Brain asymmetry in directing attention during dichotic listening test: An fNIRS study.

In a classical dichotic listening paradigm, besides auditory brain asymmetry, cognitive functions such as attention and conflict resolution play a major role. The aim of this study is to reveal the possible haemodynamic mechanisms of higher attentional performance in prefrontal cortex during dichotic listening test. Twenty-six healthy participants underwent a dichotic listening task in three sessions; non-forced attention, attention focused to right ear, and attention focused to left ear. In each session, haemodynamic activity of prefrontal brain area was recorded using functional near-infrared spectroscopy (fNIRS). Effects of focused attention and performance level of the task on oxy-, deoxy-, and total haemoglobin levels were investigated. Oxy- and total haemoglobin levels in right prefrontal regions during forced-right and forced-left sessions were significantly higher than levels of the non-forced session. This might be an indicator of inhibition and orienting attentional functions of right inferior frontal gyrus. High performers had significantly higher deoxyhaemoglobin levels in the forced-left session compared to the non-forced session, while low performers' deoxyhaemoglobin levels did not differ among these sessions. Observing this difference only in the forced-left session but not in the forced-right session might suggest conflict resolution in top-down and bottom-up processes during the forced-left session for right-handed participants.