Michael S. Gazzaniga

The dynamic integration of the lateralized and specialized capacities of the two cerebral hemispheres constitutes a hallmark feature of human brain function. This interhemispheric exchange of information critically depends upon the corpus callosum. Classical anatomical descriptions of callosal organization outline a topographic gradient from front to back, such that specific transcallosal fibers support distinct aspects of integrated brain function. Here, we present a challenge to this conventional model. Using neuroimaging data obtained from a new cohort of adult corpus callosotomy patients, we leverage modern network neuroscience techniques to show that full interhemispheric integration can be achieved via a small proportion of posterior callosal fibers. Partial callosotomy patients with spared callosal fibers retained widespread patterns of interhemispheric functional connectivity and showed no signs of behavioral disconnection, even with only 1 cm of the splenium intact. Conversely, only complete callosotomy patients demonstrated sweeping disruptions of interhemispheric network architectures, aligning with disconnection syndromes long-thought to reflect diminished information propagation and communication across the brain. These findings motivate an evolving mechanistic understanding of synchronized interhemispheric neural activity for large-scale human brain function and behavior.

Sensorimotor processing in the human brain is largely lateralized, with the corpus callosum integrating these processes into a unified experience. Following complete callosotomy, this integration breaks down, resulting in disconnection syndromes. We asked how much of the corpus callosum is sufficient to support functional unity—the absence of disconnection syndrome—by comparing three complete callosotomy patients with one retaining only the splenium. Using lateralized tasks across visual, tactile, visuospatial, and language domains, we predicted domain-specific deficits in the splenium-only patient based on established anatomical models of callosal topography. Strikingly, while complete callosotomy patients exhibited disconnection syndromes, the splenium patient demonstrated functional unity across all domains—as if his entire corpus callosum were intact. Our findings highlight the brain’s remarkable capacity to maintain behavioral integration through minimal preserved pathways, highlighting how the structure-dependent reorganizational capacity of the human brain allows to preserve functional unity.

The dynamic integration of the lateralized and specialized capacities of the cerebral hemispheres constitutes a hallmark feature of human brain function. This inter-hemispheric exchange of information critically depends upon the corpus callosum. Classical descriptions of callosal organization outline a topographic gradient, such that specific fibers integrate distinct aspects of brain function. Here we present a challenge to this conventional model. Using neuroimaging data obtained from a new cohort of adult corpus callosotomy patients, we leverage modern network neuroscience techniques to show - for the first time - that full inter-hemispheric integration can be achieved via a small proportion (~1 cm) of intact posterior callosal fibers. Only complete callosotomy patients demonstrated the expected dissolution of typical inter-hemispheric network architectures, aligning with disconnection syndromes long-thought to reflect diminished information propagation and communication across the brain. These findings motivate a novel mechanistic understanding of synchronized inter-hemispheric neural activity for large-scale human brain function and behavior.

A recently published study by the present authors reported evidence that functional changes in the anterior cingulate cortex within a sample of 96 criminal offenders who were engaged in a Go/No-Go impulse control task significantly predicted their rearrest following release from prison. In an extended analysis, we use discrimination and calibration techniques to test the accuracy of these predictions relative to more traditional models and their ability to generalize to new observations in both full and reduced models. Modest to strong discrimination and calibration accuracy were found, providing additional support for the utility of neurobiological measures in predicting rearrest.

Let's face it. We have to wonder whether the totally deterministic view of life mandates us to believe that we are all simply along for the ride. Since everything is determined by prior states of the physical world, does that not mean that although we may think we are “freely” making our life's decisions, we are not? Albert Einstein believed that we are not, Francis Crick believed that we are not, and so did Baruch Spinoza. So why should we not believe the same, along with some of the scores of interpretive caveats that even proponents of determinism attach to it? We could, perhaps, but the notion grates on most of us as being somewhere between preposterous and downright depressing. With determinism, where are human worth, value, and responsibility?

This chapter argues that cognitive neuroscience has three main issues with respect to the current field of neuroethics. First, cognitive neuroscience can help with some current ethical dilemmas such as whether the embryo has the moral status of a human being. Secondly, there are important ethical areas to which neuroscientists are being asked to contribute when, in fact, they should not be. For instance, neuroscience has nothing to say about concepts such as free will and personal responsibility, and it probably also has nothing to say about such things as antisocial thoughts. Finally, cognitive neuroscience is building an understanding of how brain research will instruct us on ideas like universal morals possessed by all members of our species. This fundamental development will find cognitive neuroscience becoming central to the modern world's view of ethical universals.