Functional Near-InfraRed Spectroscopy (fNIRS) measures cerebral hemodynamics associated with brain activation. Non-invasive optical measurements of cerebral hemodynamics are often confounded by superficial, extra-cerebral hemodynamics and by instrumental and motion artifacts. These confounds are especially prominent in optical intensity data collected at a single source-detector distance. Alternatively, slope methods and frequency-domain measurements of the phase of photon-density waves have been proposed. Here, we first demonstrate the ability of a special slope method (dual-slope) to efficiently suppress instrumental artifacts. Then, a dual-slope imaging array is utilized to generate and compare single-distance and dual-slope intensity and phase data collected on the visual cortex of a human subject during a contrast reversing visual stimulation protocol. The measured hemodynamic traces associated with visual stimulation exhibit a larger amplitude when they are derived from dual-slope versus single-distance data, and from phase versus intensity data. In particular, the functional hemodynamics obtained from dual-slope phase data feature the largest amplitude. These results indicate the greater sensitivity to brain tissue achieved by dual-slope versus single-distance data, and by phase versus intensity data. The conclusion of this work is that dual-slope intensity (in continuous-wave fNIRS) and dual-slope or single-distance phase (in frequency-domain fNIRS) appear to be most effective for functional brain measurements, with the significant practical advantage offered by the minimal sensitivity of dual-slope measurements to a variety of artifacts.
|