Slower transitions between control states lead to reductions in cognitive flexibility over the lifespan

Older adulthood is associated with declines across a wide range of cognitive functions, limiting the ability to perform tasks necessary for independent living. Changes in cognitive flexibility are a promising candidate mechanism underlying age-related changes in cognition, but the causes of inflexibility in older adulthood remain elusive. Here we focus on a core feature of flexible cognition: the ability to rapidly transition between different cognitive states when required to do so by changes in the environment or our goals (e.g., moving between states of low vs. high attentional focus). We put forward a dynamical systems model proposing that aging-related cognitive inflexibility arises in part from slowing of transitions between distinct configurations of cognitive control, even when the task is held constant. To test this model, we had participants across the lifespan perform a cognitive task under different performance goals, which induced different control configurations. Using computational modeling, we were able to measure dynamic changes in control configurations to meet different performance goals. This allowed us to simultaneously test three potential sources of age-related decreases in cognitive flexibility: 1) diminished control capacity in environments that require more switching; 2) diminished range of control adjustments; and 3) slower transitions between control states. Of these, we found that age was only associated with transition speed. When given sufficient time to maintain a given goal, older adults were able to adjust control to a similar extent as younger adults; however, when goals changed more frequently, they were more likely to undershoot their target control configuration for that goal, consistent with predictions from our model for longer transition times. Our findings demonstrate that cognitive dynamics, rather than the overall reductions in cognitive ability, are critical for understanding the mechanisms through which cognitive inflexibility arises in older adulthood.