PURPOSE: Visual scenes contain local and global properties. Numerous behavioral studies demonstrate that the global features are processed more rapidly than local features, namely global precedence effect. However, the neuronal temporal dynamics underlying the global and local processing as well as their interactions remains largely unknown.
METHODS: To address the issue, we recorded magnetoencephal-ography (MEG) from 20 human subjects as they viewed 5-sec long glass pattern stimuli, the luminance of which was randomly modulated continuously. Critically, the form coherence of glass pattern was also randomly and independently modulated simultaneously. Based on the two random sequences (luminance and form coherence) within one trial, we were able to calculate and separate the brain response that specifically tracks changes in local and global property from the same MEG responses, by employing a temporal response function technique (TRF).
RESULTS: The TRF for local and global property processing showed quite distinct spatiotemporal patterns. Specifically, local feature changes elicited activations in early visual area (EVA) around 100 msec. Global features, on the other hand, elicited much earlier responses (< 50 msec), which first developed in orbitofrontal cortex (OFC), then in V3A, V1, and TPJ. We further examined the interactions between global and local processing and found a two-stage course. In particular, within 100~200 msec, the OFC activations in global processing negatively correlated with EVA activities in local processing, suggesting a global-local competition relationship, whereas within 200-300 msec, OFC and TPJ responses in global processing and EVA responses in local processing showed a positive relationship, implicating a global-local integration process.
CONCLUSIONS: Our experiments, by employing a new technique, successfully enables the dissociation of global and local processing within the same trial response. Commensurate with previous findings, our results support that OFC initiates the coarse-to-?ne process in visual perception. The global and local process undergoes competition and then integration.