The brain stem may orchestrate the basics of awareness
In October 2004, Swedish neuroscientist Bjorn Merker packed up his video camera and joined five families for a 1-week get-together in Florida that featured several visits to the garden of childhood delights known as Disney World. For Merker, though, the trip wasn't a vacation. With the parents' permission, he came to observe and document the behavior of one child in each family who had been born missing roughly 80 percent of his or her brain.
These children, 1 to 5 years old at the time of their Disney adventure, had suffered strokes as fetuses or had experienced other medical problems shortly before or after birth that destroyed nearly all of the brain's outer layer, or cortex. In this rare condition, called hydranencephaly, cerebrospinal fluid fills the gaping hole within the child's head.
Such youngsters often die in the first year of life as a result of seizures, cerebral palsy, lung abnormalities, and a variety of other physical ailments. With proper medication and the installation of shunts to drain fluid from the braincase, however, some individuals live 20 years or more.
Neurologists typically regard hydranencephaly as an anatomical sentence to a lifelong "vegetative state." Such children supposedly validate a brutally simple equation: Little or no cortex equals no awareness of any kind.
In family activities observed in the Magic Kingdom and elsewhere, the kids quickly cast doubt on that standard assumption. Merker noted that these cortex-deprived, nonverbal children remained alert for much of the day. They reacted to what happened around them and expressed a palette of emotions. A 3-year-old girl's mouth opened wide and her face glowed with a mix of joy and excitement when her parents placed her baby brother in her arms.
The youngsters displayed good hearing but limited eyesight, a curious pattern given that they typically retained small parts of the visual cortex but none of the auditory cortex.
In observations at each child's home, Merker noted that these youngsters recognized familiar adults, liked familiar settings, and preferred specific toys, tunes, or video programs. Although saddled with limited mobility, some kids took behavioral initiatives, such as learning to activate a toy by throwing a switch.
In the February Behavioral and Brain Sciences, Merker, an independent neuroscientist in Segeltorp, Sweden, described how the accomplishments of these children relate to behaviors recorded in prior studies of human-brain function and of animals after surgical removal of the cortex. His analysis generates a provocative proposal: Basic awareness of one's internal and external world depends on the brain stem, the often-overlooked cylinder of tissue situated between the spinal cord and the cortex.
Merker argues that the brain stem supports an elementary form of conscious thought in kids with hydranencephaly. It also contains auditory structures capable of preserving hearing in someone without a cortex. In contrast, optic nerve damage in hydranencephaly frequently impairs vision, regardless of what the brain stem does.
Self-awareness and other "higher" forms of thought may require cortical contributions. But Merker posits that "primary consciousness," which he regards as an ability to integrate sensations from the environment with one's immediate goals and feelings in order to guide behavior, springs from the brain stem.
If he's right, virtually all vertebrates—which share a similar brain stem design—belong to the "primary consciousness" club. Moreover, medical definitions of brain death as a lack of cortical activity would face a serious challenge. At the very least, physicians could no longer assume that individuals with hydranencephaly don't need pain medication or anesthesia during invasive medical procedures.
"To be conscious is not necessarily to be self-conscious," Merker says. "The tacit consensus concerning the cerebral cortex as the 'organ of consciousness' ... may in fact be seriously in error."
The roots of Merker's thesis emerged more than 50 years ago in the operating room of Canadian neurosurgeons Wilder Penfield and Herbert Jasper. The surgeons pioneered the removal of large chunks of cortex as a treatment for severe, uncontrolled epilepsy. To identify and avoid damaging still-functional brain areas, Penfield and Jasper kept patients awake during the surgery and administered only local anesthesia.
Various mental abilities suffered during and after the operations, depending on the site and extent of the neural loss. Nevertheless, patients maintained a conscious stream of thought, Penfield and Jasper found.
In the course of electrically stimulating various brain areas during operations to identify key functional areas, they noted that current delivered to the right spots could produce every kind of seizure except one—so-called "absence epilepsy" characterized by a sudden loss of consciousness for a few seconds. On the basis of what they knew about the brain, the researchers theorized that structures within and just above the brain stem typically trigger absence epilepsy and collaborated with the cortex to regulate conscious thought and intentional acts.
Animal research, predominantly with rats, has since indicated that three adjacent parts of the brain stem comprise a "neural reality simulator" that gives rise to a fundamental form of consciousness, Merker asserts.
Along the top of the midbrain, which represents the roof of the brain stem, layers of cells interpret the spatial layout of an animal's surroundings relative to its body. Just below, a patch of gray tissue influences emotion-related behaviors, such as aggression, sex, defensive maneuvers, and pain reactions.
Farther down the brain stem lie interconnected regions that regulate the direction of eye gaze and organize decisions about what to do next, such as reaching for a piece of food or pursuing a potential mate.
Together, these structures surround brain stem tissue that connects to sensory areas throughout the cortex.
Merker proposes that, in creatures with a brain stem but little cortex, the neural reality simulator produces a two-dimensional, screenlike map of the world featuring moving shapes. A large cortical endowment beefs up the neural reality simulator, creating an ability to perceive a three-dimensional world composed of solid objects. Neural expansion also allows people to reflect about what they think and feel.
In support of his theory, Merker cites studies conducted over the past 40 years in which rats and cats showed relatively few behavioral problems after surgical removal of the cortex, either in infancy or adulthood. These cortex-deprived animals use vision and touch to orient to their surroundings, learn where to find food in mazes, and remain capable of standing, climbing, grooming, mating, and caring for offspring.
Merker also cites an unusual phenomenon known as the Sprague effect. Complete removal of the visual cortex on one side of the brain renders animals unable to see anything in the half of the visual field opposite the surgical site. Yet a tiny cut to the midbrain restores the animal's ability to detect and approach moving entities, even though it still can't distinguish one object from another.
The Sprague effect underscores the brain stem's visual influence, Merker argues. Visual-cortex removal derails brain stem activity via numerous neural links to the midbrain's spatial cells that suddenly lack meaningful input. A well-placed midbrain cut halts activity by some of those wayward connections, allowing a partial return of sight, in his view.
Any entity with the equivalent of a neural reality simulator, "whether cast in a neural medium or eventually in silicon," would experience consciousness, Merker theorizes.
Of 27 comments by mind and brain researchers published with Merker's article, nearly half agreed that the inner workings of consciousness lie in the brain stem.
"The roots of consciousness exist in ancient neural territories we share with all vertebrates," says neuroscientist Jaak Panksepp of Washington State University in Pullman. "By the weight of empirical evidence, all mammals are sentient beings."
In his own research, Panksepp studies the ability of animals to experience biologically based states of mind or feelings that range from hunger and thirst to emotional delight and distress. For instance, Panksepp and a coworker reported in a controversial 2003 paper that rats express "joy" while playing with other rats by making ultrasonic sounds that represent an ancestral form of laughter.
Psychologist Carroll Izard of the University of Delaware in Newark emphasizes that this form of primary consciousness, as Merker would put it, or "primary affect," as Panksepp terms the rats' consciousness, consists of sensory activity in the brain stem. This capacity generates emotions and an awareness of one's surroundings but not an ability to talk about what one has experienced, Izard continues. In the same way, people can become conscious of a feeling that they can't label or describe, a phenomenon that's especially common in healthy infants and in children lacking a cortex, Izard says.
The existence of primary consciousness challenges widespread assumptions among physicians that newborns and fetuses can't feel pain, adds pediatric neurologist K.J.S. Anand of the University of Arkansas for Medical Sciences in Little Rock. Evidence now suggests that adult and immature brains use different systems to process pain, Anand says.
The brain stem and the thalamus, a relay station for sensation just above the brain stem, foster pain responses in babies before and after birth, he asserts. The cortex takes over pain perception as it greatly expands during childhood and adolescence, Anand hypothesizes.
Other investigators criticize Merker for denying the cortex its traditional position as the brain's engine of consciousness. Even if a basic form of consciousness exists, they regard it as at least a partial product of the cortex, not just the brain stem as Merker argues.
Conscious thought probably relies on the workings of connected brain areas within and outside the cortex, contend Susanne Watkins and Geraint Rees, neuroscientists at University College London. "It seems unlikely that activity in any single area of the human brain will be sufficient for consciousness," they write.
Children with hydranencephaly studied by Merker possess remnants of cortical tissue that could have triggered states of awareness, the researchers suggest.
Other commenters, including philosopher Gualtiero Piccinini of the University of Missouri–St. Louis, cite prior evidence that the cortex by itself regulates visual awareness. Following visual-cortex damage, certain patients report no conscious ability to see on one side of their visual field but still unconsciously perceive the identity and location of items in that same visual field. Scientists call this phenomenon blindsight.
The most extensively studied blindsight patient has frequently reported being aware of "something" in his blind visual field, Merker notes. This man retains primary visual consciousness of his surroundings but can't describe what he sees in words, the Swedish researcher contends.
In a 1999 report, D. Alan Shewmon, a pediatric neurologist at the University of California, Los Angeles Medical Center, and his colleagues described home observations of three children, ages 6 to 17, who had been born with hydranencephaly and raised by loving, attentive parents. Each child displayed comparable signs of conscious mental activity, the researchers reported.
For instance, shortly after birth, a newborn girl's brain scan revealed an almost total lack of cortical tissue. Physicians told the girl's mother that the child would live no more than 2 years as a "vegetable." A neurologist concluded that the girl's brain was "like that of a reptile" and that she would never interact with other people.
Shewmon first visited the girl at age 5, observing her behavior at home. Despite difficulty sitting up or walking without aid, she exhibited excellent health. She smiled in response to Shewmon's friendly overtures and immediately looked at objects brought close to her. In a videotaped play session with her mother, the girl uttered "ah-ah" when encouraged to say "mama."
She brightened upon hearing happy songs, but often cried during sad songs. She enjoyed the sensory stimulation of car rides, crying at stops and calming down as motion resumed. She disliked the loud noises of vacuum cleaners and hair dryers. She demonstrated understanding of a few words, including "bunny rabbit" for one of her stuffed toys.
"If these children had been kept in institutions or treated at home as 'vegetables,' there can be little doubt that they would have turned out exactly as predicted," Shewmon says.
After making his own observations of children with hydranencephaly and their families, Merker seconds that point. He notes that well-treated youngsters born with little or no cortex regularly display brief losses of consciousness due to absence epilepsy, a clear sign that at other times they're conscious.
Parents described these lapses of awareness in their children to Merker with phrases such as "she is off talking with the angels."
Perhaps most intriguingly, kids with hydranencephaly demonstrate that the brain stem is not simply a reptilian relic stashed in the brain's basement. "The human brain stem is specifically human," Merker says. "These children smile and laugh in the specifically human manner, which is different from that of our closest relatives among the apes."
For now, the neural puzzle of consciousness remains unsolved. But cortically endowed investigators may have much to learn from cortically deprived kids.
Department of Pediatrics
University of Arkansas for Medical Sciences
College of Medicine
Little Rock, AR 72202
Department of Psychology
University of Delaware
Newark, DE 19716
Gamla Kyrkvagen 44
Department of Veterinary Comparative Anatomy, Physiology, and Pharmacology
Washington State University
College of Veterinary Medicine
Pullman, WA 99164-6520
Department of Philosophy
University of Missouri, St. Louis
St. Louis, MO 63121-4400
Wellcome Trust Centre for Neuroimaging
Institute for Cognitive Neuroscience
University College London
London WC1 N 3AR
D. Alan Shewmon
University of California, Los Angeles
Medical Center, MDCC 22-474
P.O. Box 951752
Los Angeles, CA 90095-1752
Wellcome Trust Centre for Neuroimaging
Institute for Cognitive Neuroscience
University College London
London WC1 N 3AR