Noncanonical spike‐related BOLD responses in focal epilepsy

Abstract

Till now, most studies of the Blood Oxygen Level-Dependent (BOLD) response to interictal epileptic discharges (IED) have assumed that its time course matches closely to that of brief physiological stimuli, commonly called the canonical event-related haemodynamic response function (canonical HRF). Analyses based on that assumption have produced significant response patterns that are generally concordant with prior electroclinical data. In this work, we used a more flexible model of the event-related response, a Fourier basis set, to investigate the presence of other responses in relation to individual IED in 30 experiments in patients with focal epilepsy. We found significant responses that had a noncanonical time course in 37% of cases, compared with 40% for the conventional, canonical HRF-based approach. In two cases, the Fourier analysis suggested activations where the conventional model did not. The noncanonical activations were almost always remote from the presumed generator of epileptiform activity. In the majority of cases with noncanonical responses, the noncanonical responses in single-voxel clusters were suggestive of artifacts. We did not find evidence for IED-related noncanonical HRFs arising from areas of pathology, suggesting that the BOLD response to IED is primarily canonical. Noncanonical responses may represent a number of phenomena, including artefacts and propagated epileptiform activity. Hum Brain Mapp 2008. © 2007 Wiley-Liss, Inc.

Continuous EEG source imaging enhances analysis of EEG-fMRI in focal epilepsy

“…The BOLD changes appeared to take place at the epileptogenic zone, « upstream » from bilateral anterior temporal IED, which reflect propagation areas and an irritative zone with a much larger extent than the epileptogenic zone. This effect may be related to BOLD changes preceding IED as has been shown in some cases of focal epilepsy, mostly in relation to generalised IED (Hawco, et al 2007;Jacobs, et al 2009;Lemieux, et al 2008;Vaudano, et al 2009). Other studies rather suggested a "downstream" effect, with IED modulating BOLD signal and presumably neuronal activity in distant regions involved in the epileptogenic network, such as the DMN (Kobayashi, et al 2006b;) and the limbic network (Kobayashi, et al 2006a) in medial temporal epilepsy.…”

Independent component analysis reveals dynamic ictal BOLD responses in EEG-fMRI data from focal epilepsy patients

“…Modeling the residuals as an autoregressive process cannot accommodate large artifactual BOLD signal transients that coincidentally occurred during the ictal event, which may result in statistically significant non-canonical HRFs. A study on epileptic spikes revealed that most non-canonical shapes tended to occur remotely from the presumed epileptogenic focus and were likely artifacts (Lemieux et al, 2008). On the other hand, restricting the HRF to the canonical shape, such as in the standard GLM method, may prevent the detection of real activations.…”

Imaging haemodynamic changes related to seizures: Comparison of EEG-based general linear model, independent component analysis of fMRI and intracranial EEG

“…Furthermore, while the canonical HRF appears to be a suitable model in normal physiological conditions (Friston et al, 1995) and for IED-related BOLD signal changes (Lemieux et al, 2008), this has not been demonstrated for seizures. Flexible modelling methods have been used, such as those based on Fourier basis sets or series of short blocks (Donaire et al, 2009;Tyvaert et al, 2009) addressing the potential complexity of ictal BOLD signal changes but they do not address the issue of how best to model these changes based on the available scalp EEG.…”