Role of subthreshold stimuli in task-performance and its underlying mechanism

Abstract

Considerable evidence exists indicating that a stimulus which is subthreshold and thus consciously invisible, influences brain activity and behavioral performance. However, it is not clear how subthreshold stimuli are processed in the brain. We found that a task-irrelevant subthreshold coherent motion leads to stronger disturbance in task performance than suprathreshold motion. With the subthreshold motion, fMRI activity in the visual cortex was higher, but activity in the dorsolateral prefrontal cortex (DLPFC) was lower, than with suprathreshold motion. The results of the present study demonstrate two important points. First, a weak task-irrelevant stimulus feature which is below but near the perceptual threshold more strongly activates visual area (MT+) which is highly related to the stimulus feature and more greatly disrupts task performance. This contradicts the general view that irrelevant signals that are stronger in stimulus properties more greatly influence the brain and performance and that the influence of a subthreshold stimulus is smaller than that of suprathreshold stimuli. Second, the results may reveal important bidirectional interactions between a cognitive controlling system and the visual system. LPFC, which has been suggested to provide inhibitory control on task-irrelevant signals, may have a higher detection threshold for incoming signals than the visual cortex. Task-irrelevant signals around the threshold level may be sufficiently strong to be processed in the visual system but not strong enough for LPFC to “notice” and, therefore, to provide effective inhibitory control on the signals. In this case, such signals may remain uninhibited, take more resources for a task-irrelevant distractor, and leave fewer resources for a given task, and disrupt task performance more than a suprathreshold signal. On the other hand, suprathreshold coherent motion may be “noticed”, given successful inhibitory control by LPFC, and leave more resources for a task. This mechanism may underlie the present paradoxical finding that subthreshold task-irrelevant stimuli activate the visual area strongly and disrupt task performance more and could also be one of the reasons why subthreshold stimuli tend to lead to relatively robust effects.