These changes in connectivity enable the world to shape the brain by experience because experience keeps the neurons active through repeated mutual activation. This shaping process is called plasticity after the Greek word plassein, meaning ‘to mould’. Synapses between cells that are in constant communication change in their sensitivity so that messages transfer more easily between them. At its most basic level, this is how information is stored in the brain – as changing patterns of neuronal activity. This crucial role for reciprocal neuronal activity is captured in the first of the neuroscientist’s principles of plasticity, ‘cells that fire together, wire together’.9
Most brain plasticity occurs during child development, with some areas continuing to change well into the late teenage years. The front part of the brain, associated with decision-making, does not become fully mature until the child reaches adulthood. Of course, there is plasticity in the adult brain as we constantly learn throughout our lifetime. However, connectivity in some brain systems seems to be time sensitive, requiring input much earlier in development. Remember neural activity is metabolically expensive. If neural connections are not active, then why keep them? In many ways it is similar to pruning your favourite rose bush. You cut away the weaker branches in order to allow the stronger branches to flourish.
These windows of opportunity, which are sometimes called critical periods, reflect the way that Nature has produced a brain that anticipates specific experiences at certain times; if these are denied or impoverished, there may be long-term impairment. This is true for the sensory systems such as vision and hearing, but as we will read in the next chapter, there appear to be critical periods for social skills as well. This loss of function due to deprivation is a second principle of plasticity, where you have to ‘use it or lose it’ when it comes to keeping neural mechanisms functional.
Core knowledge
In the same way that our brain is pre-configured to experiences before we have even had the chance to encounter the relevant sensations, some scientists believe that we are also wired to interpret the world in particular ways before we have had the chance to think about it. The speed at which babies acquire and understand aspects of the world around them before they are capable of comprehending spoken language indicates that they must be working things out fofigr themselves. As adults, we take it for granted that the world is made up of objects, spaces, dimensions, plants, animals and all manner of complex ideas that we rarely take the time to consider because we have had a lifetime of exposure to them. But how do young babies come to appreciate these concepts in the absence of language? When a baby looks around its new blurry world, what does it make of it all? Even if they are learning by themselves, how do they know what to pay attention to and what is relevant? These sorts of problems have led to the proposal that some key components of understanding the world, especially those related to the physical nature of objects, numbers and space, must be programmed into the brains of infants from birth. But how do we know what babies are thinking when they are not even capable of telling us what is on their minds? The answer comes down to showing them magic tricks.
The reason that we find magic tricks so entertaining is that they violate our expectations. When a magician makes an object vanish into thin air, we are first surprised and then set about trying to figure out how they achieved the illusion. As adults, we know that a physical law has only apparently been violated because if we did not have that understanding, then we would not be surprised. That is why it is a trick. The same is true for babies. When they are shown magic sequences where objects appear to vanish, infants look longer. They do not applaud or gasp as an adult audience would, but they notice that something is not quite right.
This magic-trick technique, known as violation of expectancy, has spawned hundreds of experiments used to tap into the minds of infants who cannot tell us what they are thinking. Harvard psychologist Elizabeth Spelke has been using violation of expectancy to probe the rules infants apply when understanding the physical world.10 From very early on, infants recognize that solid objects do not pass through other solid objects, move from one position to another without appearing in between, move by themselves unless contacted and nor do they dissolve or fall apart when touched. When we say that something is ‘solid as a rock’, it is so because it obeys Spelke’s rules for physical objects. These rules do not have to be learned and for most objects that the child will encounter throughout the rest of their life, these principles will hold true, which is why they are referred to as core knowledge, because they are programmed into the mind from birth.
Of course, there are some exceptions to these rules, such as in the case of magnets, where an iron object will move in the absence of direct contact with another object. Soft bananas dipped in liquid nitrogen become hard as nails. These exceptions to the normal rules are enchanting because they violate our expectations of how physical objects should behave. Many toys that you find in science museums are counter-intuitive examples that amaze and amuse precisely because they do not behave like most ordinary objects.
It’s alive!
Babies appreciate that people are also another type of object, but one with a special set of properties. For a start, people can move by themselves. If an inanimate object is left behind a screen then it should still be there unless someone has moved it. A person, on the other hand, can leave the room when you are not looking, so may not necessarily remain stationary when they are out of sight.11 Also people do not have to move in a straight line. Five-month-olds who watched a video of a box sliding across a stage and passing behind two screens were surprised if it did not reappear in the gap in between. However, they were not surprised when a person moving across the same stage did not reappear in between the screens, suggesting that the infants could draw a distinction between how a box and a person can behave when moving in between screens.
Living things also move in particular ways. Objects that are not alive tend to move in a rigid way, whereas living things have ‘biological motion’ which is much more fluid and flexible. These types of movement are processed by neurons that are tuned to directions and speed in the visual area at the back of the brain known as MT. Biological motion is less rigid and activates a different region which is closer to the area behind your ears that is activated by faces. This area, the fusiform gyrus, also registers the shape of the human body, which suggests it might be a region that stores general information about others like us.12 When we think of others, we expect them to be a certain shape and move in a certain way. By six months of age, babies are surprised to see a female who appears to have arms growing out of her hips that swing as she walks.13
How do babies decide what is human? We know that babies like to look at other people. They prefer biological motion at birth.14 We also know they prefer the sound of the human voice and their mother’s voice in particular.15 They prefer the smell of their mother compared to the smell from another mother.16 Just about every capability of the newborn’s senses seems to be tuned into their mums.
Over time, infants gradually start paying attention to others and noticing what they are doing. When you think about it, the sheer volume of information contained in just a minute or two of a typical everyday action that an adult might perform is staggering.17 Consider the individual steps involved in making a cheese sandwich. Every sequence requires complex motor skills that must be performed in a way that is beyond the capabilities of robots. Ingredients and implements must be retrieved from various locations in the kitchen and then prepared and assembled in the correct pre-planned order. There is no point trying to butter the slice of bread after the cheese has been inserted. How do babies begin to make sense of what they see when watching others? It turns out that in just the same way that baby brains are wired to chop language up into different segments, they are programmed to observe and learn different actions. Infants as young as six months are sensitive to the statistical regularities in action sequences and by ten to twelve months readily segment complex actions up into their constituent parts based on the flow of movements as they start and stop.18