It turns out that the over-production and subsequent cull of connections may be a cunning strategy to shape the brain to its environment. A massive connectivity means that the brain is wired up for every potential pattern of activation that it may encounter from experience. But remember, only neurons that fire together, wire together. When neurons are not reciprocally activated, nature prunes their connections through inactivity. Returning to the metaphor of our extended neighbourhood, ‘If you don’t return my call, I am not going to bother contacting you later.’ Or for those of you familiar with social networking such as Facebook or Twitter, then it’s the case of ‘un-following’ followers who do not follow you back.
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.