The physiological origin of task-evoked systemic artefacts in functional near infrared spectroscopy.

A major methodological challenge of functional near-infrared spectroscopy (fNIRS) is its high sensitivity to haemodynamic fluctuations in the scalp. Superficial fluctuations contribute on the one hand to the physiological noise of fNIRS, impairing the signal-to-noise ratio, and may on the other hand be erroneously attributed to cerebral changes, leading to false positives in fNIRS experiments. Here we explore the localisation, time course and physiological origin of task-evoked superficial signals in fNIRS and present a method to separate them from cortical signals. We used complementary fNIRS, fMRI, MR-angiography and peripheral physiological measurements (blood pressure, heart rate, skin conductance and skin blood flow) to study activation in the frontal lobe during a continuous performance task. The General Linear Model (GLM) was applied to analyse the fNIRS data, which included an additional predictor to account for systemic changes in the skin. We found that skin blood volume strongly depends on the cognitive state and that sources of task-evoked systemic signals in fNIRS are co-localized with veins draining the scalp. Task-evoked superficial artefacts were mainly observed in concentration changes of oxygenated haemoglobin and could be effectively separated from cerebral signals by GLM analysis. Based on temporal correlation of fNIRS and fMRI signals with peripheral physiological measurements we conclude that the physiological origin of the systemic artefact is a task-evoked sympathetic arterial vasoconstriction followed by a decrease in venous volume. Since changes in sympathetic outflow accompany almost any cognitive and emotional process, we expect scalp vessel artefacts to be present in a wide range of fNIRS settings used in neurocognitive research. Therefore a careful separation of fNIRS signals originating from activated brain and from scalp is a necessary precondition for unbiased fNIRS brain activation maps.

Hemodynamic signals correlate tightly with synchronized gamma oscillations.

Functional imaging methods monitor neural activity by measuring hemodynamic signals. These are more closely related to local field potentials (LFPs) than to action potentials. We simultaneously recorded electrical and hemodynamic responses in the cat visual cortex. Increasing stimulus strength enhanced spiking activity, high-frequency LFP oscillations, and hemodynamic responses. With constant stimulus intensity, the hemodynamic response fluctuated; these fluctuations were only loosely related to action potential frequency but tightly correlated to the power of LFP oscillations in the gamma range. These oscillations increase with the synchrony of synaptic events, which suggests a close correlation between hemodynamic responses and neuronal synchronization.

Precise placement of multiple electrodes into functionally predefined cortical locations.

Current research on topics such as effective connectivity, neuronal coding strategy or signal propagation in the central nervous system requires simultaneous recordings from multiple sites within functionally grouped but topologically distributed neuronal clusters. We have addressed this issue by characterization of the cortical functional architecture using optical imaging of intrinsic signals (OI) and subsequent placement of multiple, individually adjustable electrodes into pre-selected domains. In order to achieve maximum precision and flexibility for the positioning of electrodes, a plastic cylinder containing channels of an extremely high aspect ratio (density >20 channels/mm(2)) was fixed above the cortex and individual channel positions were superimposed onto the functional maps of orientation columns obtained previously with OI. Subsequently, channels corresponding to the desired locations in the functional map were used as guide tubes for electrode insertion. The spatial precision of this approach was in the range of 100 microm and experiments in cat primary visual cortex revealed a close correlation between the desired and the actually recorded orientation preferences of the targeted columns. The method is applicable to all cortical areas in which OI is feasible and offers a high degree of flexibility with respect to the number and geometry of applicable probes. It is, thus, an excellent tool for studying distributed codes and interactions between multiple predefined recording sites.