"Your brain is made of up millions of cells called neurons. These neurons work together in large networks to coordinate and control processes in your body, from understanding what you see, hear, smell, taste, and feel, to executing movements, to regulating your breathing, heart rate, and metabolism. Neurons communicate using electrochemical signals. When a neuron is activated, it generates an electrical signal that it can then transfer to the next neuron in the network, which will transfer it to the next, and so on. In this way, information can rapidly travel throughout the brain, connecting different areas that are responsible for different processes and body parts...
A single neuron on its own doesn't generate much electrical activity, but the collective activity of neurons within these networks produce enough electrical activity to be detected outside of the head. We can measure this electrical activity by placing special sensors called electrodes onto the head. We often put many electrodes on a cap, headset or headband in order to record the electrical activity coming from different regions of the brain. This method of measuring the brain's electrical activity externally is called electroencephalography, or EEG. Many BCI (brain-computer interface) systems use EEG to record brain activity, although this is not the only method...
A BCI cannot read your thoughts. Even with implantable, invasive BCI systems (the ones that can record stronger electrical signals since they are placed closer to the brain), the technology is still very, very far from being able to "read" anyone's thoughts. Instead, what BCIs look for are patterns that occur in brain activity in response to certain external events or stimuli, or that are generated during certain cognitive processes. These patterns are typically neurophysiological phenomena that have been well documented and studied by neuroscientists and doctors. For example, if you stare at a flashing light, your brain activity will start to synchronize with the rate of that flashing light, and this synchronizing pattern can be detected by the BCI. There are several types of BCIs paradigms, which are all based on their own neurophysiological pattern."
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