Modulation of human global/local perception by low spatial frequency filtering

We investigated the effect of low spatial frequency (SF) filtering on neural substrates underlying global and local processing in the peripheral vision by measuring hemodynamic responses with functional magnetic resonance imaging (fMRI). Subjects identified global or local shapes of compound letters that were either broadband in spatial-frequency spectrum or contrast balanced (CB) to removed low SFs and displayed randomly in the left or right visual fields. Attention to both broadband and CB global shapes generated stronger activation over the medial occipital cortex relative to local attention. Lateralized activations in association with global processing were observed over the right temporal-parietal junction for broadband stimuli whereas over the right fusiform gyrus for CB stimuli. Attention to CB local shapes resulted in activations in the medial frontal cortex, bilateral inferior frontal and superior temporal cortices. The results were discussed in terms of the competition between global and local information in determining brain activations in association with global/local processing of compound stimuli.