Activity of the human visual cortex measured non-invasively by diffusing-wave spectroscopy

Abstract

Activity of the human visual cortex, elicited by steady-state flickering at 8 Hz, is non-invasively probed by multi-speckle diffusingwave spectroscopy (DWS). Parallel detection of the intensity fluctuations of statistically equivalent, but independent speckles allows to resolve stimulation-induced changes in the field autocorrelation of multiply scattered light of less than 2%. In a group of 9 healthy subjects we find a faster decay of the field autocorrelation function during the stimulation periods for data measured with a long-distance probe (30 mm source-receiver distance) at 2 positions over the occipital cortex (t-test: t(8) = −2.672, p = 0.028 < 0.05 for position 1, t(8) = −2.874, p = 0.021 < 0.05 for position 2). In contrast, no statistically significant change is seen when a short-distance probe (16 mm source-receiver distance) is used (t-test: t(8) = −2.043, p = 0.075 > 0.05 for position 1, t(8) = −2.146, p = 0.064 > 0.05 for position 2). The enhanced dynamics observed with DWS is positively correlated with the functional increase of blood volume in the visual cortex, while the heartbeat rate is not affected by stimulation. Our results indicate that the DWS signal from the visual cortex is governed by the regional cerebral blood flow velocity.

Direct measurement of tissue blood flow and metabolism with diffuse optics

“…These results matched up quantitatively with similar experiments performed with other functional neuroimaging techniques, such as MRI and positron emission tomography (PET) [61][62][63]. Since this early research, many different functional activation paradigms have been studied with diffuse optics [41][42][43][44][45][46][47]. As noted above, the addition of DCS is critical, because it unambiguously permits quantification of changes in oxygen metabolism in all of these experiments.…”

Fiber-based multispeckle detection for time-resolved diffusing-wave spectroscopy: characterization and application to blood flow detection in deep tissue

“…In turn, the temporal resolution may be traded in for largely enhanced sensitivity in experiments with long integration times. This allows us to resolve very small functional changes of the DWS signal, as was recently demonstrated for the visual cortex [20].…”

“…The correction for receiver nonidealities by Eq. (1) is particularly important for detecting physiological DWS signals with very small functional changes of d , such as from the human visual cortex [20]; here, the collection tip is essential for optimal signal collection through hair.…”

Extraction of diffuse correlation spectroscopy flow index by integration of Nth-order linear model with Monte Carlo simulation

“…2,5 Conventionally, blood flow index (BFI) was extracted by fitting the autocorrelation function measured by DCS to analytical solutions of correlation diffusion equation under regular tissue boundaries (e.g., semi-infinite homogenous media, [6][7][8][9] multi-layer slabs, 10,11 a sphere inside a slab 12 ). Some of those analytical solutions were proposed to account for the influence of non-scattering layer tissues 11 or to improve the signal-to-noise ratio in deep tissues.…”

“…Some of those analytical solutions were proposed to account for the influence of non-scattering layer tissues 11 or to improve the signal-to-noise ratio in deep tissues. 9 However, the commonly used semi-infinite approximation may lead to BFI estimation errors in small volume tissues with large curvature. 13 Seeking analytical solutions is mathematically complicated 10,12 and likely impossible for irregular geometries.…”