If you think about electrochemical signals moving around the brain as a giant relay race, the synapses are the points at which the baton is handed off from one runner (or neuron) to another. By lighting up those junction points in the brain of the fruit fly Drosophila melanogaster, neuroscientists at Northwestern University were able to peer into the past to see what the insect had experienced hours before.

In a technique that could prove useful to understanding the human brain, the scientists engineered three different colored fluorescent molecules -- green, yellow and blue -- from a protein found in jellyfish. They then split these molecules in half, and using methods from molecular biology, attached one side to one neuron and the other to the neuron across the synaptic gap. When a signal leaped across the synapse, the molecule came together and lit up. What's more, it stayed lit for between one to three hours.

To conduct their study, the researchers applied their fluorescent molecules to the neural connections involved in the fly's three main biological systems: its sense of smell, temperature and sight. They then exposed the insects to different sensory experiences -- the smells of a caraway seed and a rose, for example -- and then looked under the microscope to see which synapses had were activated, as indicated by which color markers lit up.

"Different synapses are active during different behaviors, and we can see that in the same animal with our three distinct labels," said Marco Gallio, lead author on a paper about the research published Friday in the journal Nature Communications.

Albert Lleal/Minden Pictures/Corbis

Gallio, an assistant professor of neurobiology in Northwestern's Weinberg College of Arts and Sciences, believes his work with the fly -- which he says has only 100,000 neurons -- could lead to greater understanding of the human brain, which has about 100 billion neurons.

"Once we understand how a simpler brain computes decisions and processes behavior, we can use these principles as a starting point to understand the more complex, human brain" he told CNET's Crave blog. "The idea is that some of the logic will have been conserved through evolution."

Gallio also said the fluorescent technique can provide a more detailed, less invasive look at the brain than methods currently available. fMRI (functional magnetic resonance imaging) machines, for example, don't offer a cell-by-cell look at our gray matter, but rather broadly indicate what region of the brain is active during certain processes. Neuron-to-neuron communication can be monitored through injectable electrodes, but such a process is inherently invasive.

"So we are indeed in need of tools that allow us to look at activity at the single-synapse level," Gallio said. "This is what our system can do for the first time in a retrospective fashion. We can let the fly go about its business, and after the fact see which synapses were active as a result."