Fast optical signal not detected in awake behaving monkeys

Abstract

While the ability of near-infrared spectroscopy (NIRS) to measure cerebral hemodynamic evoked responses (slow optical signal) is well established, its ability to measure non-invasively the ‘fast optical signal’ is still controversial. Here, we aim to determine the feasibility of performing NIRS measurements of the ‘fast optical signal’ or Event-Related Optical Signals (EROS) under optimal experimental conditions in awake behaving macaque monkeys. These monkeys were implanted with a ‘recording well’ to expose the dura above the primary visual cortex (V1). A custom-made optical probe was inserted and fixed into the well. The close proximity of the probe to the brain maximized the sensitivity to changes in optical properties in the cortex. Motion artifacts were minimized by physical restraint of the head. Full-field contrast-reversing checkerboard stimuli were presented to monkeys trained to perform a visual fixation task. In separate sessions, two NIRS systems (CW4 and ISS FD oximeter), which previously showed the ability to measure the fast signal in human, were used. In some sessions EEG was acquired simultaneously with the optical signal. The increased sensitivity to cortical optical changes with our experimental setup was quantified with 3D Monte Carlo simulations on a segmented MRI monkey head. Averages of thousands of stimuli in the same animal, or grand averages across the two animals and across repeated sessions, did not lead to detection of the fast optical signal using either amplitude or phase of the optical signal. Hemodynamic responses and visual evoked potentials were instead always detected with single trials or averages of a few stimuli. Based on these negative results, despite the optimal experimental conditions, we doubt the usefulness of non-invasive fast optical signal measurements with NIRS.

Introduction

Near-infrared spectroscopy (NIRS) has been used for more than a decade to study functional cerebral activation in humans non-invasively (Hoshi and Tamura, 1993, Villringer and Chance, 1997). Similar to fMRI, NIRS is sensitive to the vascular changes following neuronal activity. The vascular responses consist of increases in cerebral blood flow and cerebral blood volume, which cause an increase in oxy-hemoglobin concentration (HbO) and a decrease in deoxy-hemoglobin concentration (HbR). These latter concentration changes are detected by NIRS as changes in light absorbance. While the neuronal electrical signal starts tens of ms after stimulation, the hemodynamic response is delayed and has a latency of a few seconds after stimulation. The spatial and temporal characteristics of the slow optical signals are in agreement and have been validated with fMRI (Huppert et al., 2006a, Huppert et al., 2006b, Sassaroli et al., 2006, Toronov et al., 2007). A less robust but more appealing optical signal is the so-called ‘fast signal’ or ‘event-related optical signal’ (EROS) (Gratton et al., 1997b), which detects cortical changes of unclear origin in the ms time scale. The tremendous appeal of the fast signal is due to its short temporal latency, which implies a more direct relationship between neuronal activation and the fast signal compared to the relation that exists with the slow hemoglobin signal. The ability to measure optically both the neuronal and vascular responses non-invasively in humans with reasonable spatial localization puts NIRS in the spotlight as an optimal technique to investigate neurovascular coupling.

Fast light scattering changes possibly induced by cell conformational changes and swelling were originally measured in cell cultures and tissue preparations (Carter et al., 2004, Cohen et al., 1968, Salzberg and Obaid, 1988, Stepnoski et al., 1991, Tasaki and Byrne, 1992, Yao et al., 2005). Detecting back-scattered or cross-polarized light, Rector et al. were able to measure fast optical changes in an animal's exposed cortex (Rector et al., 2001, Schei et al., 2008). As stressed by Steinbrink et al. (2005) the existence of fast optical changes in neuronal tissue and the feasibility of using invasive optical methods, like cross-polarized and back-scattered light detection, to detect such changes are not in question. The controversy pertains to the reliability of such measurements obtained with non-invasive optical methods, like NIRS. The first measure of the ‘fast’ signal in humans through intact skin and skull was reported over a decade ago by Gratton et al., 1995, Gratton et al., 1997a) using a frequency-domain NIRS system (Imagent, ISS Inc.) and measuring the phase lag (or change in the time of flight) induced by evoked scattering changes (the EROS signal). In the past 10 years, Gratton et al. have published a number of very encouraging results with the EROS signal: for example (Bartholow et al., 2001, Gratton et al., 2006, Gratton et al., 1997b, Gratton and Fabiani, 2001, Maclin et al., 2004, Tse et al., 2007). Several groups, including ours, have tried to reproduce Gratton's measurements (Franceschini and Boas, 2004, Steinbrink et al., 2000, Wolf et al., 2002, Wolf et al., 2003) with moderate success. In contrast with Gratton's results, these groups have shown, both experimentally and theoretically (with Monte Carlo simulations), that an intensity measurement is better than a phase measurement for detecting the fast signal, because of the better SNR. In a critical work by Steinbrink et al. (2005), the feasibility of reliably measuring the non-invasive fast optical signal in humans is interrogated, both theoretically and experimentally: the argument being that, based on the estimate of scattering change measured invasively, these changes are too small with respect to instrumental SNR when the partial volume effect is taken into account. For fast signal detection, instrumental SNR is typically increased by averaging a very large number of stimuli and averaging responses across several subjects (Maclin et al., 2007). In addition, physiological noise such as arterial pulsation is reduced using adaptive filters (Gratton and Corballis, 1995, Maclin et al., 2003), and by presenting stimuli in an event-related fashion. Steinbrink et al. (2005), in addition, increased the SNR significantly by using an optimized NIRS system with considerably higher light power (70–150 MW) delivered to the head than with typical NIRS systems (up to 5–10 MW), thereby reducing the standard error in the average intensity to 3 × 10⁻⁶, which theoretically was of the order of magnitude of expected scattering induced changes at the head surface. Despite the extremely low noise, they were not able to detect fast optical signals.

To minimize the partial volume effect, and subsequently to further highlight the difficulty of detecting the NIRS fast signal, we performed several NIRS measurements on two macaque monkeys with a recording well with exposed dura over the right primary visual cortex (V1). The head of the animal was restrained by a headpost cemented to the skull, and the optical probe was inserted into the recording well in contact with the dura. This set up mitigates any signal contamination by motion artifacts and avoids signal attenuation from the scalp and skull. These experimental conditions allowed us to perform measurements at a shorter distance from the brain than in human experiments, thereby improving sensitivity to cerebral changes. In different measurement sessions we used a CW and an FD NIRS system and in several sessions we acquired EEG simultaneously with NIRS to detect visual evoked potentials (VEPs). While the NIRS hemodynamic response and the VEP were detected with optimal SNR by averaging few stimuli, we did not detect any reliable fast optical signal even after averaging thousands of stimuli during multiple measurement sessions across the two animals. We conclude that the fast optical signal measured with NIRS is too difficult to obtain to be of any practical utility.