Meltem Izzetoglu

Functional near-infrared spectroscopy

The purpose of the this article is to describe an emerging neuroimaging technology, functional near-infrared spectroscopy (fNIRs), which has several attributes that make it possible to conduct neuroimaging studies of the cortex in clinical offices and under more realistic, ecologically valid parameters. fNIRs use near-infrared light to measure changes in the concentration of oxygenated and deoxygenated hemoglobin in the cortex. Although fNIR imaging is limited to the outer cortex, it provides neuroimaging that is safe, portable, and very affordable relative to other neuroimaging technologies. It is also relatively robust to movement artifacts and can readily be integrated with other technologies such as EEG

fNIRS Study of Walking and Walking While Talking in Young and Old Individuals

BACKGROUND Evidence suggests that gait is influenced by higher order cognitive and cortical control mechanisms. However, less is known about the functional correlates of cortical control of gait. METHODS Using functional near-infrared spectroscopy, the current study was designed to evaluate whether increased activations in the prefrontal cortex (PFC) were detected in walking while talking (WWT) compared with normal pace walking (NW) in 11 young and 11 old participants. Specifically, the following two hypotheses were evaluated: (a) Activation in the PFC would be increased in WWT compared with NW. (b) The increase in activation in the PFC during WWT as compared with NW would be greater in young than in old participants. RESULTS Separate linear mixed effects models with age as the two-level between-subject factor, walking condition (NW vs WWT) as the two-level repeated within-subject factor, and HbO2 levels in each of the 16 functional near-infrared spectroscopy channels as the dependent measure revealed significant task effects in 14 channels, indicating a robust bilateral increased activation in the PFC in WWT compared with NW. Furthermore, the group-by-task interaction was significant in 11 channels with young participants showing greater WWT-related increase in HbO2 levels compared with the old participants. CONCLUSIONS This study provided the first evidence that oxygenation levels are increased in the PFC during WWT compared with NW in young and old individuals. This effect was modified by age suggesting that older adults may under-utilize the PFC in attention-demanding locomotion tasks.

Motion artifact cancellation in NIR spectroscopy using Wiener filtering

We present a Wiener filtering based algorithm for the elimination of motion artifacts present in Near Infrared (NIR) spectroscopy measurements. Until now, adaptive filtering was the only technique used in the noise cancellation in NIR studies. The results in this preliminary study revealed that the proposed method gives better estimates than the classical adaptive filtering approach without the need for additional sensor measurements. Moreover, this novel technique has the potential to filter out motion artifacts in functional near infrared (fNIR) signals, too.

Functional brain imaging using near-infrared technology

0739-5175/07/

Neuroimaging of Mobility in Aging: A Targeted Review

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

Motion artifact cancellation in NIR spectroscopy using discrete Kalman filtering.

BACKGROUND The relationship between mobility and cognition in aging is well established, but the relationship between mobility and the structure and function of the aging brain is relatively unknown. This, in part, is attributed to the technological limitations of most neuroimaging procedures, which require the individual to be immobile or in a supine position. Herein, we provide a targeted review of neuroimaging studies of mobility in aging to promote (i) a better understanding of this relationship, (ii) future research in this area, and (iii) development of applications for improving mobility. METHODS A systematic search of peer-reviewed studies was performed using PubMed. Search terms included (i) aging, older adults, or elderly; (ii) gait, walking, balance, or mobility; and (iii) magnetic resonance imaging, voxel-based morphometry, fluid-attenuated inversion recovery, diffusion tensor imaging, positron emission tomography, functional magnetic resonance imaging, electroencephalography, event-related potential, and functional near-infrared spectroscopy. RESULTS Poor mobility outcomes were reliably associated with reduced gray and white matter volume. Fewer studies examined the relationship between changes in task-related brain activation and mobility performance. Extant findings, however, showed that activation patterns in the cerebellum, basal ganglia, parietal and frontal cortices were related to mobility. Increased involvement of the prefrontal cortex was evident in both imagined walking conditions and conditions where the cognitive demands of locomotion were increased. CONCLUSIONS Cortical control of gait in aging is bilateral, widespread, and dependent on the integrity of both gray and white matter.

Registering fNIR Data to Brain Surface Image using MRI templates

BackgroundAs a continuation of our earlier work, we present in this study a Kalman filtering based algorithm for the elimination of motion artifacts present in Near Infrared spectroscopy (NIR) measurements. Functional NIR measurements suffer from head motion especially in real world applications where movement cannot be restricted such as studies involving pilots, children, etc. Since head movement can cause fluctuations unrelated to metabolic changes in the blood due to the cognitive activity, removal of these artifacts from NIR signal is necessary for reliable assessment of cognitive activity in the brain for real life applications.MethodsPreviously, we had worked on adaptive and Wiener filtering for the cancellation of motion artifacts in NIR studies. Using the same NIR data set we have collected in our previous work where different speed motion artifacts were induced on the NIR measurements we compared the results of the newly proposed Kalman filtering approach with the results of previously studied adaptive and Wiener filtering methods in terms of gains in signal to noise ratio. Here, comparisons are based on paired t-tests where data from eleven subjects are used.ResultsThe preliminary results in this current study revealed that the proposed Kalman filtering method provides better estimates in terms of the gain in signal to noise ratio than the classical adaptive filtering approach without the need for additional sensor measurements and results comparable to Wiener filtering but better suitable for real-time applications.ConclusionsThis paper presented a novel approach based on Kalman filtering for motion artifact removal in NIR recordings. The proposed approach provides a suitable solution to the motion artifact removal problem in NIR studies by combining the advantages of the existing adaptive and Wiener filtering methods in one algorithm which allows efficient real time application with no requirement on additional sensor measurements.

Sliding-window motion artifact rejection for Functional Near-Infrared Spectroscopy

Functional near-infrared spectroscopy (fNIR) measures changes in the relative levels of oxygenated and deoxygenated hemoglobin and has increasingly been used to assess neural functioning in the brain. In addition to the ongoing technological developments, investigators have also been conducting studies on functional mapping and refinement of data analytic strategies in order to better understand the relationship between the fNIR signal and brain activity. However, since fNIR is a relatively new functional brain imaging modality as compared to positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), it still lacks brain-mapping tools designed to allow researchers and clinicians to easily interact with their data. The aim of this study is to develop a registration technique for the fNIR measurements using anatomical landmarks and structural magnetic resonance imaging (MRI) templates in order to visualize the brain activation when and where it happens. The proposed registration technique utilizes chain-code algorithm and depicts activations over respective locations based on sensor geometry. Furthermore, registered data locations have been used to create spatiotemporal visualization of fNIR measurements

Online fronto-cortical control of simple and attention-demanding locomotion in humans

Functional Near-Infrared Spectroscopy (fNIR) is an optical brain monitoring technology that tracks changes in hemodynamic responses within the cortex. fNIR uses specific wavelengths of light, introduced at 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 that can be used to measure brain activity in natural environments, ambulatory and field conditions. However, for such applications fNIR signals can get prone to noise due to motion of the head. Improving signal quality and reducing noise, can be especially challenging for real time applications. Here, we study motion artifact related noise especially due to poor and changing sensor coupling. We have developed a simple and iterative method that can be used to automate the preprocessing of data to identify segments with such noise for exclusion and this method is also suitable for real time applications.

A Methodology for Validating Artifact Removal Techniques for Physiological Signals

Knowledge of online functional brain mechanisms of locomotion is scarce due to technical limitations of traditional neuroimaging methods. Using functional Near Infrared Spectroscopy (fNIRS) we evaluated task-related changes in oxygenated hemoglobin levels (HbO2) in real-time over the pre-frontal-cortex (PFC) regions during simple (Normal Walk; NW) and attention-demanding (Walking While Talking; WWT) locomotion tasks in a large cohort of non-demented older adults. Results revealed that the assessment of task-related changes in HbO2 was internally consistent. Imposing greater demands on the attention system during locomotion resulted in robust bilateral PFC increases in HbO2 levels during WWT compared to NW and the cognitive interference tasks. Elevated PFC oxygenation levels were maintained throughout the course of WWT but not during the NW condition. Increased oxygenation levels in the PFC were related to greater stride length and better cognitive performance but not to faster gait velocity in WWT. These findings elucidate online brain mechanisms of locomotion, and confer significant implications for risk assessment and intervention for major mobility outcomes.