by Bruce Hood
Reciprocal communication enables experience to change the brain’s architecture. We know this from animal research in which the effects of early environments have been shown to influence the connectivity of the brain. For example, if you raise rat pups in isolation without much to see or do, their brains are lighter and have few cortical connections compared to the brains of pups raised in an enriched environment where there are lots of other rats with which to play. Nobel Prize winners David Hubel and Torsten Wiesel found that the activity of cortical neurons in the visual area was impaired in cats and monkeys raised in deprived visual environments during early development. Moreover, specific types of visual deprivation produced selective impairments. For example, animals raised in a stroboscopic world had relatively normal vision for objects but could not see smooth movement in the same way that you cannot see continuous motion in a bad 1970s disco when the strobe light is on. One unfortunate woman who acquired damage to this part of her visual brain late in life described how difficult it was for her to cross the road because she could not judge the speed of approaching cars. When she poured a cup of tea, it looked like a series of snapshots of still photographs with the cup empty, half-full and then overflowing.22
Sometimes the ability to see certain patterns is lost. Animals raised in environments without straight lines end up not being able to see straight. In short, early deprivation studies reveal that the punishment fits the crime.23 If you remove some experience during early development, it has long-term effects later in life. Children raised with faulty vision grow up with permanent visual loss known as amblyopia. Amblyopia is not a problem of the eyes but of the brain regions that produce vision. That’s why putting glasses on someone with amblyopia late in life makes no difference. It’s also why amblyopes cannot fully appreciate 3D movies because they have lost stereovision, which needs good input from both eyes early on in life. If you want to make a difference, you have to correct the problem when it first arises so that the developing connections in the brain are not permanently ruined.24 This leads on to discussion of another fundamental principle of brain development – sensitive periods.
Windows of Opportunity
Timing is everything, be it golf, sex or comedy. This turns out to be true for many basic aspects of brain development when input from the environment is required. Our brains have evolved to be malleable through experience but some experiences are required and expected at certain times during our lifetime. As noted above, deprivation can lead to permanent problems in later life but it turns out that these effects are most pronounced at certain times. Once the connections have been pruned due to inactivity, it is increasingly difficult to re-establish communication between the relevant parts of the brain. The window of opportunity has slammed shut.
These episodes of time-limited brain development are sometimes called ‘critical periods’ because no amount of remedial exposure after the window of opportunity has passed can reinstate the lost function. In truth, ‘sensitive period’ is probably more accurate as the brain has a remarkable capacity to recover, although it is worth noting that sensitive periods apply only to some of our human abilities and not others. Natural selection has evolved brains to expect certain experiences at certain times in development.25 Why would nature hedge her bets that way? Surely blank slates are the best solution for uncertain worlds.
The reason is quite simple: like any successful manufacturer, nature always seems optimized to cut the cost of production. Nature prefers to build machines that are tailored to work without being over-specialized. For example, there is no point building an all-purpose machine when some purposes are unlikely or redundant – that would be too costly. It is much better and more efficient to anticipate the most likely world rather than having the machine specified in advance. This is how evolution selects for the best fit. Those with systems that are not optimized for their environment are not as efficient and will eventually lose the race to reproduce. This explains why babies’ brains are pre-wired loosely to expect certain worlds they have not yet encountered and then become streamlined and matched to their own world through experience.
Although the modern world appears complex and confusing, the basic building blocks of how we see it are fairly predictable and unchanging from one generation to the next. Experience simply fine-tunes the system. However, if you remove the experience during the critical time when it is expected, then this creates permanent problems. One of the first demonstrations of critical period loss comes from the Nobel Prize-winning work of Konrad Lorenz who showed that newborn goslings would follow the first moving thing they saw – even if that happened to be an elderly Austrian bird expert.26 The early films of Lorenz show this bearded gent walking around smoking his pipe, being loyally followed by a line of goslings. Their bird-brains were equipped with a built-in mechanism to imprint on, and follow, the first big moving thing, whatever or whoever that was. For many animals, nature has produced a similar strategy to get them up and running as fast possible and to follow the important others in their gang. In the case of geese (and many other birds), nature gambled that the first moving thing was usually Old Mother Goose so there was no need to be too discerning. Austrian ornithologists would do fine. However, if the goslings were raised so that they did not see any large moving thing at all for the first ten days, then they did not later imprint because the window of opportunity had passed. In their natural state with no one to follow, these goslings would have perished, as their mother moved on.
Humans are more complicated than birds and our period of growth and nurturing is the longest in the animal kingdom, so there is less pressure to adapt as quickly. Nevertheless, there does appear to be evidence that we too have windows of opportunity and are preconfigured to attend to certain information from the environment. For example, human language development is usually trumpeted as one of the best examples of a brain-based ability that is both uniquely human and biologically anchored. In The Language Instinct,27 Steven Pinker points out that just about every child, irrespective of where they are raised, learns to speak a language almost effortlessly at roughly the same time, whereas their pet hamster raised in the same household does not. It doesn’t matter how much you talk to your pet, you won’t get them answering you back. The only sensible explanation for this is that the human brain is pre-programmed to learn a language, whereas pet hamsters’ brains are not. Any infant raised in any environment can learn the language to which they are exposed. This proves that there is a built-in, uniquely human capacity to learn language, which must be genetically encoded, but that the actual language acquired is determined by the environment.
The human baby’s remarkable ability effortlessly to acquire language is only one line of evidence for the biological basis of language. Have you ever noticed how difficult it is to learn a second language the older you get? For example, I do not seem to be readily able to learn a foreign language and it is not through lack of trying. Despite hours of effort with Linguaphone learning tapes, I am unable to break the British stereotype of only being able to speak English. This is because the plasticity in the neural circuits in my brain that support language learning has been progressively lost. Some of us do not have such a problem but it may be related to whether we were exposed to other languages at a young enough age. This is one of the reasons that foreign-language learning is much easier before the age of seven. For example, when Korean immigrants to the United States were tested on their ability to learn English, individuals had no problem if they arrived before they were seven. For older immigrants, it became increasingly hard for them to learn English, even though they attended night classes and were highly motivated to learn.28 This indicates there are biological limits to learning languages.
For many, just hearing the difference between languages becomes hard. In a classic study, Canadian infant researcher Janet Werker demonstrated that all babies could hear the different sound structures that exist in spoken Inuit and English languages before the age of ten months. However, the
longer they were immersed in their own language environment, the more difficult it was for them to hear differences in the structure of other languages.29 As we age, we lose the ability to detect the subtle differences between spoken languages. The best explanation is that our brains are tuning into the experience from our environments and losing the ability to process experiences that we do not encounter. Our brains are becoming less plastic for language learning. This is why, for Japanese speakers, English words that have ‘l’ and ‘r’ sounds are often confused, which can lead to comical miscommunication. Pinker wrote about his visit to Japan where he described how the Japanese linguist Masaaki Yamanashi greeted him with a twinkle in his eye when he said, ‘In Japan, we have been very interested in Clinton’s erection.’ This was several years before the US President would face impeachment in 1998 due to the Monica Lewinsky scandal.
Windows of opportunity exist in language and, as we shall see, even extend into other human qualities. But before we look into this, we should exercise against caution in over-interpreting the research on brain plasticity and critical periods described so far. This is because the discovery of critical periods in many animals led to some extreme beliefs and practices about human plasticity, especially when it came to how we should raise our children and what was the best parental practice. During the 1990s, there was a general panic that we were raising children in impoverished environments. The fear was that if we did not expose our children to a stimulating early environment, especially during the first three years, they would end up brain damaged. Suddenly, there was a public appetite for infant brain training and every parent and grandparent felt compelled to buy brain-enhancing devices from jazzy mobiles to hang over the crib, videos and DVDs to stimulate the brain, tapes of Mozart to play to pregnant mothers30 and every other kooky notion that was ‘proven by research’ to improve your child’s chances of getting into one of the Ivy League or Oxbridge universities. The marketers even had the audacity to name their various products Baby Einstein and Baby Bach. John Bruer, then director of the James S. McDonnell Foundation that supported much of the neuroscience research behind the original animal work, even wrote a book, The Myth of the First Three Years, to try to counter this hysteria based on the over-extrapolation of animal deprivation studies to human development.31
The truth is that deprivation has to be quite severe before permanent loss occurs because most daily environments are sufficiently complex to provide enough input for hungry young brains to process. Parents should not be conned into thinking that they can enhance a process that has taken millions of years to evolve. In fact, some products such as baby training DVDs to enhance language have been found actually to impair language development because parents were relying on the television rather than the richness of normal social interaction.32
Concerned educators and shrewd companies have either naively or deliberately misinterpreted the extent to which brain plasticity operates during sensitive periods. More importantly, there is little evidence that we can improve upon Mother Nature to supersize the early learning environment for a better intellectual outcome. But such messages fall on deaf ears. When it comes to doing what’s best for their kids, most parents err on the side of caution and so I suspect that the baby-brain boosting industry will always flourish. If only they would understand that the human brain has not evolved to absorb information from technology, but rather to absorb information from other people – much more complicated and yet so familiar.
The Gossiping Brain
At around 1.5 kg, the human brain is thought to be around five to seven times larger than expected for a mammal of our body size, and it has an especially enlarged cerebral cortex.33 If our brain had the same architecture as a rodent, it would weigh just 145 g and hold a meagre twelve billion neurons.34 Why do humans have such big, complicated brains in the first place? After all, they are very expensive to run, and although they only account for 2 per cent of typical body weight, they use up 20 per cent of metabolic energy.35 It has been estimated that a chess grandmaster can burn up to 6,000 to 7,000 calories simply by thinking and moving small pieces of wood around a board.36 What could justify such a biologically expensive organ? An obvious answer is that we need big brains to reason. This is why we can play chess. After all, a big brain equals more intelligence. This may be true to some extent but evolutionary psychologist, Robin Dunbar, has been pushing a less obvious answer – one that has to do with being sociable. He makes the point that big brains are not simply useful for any problem such as chess, but rather seem to be specialized for dealing with problems that must arise out of large groups in which an individual needs to interact with others.37
This is true for many species. For example, birds of species that flock together have comparatively larger brains than those that are more isolated. A change in brain size can even occur within the lifespan of an individual animal such as the locust. Locusts are normally solitary and avoid each other but become ‘gregarious’ when they enter the swarm phase of their life cycle. This swarm phase of the locust is triggered by the build up of locusts as their numbers multiply, threatening food supply, which is why they swarm to move en masse to a new location. As they rub against each other, this tactile stimulation sets off a trigger in their brain to start paying attention to each other. Amazingly, areas associated with learning and memory quickly enlarge by one third as they begin to swarm and become more tuned in to other locusts around them to become a devastating collective mass.38
Larger brains facilitate social behaviour. The link between brain size and sociability is especially true for primates where the extent of the cortex predicts the social group size for the species even when you take body mass into consideration. For example, gorillas may be big primates but they are fairly solitary animals with small close-knit family units and so their cortex is comparatively smaller than that of chimpanzees, which are much more sociable and like to party.39
If you are a member of a species that has evolved to coexist in groups, then you are faced with some challenging decisions about how to spread your genes. To make sure that you have enough resources for your self and any offspring, you need to get sneaky. This is particularly true of primates who engage in deception and coalition formation, otherwise known as Machiavellian intelligence,40 after the medieval Italian scholar who wrote the rulebook about how to govern through cunning and strategy. Primates in highly social groups try to outsmart and outflank fellow competitors for both the attention of potential mates and the distribution of resources. They need the mental machinery to keep track of others and second-guess their intentions. To do that, they need big brains with large areas of cortex to keep track of all the potential complex behaviours and information that large groups generate. For example, consider the number of interactions that exist between a dozen friends. Not only do you have to keep track of every relationship between each pairing, but you also have to work out all the potential combinations between subgroups within the group.
Using analysis based on all the major primate groups, Dunbar has shown that the cortex to group-size ratio can be used to predict the optimum group size for humans. According to Dunbar’s calculations, humans should coexist best in groups of up to 150. Any larger and the demands on social skills exceed our best capacity. It is a radical claim, and still very contentious, but there does appear to be evidence to support the hypothesis, especially when one considers pre-industrial societies. Over the course of human civilization, technology and industrialization have changed the way that we form groups. But keep in mind that the post-agricultural age began around 10,000 years ago and, with it, human behaviour changed as our species shifted from roving hunter-gathers to sedentary subsistence farmers. When you consider only those remaining hunter-gather societies that did not adapt to agriculture, the analysis reveals that Dunbar’s ratio exists among traditional societies. Even early religious settlements in the United States, such as the Hutterites, seem to have been most successful when their communities contained n
o more than 150 individuals. When a Hutterite community grows larger than 150, a new breakaway community is formed. Finally, analysis of modern companies reveals that large workforces operate and are managed best when employees form subdivisions of around the magic 150 workers. When Malcolm Gladwell was researching Dunbar’s ratio for his bestseller, The Tipping Point, he reported that Gore-Tex, the company that manufactures the high-tech material found in many sporting clothes, expanded its operations by forming subdivisions of 150 workers each time there was a need to open a new division.41 Dunbar’s number is an intriguing idea, especially as technology develops to change the way humans interact and keep track of each other. However, what worked for earlier societies may still be operating today in the modern, socially networked world.
In line with the growing field of social cognitive neuroscience, Dunbar is correct in arguing that the human brain has evolved specialized capacity and processing capability dedicated towards social functions. We know this because why else would humans have evolved into the species that spends the longest proportion of their lives as children dependent on adults? The simple answer must be that as a species we have evolved a strategy to pass on as much information as possible from one generation to the next through our storytelling and instruction. Our ability to communicate means that our offspring can know more about the world they are to embark on by listening to and learning from others without having to rediscover everything for themselves. In short, our extended human childhood means that we do not have to reinvent the wheel with each generation.