
When the birds achieved this familiarity, the researchers waited up to a few days before playing the sound again. They found that if they allowed enough time to elapse, the NCM neurons began to fire just as often as when the birds first heard the sound. Thus Nottebohm's team concluded that the NCM had forgotten the sound. Different sounds seemed to stay in the memory for different lengths of time: The NCM "remembered" a male finch song for 48 hours, while human speech stayed familiar for only 3 hours. And the birds seemed to forgot sounds suddenly, as if their brains had pressed a delete key.
The team also observed that if the finch heard a sound fewer times during the initial learning period, or if the repetitions were close together, the memory disappeared more quickly. "It's like the reasons we shouldn't have crammed for tests when we were in school," says University of California, San Francisco, neurobiologist Allison Doupe, who was impressed with the study. But Doupe warns that the finches' apparent forgetfulness may not necessarily be linked to memory per se. Says Nottebohm, "You can't ask the bird 'Have you really forgotten this?'"