Residual functional connectivity in the split-brain revealed with resting-state functional MRI

Abstract

Split-brain patients present a unique opportunity to address controversies regarding subcortical contributions to interhemispheric coordination. We characterized residual functional connectivity in a complete commissurotomy patient by examining patterns of low-frequency BOLD fMRI signal. Using independent components analysis (ICA) and region-of-interest (ROI) based functional connectivity analyses, we demonstrate bilateral resting state networks in a patient lacking all major cerebral commissures. Compared to a control group, the patient's interhemispheric correlation scores fell within the normal range for two out of three regions examined. Thus we provide evidence for bilateral resting state networks in a patient with complete commissurotomy. Such continued interhemispheric interaction suggests that, at least in part, cortical networks in the brain can be coordinated by subcortical mechanisms.

Causal effect of disconnection lesions on interhemispheric functional connectivity in rhesus monkeys

“…More generally, the present results, taken together with the studies on human patients (26,27), indicate that even weak and indirect connections between the hemispheres are enough to maintain interhemispheric functional connectivity. This finding supports a hypothesis from modeling work: Adachi et al (30) modeled the effect of different types of indirect connections on functional connectivity in macaque monkeys.…”

“…In patients with white matter damage (multiple sclerosis), decreases in structural connectivity have been observed to correlate with increases in functional connectivity (17). Even more striking is that normal levels of interhemispheric functional connectivity persist in individuals totally lacking the normal corticocortical white matter connection between hemispheres: individuals born without a corpus callosum (callosal agenesis) (26) and patients with surgical lesions of the corpus callosum (27).…”

“…Unfortunately, evidence from studies of acallosal human subjects is equivocal: although one recent study of individuals with callosal agenesis provided particularly strong evidence for normal interhemispheric functional connectivity in a relatively large cohort (eight patients) and is supported by evidence from a case study of surgical section of the corpus callosum (27), other studies with smaller numbers of patients have reported that functional connectivity is disrupted in callosal section or agenesis (31,32). Interpretation of patient studies is complicated for several reasons.…”

“…Interpretation of patient studies is complicated for several reasons. First, these studies have used different patient groups (surgical disconnection vs. callosal agenesis; in the latter, noncallosal commissures may be enlarged), although disruption and retention of connectivity have been observed in both groups (26,27,31,32). Second, the surgical disconnection patients were not neurologically typical even before surgery since they suffered from epilepsy (17,23).…”

Brain Commissural Anomalies

“…A study has shown that lesions of the posterior or mid-body corpus callosum or complete commissurotomy conflict intermanual coordination; injuries of the posterior corpus callosum and parietal cortical areas cause the alien hand sign; and lesions of the frontal lobe or anterior corpus callosum results the anarchic hand (Aboitiz et al, 2003).Studies in acallosal and split brain patients have revealed that the absence or loss of the corpus callosum integrity contributes to impairment in sensory and cognitive integration (Fabri et al, 2001;Yamauchi et al, 1997) and large individual differences in interhemispheric transfer among split-brain patients (Zaidel et al, 2003). In split-brain patients, however, several investigators have noted that transfer of some types of visual information is usually spared (Eviatar & Zaidel, 1994;Uddin et al, 2008).The condition that cortical commissures are no longer available some information can be transferred between the hemispheres through subcortical pathways (Funnell et al, 2000) by the subcortical coordination of cortical networks (Uddin et al, 2008). In regard to involvement of the corpus callosum in lower-level visuomotor functions, split-brain research indicates that the corpus callosum acts in an inhibitory fashion within a subcortico-cortical network Roser & Corballis, 2003), while recent research on callosal degradation without disconnection have shown cooperative role for the corpus callosum (Schulte & Müller-Oehring, 2010) in conscious perception (Marzi et al, 1996;Müller-Oehring et al, 2009).…”