In the adult brain, the frontal lobes are massively interconnected to most other regions of the brain and their size is largely due to the communicating neural fibres that make up the white matter beneath the grey-matter layer. However, this connectivity increases as the frontal lobes wire up to the rest of the brain. In comparison to other structures, the frontal lobes have the longest period of development, and during childhood they expand nearly twice as much as some other regions.5 In comparison to apes and monkeys, who peak earlier in the synaptic explosion of growth that we described in Chapter 2, genes that control synaptic formation delay peak connectivity until as late as five years in humans, which explains why this brain region is the last to start wiring up.6
This delayed peak of activity in the wiring programme may have significant relevance to changes in behaviour. There is a noticeable shift in frontal brain activity between three and four years of age, when there is also a vast improvement in toddlers’ abilities to plan and control thoughts and behaviours.7 They become less impulsive, which may be a consequence of this changing brain connectivity which helps to regulate behaviours.8
Another fascinating role for the frontal lobes is that they enabled humans uniquely to imagine different possible futures – to mentally time travel and make plans for the future.9 In fact, humans may be the only species that are able to contemplate the future.10 Many animals, including rodents like hamsters and squirrels, can store and hoard food for the future, but these could simply be automatic reflexive behaviours that are triggered without much thought. Bonobos will carry around the correct tool for retrieving food for as long as fourteen hours, showing that they can anticipate the future for at least half a day.11 But that’s hardly the same as planning for next year’s harvest. In one observation, cebus monkeys that were regularly fed once a day gobbled down as much food as possible until they were no longer hungry. They were always given more than they could eat in one sitting, but once they had eaten their fill, instead of saving it for another day they behaved like frat boys in a food fight and threw it out of the cage.12
Humans on the other hand plan for all sorts of future events. Much of our daily lives are taken up in preparation for anticipated outcomes. Our routines of schooling and employment are activities that pay dividends many years down the road. We even plan for our retirement decades in advance. Unlike most other animals, we can save for a rainy day. That level of foresight requires the integrity of the frontal lobes, which explains why only about a third of three-year-olds can tell you about what they are going to do tomorrow whereas twice as many four-year-olds can.13 Immaturity and damage to these regions condemn us to living in the here and now, with little concern for how things might turn out.
The silent manager
The frontal lobes occupy an exalted position in the history of neuroscience.14 The eighteenth-century Swedish scientist Emanuel Swedenborg first proposed that they were the seat of human intellect, a proposition that was later supported by the phrenologist Franz Gall in the nineteenth century. However, the activity of the frontal lobes remained surprisingly elusive to investigation. When Canadian neurosurgeon Wilder Penfield, who pioneered brain surgery on fully conscious patients in the 1940s, applied electrical stimulation to the surface of the brain, he noted how different areas triggered specific sensations or body twitches. In contrast, stimulation of the frontal lobes remained ‘silent’. So what do the frontal lobes do?
What don’t the frontal lobes do is probably the more useful question. Rather than being the focus of function, the frontal lobes are like New York’s Grand Central Station, London’s Heathrow airport or any large communication hub, receiving and sending information, linking up all the regions from the sensory systems, motor systems, emotional and memory regions spread throughout the brain. The massive amount of interconnectivity with the other brain areas indicates that the frontal lobes play a role in just about every aspect of human thought and behaviour. Rather than being localized in the frontal lobes, complex activities are integrated throughout this region like a neural junction box.15
Behaviour that requires planning, coordination and control enlists the activity of the frontal lobes. Even those activities that are automatic, such as the urges and impulses controlled deep inside the midbrain, need to be integrated into the rest of our behavioural repertoire so that they don’t get us into trouble. One way to think about the frontal lobes is to imagine their role as like a senior executive management team overseeing a large company. To be successful, a company must operate economically without wasting too much time and resources. The company needs to be able to take stock of the market, estimate demands, monitor current resources and set into action a planned strategy. The company will need to anticipate economic changes and plan for the future. Although there may be subdivisions in the company that compete for more resources than others, they have to be regulated so that the company as a whole can be more successful. This is why we need executives to manage the various operations that make the business run more efficiently as well as competitively. These executive functions monitor, coordinate, regulate and plan our thoughts and actions. Planning, memory, inhibition and attention are four executive functions (EFs) that operate from within a region that sits back from the front part of the brain, known as the prefrontal cortex (PFC).
Hot and cold
One useful distinction that has been made when considering the role of the PFC is the difference between ‘hot’ and ‘cool’ EFs.16 Hot EFs include those impulses and urges that are biological imperatives or emotionally charged drives that threaten to take over control of our thoughts and actions, whereas cool EF refers to the logical choices that one has to make when presented with a problem to solve that requires rationality. We use cool EFs when we have to remember a telephone number or a list of things to buy from the store. Most of us will repeat the information over and over to keep it fresh in our minds before we forget. If the list of items is too long, we forget the beginning before we get to the end. The task is even harder if we have to remember two numbers or, worse still, if someone starts talking to us when we are trying to concentrate. Cool EFs enable us to keep focused on the problem. In contrast, hot EFs interrupt ongoing events and make us switch priorities. When the danger signs are detected, the hot EFs swing into action to protect us.
Developmental neuroscientist Yuko Munakata proposes that the PFC operates in two ways to regulate hot and cold decisions.17 First there is the direct suppression of those drives and impulses by pathways that block the activity of mechanisms that are associated with hot, emotional EFs. Other thoughts and behaviours that represent the cold EFs that make up the normal routines of a typical day are regulated by indirect inhibition. These also need to be coordinated but without the need to shut down behaviours in the same way that hot EFs require. Munakata argues that this control is achieved by temporarily boosting the activity of different cortical regions. These support all the different options one is presented with when faced with a decision. In this competition, options with the strongest activation win out over those that are less active and so a decision is reached by the relative strength of different choices. Inhibition is not targeted at one behaviour in particular, but arises as a collateral effect of raising the profile of some options over others.