Adaptation of stimulation duration to enhance auditory response in fNIRS block design.

Functional near-infrared spectroscopy (fNIRS) is an increasingly popular method in hearing research. However, few studies have considered efficient stimulation parameters for the auditory experimental design of fNIRS. The objectives of our study are (1) to identify the most effective paradigm for the stimulation blocks with increasing duration (8s, 10s, 15s, 20s) in terms of response amplitude, i.e., the most-efficient stimulation duration; (2) to assess the linearity/nonlinearity of the hemodynamic responses with respect to increasing block durations; and (3) to generalize results to more ecological environmental stimuli. We found that cortical activity is augmented following the increments in stimulation durations and reaches a plateau after about 15s of stimulation. The linearity analysis showed that this augmentation due to stimulation duration is not linear in the auditory cortex, non-linearity being more pronounced for shorter durations (15s and 20s). The 15s block duration that we propose as the most suitable precludes signal saturation and is associated with a high response amplitude and a relatively short total experimental duration. Moreover, the distribution of stimuli among the 15s blocks, which is the most effective for white noise stimulation, also provides a comparably strong response for environmental sounds. The sum of these findings suggests that 15s stimulation duration used in the appropriate experimental setup allows researchers to acquire optimal fNIRS signal quality.