Scientists working to understand the way the brain works have built a tiny microscope which looks inside a rat's brain as the animal moves around.
But now it is possible to see inside a rodent's head as it behaves more normally.
"The brain is the organ that controls behaviour and to study the process of controlling behaviour, you have to do this in a situation where the animal can behave - perform tasks, move around and take decisions," Winfried Denk told BBC News Online.
Memory link
Dr Denk said that he and his colleagues were able to cause parts of the rat's brain to fluoresce and then use the fluorescence to look at how the dendrites of the rat's neurons were functioning.
"The dendrites are where the cell collects the inputs from other neurons and makes the decision whether to fire or not.
"Many neuroscientists believe that that's where memory is stored," he said.
The tiny microscope was custom built at Bell Labs, part of Lucent Technologies, and is the result of several years of research.
It weighs 25g (0.9 ounces) - light enough to be carried around by an adult rat.
It is connected by a flexible tether to laser equipment and computer-based imaging technology. The researchers hope to be able to devise an even smaller version.
Technical hurdles
They are also now considering whether to extend the technique to mice.
Research scientists are interested in mice because there are many strains of laboratory mice which have been genetically modified to exhibit certain features.
Some lack key genes, or have certain genes added or replaced.
Being able to look inside the brain of a mammal and see individual cells in action as it completes everyday tasks would be a major step forward for neuroscientists.
"We have overcome most of the crucial technical hurdles in the way of such experiments," Dr Denk and his colleagues write in the journal Neuron.