When the Left Hemisphere Goes Silent
Jill Bolte Taylor's Hemorrhage, Iain McGilchrist, and the Unpartitioned World
Inside a brick apartment in Belmont, Massachusetts, on December 10, 1996, Harvard Medical School neuroanatomist Jill Bolte Taylor woke with a sharp pain behind her left eye. An investigator at the McLean Hospital Brain Tissue Resource Center who dissected post-mortem human brains to study severe mental illness, Taylor did not yet know that a congenital arteriovenous malformation in her left cerebral hemisphere had just ruptured. The abnormal tangle of thin-walled blood vessels was pouring arterial blood directly over her left language and parietal association cortices.
Stepping into the shower to prepare for work, Taylor watched her own nervous system disassemble its coordinate frame piece by piece over the next four hours. First, the background roar of her internal verbal monologue—the continuous left-hemisphere voice that plans appointments, calculates errands, and maintains the running story of who she is—began to skip like a scratched record and then went completely silent. Without that serial linguistic loop, her personal biographical memories and anxieties evaporated.
Moments later, as she reached out to adjust the water faucet, a second boundary vanished. Looking down at her left arm, she could no longer perceive where the skin of her fingers ended and the porcelain wall of the shower stall began. The visual world no longer appeared as a collection of separate, solid, named objects sitting in empty space. Instead, she perceived her body and the surrounding room as a single, continuous field of shifting fluid energy. Though paralyzed on her right side and unable to speak or dial a phone without spending forty-five minutes matching digit shapes on a card to buttons on her keypad, Taylor reported that the vanishing of her left-hemisphere ego was accompanied not by panic, but by a deep euphoria of feeling undivided from the universe.
Why did silencing half of the cerebral cortex dissolve both the narrative voice and the physical boundary of the body at the same time? Three decades earlier, at the California Institute of Technology and Dartmouth College, neuroscientist Michael Gazzaniga and Nobel laureate Roger Sperry had uncovered the first clue while studying split-brain patients. In these patients, the corpus callosum—the two-hundred-million-axon cable linking the left and right hemispheres—had been surgically severed to halt intractable epilepsy. When Gazzaniga flashed a command exclusively to a patient's non-verbal right hemisphere, the patient's left hand would stand up or pick up an object without the left hemisphere knowing why. Asked why they had just stood up, the patient's verbal left hemisphere never admitted ignorance. Within half a second, it fabricated a story about wanting to stretch or fetch a drink. Gazzaniga named this left-hemisphere module the Interpreter: a compulsive narrative engine that weaves fragmented events into the illusion of a single ego in total control.
Beginning in 2009 with his synthesis The Master and His Emissary, Oxford scholar and neuropsychiatrist Iain McGilchrist demonstrated that this hemispheric division runs far deeper than language versus pictures. Every vertebrate animal—from a chaffinch pecking for seeds on a forest floor to a human navigating a city—must solve two incompatible attentional problems simultaneously. On one hand, the animal must lock narrow, focused attention onto an isolated target, such as a single seed against a background of pebbles, in order to grasp and consume it. That narrow, grasping, parts-based beam is the specialization of the left hemisphere, which controls the dominant right hand and builds simplified maps of manipulable objects.
At the same time, if the bird's entire brain is locked onto the seed, a hawk will strike it from above. The right hemisphere therefore sustains broad, open, vigilant, relational attention toward the living whole. It tracks continuous space, emotional tone, bodily grounding, and the implicit connections between things before they are carved into named categories. In modern culture, McGilchrist argued, the left hemisphere's gift for division and control often usurps the primacy of the right, convincing us that the map of separated parts is reality itself.
In February 2010, cognitive neuroscientist Cosimo Urgesi and neurosurgeon Franco Fabbro at the University of Udine in Italy tested this architecture in the operating room, tracking whether excising cortical tumors altered a person's baseline self-boundary. When Cosimo Urgesi and his neurosurgical team at the University of Udine tested eighty-eight brain-tumor patients three days before and five days after surgery in 2010, patients whose tumors were excised from the inferior posterior parietal cortex showed an immediate, statistically significant jump in trait Self-Transcendence scores less than one week after leaving the operating room.