Detection of neural light-scattering activity in vivo: optical transmittance studies in the rat brain

Abstract

Optical studies of ex vivo brain slices where blood is absent show that neural activity is accompanied by significant intrinsic optical signals (IOS) related to activity-dependent scattering changes in neural tissue. However, the neural scattering signals have been largely ignored in vivo in widely-used IOS methods where absorption contrast from hemoglobin was employed. Changes in scattering were observed on a time scale of seconds in previous brain slice IOS studies, similar to the time scale for the hemodynamic response. Therefore, potential crosstalk between the scattering and absorption changes may not be ignored if they have comparable contributions to IOS. In vivo, the IOS changes linked to neural scattering have been elusive. To isolate neural scattering signals in vivo, we employed 2 implantable optodes for small-separation (2 mm) transmission measurements of local brain tissue in anesthetized rats. This unique geometry enables us to separate neuronal activity-related changes in neural tissue scattering from changes in blood absorption based upon the direction of the signal change. The changes in IOS scattering and absorption in response to up-states of spontaneous neuronal activity in cortical or subcortical structures have strong correlation to local field potentials, but significantly different response latencies. We conclude that activity-dependent neural tissue scattering in vivo may be an additional source of contrast for functional brain studies that provides complementary information to other optical or MR-based systems that are sensitive to hemodynamic contrast.

Keywords: In vivo; Intrinsic optical signal; Neural scattering activity; Neurovascular coupling; Optical transmission measurement.

Fig. 1

Simultaneous electrophysiology and IOS recordings with transmission measurement. The schematic illustration of simultaneous LFP and IOS recordings is shown in the top panel. The LED light is coupled into the optical fiber to targeted brain sites and transmitted through the sampled tissue. The detection fiber of the transmission probe collects the light to a photodiode. Both photodiode and LFP recording signals from the same site were ×10 pre-amplified respectively, then amplified and digitized. The animal and recording system were enclosed in a full-band electromagnetic shield during data collection. (Left bottom)Schematic representation of transmission measurement for the target area’s tissue-level neural activity using the minimally-invasive fiber pair. The mirror-coating of the 45° tips that allows light to turn 90° between source emission and detector collection with the parallel fiber-pair configuration is illustrated. (Middle bottom) Lighting (full-band wavelengths) path is demonstrated, captured in the turbid liquid of a tissue-simulating phantom. The fiber pair was assembled on a holder, shown in the right bottom panel, which was calibrated to ensure 2-mm separation and equal length.

Fig. 2

Optical intrinsic signals of scattering vs. absorption in response to neuronal activation. The optical responses to local field potentials for different wavelengths, 810nm/525nm/660nm, are estimated respectively by least-squares fitting of simultaneous LFP/IOS recording data sets. The group results expressed as tissue optical density are plotted in first column (a and d, mean+/−SEM: S1, n=7 rats; CP, n=6 rats). Using the modified Beer-Lambert law and considering both tissue absorption and scattering changes that occur in response to neural activity, the concentration changes in HbO2, Hb (tHb=HbO2+Hb) and scattering per centimeter were calculated and plotted in mean+/−SEM (scattering: b and e; absorption: c and f from S1 and CP respectively). The results from the S1 group are shown in the first row and CP in the second row.

Fig. 3

Short latency of neuro-scattering responses relative to neurovascular coupling and their wavelength representatives. The peak times of scattering (neural tissue) and tHb (vascular volume) are plotted individually along with the group mean+/−SEM (a: S1, 1.41+/−0.30 vs 3.41+/−0.21 and b: CP, 1.22+/−0.16 vs 2.96+/−0.44). Paired t-tests between scattering and tHb for both groups, found a significantly faster response in scattering activity than vascular activity (*: p<0.05, **: p<0.001, two-tailed).

Fig. 4

Strong correlation between local field potentials and neuro-scattering as well as neurovascular coupling. The time courses of neural scattering activity are represented by 810nm of NIR light, shown in black in the plotted time courses; the neurovascular activity are represented by 525nm of green light, shown in green in the plotted time courses. The simultaneous recorded LFPs were convolved with the response function (group mean, shown at the end of each row) to predict optical responses (red time courses). Samples are shown in a and b from S1, c and d from CP. The experimental measurements, whether made with 810nm or 525nm, exhibit high correlation coefficients with the predicted time courses respectively. The group results of correlation coefficients are shown in e, S1: 0.69+/−0.09 and 0.74+/−0.03; CP:0.71+/−0.07 and 0.74+/−0.03 for 810nm (scattering) and 525nm (tHb) respectively.