As described in the previous chapter, the brain communicates using spikes, which are all-or-none electrical pulses produced when the neuron receives a sufficient amount of input current from other neurons via synaptic connections. It is therefore not surprising that some of the first technologies for recording brain activity were based on detecting changes in electrical potentials in neurons (invasive techniques based on electrodes) or large populations of neurons (noninvasive techniques such as electroencephalography or EEG). More recent techniques have been based on detecting neural activity indirectly by measuring changes in blood flow that result from increased neural activity in a region (fMRI) or by measuring the minute changes in the magnetic field around the skull caused by neural activity (MEG).

In this chapter, we review some of these technologies that serve as sources of input signals for BCIs. We also briefly describe technologies that can be used to stimulate neurons or brain regions, thereby allowing BCIs to potentially provide direct feedback to the brain based on interactions with the physical world.