Event-related potential evidence for a dual-locus model of global/local processing

Abstract

We investigated the perceptual time course of global/local processing using event-related potentials (ERPs). Participants discriminated the global or local level of hierarchical letters of different sizes and densities. Participants were faster to discriminate the local level of large/sparse letters and the global level of small/dense letters. This was mirrored in early ERP components: The N1/N2 had smaller peak amplitudes when participants made discriminations at the level that took precedence. Only global discriminations for large/sparse letters led to amplitude enhancement of the later P3 component, suggesting that additional attention-demanding processes are involved in discriminating the global level of these stimuli. Our findings suggest a dual-locus time course for global/local processing: (a) Level precedence occurs early in visual processing; (b) extra processing is required at a later stage, but only for global discriminations of large, sparse, stimuli, which may require additional attentional resources for active grouping.

Keywords:

• Global/local processing
• Event-related potentials
• Hierarchical stimuli
• Attention

Acknowledgments

K.A.D. was supported by the National Sciences and Engineering Research Council (NSERC) and the Michael Smith Foundation for Health Research (MSFHR). A.K. was supported by NSERC, the Social Sciences and Humanities Research Council, and the Canadian Institutes of Health Research. T.C.H. was supported by NSERC and MSFHR.

Notes

¹Thank you to L. Shalev for pointing out a second alternative interpretation of our results, that the P3 effect reflects the controlled spreading of attention across a large area. This is an interesting possibility for future research, which could be tested by comparing P3 modulation in response to our large/sparse stimuli to P3 modulation in response to new, large/dense, stimuli.