The brain is really hard to see. The whole thing is very large - the human brain is several pounds in weight - but the connections between brain cells, known as synapses, are really tiny. They're nanoscale in dimension. So if you want to see how the cells of the brain are connected in networks, you have to see those connections, those synapses.
Edward BoydenFor the last century of neuroscience, lots of people have tried to control neurons using all sorts of different technologies - pharmacology (drugs), electrical pulses, and so on. But none of these technologies are precise. With optogenetics, we can aim light at a single cell, or a set of cells, and turn just that set of cells on or off.
Edward BoydenRemember, when we're conscious of something, that state is quite often generated by unconscious processes that happen right before it.
Edward Boyden