Right Hemisphere Dominance in Nonconscious Processing

Abstract

Here we examined hemispheric differences in conscious and nonconscious perception using a repetition priming paradigm. In experiment 1, participants judged the direction of a target arrow (either left- or right-pointing), which was preceded by a prime arrow in either the left visual field (LVF) or the right visual field (RVF). The congruency effect was assessed as the response time difference between the congruent condition (the prime and the target pointed to the same direction) and the incongruent condition (they pointed to opposite directions). The prime was either masked or unmasked. Participants reported unaware of the prime in the masked condition. We found a significant congruency effect when the prime was presented in the LVF/right hemisphere (RH) but not the RVF/left hemisphere (LH) in the masked (subliminal) condition. In contrast, in the unmasked (supraliminal) condition, the RVF prime had a stronger congruency effect than the LVF prime. In experiment 2, the same procedure was used; we manipulated the prime duration from 10, 20, 30, 40 to 50 ms. A backward mask was used in all trials. Subliminal conditions were those in which d-prime measures were not significantly above zero (10, 20, and 30ms conditions for LVF primes; 10 and 20ms conditions for RVF primes). An interaction between visual field and awareness was found: LVF primes but not RVF primes generated a congruency effect in the subliminal condition, whereas in the supraliminal condition, RVF primes had a bigger congruency effect than LVF primes. Taking together, our results revealed a LH/RH distinction in conscious/nonconscious perception of directional information. This result suggests a dominant role of the RH in nonconscious processing, and is consistent with Baynes and Gazzaniga’s (2000) argument that the RH has an advantage in shaping behavior with implicit information whereas the LH plays a greater role in expressing explicit knowledge.