Albert Einstein comes near the top of any list of famously clever people, even 60 years after his death. His face is likewise instantly recognisable: the unruly white hair and grey moustache have adorned countless t-shirts, posters and popular science articles about the nature of genius. Fewer would recognise a photograph of the 26-year-old Einstein, at his remarkable peak of scientific productivity.
At that age, in 1905, he produced four groundbreaking papers on physics, all making crucial, independent contributions to the scientific revolutions of the twentieth century. These findings ultimately shaped the technology of our modern world, from solar and nuclear power to mobile computing and satellite navigation.
Einstein’s youthful brilliance was exceptional, but it is generally true that brainpower peaks in early adulthood. In particular, fluid intelligence – problem solving, adaptability and learning power – declines gradually from our twenties onwards.
Anyone who aspires to live into a happy and fulfilled old age should have a personal interest in the lifespan trajectory of our mental powers. Indeed, understanding what causes these changes holds the key to potential medical interventions for dementia, and possibly even brain-boosting for typical ageing adults.
Recent research by Rogier Kievit and colleagues suggests that two key aspects of our cognitive abilities decline at different rates. “Fluid intelligence” is the ability to solve unfamiliar problems, and reflects our power to acquire knowledge and learn new skills. In line with the “old dog, new tricks” proverb, it is well known that fluid intelligence diminishes as we get older. “Multitasking” is, unsurprisingly, the ability to effectively perform several brain-loading jobs simultaneously. Declines in multitasking are associated with age-related difficulties from recurrent falls to memory problems.
The study, performed in close collaboration with the University of Cambridge, was part of the Cambridge Centre for Ageing and Neuroscience initiative (funded by the Biotechnology and Biological Sciences Research Council). The team looked for connections between these cognitive functions and the composition of different brain regions, assessed through Magnetic Resonance Imaging and other brain scanning techniques. They focused on specific areas of the prefrontal cortex, which has long been associated with planning and voluntary control of action (see the famous case of Phineas Gage, who survived a metal rod blasting through the front of his skull, but with his personality significantly altered).
People from 18 to almost 90 years of age took part in cognitive tests (see Figure 1 from Kievit et al., 2014) and brain imaging. As expected, both multitasking and fluid intelligence declined with age, but the latter – as measured by problem solving on novel tasks – dropped more quickly. Furthermore, these changes were not uniformly associated with a decline in the amount of brain tissue.
The researchers measured the volume of grey matter (containing the bodies of brain cells) and white matter (long-range connections between brain regions) in specific areas of prefrontal cortex. The volume of the prefrontal cortex is known to decrease with age, and it had been thought that this could be associated with general cognitive decline. Contrary to this proposal, Kievit and colleagues found that levels of fluid intelligence and multitasking were associated with distinct areas of white and grey matter. Fluid intelligence was related to both the volume of grey matter in Brodmann area 10 in the prefrontal cortex and the functional effectiveness of the white matter connecting these regions in the left and right side of the brain. Multitasking was associated particularly with the functioning of white matter connecting the prefrontal cortex to deeper brain structures.
The results suggest that cognitive decline with age is not a simple, unified process. Fluid intelligence and multitasking ability fall at different rates, associated with structural changes in different parts of the brain. This study is, of course, just a step in understanding the associations between our brains and our intelligence: there are many more brain areas and connections to be thoroughly explored before we can predict mental ability from a brain scan with complete confidence.
Finally, if it seems a little gloomy to consider the effects of passing time on brainpower, you might be comforted that “crystallised intelligence” – the sum of our available knowledge – is on an upward curve until late in life. Furthermore, engaging our prefrontal cortex – through regular learning of difficult unfamiliar skills – may actually slow the decline in fluid intelligence. Old dogs, it seems, should keep trying out new tricks.