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

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. © 2007 Optical Society of America OCIS codes: (030.6140) Speckle; (170.0170) Medical optics and biotechnology; (170.5270) Photon density waves; (170.5280) Photon migration; (290.1990) Diffusion; (290.1350) Backscattering; (290.4210) Multiple Scattering; (290.7050) Turbid media References and links 1. K. Dörschel and G. M̈uller, “Velocity resolved laser Doppler blood flow measurements in skin,” Flow Meas. Instrum.7, 257–264 (1996). 2. G. Maret and P. E. Wolf, “Multiple light scattering from disordered media: The effect of Brownian motion of scatterers,” Z. Phys. B65, 409–413 (1987). 3. D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. 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Introduction
When tissue is illuminated by light with large coherence length, a speckle pattern forms on the surface which arises from the interference of multiply scattered photons which have travelled through the tissue along different paths.Fluctuations of the speckle intensity I(r r r,t) at the position r r r yield information on microscopic motions of scatterers within the volume swept by the diffuse photon cloud.In contrast to laser Doppler velocimetry where the power spectrum of speckle fluctuations measured in near-backscattering geometry is dominated by skin blood flow [1], diffusing-wave spectroscopy (DWS [2,3]; also called diffuse correlation spectroscopy, DCS) measures the temporal intensity autocorrelation function g (2) (τ) = I(0)I(τ) / |I(0)| 2 at large source-receiver distances of up to several centimeters.The high sensitivity to even minute scatterer displacements has lead to several applications of DWS in the fields of tumor and skeletal muscle perfusion [4,5,6,7,8,9,10].
Recently, DWS was used to non-invasively detect functional activity in the human somatomotor cortex [11,12].The analysis of DWS autocorrelation functions yielded a increase of the cortical dynamics of about 40% upon contralateral stimulation by a finger opposition exercise.The enhanced dynamics in the somato-motor cortex observed with DWS is consistent with the activation-induced increase of cerebral blood flow (CBF) observed with positron emission tomography (PET) [13,14].While the coupling of blood flow velocity and the DWS signal has been validated in skeletal muscle [10], the origin of the DWS signal from the cortex is not understood in detail.As cortical tissue is strongly scattering, DWS could, in addition to blood flow velocity, also be sensitive to shear deformations within the tissue surrounding the vessels which are induced by pulsation.Motor stimulation is not suited to assess the contributions of blood flow velocity and tissue shearing to the DWS signal separately since both the blood flow velocity and the heartbeat rate (which governs the shear rate within tissue) change simultaneously [12,15,16].
Steady-state flickering elicits a strong oscillatory response in the primary visual cortex and has been extensively studied with electroencephalography (EEG) [17,18].PET experiments show that 8 Hz flickering may induce an increase of the cerebral blood flow of 68%, and an increase of the cerebral blood volume (CBV) of as much as 21% [19].In contrast to motor stimulation, heartbeat rate changes during flickering stimulation are very small [19], which makes this protocol a good candidate for studying the effect of functionally enhanced blood flow on the DWS signal without interfering effects from tissue shearing.The primary visual cortex is located around the calcarine fissure in the occipital lobe, with a substantial portion inside the sulcus and the medial aspects of both hemispheres, i.e., at a considerable distance from the surface.Functional MRI data indicate that the spatial dimension of activated areas in the primary visual cortex increases with the intensity contrast of the stimulus [20].Given the low signal-to-noise ratio of DWS autocorrelation functions resulting from the large sourcereceiver distance required for probing the cortex, the detection of activity in the primary visual cortex areas with DWS presents a serious challenge to currently used fiber-based detection schemes.
In this paper, we report on the non-invasive detection of functional activity in the primary visual cortex in adult humans induced by flickering at 8 Hz.Using a novel multi-speckle detection system which allows to resolve small functional changes of the decay time of the DWS autocorrelation function, we find enhanced dynamics in the visual cortex upon stimulation in a group of 9 subjects.

Methods
DWS experiments were performed in transmission geometry with two fiber-optic probes with source-receiver distances of 16 mm (short-distance probe) and 30 mm (long-distance probe) which were placed on the head of the subject over the occipital lobe (between positions O1 and O2 in the international 10-20 system for EEG [21]; see Fig. 1).Light from a diode laser operating at a wavelength of 802 nm (Toptica TA 100) was coupled into a multi-mode fiber whose output was expanded to reach 4 mW/mm 2 on the scalp surface.Multiply scattered light was collected with bundles of N = 23 and N = 2 few-mode fibers [22] (for the long and the short source-receiver distances, respectively).The output of each fiber was coupled to an avalanche photodiode (Perkin-Elmer SPCM-AQ4C) whose TTL pulse output was used to compute the intensity autocorrelation function by a custom-built 32-channel autocorrelator (correlator.com)with an integration time of 26 ms.The experimental setup is schematically shown in Fig. 1.From the intensity autocorrelation function g was calculated, using experimentally determined coherence factors β i (G.Dietsche et al., in preparation).
Nine female, healthy volunteers (age between 20 and 25 years, right-handed, without epilepsy history) recruited from students of the University of Konstanz took part in this experiment.The visual stimulation consisted in watching a CRT screen flickering at 8 Hz for 30 s.The distance between eyes and screen was approximately 80 cm (screen diagonal: 43.2 cm).During the baseline period of 30 s the subjects closed their eyes.Each baseline-stimulation block was repeated 5 times in a darkened room.During the experiment, the heartbeat rate was recorded with a pulse oxymeter (Nellcor N595).The study protocol was approved by the University's Ethical Review Board.
b (τ) dτ. (1) The lower integration limit is τ 1 = 4 × 10 −7 s, and τ 2 is defined by g b (τ 2 ) = 0.1.Our choice of the upper integration limit τ 2 aims at suppressing contributions from short photon paths which do not carry information on the cortex, and, secondly, at reducing the noise in the decay time [23].b (τ), measured over the primary visual cortex, from one subject for baseline (blue circles) and visual stimulation periods (red squares).Bottom: difference ∆g

Results
Figure 2 shows the bundle-averaged field autocorrelation function g (1) b (τ) from one subject, as a function of the lag time τ, measured with the long-distance receiver for baseline and stimulation periods.During the stimulation period, the decay of the autocorrelation function is faster, indicating that the dynamics in the visual cortex area probed by the DWS experiment is accelerated, such as by an increase in CBF.The difference of field autocorrelation functions ∆g The measurements were carried out successively on two positions above the inion: position 1 located at 10% of the inion-nasion distance and position 2 at 15% of this distance (see Fig. 1b).In the group average, the relative decay time τ s/b is smaller than unity (see Fig. 3).For the long-distance probe, we find τ s/b = 0.962 and τ s/b = 0.970 at position 1 and 2, respectively.For the short-distance probe, the ratios are τ s/b = 0.953 and τ s/b = 0.950 for position These changes are roughly in line with model calculations with a three-layer head model (modelling the cortex as semi-infinite) [12]: an increase of the cortical absorption coefficient by 21% is predicted to lead to a 2.88% decrease of transmitted light intensity recorded with a source-receiver distance of 30 mm.
Modelling the dynamics in scalp and cortex by Brownian motion, an increase in the cortical diffusion coefficient by 68%, together with a 21% increase in cortical absorption, results in a decrease of the decay time by about 2.37%, slightly smaller than the experimental decrease (3.77% for position 1, and 3.01% for position 2).In our model calculation, optical parameters for each layer were taken from the literature [24], and values 0.56 cm and 0.63 cm for the thickness of scalp and skull, respectively, were assumed.Baseline effective diffusion coefficients for scalp and cortex were assumed to be 5.0 × 10 −9 cm 2 /s.The differences between the decay time calculated with the 3-layer model and the measured ones might be due to inaccurate values of the thicknesses of scalp and scull, or due to the modelling of erythrocyte motion by diffusion.It should be noted that that a CBV increase has two effects with opposite directions on the DWS decay time: an increase in CBV gives rise to increased cortical absorption, which changes the path-length distribution in tissue by cutting off the long paths.This causes the decay time to increase.On the other hand, as more erythrocytes enter the sampled volume, the number of scattering events contributing to the autocorrelation function increases by the same percentage as the CBV.This increase in the 'effective scattering events' results in a faster decay of the autocorrelation function.
The correlation coefficient r between the relative decay time of the autocorrelation function and the relative photon count rate were also computed for the long-distance probe data across the 9 subjects: r = 0.688 for position 1, and r = 0.585 for position 2.This positive correlation between changes in CBF and CBV is consistent with the hemodynamical response pattern observed in the activated visual cortex with PET [19].
During stimulation periods, we observed a slight increase in the average heartbeat rate.Nevertheless, this does not account for the functional reduction of the DWS decay time observed with the long-distance probe, as (i) the heartbeat change is not significant (t(8) = 0.991, p = 0.351 for position 1; t(8) = 0.853, p = 0.419 for position 2); (ii) not all subjects showed an increased heartbeat rate during stimulation: in 3 of 9 subjects the heartbeat rate was found to slightly decrease during stimulation periods, yet the faster decay was still observed; (iii) the correlation between the relative heartbeat rate and the relative decay time τ s/b is not significant (t(7) = −2.104,p = 0.074 for position 1; t(7) = −1.018,p = 0.455 for position 2).The absence of heartbeat rate changes upon stimulation is consistent with the DWS decay times of the superficial layers measured with the short-distance probe.This is in contrast to motor stimulation by finger opposition where the peripheral perfusion and the heartbeat rate are accelerated along with the cortical dynamics [12,15].The observation that the dynamics within the visual cortex is enhanced during the flickering periods even though the heartbeat rate remains constant indicates that the acceleration of the DWS signal from the visual cortex is mainly determined by the functionally increased blood flow velocity and not by shear deformations within cortical tissue induced by pulsatile volume variations of the vessels.DWS signal is determined by the regional blood flow velocity and not by shear deformations within cortical tissue.

Conclusions
In conjunction with near-infrared spectroscopy, the high sensitivity and temporal resolution of DWS to cortical perfusion could be used for mapping metabolism in the human brain noninvasively in real time.This might be useful for situations not allowing the use of conventional methods such as MRI or PET, such as in neuro-intensive care or in neuro-rehabilitation.

Fig. 1 .
Fig. 1.(a) Schematic view of the experimental setup showing the positioning of the source fiber (S; red circle) and the receiver fiber bundles (R 1 and R 2 for short-and long-distance probes, respectively; blue circles).Light and dark grey shaded areas indicate the tissue regions sampled by the long-and by the short-distance probes, respectively.(b) Positioning of the DWS probes on the occipital cortex.I: inion, Cz: vertex, N: nasion.

Fig. 2 .
Fig. 2. Top: bundle-averaged field autocorrelation function g (1) (τ) as a function of lag time τ.The probe with 30 mm source-receiver spacing was located above the inion at 10% of the inion-nasion distance.The error bars represent the standard deviation over 5 blocks.For clarity, only data for lag times τ < 1.3 × 10 −4 s are shown.The average relative decay time τ s/b is 0.955.Total integration time: 150 s.Count rate per fiber mode: 3.5 kHz.

Fiber-based
multi-speckle detection allows to detect the functional activation in the human visual cortex elicited by 8 Hz flickering in an entirely non-invasive way in individual subjects.Flickering leads to a reduction of the decay time of the DWS autocorrelation function of about 4% in a group of 9 subjects.This effect is by about 42 % larger than the change of transmitted intensity reflecting the functional change of blood volume.Our data indicate that the functional #81341 -$15.00USD Received 22 Mar 2007; revised 5 May 2007; accepted 9 May 2007; published 15 May 2007 (C) 2007 OSA 28 May 2007 / Vol. 15, No. 11 / OPTICS EXPRESS 6649