Biphasic attentional orienting triggered by invisible social signals

- Novel continuous flash suppression (CFS) masks render motion stimuli invisible.
- Invisible biological motion (BM) under CFS attracts spatial attention.
- Attentional orienting triggered by invisible BM shows a biphasic temporal profile.
- This dynamic attentional orienting is specific to invisible, upright BM cues.
- Objective tests of suppression effectiveness show BM under CFS is truly invisible.

Biological motion (BM) is one of the most important social cues for detecting conspecifics, prey, and predators. We show that unconscious BM processing can reflexively direct spatial attention, and that this effect has a biphasic temporal profile. Participants responded to probes that were preceded by intact or scrambled BM cues rendered invisible through continuous flash suppression. With a short inter-stimulus interval (ISI, 100 ms) between the invisible BM cues and the probe, responses to probes at the same location as the invisible, nonpredictive BM cue were faster than to probes at the location of the scrambled BM cue. With a longer ISI (800 ms) this effect reversed, with slower responses to probes at the location of the invisible, nonpredictive BM. These effects were absent when BM and its scrambled control were made visible with both short and long cue durations across variable length of ISIs, indicating that the saliency of BM itself cannot account for the dynamic orienting effects from invisible social cues. Moreover, this dynamic attentional shifts were specific to upright BM cues and not obtained for inverted stimuli. Thus, this reflexive and dynamic attentional modulation triggered by invisible BM, with initial facilitation followed by inhibition, demonstrates that in the complete absence of conscious awareness, cue predictiveness, and saliency differences, attentional systems promote exploration of our visual environment for social signals.