MIT‘s Kwanghun Chung and Stanford‘s Karl Deisseroth‘s CLARITY allows researchers to see directly into optically transparent whole brains or thick blocks of mouse brain tissue.

Such studies in living people are impossible, because most neuron-tracing methods require genetic engineering or injection of dye in living animals.

The hope is that this will one day lead to a way to help people with severe mental illness or brain diseases.

Applying CLARITY to whole mouse brains, the researchers viewed fluorescently labeled neurons in areas ranging from outer layers of the cortex to deep structures such as the thalamus. They also traced individual nerve fibers through 0.5-millimetre-thick slabs of formalin-preserved autopsied human brain — orders of magnitude thicker than slices currently imaged.