Intra- and extra-cranial effects of transient blood pressure changes on brain near-infrared spectroscopy (NIRS) measurements

Ludovico Minati,Inge U Kress,Elisa Visani,Nick Medford,Hugo D Critchley

doi:10.1016/j.jneumeth.2011.02.029

Abstract

Brain near-infrared spectroscopy (NIRS) is an emerging neurophysiological tool that combines straightforward activity localization with cost–economy, portability and patient compatibility. NIRS is proving its empirical utility across specific cognitive and emotional paradigms. However, a potential limitation is that it is not only sensitive to haemodynamic changes taking place in the cortex, and task-related cardiovascular responses expressed in the perfusion of extracranial layers may be confounding. Existing literature reports correlations between brain NIRS and systemic blood pressure, yet it falls short of establishing whether in normal participants the blood pressure changes encountered in experimental settings can have confounding effects. Here, we tested this hypothesis by performing two experimental manipulations while recording from superficial occipital cortex, encompassing striate and extrastriate regions. Visual stimulation with reversing chequerboards evoked cortical haemodynamic responses. Simultaneously and independently, transient systemic blood pressure changes were generated through rapid arm-raising. Shallow-penetration NIRS recordings, probing only extra-cerebral tissues, highlighted close haemodynamic coupling with blood pressure. A different coupling pattern was observed in deep-penetration recordings directed at haemodynamic signals from visual cortex. In absence of blood-pressure changes, NIRS signals tracked differences in visual stimulus duration. However when blood pressure was actively manipulated, this effect was absent and replaced by a very large pressure-related response. Our observations demonstrate that blood pressure fluctuations can exert confounding effects on brain NIRS, through expression in extracranial tissues and within the brain itself. We highlight the necessity for continuous blood pressure monitoring alongside brain NIRS, and for further research on methods to correct for physiological confounds.

Highlights

Near-infrared spectroscopy (NIRS) of the brain is receiving increasing interest as a functional neuroscientific technique, complementing neuroelectric approaches such as electroencephalography (EEG) and high-cost neuroimaging using functional magnetic resonance imaging or positron emission tomography (PET).Like fMRI, brain NIRS is typically used to measure haemodynamic changes related to cortical neural activity
The key findings of this study are that (1) transient systemic blood pressure changes are reflected in brain NIRS measurements, and (2) the coupling appears to take place in both extracranial tissues and the brain itself, with different patterns
There are a number of possible mechanisms for this including the central command driving local and systemic facilitatory cardiovascular responses in anticipation and in response to effort (Smith et al, 2000), and the sudden shift between two hydrostatic states, i.e. when the arm is lowered all its blood volume is below the level of the shoulders whereas the opposite holds when it is raised

Summary

Introduction

Near-infrared spectroscopy (NIRS) of the brain is receiving increasing interest as a functional neuroscientific technique, complementing neuroelectric approaches such as electroencephalography (EEG) and high-cost neuroimaging using functional magnetic resonance imaging (fMRI) or positron emission tomography (PET).Like fMRI, brain NIRS is typically used to measure haemodynamic changes related to cortical neural activity. There is growing evidence for its utility across a range of active experimental paradigms probing motor and language function, executive control and appraisal of affective stimuli (Strangman et al, 2002; Wolf et al, 2007; Lloyd-Fox et al, 2010). These cognitive and emotional processes are known to affect peripheral physiology, influencing heart rate, respiration, blood pressure and skin perspiration (Jänig et al, 2006; Critchley, 2009). Transient blood-pressure increases are frequently observed, e.g., in response to positive or negative emotions, mental effort and response conflict (Hanson et al, 1993; Minati et al, 2009)

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