Flicker helps flutter: visual-tactile integration benefits tactile frequency perception even in the absence of visual awareness

Humans cannot consciously perceive visual flicker faster than comparatively low threshold frequencies. The frequency information of perceptually fused flicker might be lost during early processing stages or processed sufficiently to impact perception in a different modality. Here, we tested whether fused visual flicker nevertheless boosts the perception of congruent tactile flutter. To this aim, we measured tactile frequency discrimination performance under unisensory tactile and multisensory visuo-tactile conditions. Tactile and visuo-tactile test frequencies ranged from below to above participants’ critical flicker-fusion frequency (CFFF; measured separately). Thus, in about half of the visuo-tactile trials, participants perceived the congruent visual flicker accompanying the task-relevant tactile flutter as a steady light. Yet, tactile discrimination performance was improved in visuo-tactile compared to unisensory tactile trials for test frequencies below and above the CFFF. This bimodal advantage across frequencies reveals that visual flicker alternating too fast to be consciously perceived is still processed and that multisensory integration benefits perception even if stimulus information in one modality does not reach awareness.