Task complexity relates to activation of cortical motor areas during uni- and bimanual performance: A functional NIRS study

Hand motor tasks are frequently used to assess impaired motor function in neurology and neurorehabilitation. Assessments can be varied by means of hand laterality, i.e. unimanual or bimanual performance, as well as by means of task complexity, i.e. different degrees ranging from simple to complex sequence tasks. The resulting functional activation in human primary motor cortex (M1) has been studied intensively by traditional neuroimaging methods.Previous studies using functional near-infrared spectroscopy (fNIRS) investigated simple hand motor tasks. However, it is unknown whether fNIRS can also detect changes in response to increasing task complexity. Our hypothesis was to show that fNIRS could detect activation changes in relation to task complexity in uni- and bimanual tasks.Sixteen healthy right-handed subjects performed five finger-tapping tasks: unimanual left and right, simple and complex tasks as well as bimanual complex tasks. We found significant differences in oxy-hemoglobin (O₂Hb) and deoxy-hemoglobin (HHb) concentration in the right hemisphere over M1. Largest O₂Hb concentration changes were found during complex (0.351 ± 0.051 μmol/l) and simple (0.275 ± 0.054 μmol/l) right hand tasks followed by bimanual (0.249 ± 0.047 μmol/l), complex (0.154 ± 0.034 μmol/l) and simple (0.110 ± 0.034 μmol/l) left hand tasks. Largest HHb concentration changes were found during bimanual (− 0.138 ± 0.006 μmol/l) tasks, followed by simple right hand (− 0.12 ± 0.016 μmol/l), complex left (− 0.0875 ± 0.007 μmol/l), complex right (− 0.0863 ± 0.005 μmol/l) and simple left (− 0.0674 ± 0.005 μmol/l) hand tasks.We report for the first time that fNIRS detects oxygenation changes in relation to task complexity during finger-tapping. The study aims to contribute to the establishment of fNIRS as a neuroimaging method to assess hand motor function in clinical settings where traditional neuroimaging methods cannot be applied.