Effect of confounding variables on hemodynamic response function estimation using averaging and deconvolution analysis: An event-related NIRS study.

Slow and rapid event-related designs are used in fMRI and functional near-infrared spectroscopy (fNIRS) experiments to temporally characterize the brain hemodynamic response to discrete events. Conventional averaging (CA) and the deconvolution method (DM) are the two techniques commonly used to estimate the Hemodynamic Response Function (HRF) profile in event-related designs. In this study, we conducted a series of simulations using synthetic and real NIRS data to examine the effect of the main confounding factors, including event sequence timing parameters, different types of noise, signal-to-noise ratio (SNR), temporal autocorrelation and temporal filtering on the performance of these techniques in slow and rapid event-related designs. We also compared systematic errors in the estimates of the fitted HRF amplitude, latency and duration for both techniques. We further compared the performance of deconvolution methods based on Finite Impulse Response (FIR) basis functions and gamma basis sets. Our results demonstrate that DM was much less sensitive to confounding factors than CA. Event timing was the main parameter largely affecting the accuracy of CA. In slow event-related designs, deconvolution methods provided similar results to those obtained by CA. In rapid event-related designs, our results showed that DM outperformed CA for all SNR, especially above -5 dB regardless of the event sequence timing and the dynamics of background NIRS activity. Our results also show that periodic low-frequency systemic hemodynamic fluctuations as well as phase-locked noise can markedly obscure hemodynamic evoked responses. Temporal autocorrelation also affected the performance of both techniques by inducing distortions in the time profile of the estimated hemodynamic response with inflated t-statistics, especially at low SNRs. We also found that high-pass temporal filtering could substantially affect the performance of both techniques by removing the low-frequency components of HRF profiles. Our results emphasize the importance of characterization of event timing, background noise and SNR when estimating HRF profiles using CA and DM in event-related designs.

Shedding light on interictal epileptic spikes: An in vivo study using fast optical signal and electrocorticography.

OBJECTIVE: Interictal epileptic spikes (IESs), apart from being a key marker of epileptic neuronal networks, constitute a nice model of the widespread endogenous phenomenon of neuronal hypersynchronization. Many questions concerning the mechanisms that drive neurons to hypersynchronize remain unresolved, but synaptic as well as nonsynaptic events are likely to be involved. In this study, changes in optical properties of neural tissues were observed in rats with penicillin-induced IES using fast optical signal (FOS) concomitantly with electrocorticography (ECoG). METHODS: In this study, near-infrared optical imaging was used with ECoG to investigate variations in the optical properties of cortical tissue directly associated with neuronal activity in 15 rats. FOS changes correspond to variations of scattered light from neuronal tissue when neurons are activated. To independently evaluate our method, a control experiment on somatosensory was designed and applied to seven different rats. Time-frequency analysis was also used to track variations of (de)synchronization concomitantly with changes in optical signals during IES. RESULTS: FOS responses revealed that changes in optical signals occurred 320 msec before to 370 msec after the IES peak. These changes started before any changes in ECoG signal. In addition, time-frequency domain electrocorticography revealed an alternating decrease-increase-decrease in the ECoG spectral power (pointing to desynchronization-synchronization-desynchronization), which occurred concomitantly with an increase-decrease-increase in relative optical signal during the IES. These results suggest a relationship between (de)synchronization and optical changes. SIGNIFICANCE: These changes in the neuronal environment around IESs raise new questions about the mechanisms that induce changes in optical properties of neural tissues before the IES, which may provide suitable conditions for neuronal synchronization during IESs. FOS-ECoG constitutes a multimodal approach and opens new avenues to study the mechanisms of neuronal synchronization in the pathologic brain, which has clinical implications, at least in epilepsy.

NIRS-measured oxy- and deoxyhemoglobin changes associated with EEG spike-and-wave discharges in a genetic model of absence epilepsy: the GAERS.

PURPOSE: Absence epilepsy may be severe and is frequently accompanied by cognitive delay, yet its metabolic/hemodynamic aspects have not been established. The Genetic Absence Epilepsy Rats from Strasbourg (GAERS) are an isomorphic, predictive, and homologous model of human absence epilepsy. We studied hemodynamic changes related to generalized spike-and-wave discharges (GSWDs) in GAERS by using a technique with high temporal resolution: near-infrared spectroscopy (NIRS). We hypothesized that conflicting results from other techniques might be due to the averaging of a biphasic response such as the one we described in children. METHODS: NIRS is particularly suitable for monitoring changes in the concentrations of oxy-, deoxy-, and total hemoglobin (HbO2, HHb, and HbT), using the specific absorption properties of living tissues in the near infrared range. We obtained concomitant high quality electroencephalography (EEG)-NIRS recordings in six GAERS (total of 444 seizures), and tested whether the discharges were related to changes in cardiac or respiration rates. RESULTS: The onset of GSWDs was preceded by a deactivation, followed by an activation that was possibly due to seizure-suppression mechanisms. The end was marked by a deactivation. The onset of GSWDs was associated with a decrease and the end with a brief increase in respiratory rate. DISCUSSION: Our results differ partially from those of previous studies on hemodynamic aspects of GSWDs (many of which describe a simple deactivation), probably due to differences in temporal resolution and data processing; however, they are consistent with metabolic studies, functional magnetic resonance imaging (fMRI) studies on WAG/Rij rats, and some results in children with absence epilepsy.