The most fundamental difference between the human brain and those of other mammals is the greater extent to which development of its structure and function is influenced by sensory input.(1) Enduring changes in structure result from repeated activation of some cells and pathways more than others, following the principle that “neurons that fire together wire together.” Neuroplasticity refers to change in neural structures, and is usually the result of activity-dependent change in the interconnections among cells that constitute the structures. While the multineuronal systems of all mammals are constituted by connections among neurons that result from sensory input—meaning that environmentally induced activity shapes both the structure and function of the brain—only human beings shape the environments that, in turn, shape their brains.
The neuroplastic potential of the human brain is greater than that of our nearest primate cousins owing to changes in two parameter settings in pre- and post-natal neurodevelopment. One is a further increase in the number of neurons. The second is an increased length of time after birth during which interconnections among neurons are easily shaped by environmental input. These two changes make it possible for environmental input to create more elaborate and powerful neural functional structures.
Understanding the centrality of neuroplasticity in human brain development, and the power of cultural evolution that rests upon it, provides a new biologically-based understanding of the relationship between human beings and the environment. The first phase of that relationship is the transgenerational dialectic between human capabilities, human actions, and human-created expectations based on the influence of the human-made environment on neurocognitive development. In this phase, developing individuals have limited ability to act on the environment but are profoundly affected by it. A homology is created between the external environment and internal structures because the brain shapes itself to the recurring features of the specific environment within which it develops.
But cultural evolution differs from Darwinian biological evolution in several important ways. Cultural evolution creates more rapid, more incremental, and more widespread population variability. Cultural and biological evolution also differ in the way in which information is stored so as to provide continuing influence on function. In biological evolution, information is stored in the largely stable base sequence of DNA molecules. In cultural evolution, information is stored in the minds and behavior of adult members of society; in cultural artefacts such as books, architecture, and works of art; and in social institutions including laws, customs, and schools. In biological evolution, the information is stored in identical and complete form in many individuals. In cultural evolution, the information is distributed in different and incomplete form across many individuals and artefacts.
There are 100 billion neurons in the human brain, each directly connected to over 1000 other neurons. Consistent with this massive interconnectivity, learning simple associations between stimuli leads to altered responses in millions of cells distributed across wide expanses of cortical territory.(2) When people perform simple cognitive operations, multiple brain areas in both cerebral hemispheres become more active, and others decrease their activity.(3) The early twentieth-century Russian neuropsychologist A.R. Luria noted that localized injuries rarely affected only one cognitive operation, but usually affected multiple such operations.(4) He also noted that individual cognitive operations were affected by injuries in multiple different areas of the brain. Luria concluded that, while groups of cells in a specific anatomic location might collectively have some elementary tissue function, such functions do not correspond to mental functions like perception, memory, or cognition. Mental operations are instead properties of multicomponent functional systems. Like other systems, cerebral functional systems perform constant functions through means or components that vary from instance to instance. Functions are properties of a system and not of a specific anatomic location. Most contemporary views of the functional organization of the human brain are based on such systems.
This modern view contrasts with nineteenth-century concepts of phrenology and related twentieth-century concepts of modularity.(5) Phrenology and modularity posit that specific cognitive operations are performed at circumscribed, localized anatomic sites, and that the function of these sites is the specific cognitive operation. In contrast, the twenty-first century systems view posits that ensembles of cells at different locations have different characteristics, like different letters of an alphabet. Cognitive functions emerge from combinations of different local units just as words emerge from combinations of letters.
The dynamic systems view helps explain the striking fact that when one hemisphere of the brain must be surgically removed in very young infants, their subsequent cognitive development is largely normal, and all cognitive operations are performed with the remaining hemisphere.(6) Even when the left or language hemisphere is removed, near-normal language function is supported by the right hemisphere. As with the other examples of developmental neuroplasticity discussed below, these relocations and reconfigurations of the brain’s functional architecture are more easily understood in the systems/emergent property view than in the phrenology/modularity view.
The brain requires sensory stimulation in order to maintain structural integrity. Information-processing structures along afferent pathways from peripheral sensory receptors to cortical processing centers atrophy without sensory input. Effects of sensory deprivation on the development of the brain’s functional organization follow from these effects on cell viability and growth. Neurons at each stage of processing compete for connections with neurons at each subsequent stage, with those that fire more often gaining territory.
The extent of neuroplastic potential in the developing mammalian brain is remarkable. In adult rats that had an eye removed at birth, stimulation of their whiskers led to electrophysiological and metabolic activity within the visual cortex.(7) Apparently, neurons in what is normally a visual processing area came instead to respond to input from the whiskers when deprived of input from the eye. In perhaps the most dramatic demonstration of plasticity, the optic nerve in one-day-old ferrets was rerouted to provide visual rather than auditory input to what is normally the auditory cortex. The auditory cortex developed a functional organization of ocular dominance columns highly similar to the normal visual cortex, rather than its usual tonotopic structure, and the ferrets saw with what would normally have been the auditory regions of the brain.(8)
Studies demonstrate that mammalian brains (and minds) develop concrete perceptual structures, capabilities, and sensitivities based on prominent features of the rearing environment, and then are more able and more likely to see those features in the sensory mix of new environments encountered subsequently. Or, to turn it around, mammals have limited ability to see even prominent features of a new environment if those features were absent from their rearing environment.
In their studies of infant monkeys and wire mesh surrogate mothers, Harry Harlow and Clara Mears added a key feature to the definition of mammals, one that is in many ways more important than their well-established characteristics. Infant monkeys were separated from their mothers and raised in cages with access to both a wire mesh and a cloth surrogate mother.(9) Both surrogate mothers were kept at the same temperature as normal monkey mothers. One-half of the monkeys received milk from the wire mesh mother and one-half from the cloth mother. Both groups spent much more time on the cloth than on the wire mesh mother. The differential was greater by only a small amount when the cloth mother was the source of milk. The preference for the cloth mother became greater over time in both groups, the opposite of what would be expected from a food/hunger reduction conditioning model, which would predict increasing preference over time for the food-providing surrogate mother. Harlow and Mears concluded that the disparity in favor of selecting the cloth mother independently of which mother provides milk “is so great as to suggest that the primary function of nursing as an affectional variable is that of ensuring frequent and intimate body contact of the infant with the mother.”(10) In other words, instead of the provision of milk being in and of itself the end goal of mother-infant interaction, it is a means of ensuring contact between the mother and the infant, because this contact is essential for provision of the sensory stimulation necessary for brain development, and for the production of population variability through variability in that stimulation.
Real living mothers and other parenting figures vary in the ways in which they stimulate their infants and children. Naturally occurring differences in these parenting behaviors have lifelong and specific effects on the brains and behavior of their offspring, and changes in DNA structure that mediate these effects have been identified in studies of rats.(11) Mother rats differ in the amount of time they spend licking and grooming their pups, and in the ways in which they position themselves for nursing. Michael Meaney and colleagues found that adult rats that had been licked more as pups had decreased behavioral and hormonal responses to stress, and greater spatial learning abilities—a capacity in which areas of the hippocampus play an important role.(12) Examining brain chemistry and structure, they found greater levels of specific types of messenger RNA that carry information from the DNA to parts of the cells that synthesize the glucocorticoid receptors important in regulating stress responses, and the NMDA receptors important in promoting neuroplasticity. Direct examination of the hippocampus revealed that offspring of high-licking mothers had longer neurons with more branches and interconnections. Furthermore, direct examination of the DNA identified actual changes in the genes associated with stress response as a result of the degree of maternal licking. Shortly after birth, the surface of DNA is largely covered by small chemical complexes called methyl groups. These methyl groups limit access to the DNA and thereby limit activation or expression of genes. The effects of experience on methylation are much greater during the first three weeks of a rat’s life than thereafter, and changes induced by experience during this critical period usually remain relatively unchanged throughout the rat’s adult life. Maternal licking initiates a series of neurochemical processes that selectively demethylate genes that produce the glucocorticoid receptors in the hippocampus and frontal lobes that turn off the stress response. To ensure that their observations were due to the differences in maternal behavior, and not to genes that high-licking mothers passed on to their offspring, Meaney and colleagues had pups born to low-licking mothers raised from birth by high-licking mothers, and vice versa.(13) When these rats became adults, their stress responses and the methylation of their DNA were both consistent with the type of mother that had reared them, and not with the type of their biological mother.(14)
Some of the persistent neurochemical and behavioral effects of maternal care of female infants affect the way the infant functions as a mother herself when she becomes an adult. Females that had been separated from their mothers when they were infants, showed lower than normal gene expression in areas of the brain associated with maternal behaviors when they themselves became mothers.(15) Such intergenerational effects are potentially self-propagating and even self-amplifying. Moreover, since litter size and food availability can influence the amount of licking and other behavioral interactions between mother and infant, a variety of environmental factors can influence maternal behaviors and their impact, across generations, on a range of individual and group behaviors.(16) All of this depends on the postnatal sensitivity of the mammalian brain to sensory stimulation, and on the proximity of mammalian infants and mothers ensured by nursing.
Human rearing behaviors are more complex and more varied than those of other mammals, and include massive social components and influences from extended families, communities, and nation states. The extrafamilial influences include schools, mass media, arts, laws, and customs. The human social and economic environments also affect the states of mind, time, and energy of the parents, thus affecting their interactions with their offspring in a manner analogous to the effects of food supply on rat maternal behavior. As Kenneth Kaye has remarked, “social interference in the object-directed activities of babies is such a commonplace occurrence that few authors have remarked on its absolute uniqueness to our own species.”(17) The brains and minds of human infants and children develop while closely linked to the minds and brains of their biobehaviorally mature caregivers.
The characteristics of adults shape the stimulation that shapes the growing brains of children through the small details and general rhythms of the child’s experiences. The child integrates input from progressively larger circles of direct interaction, beginning with primary caregivers and growing to include extended family members and then members of the community and society more broadly. While some of the social input is actively shaped and provided by others, much is just absorbed through essentially constant imitation. From infancy on, children learn how to do things simply by watching them being done. They imitate the goals of action even by different means, and imitate a parent’s affective response to new stimuli.(18) Within two days of birth, infants will stick out their tongues and move their heads in imitation of an adult doing so.(19) Mirror neurons fire when people (and monkeys) watch an act being done, and many times these same neurons are then active when the individual performs the action previously observed.(20) Similarly, looking at someone else in pain activates the same regions of the brain as are active when the observer experiences pain him or herself.(21) David H. Hubel and Torten Wiesel demonstrated that environmentally induced neuronal activity shaped the development of cerebral functional structures, following the principle that “neurons that fire together wire together.”(22) In human development, active parental and community interventions and nearly constant imitation of what is seen and heard produce intensive and repetitive firing of neuronal ensembles and circuits.
Well before the relevant neuroscience research, psychologists were aware of the role of the social environment in shaping mental development, describing the processes in language remarkably similar to what would be suggested by the subsequent work of David H. Hubel, Torsten N. Wiesel, Michael Meaney, and others. Writing in 1926, Otto Fenichel states that “changes in the ego, in which characteristics which were previously perceived in an object [usually an important person] are acquired by the perceiver of them, have long since been familiar to psychoanalysis.”(23) Sigmund Freud described identification as “the assimilation of one ego to another one, as a result of which the first ego behaves like the second in certain respects, imitates it and in a sense takes it up into itself.”(24) And Ralph R. Greenson stated that “identification with an object means that […] a transformation of the self has occurred whereby the self has become similar to the external object […]. [O]ne can observe behavior, attitudes, feelings, posture, etc., which are now identical to those characteristics belonging to the external object,” and that, at early stages of development, “perception implies transformation of the self.”(25)
Brain imaging studies have now demonstrated changes in brain structure and function that result from unusual motor activity or sensory input during childhood and persist into adulthood. One set of studies has examined differences in brain structure and function as a result of practicing a musical instrument during childhood.(26) A socially and culturally created and induced activity on multiple levels, intensive practice of string instruments leads to selective increase in volume of the right somatosensory and motor areas associated with the rapid, fine motor movements of the fingers of the left hand that provide intricate and fast-moving sequences of pressure to the strings. The changes in the brain are greater in adults who practiced for more hours and began practicing at younger ages.
The second set of studies looked at brain activations in the normal visual areas of the brain in adults who were blind at birth or shortly thereafter, or the normal auditory areas of the brain in adults who were deaf at or shortly after birth. Directly analogous to the selective sensory deprivation experiments of Hubel and Wiesel, these experiments’ findings were also analogous. In early blind subjects, the area of the brain that is normally the site of early visual processing is activated instead by auditory and tactile stimulation;(27) and it is also more active during language processing tasks than is the case in sighted people.(28) Apparently, when the normal sensory input to the area was absent, other sensory input and cognitive operations moved into the territory. Moreover, among the blind individuals, memory performance was higher in the individuals who made more use of the “visual” areas during the memory task.
The demonstration of changes in brain morphology as a result of practicing a musical instrument extends things a bit beyond the animal studies, in that practicing music is clearly a socially constructed human activity. It is impressive that the environmentally induced changes can be seen with the naked eye when data from multiple individuals is averaged together. But the demonstration of changes is at a gross anatomic level, and does not reveal more fine-grained changes in structure and function. By linking the data and theory from animal studies to human beings via the imaging studies cited above, scientists complete an evidentiary loop and increase confidence in the application to human beings of principles based on the data from animals.
Functional properties of individual neurons in the human brain differ little from those of individual neurons in the brains of other primates. The large differences in function between the human brain and other primate brains result instead from the increased number of cells and interconnections among them, the extended period after birth during which the brain is highly susceptible to shaping by environmentally induced neuronal activity, and the fact that humans alone alter the environment that produces the neuronal activity that shapes the brains of their offspring. Together these factors constitute neuroplasticity and cultural evolution. Cultural evolution produces changes in human capabilities, desires, and expectations much more rapidly and through very different mechanisms than does Darwinian biological evolution. It is a cross-generational and social process that shapes individual actions; and these actions then in turn contribute to the social and cross-generational influences that shape other individuals.
We humans are not handed a set of fixed capabilities, developed desires and inclinations, and standards for ethical conduct. All three of these critical aspects of human being are in dynamic interplay through human history. Our ability to shape our environments, and through that to shape our minds, brains, and behavior, begets complex responsibility, promising both opportunity and political contestation. Such is our neurobiological relationship with our natural and human-made environments.
The self is shaped by and uses the material world to construct itself, even at the biological level of neural and genetic plastic transformation. The internet is one of the largest changes in the environment ever made by human beings. It has transformed the ways in which we think, learn, work, socialize, and buy and sell. Scientists have only begun to consider the impact of the internet on brain development, but studies exist on internet addiction and the effects of computer games on thinking and behavior. The apparent immateriality of the internet cloud seems to increase the existential challenge to the self. Is it possible to make visible the technical and social processes, and the associated social and psychological changes of human concern, in such a way as to promote thought about and understanding of the viewer’s relationship to the expanded technology space in which the human individual and species is now integrated? Is there some incongruity between the brain-based self—so fundamentally physical—and this new technology space? If the worry here is that the individual self is being swept along in a rapid environmental change that has distorted and threatened her person, digital natives on the other hand comfortably state that “we live on the internet.” They refer to the material world around them as RL (real life)—just one of several options in which to spend time and energy. Perhaps it is just that our neurally, psychosymbolically, and biologically constituted selves are simply experiencing the stress of having been shaped by one environment and now being swept into another.
(1) Editors’ note: an earlier version of Bruce Wexler’s contribution was published as “Neuroplasticity, Culture and Society,” in Arne De Boever and Warren Neidich (eds), The Psychopathologies of Cognitive Capitalism: Part One (Berlin: Archive Books 2013).
(2) E.R John, Y. Tang, A.B. Brill, R. Young, and K. Ono, “Double-labeled metabolic maps of memory,” Science, 233 (1986): 1167–75.
(3) M. D’Esposito, “From cognitive to neural models of working memory,” Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences, 362 (2007): 761–72.
(4) A.R. Luria, The Working Brain, trans. B. Haugh (New York: Basic Books, 1973). See also L.S. Vygotsky, Mind in Society: The Development of Higher Psychological Processes (Cambridge MA: Harvard University Press, 1978).
(5) B.E. Wexler, “Using fMRI to study the mind and brain,” in R. Shulman and D. Rothman (eds), Brain Energetics and Neuronal Activity (West Sussex: John Wiley and Sons, 2004), 279–94; B.E. Wexler, Brain and Culture: Neurobiology, Ideology and Social Change (Cambridge, MA: MIT Press, 2006).
(6) J.A. Ogden, “Phonological dyslexia and phonological dysgraphia following left and right hemispherectomy,” Neuropsychologia, 34 (1996): 905–18; R. Werth, “Visual functions without the occipital lobe or after cerebral hemispherectomy in infancy,” European Journal of Neuroscience, 24 (2006): 2932–44.
(7) J. Toldi, I. Rojik, and O. Feher, “Neonatal monocular enucleation-induced cross-modal effects observed in the cortex of adult rat,” Neuroscience, 62 (1994): 105–14.
(8) J. Sharma, A. Angelucci., and M. Sur, “Induction of visual orientation modules in auditory cortex,” Nature, 404 (2000): 841–7.
(9) Harry F. Harlow and Clara Mears, The Human Model: Primate Perspectives (Washington: V. H. Winston & Sons, 1979).
(10) Harlow and Mears, Human Model, 108.
(11) I.C.G. Weaver, N. Cervoni, F.A. Champagne, et al., “Epigenetic programming by maternal behavior,” Nature Neuroscience, 7 (2004): 847–54; I.C.G. Weaver, J. Diorio, J.R. Seckl, M. Szyf, and M.J. Meaney, “Early environmental regulation of hippocampal glucocorticoid receptor gene expression: characterization of intracellular mediators and potential genomic sites,” Annals of the New York Academy of Sciences, 1024 (2004): 182–212.
(12) Weaver et al., “Early environmental regulation.”
(13) Ibid.; Weaver et al., “Epigenetic programming.”
(14) Weaver et al., “Epigenetic programming.”
(15) A.S. Fleming, G.W. Kraemer, A. Gonzalez, V. Loveca, S. Reesa, and A. Meloc, “Mothering begets mothering: the transmission of behavior and its neurobiology across generations,” Pharmacology Biochemistry and Behavior, 73 (2002): 61–75.
(16) On litter size, see Fleming et al., “Mothering,” and J.E. Jans, and B. Woodside, “Effects of litter age, litter size, and ambient temperature on the milk ejection reflex in lactating rats,” Developmental Psychobiology, 20 (1987): 333–44. On food availability, see D.M. Lyons, H. Afariana, A.F. Schatzberg, A. Sawyer-Glover, and M.E. Moseley, “Experience-dependent asymmetric variation in primate prefrontal morphology,” Behavioural Brain Research, 136 (2002): 51–9.
(17) K. Kaye, “Organism, apprentice, and person,” in E. Tronick (ed.), Social Interchange in Infancy: Affect, Cognition, and Communication (Baltimore: University Park Press, 1982), 193.
(18) K. Kaye, The Mental and Social Life of Babies: How Parents Create Persons (Chicago: University of Chicago Press, 1982); M. Klinnet, R.N. Emde, P. Butterfield, and J.J. Campos, “Social referencing: the infant’s use of emotional signals from a friendly adult with mother present,” Developmental Psychology, 22 (1986): 427–32.
(19) A.N. Meltzoff and M.K. Moore, “Imitation of facial and manual gestures by human neonates,” Science, 198 (1977): 74–8; A.N. Meltzoff and M.K. Moore, “Imitation in newborn infants: exploring the range of gestures imitated and the underlying mechanisms,” Developmental Psychology, 25 (1989): 954–62.
(20) M. Iacoboni, R.P. Woods, M. Brass, H. Bekkering, J.C. Mazziota, and G. Rizzolatti, “Cortical mechanisms of human imitation,” Science, 286 (1999): 2526–8; G. Rizzolatti, L. Fadiga, V. Gallese, and L. Fogassi, “Premotor cortex and the recognition of motor actions,” Cognitive Brain Research, 3 (1996): 131–41; M.A.,Umilta, E. Kohler, V. Gallese, et al., “I know what you are doing: a neurophysiological study,” Neuron, 31 (2001): 155–65.
(21) X. Gu and S. Han, “Attention and reality constraints on the neural processes of empathy for pain,” NeuroImage, 36 (2007): 256–67; P.L Jackson, A.N. Meltzoff, and J. Decety, “How do we perceive the pain of others? A window into the neural processes involved in empathy,” NeuroImage, 24 (2005): 771–9; T. Singer, B. Seymour, J. O’Doherty. H. Kaube, R.J. Dolan, and C.D. Frith, “Empathy for pain involves the affective but not sensory components of pain,” Science, 303 (2004): 1157–62.
(22) D.H. Hubel, and T.N. Wiesel, “The period of susceptibility to the physiological effects of unilateral eye closure in kittens,” Journal of Physiology, 206 (1970): 419–36.
(23) O. Fenichel, (original German publication: 1926) “Identification,” in G. Pollock (ed.), Pivotal Papers on Identification (Madison, CT: International Universities Press, 1993), 57–74.
(24) S. Freud, (1933) “Excerpt from Lecture XXXI: The dissection of the psychical personality,” in Pollock (ed.), Identification, 47.
(25) R. R. Greenson, (1954) “The struggle against identification,” in Pollock (ed.), Identification, 160–1.
(26) G. Schlaug, “The brain of musicians: a model for structural and functional adaptation,” Annals of the New York Academy of Sciences, 930 (2001): 281–99.
(27) K.E. Weaver, and A.A. Stevens, “Attention and sensory interactions within the occipital cortex in the early blind: an fMRI study,” Journal of Cognitive Neuroscience, 19 (2007): 315–30.
(28) A. Amedi, N. Raz, P. Pianka, R. Malach, and E. Zohary, “Early ‘visual’ cortex activation correlates with superior verbal memory performance in the blind,” Nature Neuroscience, 6 (2003): 758–66.
Dr. Bruce Wexler is Professor of Psychiatry at Yale University. He is the author of a number of scientific articles and books, among them Brain and Culture: Neurobiology, Ideology and Social Change (2006).