Functional transcranial Doppler ultrasonography (fTCD) is a non-invasive, reliable and valid method for measuring event-related changes in blood flow velocity in the middle cerebral arteries (MCA). It has been used for more than 15 years for the assessment of hemispheric dominance for a number of cognitive functions, mainly language.

FTCD is based on the Doppler effect, which is the change in the frequency of a wave (or other periodic event) for an observer moving relative to its source. Using fTCD blood flow velocity is computed by measuring the frequency change of the emitted sound wave caused by its bouncing off the blood cells circulating in the MCAs. By comparing the changes in velocity between the right and left MCAs, the dominant hemisphere for the function in question can be determined.

FTCD, by measuring blood flow velocity changes in the MCAs, measures changes in cerebral perfusion during cognitive tasks compared to rest periods. These cerebral blood flow velocity changes in the MCAs are taken to indicate the downstream increase of regional metabolic activity during a task. FTCD thus adds to the perfusion-sensitive techniques of functional imaging (fMRI, PET, etc). These techniques are based on the fact that cerebral perfusion is closely coupled to cerebral metabolism and neural activation.

Electroencephalography (EEG) is one of the most widely used non-invasive techniques for studying the human brain. Using EEG one can amplify, record and analyse the electrical activity of the brain caused by the firing of neurons. The recording is obtained by placing electrodes on the scalp with a conductive gel or paste.

Using the EEG, one can also record event-related potentials (ERPs). ERPs are electrical potentials recorded from the neurons following the presentations of visual, auditory, or tactile stimuli, in order to assess cognitive functions such as attention, memory, perception language and emotions. The main advantage of ERPs over other brain imaging techniques, is that ERPs allow for a temporal resolution down to the millisecond scale. Therefore, it allows for the real-time measurement of neuronal activity during cognitive functions.