Task-Evoked Metabolic Balance and Intrinsic Network Organization Relate to Processing Speed

Abstract Processing speed is a core component of cognition that declines with aging and is impaired across neurological and psychiatric disorders. We combined functional proton magnetic resonance spectroscopy at 7 T with resting-state functional MRI to examine neurochemical and functional correlates of processing speed in 26 young and eight older adults. During a symbol–digit matching task, lactate showed a significant age-by-condition interaction, with a larger stimulation-related increase in older adults. Glutamate showed no significant group-level modulation, although its time course suggested a delayed response in older adults. The glutamate-to-lactate ratio decreased during stimulation in older adults, with an apparent rise during subsequent control periods, suggesting a possible temporal offset between the metabolite responses. Across participants, larger task-evoked lactate changes were associated with slower processing, whereas glutamate responses were heterogeneous and nonsignificant at the group level. Older adults also showed lower intrinsic functional coherence in the dorsal medial anterior cingulate cortex and executive and attention networks. Exploratory analyses linked greater coherence to better processing speed and smaller lactate responses. Together, these findings link individual differences in processing speed to both task-evoked neuroenergetic responses and intrinsic functional organization. The greater lactate recruitment observed with aging and poorer cognitive performance may reflect increased reliance on non-oxidative metabolism or compensatory metabolic engagement to meet cognitive demands. These results highlight fMRS as a complementary approach for characterizing the metabolic mechanisms underlying individual differences in cognition and cognitive aging.