The Myth of Cognitive Decline

The myth of cognitive decline Michael Ramscar, Peter Hendrix & Harald Baayen Seminar fur Sprachwissenschaft, Wilhelmstrase 19 Universitat Tubingen, Tubingen, Germany Abstract correctly identify greater variation in the cognitive performance of older adults, successfully predicting that older adults will show more sensitivity to fine-grained differences in test items than younger adults. The models run (and can be rerun) on computers, eliminating the possibility that aging hardware influences their performance, which instead reflects the information- processing costs incurred as knowledge increases. Once the demands of processing this extra information are taken into account, it becomes clear that much of the evidence for age- related declines in cognitive capacity better supports the idea that information processing costs rise as the amount of information in a system increases. Across a range of psychometric tests, reaction times slow as adult age increases. These changes have been widely taken to show that cognitive-processing capacities decline across the lifespan. Contrary to this, we suggest that slower responses are not a sign of processing deficits, but instead reflect a growing search problem, which escalates as learning increases the amount of information in memory. A series of computational simulations show how age-related slowing emerges naturally in learning models, as a result of the statistical properties of human experience and the increased information-processing load that a lifetime of learning inevitably brings. Once the cost of processing this extra information is controlled for, findings taken to indicate declines in cognitive capacity support little more than the unsurprising idea that choosing between or recalling items becomes more difficult as their numbers increase. We review the implications of this for scientific and cultural understanding of aging. The problem with “processing speed” Keywords: Learning; Language; Memory; Psychometric Testing; The Age of Tithonus More and more people live longer and longer lives. Outside of 18 countries the UN describes as ‘outliers’ (Watkins et al, 2005), increased life expectancy and declining birth rates are raising median ages in populations across the globe. By 2030, 72 million Americans will be aged 65 or older, a twofold increase from 2000. The world’s population is more aged than ever before in history, and its rate of aging is increasing. People are clearly living longer; it is less clear that this is a blessing. In Greek mythology, Tithonus was the mortal lover of Eos, goddess of the dawn. While asking Zeus to make Tithonus immortal, Eos forgot to mention “eternal youth,” dooming Tithonus to an eternity of decrepit babbling. The psychological and brain-sciences endorse the Tithonean view of aging, portraying adulthood as an extended period of mental decline: memories dim; thoughts slow; problem-solving abilities diminish (Naveh-Benjamin & Old, 2008; Deary et al, 2009); and each year, the onset of cognitive decrepitude is set ever younger (Salthouse, 2009; Singh-Manoux et al., 2012). One crumb of comfort is that older adults are, on average, happier (Charles & Carstensen, 2010), although in the circumstances, this might be taken as further evidence of their declining mental prowess. In what follows, we show how the slowing response speeds that are taken as evidence of “cognitive decline” in adults emerge naturally in learning models (Baayen et al, 2011) as knowledge increases. These models, which are supported by a wealth of psychological (Ramscar et al, 2010) and neuroscientific (Schultz, 2006) evidence, Findings from a range of psychometric tests suggest that the rates at which the mind processes information increase from infancy to young adulthood, and decline steadily thereafter (Salthouse, 2011). Increasing reaction times are a primary marker for age-related cognitive decline (Deary et al, 2010), and are even considered its cause (Salthouse, 1996), yet they are puzzling. Practice improves speed and performance on individual cognitive tasks at all ages (Dew & Giovanello, 2011). Since we get more practice using our cognitive capacities as we age, why does our performance on tests of them decline? The answer lies in the way that psychometric tests neglect learning, and its relationship to the statistical patterns characteristic of human life. Learning is a discriminative process that serves to locally reduce the information processing costs associated with various aspects of knowledge and skill (Rescorla & Wagner, 1972). However, age increases the range of knowledge and skills individuals possess, which increases the overall amount of information processed in their cognitive systems. This extra processing has a cost. Learning and the long tail of linguistic experience Statistically, the distribution of human experience is highly skewed: Much of our day-to-day life is fairly repetitive, involving a small repertoire of common occurrences, such as reading the newspaper and going to work. At the same time, we experience a far more diverse repertoire of infrequent or even unique occurrences (we rarely read the exact same newspaper twice). When data is distributed like this, comparisons of means are often meaningless. Consider the problem of remembering birthdays: We are reminded of the birthdays of family members on an annual basis, and this usually makes us expert at remembering them. However, as we move through