As infants, we live without a sense of the past as adults, we can recall events from decades ago. Scientists have only a vague understanding of this remarkable transition, when our sense of time expands beyond this morning's feeding and last week's bath, but now they know a bit more: Conor Liston of Harvard University has determined that the beginnings of long-term recall arise between the ninth and the 17th month of a baby's life, coinciding with structural changes in the memory- processing regions of the brain. Besides explaining why Junior doesn't remember last month's trip to Disney World, these results should help guide future research on the link between early behavioral development and changes in the infant brain. 'It wasn't clear how long children in the first year of life could retain a memory of an event,' Liston says. We were interested in testing the hypothesis that neurological developments at the end of the first year and the beginning of the second would result in a significant Enhancement in this kind of memory. Liston showed a simple demonstration to infants ages 9, 17, or 24 months old. The test results showed a huge difference between the test children Who had been 9 months old when they saw the first demonstration and those who had been older. 'Whereas 9-month-olds don't I really remember a thing after four months, 17-and 24-month-olds do,' Liston says. 'Something is happening in the brain between 9 and 17 months old that enables children to encode these memories efficiently and in such a way that they can be retained and retrieved after a long period of time,' Liston says. Researchers believe that changes in certain regions of the brain's frontal lobe and the hippocampus, which axe associated with memory retention and retrieval, drive the rapid expansion of childhood recall. Previous studies have shown that the frontal lobes in humans begin to mature during the last quarter of the first year of life. Liston's work may help explain why adults can rarely remember anything from before their second birthday or so. Most people simply accept this 'infant amnesia' as a fact of life. 'But it's not clear why a 40-year-old has plenty of memories for something that happened 20 years ago, but a 20- year-old has basically no memories for something that happened when he was 2 or 3 ,' Liston says. He suggests that the same brain mechanisms that were not yet able to encode long-term memories in 9-month-olds may also play some role in adults' inability to remember events of infancy. Researchers still need to look at other areas of cognition -- such as what role language ability plays in memory -- to really fully understand why people can' t remember anything that happened before 2--3 years of age. But one thing is clear: When l-year-old Snookums claims he doesn't remember breaking the heirloom chitin five months ago, he's almost surely telling the truth. Conor Liston ______.