• confidence
• consciousness
• interhemispheric transfer
• split brain
• visual matching

• European Research Council

In common sense experience based on introspection, consciousness is singular. There is only one ‘me’ and that is the one that is conscious. This means that ‘singularity’ is a defining aspect of ‘consciousness’. However, the three main theories of consciousness, Integrated Information, Global Workspace and Recurrent Processing theory, are generally not very clear on this issue. These theories have traditionally relied heavily on neuropsychological observations and have interpreted various disorders, such as anosognosia, neglect and split‐brain as impairments in conscious awareness without any reference to ‘the singularity’. In this review, we will re‐examine the theoretical implications of these impairments in conscious awareness and propose a new way how to conceptualize consciousness of singularity. We will argue that the subjective feeling of singularity can coexist with several disunified conscious experiences. Singularity awareness may only come into existence due to environmental response constraints. That is, perceptual, language, memory, attentional and motor processes may largely proceed unintegrated in parallel, whereas a sense of unity only arises when organisms need to respond coherently constrained by the affordances of the environment. Next, we examine from this perspective psychiatric disorders and psycho‐active drugs. Finally, we present a first attempt to test this hypothesis with a resting state imaging experiment in a split‐brain patient. The results suggest that there is substantial coherence of activation across the two hemispheres. These data show that a complete lesioning of the corpus callosum does not, in general, alter the resting state networks of the brain. Thus, we propose that we have separate systems in the brain that generate distributed conscious. The sense of singularity, the experience of a ‘Me‐ness’, emerges in the interaction between the world and response‐planning systems, and this leads to coherent activation in the different functional networks across the cortex.

• anosognosia
• consciousness
• covert knowledge
• ego dissolution
• singularity

• Voice gender categorization
• gender bias
• hemispheric asymmetries
• split-brain patient
• temporal voice area (TVA)

• Consciousness
• Epilepsy
• Lateralisation
• Split-brain
• Tactile perception

• Corpus Callosum
• Functional Laterality
• Hand
• Humans
• Male
• Split-Brain Procedure
• Touch

This short review examines the most recent functional studies of the topographic organization of the human corpus callosum, the main interhemispheric commissure. After a brief description of its anatomy, development, microstructure, and function, it examines and discusses the latest findings obtained using diffusion tensor imaging (DTI) and tractography (DTT) and functional magnetic resonance imaging (fMRI), three recently developed imaging techniques that have significantly expanded and refined our knowledge of the commissure. While DTI and DTT have been providing insights into its microstructure, integrity and level of myelination, fMRI has been the key technique in documenting the activation of white matter fibers, particularly in the corpus callosum. By combining DTT and fMRI it has been possible to describe the trajectory of the callosal fibers interconnecting the primary olfactory, gustatory, motor, somatic sensory, auditory and visual cortices at sites where the activation elicited by peripheral stimulation was detected by fMRI. These studies have demonstrated the presence of callosal fiber tracts that cross the commissure at the level of the genu, body, and splenium, at sites showing fMRI activation. Altogether such findings lend further support to the notion that the corpus callosum displays a functional topographic organization that can be explored with fMRI.

• Corpus callosum
• Interhemispheric transfer
• Functional magnetic resonance imaging and diffusion tensor imaging
• Brain imaging
• Topographic organization

• Callosal commissure
• DTI
• functional brain imaging
• interhemispheric transfer
• split-brain patients

• Anatomy
• Brain
• Diffusion tensor imaging
• Magnetic resonance imaging
• Physiologic imaging
• White matter

• Adult
• Auditory Perception/physiology
• Brain Mapping/methods
• Corpus Callosum/physiology
• Female
• Humans
• Magnetic Resonance Imaging/methods
• Male
• Middle Aged
• Psychomotor Performance/physiology
• Taste/physiology
• Touch/physiology
• Visual Perception/physiology
• Young Adult

With the advances in diffusion magnetic resonance (MR) imaging techniques, diffusion tensor imaging (DTI) has been applied to a number of neurological conditions because DTI can demonstrate microstructures of the brain that are not assessable with conventional MR imaging. Tractography based on DTI offers gross visualization of the white matter fiber architecture in the human brain in vivo. Degradation of restrictive barriers and disruption of the cytoarchitecture result in changes in the diffusion of water molecules in various pathological conditions, and these conditions can also be assessed with DTI. Yet many factors may influence the ability to apply DTI clinically, so these techniques have to be used with a cautious hand.

• Diffusion tensor imaging
• Diffusion weighted imaging
• Tractography

• Adolescent
• Brain Injuries/pathology
• Brain Injuries/physiopathology
• Child
• Corpus Callosum/injuries
• Corpus Callosum/pathology
• Corpus Callosum/physiopathology
• Diffusion Tensor Imaging
• Humans
• Image Interpretation, Computer-Assisted
• Intracranial Hypertension/complications
• Intracranial Hypertension/pathology
• Intracranial Hypertension/physiopathology
• Prosencephalon/injuries
• Prosencephalon/pathology
• Prosencephalon/physiopathology
• Recovery of Function
• Young Adult

• G0001237 Medical Research Council
• G0600986 Medical Research Council
• G9439390 Medical Research Council