Hemispheric Asymmetry in Nonconscious Processing

Hemispheric Asymmetry in Nonconscious Processing Jing Chen (jinghku@hku.hk) Janet Hui-wen Hsiao (jhsiao@hku.hk) Department of Psychology, University of Hong Kong 691 The Jockey Club Tower, Centennial Campus, Pokfulam Road, Hong Kong SAR Abstract Here we investigated whether hemispheric asymmetry effects can be observed in nonconscious processing with a basic- level animal categorization (cat/dog) task. We found a significant nonconscious congruency priming effect when the prime was presented in the right visual field/left hemisphere but not when it was presented in the left visual field/right hemisphere when the prime duration was only 10 ms; the left- lateralized congruency priming effect was consistent with the left hemisphere superiority in processing abstract category information reported in the literature (e.g., Marsolek, 1999). This result thus showed that nonconscious processing can go beyond the sensory level to influence hemispheric asymmetry in the processing of category information. In contrast, this hemispheric difference was not observed when the prime was presented for 50 ms (nonconscious) or 150 ms (conscious). This effect may be because 10 ms subliminal information was insufficient to allow inter-hemispheric transfer/processing, allowing the hemispheric difference to emerge. It also suggests that hemispheric asymmetry may be better observed at subliminal level. Keywords: subliminal priming; hemispheric asymmetry; attention; nonconscious processing; Introduction It has been well established in the literature that subliminal stimuli are able to elicit subsequent cognitive and behavioral influences (see Kouider & Dehaene, 2007 for a review). It remains unclear, however, how a subliminally presented stimulus is processed. Here we aim to examine whether a subliminally presented stimulus can induce hemispheric asymmetry effects. In the literature of hemispheric asymmetry in perception, hemispheric asymmetry effects typically did not emerge until a late perceptual stage (Fendrich & Gazzaniga, 1990; Hsiao., Cipollini., & Cottrell., accepted; Sergent, 1982). More specifically, hemispheric asymmetry effects were typically observed in tasks involving high-level perception such as face recognition (e.g., Keenan, Whitman, & Pepe, 1989), but not in tasks more relevant to early perceptual processes such as grating detection (e.g., Fendrich & Gazzaniga, 1990). Based on these findings, it has been argued that hemispheric asymmetry effects “must result from processing taking place beyond the sensory level (Sergent, 1982). Thus, examining hemispheric asymmetry effects in subliminal priming can help us understand whether nonconscious processing can go beyond early perceptual processes to induce hemispheric asymmetry effects. Nevertheless, in the literature on subliminal priming, whether hemispheric asymmetry effects can be observed in the processing of subliminal stimuli was rarely studied. Marzouki, Grainger, and Theeuwes (2007) examined subliminal priming effects in a letter/pseudo-letter judgment task. In their experiment, the target stimulus always appeared at the center, while the preceding 45-ms prime (the same letter as the target) could be in the left visual field (LVF) or the right visual field (RVF). The results showed that in the trials with a letter target, a robust priming effect was found, however, only when the prime was presented in the RVF/left hemisphere (LH), but not when it was presented in the LVF/right hemisphere (RH). While Marzouki et al. (2009) interpreted the results as an attentional bias in favor of the RVF, their results may suggest a hemispheric asymmetry effect in subliminal letter processing as well. However, it is also possible that the RVF priming effect Marzouki et al. (2007) observed was due to perception-response compatibility, as participants always used their right hand to respond to the letter targets. Thus, it remains unclear whether hemispheric asymmetry effects can be observed in subliminal priming. Using a supraliminal priming paradigm, Marsoleck (1999) found hemispheric asymmetry effects in processing category-related information. In Marsoleck (1999)’s study, participants were asked to name pictures of objects presented for 17 ms in either the LVF or the RVF. Before the target presentation, the same object or another object from the same basic category as the target object was presented centrally for 3 seconds as the prime. While a same-object priming effect was found for both LVF and RVF targets, the same-category different-object priming effect was only observed in the RVF/LH. This result suggested that the LH may be biased towards abstract- category representations whereas the RH is biased towards exemplar-specific representations. This hemispheric asymmetry in the representation of category knowledge was further supported by several follow-up studies using various stimuli (Laeng, Zarrinpar, & Kosslyn, 2003; Studer & Hubner, 2008). Thus, in the present study, we aimed to examine whether this hemispheric asymmetry in the representation of category knowledge can be induced with subliminal stimuli in a category judgment task (Marsolek, 1999), with perception-response compatibility controlled (cf. Marzouki