But:  Spatial and temporal frequency was varied in a visual stimulation protocol in order to investigate the effect of those parameters in the fMRI signal measured in the visual cortex.

Objectifs and Methods:  Three normal healthy volunteers participated in this study. The data were acquired with a 3T clinical scanner using 2mm³ isovoxel for the functional images and 1mm³.for the high-resolution anatomical images.

In order to weight the magno and parvocellular layer response, two different checkerboard stimuli were used. The high temporal frequency checkerboard flickered at 14Hz and had a spatial frequency of 50min of arc. The low temporal frequency checkerboard flickered at 4Hz and had a spatial frequency of 5min of arc. The contrast was 100% for all checkerboard stimuli.

Each presentation cycle consisted of one stimulation and one rest condition, 18 seconds each. The whole experiment lasted for eight cycles. The data were analyzed with the Brain Voyager software.

Results:  As expected, the visual cortex responded significantly to all checkerboard stimulations. The fMRI signal measured in the occipital pole area was greater for the 4Hz/5min of arc stimulation when compared with the 14Hz/50min of arc stimulation. On the other hand, the response in the anterior calcarine fissure was greater for the temporal/spatial frequency of 14Hz/50min of arc.

Discussion:  These results indicate that the anterior calcarine fissure responded particularly to visual stimuli related to the magnocellular system, which is mainly linked to the peripheral visual field. Likewise, the most posterior areas of the occipital cortex responded most importantly to the stimulation associated to the parvo­cellular system, which is located in a more central area of the visual field.

Conclusion:  Our method permitted to locate the cortical responses related to the parvo- and magnocellular systems.