Abstracts

Rationale: The default-mode network (DMN) of the brain supports resting consciousness [1]. It is believed that anti-correlation between the functional MRI (fMRI) signals in the DMN and the epileptogenic regions in temporal lobe epilepsy (TLE) may reflect the altered mental state caused by the epileptic discharges [2]. However, few simultaneous EEG-fMRI studies have detected negative activation in the DMN in response to interictal epileptic discharges (IED) which would support such a relationship [3-4]. In this study we mapped negative fMRI functional connectivity from a region in the default-mode network to the mesial temporal lobe to find evidence of this anti-correlation in a small homogeneous group of left TLE patients and healthy controls. Methods: Five patients with left mesial temporal sclerosis or hippocampal structural abnormality [5] were reanalyzed for this study. All underwent left selective hippocampectomy and became free of disabling epileptic seizures. Ten healthy controls were also included. Subjects were imaged on a 3.0T MRI scanner including structural and fMRI scans at rest with eyes closed (64x64, FOV=240 mm, TE/TR=35/2000 ms, 200 volumes). Previous results showed that the 2dTCA [5-6] analysis performed on the patients detected transient fMRI signal spiking in the DMN [6] (Figure 1a (outline)). An average time course in an activated region in the posterior cingulate (pcc) of this map was used as a seed in the functional connectivity analysis (Figure 1a (white)). Motion and global time course signals were regressed from all data and images were low pass filtered at 0.10 Hz. A two sample t-test was used to determine significant differences between the groups. Results: The controls had increased connectivity over the patients in the left posterior hippocampus (Figure 1b (white)) adjacent to the region in the anterior hippocampus that was found to have transient spiking by 2dTCA (Figure 1b (black)). The anterior hippocampal region coincides with the region of resection and is believed to be the epileptogenic region. Figure 1c shows that the connectivity to the posterior hippocampus is negative in the patients and positive in the controls, while the connectivity to the anterior hippocampus is not statistically different from zero in either. Conclusions: This preliminary work shows evidence of a region of anti-correlation between the DMN and the left mesial temporal lobe in the patients, but not controls, which may account for the change in mental state associated with IEDs. This region of anti-correlation is located adjacent and posterior to the epileptogenic focus that was resected, suggesting a possibly indirect coupling between these effects. This work was supported by the Epilepsy Foundation, NIH EB00461 and NIH NS055822. 1. Raichle ME, et al. PNAS 2001;98(2):676. 2. Gotman J, et al. PNAS 2005;102(42):15236. 3. Kobayashi E, et al. Hum Brain Mapp 2006;27:488. 4. Laufs H, et al. Hum Brain Mapp 2007; 28:1023. 5. Morgan VL, et al., Epilepsy Research 2007;76:22. 6. Morgan VLV, et al. AES Annual Meeting Proceedings 2008; 3.180.