Spatial frequency of visual image modulates neural responses in the temporo-occipital lobe. An investigation with event-related fMRI.

Our visual environment consists of information ranging from coarse to fine patterns with respect to spatial frequency (SF). Neurophysiological studies using experimental animals have shown that there exist specific SF channels in striate and extrastriate visual cortices. In the present study, we used event-related functional magnetic resonance imaging (fMRI) and healthy subjects to investigate whether manipulation of the SF of visual images modulates neural responses in the temporo-occipital lobes. Subjects were scanned while performing "one-back task" with high-pass or low-pass filtered images of a face and house. We demonstrated that visual attention to the stimuli with high SF more specifically involves cortical activation in the left fusiform gyrus and inferior occipital gyrus as compared to that with low SF. High-SF specificity in the left fusiform gyrus was confirmed by voxel-by-voxel comparison of original images with left-right flipped images. There was no low-SF region in the right hemisphere; however, processing of low-SF images may be category-specific in face- and house-related regions. These results may shed light on the neural correlates of behavioral evidence that high-SF stimuli are handled faster and more accurately when presented to the right visual hemifield than to the left counterpart. The present results were also discussed in a viewpoint of local/global processing and functional asymmetry of cerebral hemispheres.