Abstracts

Rationale: Surgical removal of one brain hemisphere (hemispherotomy, HT) can be a definitive therapy for epilepsy in rare cases, but beyond that it offers the opportunity to study pieces of isolated, yet fully vital cortical tissue. In healthy people, a widely accepted set of seven functional brain networks has been shown to reliably describe intrinsic activity in the resting-state (Yeo et al., J Neurophysiol 2011, 106(3):1125-65). We aim, first, to demonstrate that this network parcellation is applicable to both the connected and isolated hemisphere after HT and, second, to quantify intrinsic functional connectivity compared to healthy subjects.

Methods: We included 28 patients (17 female) who had undergone HT (age at MRI 22.4 ± 9.9 years [mean ± SD]) and 24 healthy control subjects (12 female, age at MRI 31.2 ± 12.5 years). In nine patients, the isolated hemisphere consisted of sufficient intact tissue to analyze both hemispheres, in the 19 other patients only the connected hemisphere was studied. All subjects were examined using resting-state functional MRI and T1-weighted structural MRI, preprocessing was performed using FreeSurfer and fmriprep standards. Images were parcellated into seven networks according to Yeo et al. and further into a sub-parcellation with 200 parcels per hemisphere (Schaefer et al., Cereb Cortex 2018, 28:3095-114, panel A). To probe parcel homogeneity, we calculated the Pearson correlation between the time series of each voxel and the average time series of all voxels belonging either to the same parcel (“within”), to other parcels in the same network (“inside”), or to other parcels in a different network (“outside”). Furthermore, within-network and between-network connectivity were assessed comparing the average time series between parcels in the same network (“within”) and between parcels belonging to different networks (“between”).

Results: Our analyses confirm an intact pattern of homogeneity (voxel-to-parcel-correlations “within” >“inside” >“outside”, panel B/C) in control subjects, in the connected hemisphere post-HT and to a lower extent also in the isolated hemisphere. Within-network connectivity on parcel-level was lower in the isolated hemisphere than in the connected hemisphere and in controls. With regard to between-network connectivity, we found stronger correlations in the isolated hemisphere than in the connected hemisphere and in controls (panel D). This pattern is also predominantly found when each network is considered separately. However, higher within-network than between-network connectivity was preserved in all groups and across all networks (panel E/F).

Conclusions: Our data confirm results of an initial case series (Kliemann et al., Cell reports 2019, 29:2398-407) depicting that intrahemispheric functional connectivity patterns in subjects with only one remaining cerebral hemisphere after hemispherectomy largely correspond to those of healthy subjects. Furthermore, our results demonstrate that these well-studied connectivity patterns, rather than random activity, are found even in the surgically disconnected hemisphere, raising questions pertaining to possible mental states in isolated cortical hemispheres.

Funding: Please list any funding that was received in support of this abstract.: BONFOR/University of Bonn.