With evolution of language in humans, one hemisphere—the left—became specialized for dealing with symbols of experience (abstract, theoretical processing) rather than the gathering of experience itself, and the left hemisphere is systematically worse at all the ways in which we grasp hold of the world because it has been deliberately kept away from direct engagement with reality in order to do theoretical processing

The study of hemispheric lateralization was heavily discouraged in academic neuroscience during the careers of most researchers trained after the 1980s, with senior scientists warning junior researchers that studying this topic would 'taint their career' and harm their academic prospects, specifically telling them 'don't allow your career to be tainted, you can do well, don't do this'

Heublings Jackson, a great father of modern neurology in the 19th century, remarked that it is not common enough for us to wonder at the fact that the brain is divided in the way it is, suggesting that the evolutionary basis for hemispheric division deserves more scientific attention than it has received

Early in McGilchrist's career, he became troubled by the way literature was being analyzed and taught—scholars were taking embodied, implicit, unique works and making them disembodied and explicit, rendering them general rather than unique, and losing the meaning in the process.

McGilchrist came to the study of neuroscience through his reading of Oliver Sacks' 'Awakenings,' which demonstrated that attending to individual patients while drawing philosophical conclusions about personhood, mind, and consciousness could illuminate what changes in the brain do to human experience.

The popular conception that the right brain is emotional while the left brain is rational and reliable is largely inverted from the truth: the left hemisphere is actually less in touch with reality, more prone to jumping to conclusions, less reliable in judgment, and more prone to anger, while the right hemisphere is more intelligent in terms of IQ, attends more broadly to the world, perceives more, makes better judgments, and is less prone to quick conclusions

The difference between the two hemispheres is not primarily in what they do (function), but in how they do it—the manner, style, and way in which they process information—and each hemisphere is involved in all functions including reason, language, emotion, and visuospatial processing, just in characteristically different ways

Split-brain surgery, which involved dividing the corpus callosum to prevent epileptic seizures from overwhelming the entire brain, was successful in preventing seizures and provided a unique window into understanding how the two hemispheres operate independently, revealing that they can hold conflicting information and make different decisions

In split-brain patients, when information is presented to only one hemisphere, the other hemisphere—if it has no information about what was presented—will confabulate or make up a plausible explanation for why the person acted in a certain way, revealing that the left hemisphere is extraordinarily good at constructing false narratives and appears to believe its own fabrications

In split-brain patients, particularly in the early months after surgery, there are often conflicting behaviors where the two hemispheres attempt to perform incompatible actions simultaneously, such as a woman's right hand taking out one dress while the left hand puts it back and takes out a different one, or a person trying to pay money with one hand while the other hand takes it back and replaces it in the pocket

The corpus callosum's primary function appears to be inhibitory rather than facilitative—it does not simply pass information between hemispheres but rather allows one hemisphere to suppress or prevent interference from the other hemisphere, and even in a perfectly functioning intact brain much of the inter-hemispheric communication is inhibition rather than positive exchange of information

Most of what is happening in the brain—estimated at between 99.44 percent and over 99 percent of brain activity—is not within the field of consciousness, and what we call 'consciousness' is the narrow spotlight of attention that illuminates only a small portion of the brain's activity at any given moment

The left hemisphere has a very narrow beam of attention, highly clarified and precise, focused on about three degrees of the 360-degree attentional arc, whereas the right hemisphere has a very broad picture and is vigilant and on lookout all the time, surveying the full field of possible experience

McGilchrist has spent decades—approximately 30 years researching the brain's lateralization and 10 more years writing new work—really studying the hemispheric literature in depth, which means his understanding is based on extensive immersion in the research rather than superficial overview, and very few neuroscientists have invested the career cost to do this level of deep study

The reason hemispheric research was dismissed as 'pop psychology' was twofold: first, popular culture had appropriated split-brain findings in cartoonish ways (like advertising cars as 'for your right brain'), and second, some early researchers (1960s-1980s) drew conclusions too quickly from the initial split-brain data, leading serious neuroscience to overreact by dismissing the entire topic as pseudoscience

Transcranial magnetic stimulation (TMS) allows researchers to painlessly stimulate or suppress specific brain areas depending on the frequency of pulses, and when applied to knock out one hemisphere, it instantly releases full-fledged, ready-to-go decision-making patterns characteristic of that hemisphere, showing that these patterns were present and ready to manifest even in the intact brain

Alan Schneider in Sydney has found that suppressing left-frontal-cortex activity and enhancing right-frontal-cortex activity can facilitate problem-solving, including mathematical problem-solving, suggesting that the right hemisphere has superior capabilities for complex reasoning that are normally suppressed by left-hemisphere dominance

Consciousness might not be generated solely or primarily by the brain but rather the brain might be receiving a stream of consciousness that is transmitted and transduced by neural structures, functioning more like an antenna or receiver than a generator or producer of consciousness

The master and emissary parable describes a spiritual community where a wise master delegates daily business to a capable administrator, but the administrator comes to believe he is the master and loses the master's understanding, causing the whole community to fall to ruin—serving as a metaphor for the left hemisphere's usurpation of the right hemisphere's role.

The corpus callosum, which connects the two hemispheres with a band of fibers at the base of the brain, only directly connects about two percent of the brain's cortical fibers and is a mammalian invention—birds, reptiles, amphibians, and monotremes do not have a corpus callosum at all.