Functional brain imaging using near-infrared technology

0739-5175/07/$25.00©2007IEEE I n the last decade, functional near-infrared spectroscopy (fNIR) has been introduced as a new neuroimaging modality with which to conduct functional brain imaging studies [1]–[24]. fNIR technology uses specific wavelengths of light, irradiated through the scalp, to enable the noninvasive measurement of changes in the relative ratios of deoxygenated hemoglobin (deoxy-Hb) and oxygenated hemoglobin (oxy-Hb) during brain activity. This technology allows the design of portable, safe, affordable, noninvasive, and minimally intrusive monitoring systems. These qualities make fNIR suitable for the study of hemodynamic changes due to cognitive and emotional brain activity under many working and educational conditions, as well as in the field. Functional imaging is typically conducted in an effort to understand the activity in a given brain region in terms of its relationship to a particular behavioral state or its interactions with inputs from another region’s activity. The program of research in cognitive neuroscience conducted by our optical brain imaging group has utilized the current-generation fNIR system to investigate brain activity, primarily in the dorsolateral and inferior frontal cortex [20]–[24]. To date, the fNIR studies of cognition and emotion have focused on functions associated with Brodman’s areas BA9, BA10, BA46, BA45, BA47, and BA44. Recent positron emission tomography (PET) and functional magnetic resonance (fMRI) studies have shown that these areas play a critical role in sustained attention, both the short-term storage and the executive process components of working memory, episodic memory, problem solving, response inhibition, and the perception of smell (for a recent review, see [25] and [26]). In addition, word recognition and the storage of verbal materials activate Broca’s area and left hemisphere supplementary and premotor areas [25], [27], [28]. To date, studies utilizing fNIR have shown results consistent with fMRI and PET findings for working memory and sustained attention [21]–[23]. In this article, we will describe the working principles of fNIR and how the hemodynamic signals are extracted from the raw fNIR measurements using the modified Beer-Lambert Law. We will also introduce the fNIR system that we have developed and used in our studies. Current results from the augmented cognition research conducted in our laboratory are also presented, and the merits of optical imaging in augmented cognition are summarized. Working Principles Typically, an optical apparatus consists of a light source by which the tissue is radiated and a light detector that receives light after it has interacted with the tissue. Photons that enter tissue undergo two different types of interaction, namely absorption (loss of energy to the medium) and scattering [4], [5], [19]. Most biological tissues are relatively transparent to light in the near-infrared range between 700 to 900 nm, which is usually called the “optical window. ” This is mainly due to the fact that within this optical window, the absorbance of the main constituents in the human tissue (i.e., water, oxy-Hb, and deoxy-Hb) is small, allowing the light to penetrate the tissue (see Figure 1). Among the main absorbers (chromophores) in the tissue, oxyand deoxy-Hb are strongly linked to tissue oxygenation and metabolism. Fortunately, in the optical window, the absorption spectra of oxyand deoxy-Hb remain significantly different than each other, allowing spectroscopic separation of these compounds to be possible using only a few sample wavelengths. fNIR technology employs specified wavelengths of light within the optical window. Once the photons are introduced into the human head, they are either scattered by extraand intracellular boundaries of different layers of the head (skin, skull, cerebrospinal fluid, brain, etc.) or absorbed mainly by oxyand deoxy-Hb. A photodetector placed a certain distance away from the light source can collect the photons that are not absorbed and those that traveled along the “banana shaped path” between the source and detector due to scattering [9], [29] as shown Figure 2. In functional optical brain imaging studies, the attenuation (reduction in the amount of photons detected by the photodetectors) due to scattering is assumed to be constant since the amount of scatterers within different layers of the head does not change due to cognitive activity. The change in the attenuation measured as a result of cognitive activity is hence due to the changes in absorption resulting from the variation in the concentrations of oxyand deoxy-Hb in the brain tissue. This relationship is not surprising, since cerebral hemodynamic changes are related to functional brain activity through a mechanism that is called neurovascular coupling [8], [30]. In fact, this physiological relationship and the ability of fNIR Functional Brain Imaging Using Near-Infrared Technology