Abstracts

We investigated whether the hemodynamic response linked to interictal epileptiform discharges in patients with focal epilepsy, as observed using Blood Oxygen Level-Dependent (BOLD) fMRI, varied significantly from that derived from the canonical hemodynamic response function (HRF)., 63 patients with frequent interictal epileptiform discharges (IED) were studied. The patients were scanned at 1.5T (GE) using EPI BOLD. Scalp EEG was recorded simultaneously with pulse and imaging artefact subtraction. Experiments in which no IED were oberserved or in which IED were not identifiable individually were excluded, leaving data from 34 experiments. IED were marked to create a list of event onsets. Regressors of explanatory variables were obtained by convolving a time series of IED time markers with 8 cosines and sines over a 32s time-window with origin at the event onset to allow for inter-regional variations in the response shape (using SPM2). Effects of motion were modelled. Resulting activation maps were obtained by performing an F-test across the effects of interest and thresholding (P[lt]0.05, corrected). Results were compared to those obtained using models based on the canonical HRF and its time derivative (TD)., In 13/34 cases, significant activations were revealed with responses that deviated from canonical shape. In 8/13 cases, the activation pattern was remote from the presumed IED generator. Of the remainging 5/13 cases, two had generalised or bi-frontal EEG abnormalities. There was a single case of a significant non-canonical response (extent: 1 voxel) in the vicinity of the presumed (focal) generator where the standard model failed to reveal a concordant activation. The spatial extent of the cluster containing the global statistical maximum for the non-canonical responses varied between 1 voxel and 1068. The time courses of the non-canonical responses (global maximum) were rapid oscillations (8 cases; all 1-voxel clusters), sustained decreases (2 cases; 1 remote, 1 concordant), increases (1 case; remote) and bi-phasic patterns resembling the canonical response (2 cases; bilateral IED)., Using a flexible model of the IED-related response, we have revealed responses that ranged from distinct to slight variants of the canonical response, in addition to the canonical ones, in about a third of cases studied. The responses were mostly remote from the presumed IED generator. In many cases, their location and time courses are suggestive of artifacts or over-fitting. Our findings indicate that IED-related non-canonical responses are uncommon in focal epilepsy and generally do not reflect epileptiform activity in the vicinity of the generator(s); some variants of the canonical response may reflect propagation of epileptiform activity., (Supported by Medical Research Council and Wellcome Trust.)