Effects of Visually Masked Numbers and Lines on Brain Activity

A frequently used paradigm to investigate unconscious processing of visual stimuli is masked priming. Traditionally, a direct task is compared to an indirect task: In the direct task, detection of a masked prime stimulus is typically found to be close to chance-level. In the indirect task, effects of the prime on reaction times to a subsequent target stimulus are analyzed (priming effects). Here, we focus on the indirect task and use event-related potentials (ERPs) to examine the effects of the masked primes on brain activity. Previous studies found that early deflections in the lateralized readiness potential (LRP), a measure of specific motor activation, reflect correct and incorrect response activations by the prime. This supports the idea of prime-induced covert motor activations that conflict with the responses required by the target. To test this, we performed two experiments using masked priming with numbers (Exp. 1) and simple line stimuli (Exp. 2). Additionally, in Exp. 1 prime contrast was varied to modulate visibility of the numbers. Preliminary results show that ERP onset latencies for congruent vs. incongruent trials measured at electrode Cz (Exp. 1: normal contrast: M=12 ms, p<.001 / high contrast: M=21 ms, p<.001; Exp. 2: M=51 ms, p<.001) closely matched the behavioral priming effects (Exp. 1: normal contrast: M=9 ms, p<.001 / high contrast: M=21 ms, p=.003; Exp. 2: M=46 ms, p=.001). In addition, we found more negative deflections in the early LRP phase for incongruent as compared to congruent trials in Exp. 2 (albeit not in Exp. 1). Overall, the results support the idea of prime-induced pre-activated motor responses. We discuss differences between the experiments and to which extend the behavioral results are related to the ERP effects.