Patients who have undergone the “split-brain” operation to relieve severe intractable epilepsy provide a unique opportunity for cognitive neuroscientists to examine mechanisms of interhemispheric interaction, neural plasticity and compensatory reorganization. A somewhat surprising finding has been that aside from deficits visible under restricted testing conditions, these patients “show a remarkable absence of functional impairment in nearly all ordinary behavior” [1
]. Numerous studies of split-brain patients have shown evidence for continued interhemispheric transfer -- albeit degraded -- in the absence of cerebral commissures [2
]. These patients can often transfer coarse perceptual visual information between the hemispheres [3
]. The common assumption is that this is accomplished by subcortical pathways, which can compensate when low-level information transfer is required [3
It is generally accepted that some interhemispheric transfer exists in these patients, yet our understanding of the coordination of processing between the split hemispheres remains limited. While some studies of typical interhemispheric coordination in neurologically intact participants suggest entirely cortico-cortical connections, others implicate subcortical contributions [4
]. Efforts to differentiate between these possibilities have relied on measures of coherence as indexed by electroencephalogram (EEG), with conflicting findings. Coherence analyses of EEG have been used as an indicator of functional cortico-cortical connections, with coherence decreases thought to reflect decreased interhemispheric connectivity [4
]. While some report residual coherence in human split-brains as well as in surgically callosotomized animals, supporting the of idea of subcortical interhemispheric coordination [5
], others report drastically reduced interhemispheric coherence post-operatively, suggesting that coherence is entirely mediated by cortico-cortical connectivity [6
]. Unfortunately, EEG lacks the anatomic resolution required for studying specific networks. Recently developed functional connectivity measures applied to fMRI data provide an alternative method for studying interhemispheric coordination.
The discovery of resting-state functional connectivity in the motor system [7
] has provided a new method for mapping major cortical networks. To date, coherent spontaneous fMRI activity has been used to identify attention systems [8
], the default mode network [9
], as well as several other resting state networks (RSNs) [10
]. Resting state functional connectivity can also be used as a measure of interhemispheric coherence, with greater anatomic specificity than is possible with EEG. For example, a recent study has demonstrated that homologous regions of the anterior cingulate cortex in each hemisphere exhibit stronger correlations than non-homologous regions [11
While the significance of these resting state networks is not yet fully understood, they are widely thought to reflect intrinsic functional architecture. In a recent review, it is suggested that these signals serve to organize and coordinate neuronal activity [12
]. In support of a putative functional role in organizing and coordinating neural activity across distributed networks, its been shown that low-frequency oscillations (< 0.1Hz) serve to synchronize activity in large-scale networks, while higher frequency oscillations modulate more local events [13
]. Thus, current models in the literature point to organizational functional properties of resting-state networks.
Functional connectivity, whether measured by EEG or fMRI, is thought to reflect structural connectivity. Given the known structural connectivity existing between homologous regions in each cerebral hemisphere, cortical commissural fibers have long been purported to mediate the majority of interhemispheric interactions. In split-brain patients however, interhemispheric coordination, when it exists, must be mediated by extra-callosal pathways.
Here we used resting-state functional connectivity to establish the degree to which coordination of processing between the cerebral hemispheres can be supported by subcortical mechanisms. We used independent components analysis (ICA) and seed-based functional connectivity measures on fMRI data from a commissurotomy patient to test for the presence of continued patterns of bilateral connectivity in commonly observed resting state networks. This patient has been previously characterized using several behavioral assessments [14
], and surgical reports verify completeness of the commissurotomy [1