Linear aspects of changes in deoxygenated hemoglobin concentration and cytochrome oxidase oxidation during brain activation

We used near infrared spectroscopy (NIRS) to investigate the vascular and m etabolic response to brain activation in human primary and adjacent seconda ry visual cortex. NIBS is able to measure concentration changes in deoxygen ated hemoglobin ([deoxy-Hb]) (which mainly contribute to the blood oxygenat ion level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI)) as well as concentration changes of oxygenated hemoglobin ([oxy-Rb] ) and corpuscular-blood volume ([total-Hb] = [oxy-Hb] + [deoxy-Hb]) and cha nges in the redox status of the cytochrome c oxidase ([Cyt-Ox]), a putative parameter for cellular oxygenation. A sound understanding of the transfer functions between stimulus parameters, neuronal activity, and vascular/meta bolic parameters is important for interpretation of data acquired with indi rect neuroimaging techniques like fMRI, especially in event-related design studies. In the present study we tested whether the vascular/metabolic resp onse to stimulation can be described as a linear and time invariant system. Since linearity is a property attributed to systems that satisfy the scali ng and superposition properties, as a first simple test, superposition of t he responses obtained from short duration visual stimuli was used to predic t the responses of longer duration stimuli. Our results showed that the pre dictions of [deoxy-Hb] and [Cyt-Ox] responses to stimuli of 6- to 24-s dura tion were satisfactory whereas predictions of [oxy-Hb] and [total-Hb] were insufficient. Ina second step, a calculated convolution function of an assu med impulse response function and the stimulus function was fitted with the measured [deoxy-Hb] and [Cyt-Ox] curves to obtain amplitude, time delay, a nd time constant parameters. We show that predictions of cellular and vascu lar oxygenation responses to visual stimulation are good for 6- to 24-s sti muli duration under the assumption of a linear transfer characteristic. Thi s model is not valid for corpuscular volume changes which affect mainly the [deoxy-Hb] response. Noninvasive NIBS is shown to be a suitable method to get more direct information about neuronal-activity-associated changes in c erebral parameters which are partly reflected in BOLD signal but are not fu lly understood yet. (C) 2001 Academic Press.