Meditation Modulates both Early and Late Affective Processing: An ERP Study

Abstract

Past studies have reported the positive effect of meditation on attention and affective processing (Lutz et al., 2008; Lee et al., 2012). However, neurophysiological studies have only shown that meditation could modulate late affective processing (Sobolewski et al., 2011) and early pre-attentive processing (Srinivasan et al., 2007), yet little is known about its relationship with early affective processing. To address this research gap, we studied two ERP components, EPN and LPP, in both meditators and novices, when emotionally pleasant, unpleasant and neutral pictures were viewed. These components were consistently reported in past studies for indexing early selective attention and late cognitive evaluation during perceptual processing of affective information (Schupp et al., 2004; Olofsson et al., 2008; Schupp et al., 2012). We predicted that meditation would induce 'positivity-bias' effects of larger EPN and LPP for pleasant than unpleasant stimuli, and this effect would be more pronounced in meditation experts than novices. Methods: EEG was recorded by electrode cap with 64-channel at extended 10-20 system in 22 male Chinese, including 10 meditation experts (mean age = 56.60±6.98) and 12 matched novices (mean age = 58.17±6.15) during an affective processing task. These groups did not differ significantly on any of the matching variables, but the Toronto Mindfulness Scale (TMS) scores (meditators mean TMS = 34.30±7.75; novices mean TMS = 22.17±10.57; t(20) = 3.01, p = .007). Each trial in the experimental task began with a white fixation cross (varying display of 0.5-1.5 sec), followed by a 3 sec display of a picture randomly selected from 60 full-coloured images from the International Affective Picture System (IAPS) (20 for each type of pleasant, unpleasant and neutral images), and finally rating of the picture (valence or arousal). The 60 images appeared once in each block and 4 blocks were performed. The recorded data were preprocessed offline using SPM8: bandpass filtered (0.530 Hz), corrected for eye-artefacts, re-referenced to the whole-scalp average, down-sampled, epoched (−200-720 ms post stimulus onset) and baseline corrected. Epochs by pleasant, neutral, and unpleasant stimuli were averaged by each channel for each participant. Results: Significant interaction of Group (meditators, novices) and Emotions (pleasant, neutral, unpleasant) was found for the valence (F(2, 40) = 7.920, p = .008) but not arousal ratings. Pleasant stimuli, relative to unpleasant ones, elicited larger EPN (200-280 ms; F(1, 20) = 31.081, p < .001) and larger frontal LPP (300-700 ms; F(1, 20) = 28.888, P < .001) across groups, suggesting the 'positivity-bias' effect. No significant difference on the strength of 'positivity-bias' effect was observed for meditators relative to the novices; instead, independent of the stimulus type, less pronounced EPN (F(1, 20) = 5.174, p = .034) was found in meditators. While for LPP, there was significant interaction effect of Group and Location (F(2, 40) = 4.339, p = .038), with larger difference between meditators (mean = 2.700±1.595µV) and novices (mean = 1.330±1.454µV) at the parietal site than at the central or frontal locations. Conclusions: Our findings demonstrated the relationship between meditation and affective processing in both stages. It supported the hypothesis that meditation practice, regardless of the experience, promotes perceptual bias towards positive affective cues in both early and late affective processing stages. However, strength of this positivity-bias effect was not influenced by the expertise in meditation. Instead, overall impacts across all types of stimuli were observed, with less pronounced EPN but enlarged parietal LPP. This finding suggests that meditation experiences modulated affective processing by lowering the stimulus-driven attention during early attention orientation stage, but increasing the conscious processing at the later stage of cognitive evaluation.