Laboratory-made "minibrains" that show a primitive ability to think could transform research on neurological diseases and treatments, scientists say.
The tiny balls of brain cells, each about the size of a housefly's eye, have been created by US researchers who claim they could be mass-produced in large numbers.
Grown from stem cells, the brains contain many of the neurons and cells of their full-sized human equivalents.
Scientists believe they will provide far more reliable results in drug tests and other experiments than animal "models" such as mice and rats.
Lead researcher Professor Thomas Hartung from the Johns Hopkins Bloomberg School of Public Health in the US, said: "Ninety-five per cent of drugs that look promising when tested in animal models fail once they are tested in humans at great expense of time and money.
"While rodent models have been useful, we are not 150lb rats. And even though we are not balls of cells either, you can often get much better information from these balls of cells than from rodents.
"We believe that the future of brain research will include less reliance on animals, more reliance on human, cell-based models."
The brains were created using induced pluripotent stem cells (iPSCs), adult cells that have been genetically reprogrammed into an embryonic stem cell-like state.
Stimulation with the right chemical cocktail causes the iPSCs to further develop into the different cells and layers of the brain.
Potentially, thousands of exact copies can be produced - a hundred in just one laboratory petri dish, say the scientists. They can also be customised to display certain traits or genetic defects associated with diseases such as Alzheimer's, Parkinson's and multiple sclerosis, as well stroke or trauma.
Prof Hartung described the work at the annual meeting of the American Association for the Advancement of Science (AAAS), taking place in Washington DC.
He said one of the most exciting discoveries was that the brains displayed spontaneous electrophysiological activity that could be recorded with electrodes.
"It's starting to produce a primitive type of thinking," he added. "Obviously there's no input or output. It is meaningless electrical activity but the neurons are trying to communicate with each other.
"This is the beauty of the third dimension. With a normal culture, which looks a little like eggs sunny-side up, there's not a very dense cell wall where all these brain cells come together. In these new mini-brains they're starting to produce functionality, and cellular communication on the level of using these electrical signals."
After being cultivated for about two months, the brains developed four types of neurons and two kinds of support cell that nurture nerves.
One of the support cell types, oligodendrocytes, produce myelin, the fatty insulating sheath surrounding nerve fibres that is lost in multiple sclerosis.