Quick: What's the fattiest system in your body that has two sides and weighs between 2 and 4 pounds?
It's your brain -- you know, that thing that remembers stuff. But because of rapidly evolving information technology, your first impulse was probably to search for the answer on the Internet.
As we become ever more dependent on external sources of memory -- using GPS to guide our driving, smartphones to keep our schedules -- it's time to rethink our ideas about what "memory" actually is.
While we don't physically plug smartphones and other devices into our heads, in some ways we're already one with them, as evidenced by the anxiety we feel when we're without them. Would you remember to pick up milk? Would you know your parents' phone numbers?
If you've ever found yourself running late because you left your phone at home, "you might be a cyborg," says Fred Trotter, a blogger who spoke about information technology at the Health Journalism 2012 conference in April.
Brain implants that make you think of "Avatar," "The Matrix" and "Star Trek" may still be to come, and scientists are working on ways that we can control devices with thoughts alone. Researchers at Duke University last year, for instance, showed how a monkey could control a virtual arm with its brain, as well as feel sensations the appendage delivered.
But in some ways it doesn't matter that we're pushing buttons with our fingers instead of our thoughts. We have become dependent on the networked devices that live in our pockets and colorful rubber cases, rather than what's in our skulls.
"They're really external extensions of our mind," said Joseph Tranquillo, associate professor of biomedical and electrical engineering at Bucknell University.
Tranquillo and his colleague in the humanities, John Hunter, spoke about the tension between technology and memory at the Neuro-Humanities Entanglement Conference at Georgia Tech in April, where academics and thinkers from a variety of disciplines came together to discuss how their seemingly disparate areas of study might connect.
There are two models of memory, they said. One idea is that it's closed, predictive, static and stable -- such that what you put into the system never changes. The other is that it's unstable, dynamic, open and contextual -- ever changing.
In reality, memory as we know it fluctuates on the spectrum between these two extremes. And digital technology is creating ever more tension between them.
What we remember
A computer will save something for you when you hit command-S. For humans, it's more complicated.
We have two kinds of memory: short-term, or working memory, which are the most fleeting recollections, and then long-term memory, through which we can access perceptions of events in the distant past. Scientists believe a brain region called the hippocampus is involved in short-term memory. A 2009 study in the Journal of Neuroscience suggests that, by contrast, the frontal, temporal and parietal cortices -- all located on the surface of the brain -- are more active when recalling older memories.
So what makes a memory stay with you? Experts say it's all about context.
When you save a text document on your computer, your hard drive doesn't care whether it's a college application, a poem that made you cry or an angry letter to your cable company. The computer will save the file just the same, and you can retrieve the document with an ease that is unrelated to its emotional content.
But with human memory, emotional weight gives extra stickiness to experiences. An almond-shaped brain region called the amygdala, involved in the flight-or-fight response, has a big impact on memory processing connected with your feelings. You probably remember exactly where you were when you learned about the World Trade Center attacks of September 11, 2001, in a way that's more vivid than your recollection of lunch last Tuesday.
Similarly, when your computer encodes images that you upload from your camera, it does so irrespective of the subject matter of the photo. But the research of Aude Oliva, associate professor of brain and cognitive sciences at Massachusetts Institute of Technology, has shown how memorable different images are to humans varies greatly by subject matter.
For instance, you may think a photo of a landscape is pretty, but you won't remember it as well if there aren't people or animals in it. "Suddenly this will give to that scenery a high level of memorability," Oliva said.
And if there's an image of a person who's looking right at you, that's likely to be a little more memorable than if the person's gaze is averted. Similarly, an image of two people interacting will stay with you longer than if they're not interacting.
What's going on here? There are brain structures specific to facial recognition, so we are extra attuned to remember seeing faces of other people. The brain is tuned to look at other people and perhaps try to determine what they are thinking.
Studies have also found that when memories are similar, they're more likely to be confused, said Dr. Gary Small, director of the University of California, Los Angeles Center on Aging and co-author of the book "iBrain: Surviving the Technological Alteration of the Modern Mind."
But we do have the advantage of forming memories based on five senses -- sight, smell, taste, touch and hearing, notes Paul Nussbaum, clinical neuropsychologist at the University of Pittsburgh. These can also serve as reminders or provokers of past experiences.
Computers cannot at present reproduce the way your mother's salty chicken soup feels sliding down your sore throat, despite your vivid recollection of it from childhood. And that memory might come flooding back to your brain, not your phone, when you taste the soup again.
In other words, it's a lot harder for the human brain to store random strings of data that have no particular context or emotion. Computers can do this instantly, but we still need our brains to help us give our experiences meaning.