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Symposium Session 5 - Network Integrity and Disconnection Syndromes: New Insights from the Split-Brain

Chair: Michael Miller¹; ¹University of California Santa Barbara
Presenters: Edward H.F. de Haan, Michael B. Miller, Lukas J. Volz, Christof Koch

The split-brain represents one of the fundamental models of cognitive neuroscience, providing unique insights into the consequences of hemispheric severance and the nature of lateralized information processing in the human brain. Yet, these questions have largely gone uninterrogated from a network neuroscience perspective, and debate persists over the central matter of whether split-brains truly represent split-minds. This symposium will explore these current frontiers, presenting groundbreaking research from two recent cohorts of callosotomy patients. Edward de Haan (Talk 1) will begin by challenging the classical interpretation of the split-brain in light of recent findings from the well-studied Italian cohort of patients—in particular, the extent to which divided perception and control of action reflect a divided human mind. Michael Miller (Talk 2) will present a new cohort of German split-brain patients who underwent surgery in the last few years, providing novel experimental insights in support of a divided mind—and an astounding behavorial finding from a partial callosotomy patient with only a fraction of splenium intact. Lukas Volz (Talk 3) will follow up by presenting neural data from this cohort of German patients to address how callosotomy affects the organization of large-scale, intrinsic functional networks—highlighting how partial callosotomy patients reveal not only the complexity of structure-function relationships, but the remarkable ability of the brain to reorganize its functional architecture in service of robust interhemispheric communication. Finally, Christof Koch (Talk 4) will conclude with a discussion of how this recent split-brain research impacts current theories regarding the neural correlates of consciousness.

Presentations

The split-brain phenomenon revisited: two or one conscious agent?

Edward H.F. de Haan¹; ¹Donders Institute, Radboud University, Nijmegen, the Netherlands

In extensive studies with two split-brain patients, we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other. Yet, crucially, we show that the canonical textbook findings that a split-brain patient can only respond to stimuli in the left visual half-field with the left hand, and to stimuli in the right visual half-field with the right hand and verbally, are not universally true. Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally). Crucially, we used confidence ratings to assess conscious awareness. This revealed that also on high confidence trials response type did not affect performance. These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not necessarily create two independent conscious agents within one brain. Several explanations for the discrepancies with the conventional observations will be discussed.

No Disconnection Syndrome after Near-Complete Callosotomy

Michael B. Miller¹; ¹University of California, Santa Barbara

We conducted behavioral testing on a new cohort of split-brain patients in Bielefeld, Germany. Among several patients with complete callosotomies, testing confirmed robust disconnection effects across a broad battery of bedside tasks, including finger perimetry, topognosis, stereognosis, WISC-R Block Design, and tasks involving both speech comprehension and production. All patients underwent surgery in adulthood and performed within the normal range on standard neuropsychological assessments. In addition, some of these patients completed a computerized version of the Pinto et al. (2017) task with integrated eye-tracking to monitor gaze behavior. Results showed no indication of a unified control system across hemispheres. Unexpectedly, results from a couple of partial callosotomy patients—whose resections began anteriorly and proceeded posteriorly but were halted for surgical reasons—revealed no disconnection effects on any task. Based on the anatomical gradient of their resections, we hypothesized that visual integration between hemispheres would remain intact (via the splenium’s homotopic fibers), while integration in networks relying on anterior callosal pathways would be disrupted. However, even the patient with only a small remnant of the splenium intact showed no evidence of interhemispheric disconnection. Given previous (though limited) reports of some disconnection effects in partial callosotomy patients within a year of surgery, we propose that the fully intact behavioral integration observed here—more than one year post-surgery in all cases—may reflect functional reorganization across the cerebral networks.

Complete and Near-Complete Callosotomy: A Network Perspective

Lukas J. Volz¹; ¹University of Cologne, Germany

Using functional resonance magnetic imaging, we assessed how full and partial callosotomy affect the functional architecture of human brain networks. Given the classical view of callosal function and anatomy, we expected that interhemispheric functional connectivity would largely break down after full callosotomy. Conversely, partial callosotomy was assumed to only affect networks relying on severed callosal fibers for interhemispheric information integration according to anatomical topology. While we observed that interhemispheric functional network architecture was indeed severely disrupted after full callosotomy, functional network architecture was preserved after partial callosotomy, even if only very few splenium fibers (<1cm) remained intact. In other words, full inter-hemispheric integration seems to arise even if only a small proportion of posterior callosal fibers support interhemispheric information integration. These findings challenge our understanding of structure-function relationships underlying cognitive processing and highlight the adaptability of the adult human brain to reorganization its functional network architecture.

Being of One Mind

Christof Koch²; ¹Allen Institute, Seattle, United States, ²Tiny Blue Dot Foundation, Santa Monica, United States

Partial and complete split-brain patients provide evidence for a striking prediction of the Integrated Information Theory (IIT) of Consciousness. Its axiom of exclusion states that any experience is definite. At the level of physical existence, this axiom translates into demanding that the substrate of any experience be a maximum of cause-effect power, quantified as Φmax. Given the loss of causal interaction between the two cortical hemispheres in patients with complete callosotomy, the theory predicts two separate substrates of experience, that is, two minds, one in each cortical hemisphere, in line with the disconnection syndrome. IIT also predicts that as the surgeon’s scalpel slices through the axonal bundle making up the corpus callosum, there will be a point at which severing a few axons will cause Φmax across the cortex to dip below the Φmax values for either the left or the right hemisphere. At this moment, the single mind associated with the partially cut brain will fission into two independent minds, with their associated substrate in the two partial disconnected hemispheres. The evidence provided by Miller and Volz in the two preceding talks supports this conjecture. Furthermore, it is striking that patient BT, who has ~1 cm ribbon of splenium preserved, retains a single mind – this is in line with IIT’s conjecture that the topographical organized temporal-parietal-occipital grid-like cortex (posterior hot zone) is the substrate of everyday experience in neurotypicals.