This is really dependent on the signal quality. Typically for normal cortical penetration you will want anywhere from 3cm up, the larger the distance the deeper the penetration however at the sake of a reduced signal-to-noise ratio. Typically for normal adults we employ a 3.5cm interoptode distance, for those with darker skin or dark/dense hair you may need to reduce this to achieve an ideal SNR. For younger children and infants often times 2.5cm is more common.