RESEARCH
Papers and sources.
Every number on this site traces back to something you can read: the literature that established fast optical signal, the sources behind our modality comparison, and a register of the claims we make and where each one stands.
Foundational fast optical signal papers
The method is variously called the fast optical signal (FOS), the event-related optical signal (EROS), or simply the fast signal, in contrast to the slow hemodynamic signal measured by the same instruments. These are the papers we hand to new hires, in roughly the order we hand them over.
- Shades of gray matter: noninvasive optical images of human brain responses during visual stimulation.
Gratton, G., Corballis, P. M., Cho, E., Fabiani, M., & Hood, D. C. (1995).
Psychophysiology, 32(5), 505–509.The first non-invasive demonstration in humans. - Fast and localized event-related optical signals (EROS) in the human occipital cortex: comparisons with the visual evoked potential and fMRI.
Gratton, G., Fabiani, M., Corballis, P. M., Hood, D. C., Goodman-Wood, M. R., Hirsch, J., Kim, K., Friedman, D., & Gratton, E. (1997).
NeuroImage, 6(3), 168–180. doi:10.1006/nimg.1997.0298Places the optical response against EEG and fMRI in the same subjects. - The event-related optical signal: a new tool for studying brain function.
Gratton, G., & Fabiani, M. (2001).
International Journal of Psychophysiology, 42(2), 109–121. doi:10.1016/S0167-8760(01)00161-1The clearest early statement of the method and its mechanism. - Functional frequency-domain near-infrared spectroscopy detects fast neuronal signal in the motor cortex.
Wolf, M., Wolf, U., Choi, J. H., Gupta, R., Safonova, L. P., Paunescu, L. A., Michalos, A., & Gratton, E. (2002).
NeuroImage, 17(4), 1868–1875.Fast signal in motor cortex, on a frequency-domain instrument. - The fast optical signal — robust or elusive when non-invasively measured in the human adult?
Steinbrink, J., Kempf, F. C. D., Villringer, A., & Obrig, H. (2005).
NeuroImage, 26(4), 996–1008.The skeptical paper. Read it; it frames the signal-to-noise problem we work on. - Fast optical imaging of human brain function.
Gratton, G., & Fabiani, M. (2010).
Frontiers in Human Neuroscience, 4, 52. doi:10.3389/fnhum.2010.00052The review. Sub-centimeter localization, temporal resolution under 100 ms, penetration limited to about 3 cm.
Mechanism and physics
- Changes in neuron structure during action potential propagation and synaptic transmission.
Cohen, L. B. (1973).
Physiological Reviews, 53(2), 373–418.Why firing neurons change their optical properties at all. - Noninvasive detection of changes in membrane potential in cultured neurons by light scattering.
Stepnoski, R. A., LaPorta, A., Raccuia-Behling, F., Blonder, G. E., Slusher, R. E., & Kleinfeld, D. (1991).
Proceedings of the National Academy of Sciences, 88(21), 9382–9386. - Measurements of scattering and absorption changes in muscle and brain.
Gratton, E., Fantini, S., Franceschini, M. A., Gratton, G., & Fabiani, M. (1997).
Philosophical Transactions of the Royal Society B, 352(1354), 727–735. - Frequency-domain techniques for cerebral and functional near-infrared spectroscopy.
Fantini, S., & Sassaroli, A. (2020).
Frontiers in Neuroscience, 14, 300.Amplitude and phase, and why phase carries depth.
The modality table, sourced
The EEG, MEG, fMRI, fNIRS and FOS figures on the home and technology pages are typical published ranges. Each row's source is below.
| Modality | Figures we quote | Source |
|---|---|---|
| EEG | ~1 ms temporal · ~1–5 cm spatial · portable | Burle et al. (2015), Int. J. Psychophysiology 97(3), 210–220, doi:10.1016/j.ijpsycho.2015.05.004; Michel & Brunet (2019), Frontiers in Neurology 10, 325. |
| MEG | ~1 ms · ~2–5 mm · shielded room | Baillet (2017). Magnetoencephalography for brain electrophysiology and imaging. Nature Neuroscience 20(3), 327–339. |
| fMRI | ~3–6 s · ~1–3 mm · whole brain · fixed scanner | Logothetis (2008). What we can do and what we cannot do with fMRI. Nature 453, 869–878. |
| fNIRS | ~4–6 s · ~5–15 mm · ~1–3 cm depth · portable | Scholkmann et al. (2014), NeuroImage 85, 6–27; Pinti et al. (2020), Ann. N.Y. Acad. Sci. 1464(1), 5–29. |
| FOS | 5–10 ms · sub-centimeter · ~1–3 cm depth · portable | Gratton & Fabiani (2010), Front. Hum. Neurosci. 4, 52; Gratton et al. (1997), NeuroImage 6(3), 168–180; Parks et al. (2012), NeuroImage 59(3), 2504–2510; Parks et al. (2015), Neuropsychologia 78, 153–158. |
CLAIMS REGISTER
What we say, and what it rests on.
Every substantive claim on this site, where it appears, and its basis.published means a peer-reviewed source, cited above or in the basis column.ours means our own data, estimate or design specification.statement means a position we take as a company.
| Claim | Where | Basis | Type |
|---|---|---|---|
| Fast optical signal was first demonstrated non-invasively in humans in 1995. | Home, blog | Gratton et al. 1995 [1] | published |
| Neurons change shape when they fire, which changes how tissue scatters light. | Home, technology | Cohen 1973 [7]; Stepnoski et al. 1991 [8]; Gratton & Fabiani 2001 [3] | published |
| Fast signal has been recorded from motor cortex. | Home | Wolf et al. 2002 [4] | published |
| FOS penetration is up to about 3 cm. | Home, technology | Gratton & Fabiani 2010 [6] | published |
| FOS localizes to the sub-centimeter level. | Home, technology | Gratton & Fabiani 2010 [6] | published |
| EEG, MEG, fMRI and fNIRS ranges in the modality table. | Home, technology | Sources in the table above | published |
| FOS temporal resolution of 5–10 ms. | Home, technology | Parks et al. (2012), NeuroImage 59(3), 2504–2510; Parks et al. (2015), Neuropsychologia 78, 153–158. | published |
| The optical response to a motor command appears about 96 ms before the button press. | Home | DeSoto et al. (2001), J. Cogn. Neurosci. 13(4), 523–536. | published |
| Published studies: EEG 542,256 · fMRI 176,076 · fNIRS 19,593 · FOS 136. | Home, blog | Our count of OpenAlex-indexed works, 2026. | ours |
| More photons enter the skull on a sunny day than the instrument emits. | Home, technology, participate | Our estimate, from near-infrared solar irradiance and the instrument's emission. | ours |
| Our CTO built the only commercial instrument sold for FOS measurement. | Home, team | ISS markets the Imagent for EROS measurement. No other maker on the fNIRS Society's instrument list advertises a fast-signal mode (our review of manufacturers' sites, October 2026). | statement |
| The first device designed from the ground up for FOS. | Home | Our reading of the published instrumentation. | statement |
| FOS is the only non-invasive method that delivers on all four elements: speed, precision, coverage, and portability. | Home | Follows from the modality table, on the figures cited. | statement |
| Our instruments are investigational and not cleared or approved by any regulator. | Terms | Regulatory status as of the effective date. | statement |