THE BIG BRAIN QUESTIONS
The human brain is the most astoundingly complex structure in the known universe. Yet we are starting to unravel some of its mysteries, thanks to
advances in brain imaging, genetics, stem cell research and more. We explore the latest findings from the hottest topics in neuroscience.
1. What are memories made of?
MEMORIES are the basic stuff of thought. We access our stores of knowledge every time we perform a task, communicate through speech or formulate the
simplest concepts. Yet the physical form of memory has long been mysterious. What changes occur in the brain when a new memory is encoded?
One thing we do know is that memory formation involves the strengthening of synaptic connections between nerve cells. Using sea slugs, which have a
relatively simple nervous system, a team led by Kelsey Martin at the University of California, Los Angeles, last year became the first to watch
memories being made, in the form of new proteins appearing at the synapses (Science, vol 324, p 1536).
Where, though, is knowledge stored in the complex brains of mammals? Short-term memories, such as a telephone number about to be used, seem to be
stored in two small curled-up structures called the hippocampi, buried deep in the brain's two hemispheres. In 2008 Courtney Miller and David Sweatt
at the University of Alabama in Tuscaloosa showed in mice that during the first hour after a memorable event there were chemical changes to the DNA of
neurons in this area, altering the proteins produced. Over the subsequent week, there were similar changes to the genes of neurons in the cortex.
These changes seemed to be permanent, indicating that long-term memories are stored there (Neuron, volume 53, p 857). The pair think they watched
short-term memories form in the hippocampus, which then became long-term memories in the cortex.
2. Can we regenerate the brain?
YOU were born with all the brain cells you'll ever have, so the saying goes.
So much for sayings. In the 1990s, decades of dogma were overturned by the discovery that mammals, including people, make new neurons throughout their
lives. In humans, such "neurogenesis" has been seen in two places: neurons formed in the olfactory bulb seem to be involved in learning new smells,
while those born in the hippocampus are involved in learning and memory.
The discovery that new neurons can integrate into the adult brain raises intriguing possibilities. Could the process be harnessed to treat diseases of
the brain, such as Parkinson's and Alzheimer's?
The trick will be in replacing diseased cells with just the right kind of neuron, says Jeff Macklis, who studies neurogenesis at the Massachusetts
Institute of Technology. By some estimates, the nervous system is made up of 10,000 different kinds of neuron.
This complexity means you can't just hijack any old cell produced by natural neurogenesis. However, there may be other ways of growing new neurons to
3. What are mirror neurons?
"WHEN you're smilin', the whole world smiles with you," sang Louis Armstrong. He could have been referring to what some consider one of the
greatest recent discoveries of neuroscience: mirror neurons.
Discovered in macaques in the 1990s, these cells were spotted when researchers made recordings from microelectrodes placed in the animals' brains as
they performed various tasks. While many neurons fired when the animal performed an action, a subset also fired when the animals saw the researcher
perform the same action, with different groups of mirror neurons for different actions.
Neuroscientists have speculated that in people, mirror neurons could represent the neural basis of empathy. They could also contribute to imitation
and learning, and perhaps even language acquisition.
It has been hard to find out if people have mirror neurons, but MRI scans have shown that certain areas of the brain - dubbed mirror systems - "light
up" when we perform and watch the same action. Numerous studies have shown that people with more activity in their mirror systems seem to be better
at understanding other people's emotions. Conversely, less activity in mirror systems has been linked to autism and also with psychopathy - different
conditions that are both noted for low levels of empathy. Nina Bien's team at Maastricht University in the Netherlands recently identified inhibition
mechanisms that hint at how we can mentally imitate an action without actually performing it (Cerebral Cortex, vol 19, p 2338).
4. How many ways can we be conscious?
YOU might think consciousness is like a light switch, either on or off. But the true picture now seems rather murkier, challenging our notions of
awareness and free will, as well as raising issues of consent in coma patients.
People used to think just three states of consciousness existed, says Adam Zeman of the Peninsular Medical School in Exeter, UK. "You're either
awake or asleep - and if you're asleep you're either in dream or non-dream sleep."
But brain imaging suggests there are more. Take sleepwalking, which affects as many as 1 in 20 adults. "The sleepwalking brain is literally in a
half-awake, half-asleep state," says Zeman. Researchers once manoeuvred a sleepwalker into a brain scanner, and while they saw that much of the
cortex - involved in awareness and consciousness - was offline, other brain areas were active, including those linked with emotion.
Similar overlaps might explain other weird states of consciousness. In dreaming sleep - also known as REM sleep - we lie still because an area of the
brainstem called the pons blocks signals to the muscles. People with REM behaviour disorder lose this inhibition and physically act out their dreams.
The opposite condition, known as sleep paralysis, occurs if people wake up while still unable to move.
[edit on 10-4-2010 by predator0187]