Neurons talk by sending electrical signals to one another. When a neuron is sending or receiving electrical signals, we say that the neuron is active. These electrical signals are based on an imbalance of ions between the inside and outside of the neuron.
Ions are elements that either have too few or too many electrons relative to their neutral state, and that causes the ion to carry a positive or negative charge. When considering large groups of ions, the ions will spread out evenly to create an overall neutral charge. By collecting a slightly larger number negative ions inside the neuron, the membrane that divides the inside and outside takes on that negative charge. By moving around this collection of negative ions, the neuron can move this electrical charge down its branches.
When a neuron is active, calcium enters the neuron to serve as a chemical signal for lots of internal functions. So a higher number of calcium ions in the neuron is a good indicator that a neuron is active. Researchers have engineered a set of chemicals that when bound to calcium, give off a fluorescent signal. These chemicals are called calcium indicators. The most prevalent of these indicators are engineered from the proteins of fluorescent marine animals, the most famous being jellyfish. Using a camera, researchers can capture the fluorescent light that each neuron gives off whenever it’s active. So we’re not actually listening for the electrical signal; we’re looking for it.
A great feature of these calcium indicators is that we can use them in alive and behaving animals. For mice, researchers replace a part of the skull with a window so that a camera can record the light given off by the indicators. So now, researchers can control the sensory input to the animal and record the activity of its neurons at the same time.
There are some tricks we have to pull off to see deep into a tissue. You have experience with this problem, since you can’t see through your hand. By focusing two photon beams at specific layers in the brain, researchers can effectively use lasers to excite the fluorescent dyes that are absorbed by the neurons, highlighting them within a dark brain beneath the microscope in real time.
No. While we want to understand the human brain, manipulating the brain to run these types of experiments would be unethical.
Fortunately, it seems that evolution has conserved some of the best properties of the human brain in other organisms. There are many behaviors that animals can do that we can also identify in humans, like locating where we are in the world and determining how we make some basic decisions. At the microscopic level, it can be hard to tell a human brain apart from other mammal brains.