We present a 32-transmitter, 32-receiver dual-wavelength frequency-domain (FD) fNIRS system comprised of commercially available avalanche photodiodes, laser drivers and laser mounts. The custom frequency domain (FD) fNIRS system is used to interrogate cerebral tissue with optodes positioned at the posterior occipital region of the head. Data are collected from human subjects watching movie scenes with no sound. We applied cross-validated PCA to identify the number of dimensions retained in the neural signal recorded using FD-fNIRS for the magnitude, phase, and FD (magnitude and phase combined) components of the recorded signal. Importantly, a comparison of the cross-validation error for each signal allows us quantify the dimensionality of the linear subspace spanned by each data type. The number of principal components producing the minimum cross-validation error for the held-out test runs represents the number of orthogonal signal dimensions preserved across training and held-out test data runs. We find that the FD signal captures a higher dimensional space compared to the magnitude or phase signals in isolation. Previous theoretical and empirical work suggest that signals extracted using FD-fNIRS contain higher fidelity neural information than CW-fNIRS in isolation. The findings reported here further support this hypothesis and extend beyond the findings reported in the literature, demonstrating that a higher dimension linear subspace is covered by FD-fNIRS above and beyond the baseline signal captured using traditional CW-fNIRS, assuming other optical performance metrics such as optical dynamic range, noiseequivalent power and cross-talk are comparable. This work was funded by a research contract under Facebook’s Sponsored Academic Research Agreement.
Frequency-domain (FD) fNIRS is attractive for non-invasive brain imaging because phase-sensitive detection leads to increased resolution and may exhibit improved robustness to motion artifacts. We present an FD-fNIRS system with silicon photomultiplier (SiPM) receivers, where the sensitivity and dynamic range approach those of a first-class continuous-wave (CW-) fNIRS system. This represents a significant step toward fully exploiting the phase degree of freedom provided by FD-fNIRS. The transmitter subsystem includes 32 channels and each supplies 12.5 mW of coherent light at both 690 and 852 nm. A dedicated radio circuit intensity-modulates each laser, and they are independently configured to operate at frequencies up to 400 MHz. The transmitters are on-off-keyed according to a user-specified pattern to mitigate shot noise and maximize dynamic range. The receiver subsystem also includes 32 channels. Each consists of a large-area (2.16-mm diameter), high-NA (0.66) fiber bundle, which carries light to a custom photo-receiver. A three-lens assembly enhances coupling between the fiber-bundle and the SiPM, and the SiPM (ON Semiconductor MICRORB-10020) converts the signal to the electrical domain. The electrical signal is amplified and down-converted to the audio spectrum, and a transformer balances the signal and provides galvanic isolation. Each of the 32 audio waveforms is digitized at 192 kS/s in a bank of commercial audio digitizers. Using a modulation frequency of 211 MHz, swept-power measurements demonstrate that the average noise-equivalent power of the SiPM photo-receivers is 20.5 fW per square root Hz, with about 6 decades of optical dynamic range. This work was funded by a research contract under Facebook’s Sponsored Academic Research Agreement.
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