Abstract
Spatial frequency (SF) selection has long been recognized to play a role in global and local processing, though the nature of the relationship between SF processing and global/local perception is debated. Previous studies have shown that attention to relatively lower SFs facilitates global perception, and that attention to relatively higher SFs facilitates local perception. Here we recorded event-related brain potentials (ERPs) to investigate whether processing of low versus high SFs is modulated automatically during global and local perception, and to examine the time course of any such effects. Participants compared bilaterally presented hierarchical letter stimuli and attended to either the global or local levels. Irrelevant SF grating probes flashed at the center of the display 200 ms after the onset of the hierarchical letter stimuli could either be low or high in SF. It was found that ERPs elicited by the SF grating probes differed as a function of attended level (global versus local). ERPs elicited by low SF grating probes were more positive in the interval 196–236 ms during global than local attention, and this difference was greater over the right occipital scalp. In contrast, ERPs elicited by the high SF gratings were more positive in the interval 250–290 ms during local than global attention, and this difference was bilaterally distributed over the occipital scalp. These results indicate that directing attention to global versus local levels of a hierarchical display facilitates automatic perceptual processing of low versus high SFs, respectively, and this facilitation is not limited to the locations occupied by the hierarchical display. The relatively long latency of these attention-related ERP modulations suggests that initial (early) SF processing is not affected by attention to hierarchical level, lending support to theories positing a higher level mechanism to underlie the relationship between SF processing and global versus local perception.
Highlights
Previous investigations of the neural mechanisms underlying global versus local processing have established functional hemispheric differences in extrastriate, occipito-temporal, and posterior parietal regions, with right hemisphere regions demonstrating an advantage for processing global aspects of a stimulus, and left hemisphere regions demonstrating an advantage for processing local aspects
Similar to the P1, the P2 elicited by lower SFs (LSFs) probes (196–236 ms) was earlier than that elicited by higher SF (HSF) probes (250–290 ms)
In the present study participants attended to global or local levels of peripheral Navon stimuli while irrelevant Spatial frequency (SF) probes flashed in the center of the display that were either relatively LSF (0.7 cpd) or HSF (3 cpd)
Summary
Studies of global versus local processing have typically employed hierarchical “Navon” stimuli (Navon, 1977), in which a series of local objects (often letters, though studies have shown similar results using non-letter shapes, e.g., Kimchi and Merhav, 1991) are spatially arranged to form a global object (see Figures 1 and 2 for examples). Hemispheric differences in global/local processing were originally demonstrated in behavioral hemifield studies, showing that participants were facilitated (i.e., faster and more accurate) at identifying global targets in a Navon stimulus that was projected first to the right hemisphere (i.e., presented in the left visual field), and they were facilitated at identifying local targets in a Navon stimulus that was projected first to the left hemisphere (Martin, 1979). The specific brain regions implicated in global/local processing have varied across studies, and www.frontiersin.org
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