Target bottom-up strength determines the extent of attentional modulations on conscious perception.

A sufficient level of alerting, bottom-up stimulus strength, and attention have been proposed as important pre-requisites for conscious perception (Dehaene et al. in Trends Cogn Sci 10:204-211, 2006). The combination of different levels of each of these processes might differentially bias the access to consciousness, so that the impact of a specific factor on conscious access would depend on the levels of the other factors. To explore this possibility, we measured how the interaction between different levels of (phasic and tonic) alerting, stimulus bottom-up activation, and endogenous spatial attention, influences conscious perception. We observed that endogenous spatial attention affected conscious perception mainly when target bottom-up strength was low, by improving perceptual sensitivity and making the response criterion stricter. Attention-driven increases of perceptual sensitivity (without variations in response criterion) were also observed for higher levels of bottom-up strength, but only when tonic alerting decreased. Phasic alerting boosted perceptual sensitivity independently of target bottom-up strength, even though it differently affected response bias, yielding a more liberal response criterion when target bottom-up strength increases. These results suggest that a more exhaustive approach to the study of conscious perception should consider the interaction of the multiple factors that are susceptible to modulate perceptual consciousness, rather than studying their effects in isolation.