Comparison of Mutual Information and Partial Correlation for Functional Connectivity in fNIRS M. Dadgostar, Z. Einalou Department Of Biomedical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. m.dadgostar@srbiau.ac.ir, zahra_einalou@yahoo.com S. K. Setarehdan Department of Electrical and Computer Engineering, University of Tehran, Tehran, Iran. ksetareh@ut.ac.ir H. Y. Keskin-Ergen Department of Physiology, Bahcesehir University, Istanbul, Turkey. h.yaseminkeskin@gmail.com A. Akin Institute of Biomedical Engineering, Bogazici University, Istanbul, Turkey. ata.akin@boun.edu.tr Abstract— Non-invasive monitoring of human brain function by Functional near-infrared spectroscopy (fNIRS) has been applied to study of brain oxygenation and metabolism. In this study, we aimed to investigate the functional connectivity in the prefrontal cortex (PFC) during a modified version of the color-word matching Stroop task. Stroop task, which requires responding to a particular stimulus dimension while suppressing a competing stimulus dimension, is commonly used to evaluate PFC function. Partial correlation (PC) values and Mutual Information (MI) values were computed for each stimulus condition. Eventually the two methods were compared. I. INTRODUCTION Functional connectivity can be measured via several functional neuroimaging tools and with several algorithms. Among these tools and algorithms we chose the fNIRS tool due to its non-invasiveness, unobtrusiveness during cognitive tasks, rapidness and ease of application. This tool has been extensively used in cognitive tasks and its validity has been confirmed in the literature while its limitations have been recently discussed extensively [1-3]. Similarly among several algorithms, we decided to compare two connectivity metrics. In this study, we aimed to investigate the functional connectivity in the prefrontal cortex (PFC) during a modified version of the color-word matching Stroop task. The mutual information between two time-courses is a measure of general dependence and will detect both linear and non-linear dependencies [4]. Since PC analysis helps to remove the effect of indirect paths, by applying this method, the PC between two channels is correlated, with the activity at all other 14 regions regressed out [5]. II. METHODS A. Subjects Data were collected from 5 healthy volunteers at the Neuro-Optical Imaging Laboratory, Bogazici University, Istanbul, Turkey. Control subjects had no history of psychiatric or neurological disorders. B. Stroop Task Subjects were asked to perform color–word matching Stroop task whose trials are the Turkish versions of Zysset et al. [6,7]. Subjects were presented with two words, one written above the other. The top one was written in ink color whereas the bottom one was in white. Subjects were asked to judge whether the word written below correctly denotes the color of the upper word or not. If color and word match, then subjects were to press on the left mouse button with their forefinger, and if not, on the right mouse button with their middle finger. Subjects were informed to perform the task as quickly and correctly as possible. The words stayed on the screen until the response was given with a maximum time of 3 s. The screen was blank between the trials. The experiment consisted of neutral, congruent and incongruent trials. In the neutral condition, upper word consisted of four X’s (XXXX) in ink color. In the congruent condition ink color of the upper word and the word itself were the same, whereas in the incongruent condition, they were different. The trials were presented in a semi blocked manner. See Fig. 1 for the details. Each block consisted of six trials. Inter stimulus interval within the block was 4.5 s and the blocks were placed 20 s apart in time. The trial type within a block was homogeneous (but the arrangements of false and correct trials were altering). There were five blocks of each type. Experiments were performed in a silent, lightly dimmed room. Words were presented via an LCD screen that was 0.5 m away from the subjects. The task protocol is approved by the Ethics Review Board of Bogazici University [8].