Motor imagery (MI) is described as the mental rehearsal of voluntary movements. We used wireless functional near-infrared spectroscopy (fNIRS) recorded over secondary motor areas during performance of MI and motor execution (ME) in 11 healthy subjects, who either executed or imagined two drawing tasks differing in shape and frequency, i.e. simple (circle, 0.2 Hz) and complex (curved shape, 0.333 Hz). At the group level, results showed that fNIRS is capable of discriminating between the task mode, i.e. MI vs. ME, and the task complexity, i.e. simple vs. complex. At the single-subject level, we observed inverse oxygenation responses, i.e. a decrease in Δ[O2Hb] and/or increase in Δ[HHb]. These inverse responses only occurred during MI tasks and were highly correlated, in the first place, with task mode, and secondly with task complexity. Inverse Δ[O2Hb] responses are likely to reflect individual differences in performance-related signals and may contribute to the commonly observed inter-subject variability in fNIRS measurements. As MI is now widely used as a mental task in neurorehabilitative applications, the resulting oxygenation pattern may be of use for future developments. For this programme to be successful it is crucial to determine the sources of inter-subject variability. Our study presents a first effort in this direction, indicating that MI-related inverse Δ[O2Hb] responses are correlated, first, with task mode and, secondly, with task complexity.