Loss of resting interhemispheric functional connectivity after complete section of the corpus callosum

Abstract

Slow (<0.1 Hz), spontaneous fluctuations in the functional magnetic resonance imaging blood oxygen level-dependent (BOLD) signal have been shown to exhibit phase coherence within functionally related areas of the brain. Surprisingly, this phenomenon appears to transcend levels of consciousness. The genesis of coherent BOLD fluctuations remains to be fully explained. We present a resting state functional connectivity study of a 6-year-old child with a radiologically normal brain imaged both before and after complete section of the corpus callosum for the treatment of intractable epilepsy. Postoperatively, there was a striking loss of interhemispheric BOLD correlations with preserved intrahemispheric correlations. These unique data provide important insights into the relationship between connectional anatomy and functional organization of the human brain. Such observations have the potential to increase our understanding of large-scale brain systems in health and disease as well as improve the treatment of neurologic disorders.

Figure 1.

Functional connectivity computed before and after corpus callosotomy for right- and left-sided seed regions. Seed regions defined in the right hemisphere (first column of each set) were used to generate correlation maps both before (second column) and after (third column) complete transection of the corpus callosum. Seed ROIs were placed laterally in the hemispheres to avoid confusion that might arise because of partial volume averaging associated with ROIs close to the midline. Presurgery, functional connectivity maps using the selected seed regions revealed a pattern of correlations that is consistent with published literature, including near symmetric correlations with the contralateral hemisphere. For example, a seed in right lateral parietal region demonstrates correlations with the left lateral parietal as well as with medial prefrontal cortex and posterior cingulate/precuneus (second row). Postsurgery, contralateral correlations that were present before transection prominently disappear (FEF, top row; LP, second row; V1, third row). Correlations with the hippocampal formation seed and the somatomotor seed do not show complete loss of contralateral correlations. These regions have neuroanatomical connections with the opposing hemisphere through fibers that do not cross in the corpus callosum. The temporal lobe, for example, is able to communicate with the contralateral hemisphere through the anterior commissure (Schmahmann and Pandya, 2006). Findings were similar for both right- and left-sided seeds. FEF, z = 55.5; LP, seed, z = 36.5; V1, z = 12.5; HF, z = −13.5; SM, z = 51.5.

Figure 2.

z score map of right FEF seed (dorsal attention system) projected onto a smoothed cortical surface (Caret software). A, Preoperatively, FEF anticorrelations within the default network were not well developed, a result that may be developmentally normal. However, the major constituents of the dorsal attention system (FEF and IPS) were bilaterally present. B illustrates loss of interhemispheric correlations with preservation of ipsilateral dorsal attention system (FEF, IPS, and MT+) functional connectivity.