Atlanta, Georgia (CNN) -- Quick: What's the fattiest system in your body that has two halves 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.
Is memory unchanging, or ever-changing?
Regardless of the handiness of our smartphones, we still rely on our memories to recall certain images and events that we did not record. We'd like to think that those memories are, as much as possible, "true."
But here's the thing: The way you encode memories depends on the state of your brain at that moment and the environmental context. That means neural network changes forever, and will never return to that exact state, Tranquillo and Hunter said. And when you recall something, that changes your brain, too! The very act of remembering uses brain processes, so you can never go back to the self that you were in the moment you're thinking about. Pretty trippy, right?
Technologies are emerging to help people document their lives digitally as never before. For instance, Microsoft developed a camera called SenseCam, which captures photos of your visual experience all day, every day. Gordon Bell, a Microsoft researcher, wrote a book called "Total Recall" in 2009 after recording every aspect of his life for a decade.
You might think that digital memory is more stable -- at its core, it's just a collection of 0s and 1s after all.
But, like the brain, technology is a constantly evolving, open system, subject to environmental influences. On a small scale, think about how files must be filtered to change formats. With compression, images and videos lose resolution and quality. But programs can alter photos in ways that enhance them, too. The computer is reassembling and reprocessing the files in each of these cases.
And, your computer may eventually break down, purging your files (which is why hard-drive backups are recommended).
Social media also gives new meaning to digital storage. Where you share a photo, how it gets tagged and what its caption says all create a memory around the image that wouldn't exist otherwise, Tranquillo said.
And your digital memory becomes influenced in even stranger ways now that there are social networking tools that update content without your direct intervention -- for instance, posting on Facebook or Twitter when you arrive at a location. With Facebook's Timeline feature, other people can contribute to your online life history by posting photos, videos and comments.
"This is the crowdsourced self," Tranquillo said. "As the viewer changes, so does the collective construction of 'you.'"
Aiding the aging brain
You may have, at one point or another, struggled with multitasking. That's because when you move from one task to another, your brain shuts down one neural circuit in order to move on to the next. That's inefficient, and studies have shown that it's harder for older adults to re-establish that initial circuit and return to the first activity.
But if you use the Internet in ways that make your life more efficient, it could theoretically reduce the multitasking that you do. You don't have to fumble through an address book frantically, or find your way out of an unfamiliar neighborhood unguided, while thinking about other things.
That's important as the number of Americans with Alzheimer's disease -- currently 5.5 million -- continues to climb. As people get older and their memory begins to decline, they are still able to access information through search, helping to compensate for their own memory deficits.
Small is working with computer scientists at UCLA on games that can help older people improve their ability to remember names and faces. Plenty of research is in the works to find brain-boosting pharmaceuticals, supplements and foods (most recently, berries), although nothing is a certain supplement.
The dark side
The downside of technology is that it can make us less thoughtful and less creative. And we may spend less time communicating face to face, which can reduce the quality of relationships. A 2012 survey of American girls found that spending time multitasking with various digital devices, watching video or communicating online is associated with abnormal social tendencies.
It's also not certain that freeing our memory from addresses and phone numbers frees up room to be more creative, Tranquillo said.
What's more, a lot of creative thinking depends on having ideas accessible in a way that preserves their context, so merely writing them down to keep externally -- whether on a computer or a notebook -- doesn't necessarily allow you to do novel things with them.
Tapping storage potential
It's not hard to appreciate how, in terms of sheer volume, computer memory has outpaced humans. Google's Gmail offers 10 gigabytes of storage. Tom Landauer, professor of psychology at the University of Colorado at Boulder, estimated in 1986 that the human brain holds 200 megabytes of information.
And what if we stored all of an individual's experience? Landauer calculated that if a person only takes in one byte per second, and lives about 25,000 days, that's still only 2 gigabytes.
But that's just a tiny fraction of estimates for the brain's total storage capacity, which is as high as 2.5 petabytes (2.6 million gigabytes), based on the number of neurons (1 billion) and connections to other neurons.
That sounds like a lot, but McKinsey Global Institute estimated that consumers stored 7 exabytes (7.5 billion gigabytes) of new data on PCs, laptops and other devices in 2010.
Obviously, there is too much information out there to hope to compete with our minds against machines. But there are certain things you can do to help your brain live a long, healthy life so that you are using it to its fullest potential.
Engaging in activities that are new, difficult and complex forces your brain to lay down new cellular connections, Nussbaum said. Exercise and healthy diet are also important to brain health. Emerging research in meditation and spirituality has indicated that mindfulness -- practices associated with being in the present moment -- help, too.
"We tend to really get so impressed with the latest gadget, the latest phone, the latest whatever it is," Nussbaum said, "and we forget that all of the technology that built it came from the human brain."
What is there left to remember that's not out there on the Internet? You'd better know your most valuable digital secrets by heart: Your passwords!
Share your thoughts about how digital technology helps or hurts your memory in the comments.