Neural correlates of a standardized version of the trail making test in young and elderly adults: a functional near-infrared spectroscopy study

The trail making test (TMT) is a widely applied diagnostic tool measuring executive functioning in order to discriminate between healthy and pathological aging processes. However, due to its paper-and-pencil nature it is difficult to adapt for functional brain imaging. Related neural underpinnings even in healthy aging are mostly unknown since no consistent administration for imaging is available. In this study a standardized implementation of the TMT for functional near-infrared spectroscopy (fNIRS) is proposed to investigate associated frontal cortex activation in healthy young (mean age 25.7 ± 3.02 years) and elderly adults (mean age 70.95 ± 3.55 years). The TMT consisted of a number condition (TMT-A), an alternating number and letter condition (TMT-B) as well as a control task. Behavioral results demonstrated that elderly participants performed slower but committed a similar number of errors compared to younger adults. The fNIRS results showed that particularly the TMT-B provoked bilateral activation in the ventro- and dorsolateral prefrontal cortex (vlPFC and dlPFC) as well as in premotor regions. Elderly participants displayed more significantly activated channels and a different activation pattern compared to younger participants especially manifesting in more bilateral dlPFC activation. In line with the hemispheric asymmetry reduction in elderly adults (HAROLD) model, the results were interpreted as an additional need for cognitive control resources in elderly participants. This study succeeded in implementing an appropriate version of the TMT for fNIRS and helps elucidating neural aging effects associated with this task.