Optical imaging of the layout of functional domains in area 17 and across the area 17/18 border in cat visual cortex.

Optical imaging based on intrinsic signals was used to investigate the functional architecture of cat area 17 and the border between areas 17 and 18. The visual stimuli were gratings of different spatial frequencies moving at different angles, in different directions and with different speeds. In area 17 the iso-orientation domains were usually organized in patches rather than as elongated bands. Patches with different orientation preferences were arranged radially forming 'pinwheels' around 'orientation centres'. The pinwheel density was approximately 1.7-fold higher than in area 18. To explore clustering according to direction of motion, stimuli having the same orientation but moving in opposite directions were used. These two stimuli yielded very similar activity maps giving no indication of robust directionality clustering. Using near infrared light we were able to simultaneously image ocular-dominance and iso-orientation domains. A quantitative assessment of the relative strengths of the two subsystems showed that in upper cortical layers clustering according to orientation preference was three-fold stronger than clustering according to ocular dominance. The functional organization of spatial frequency was also examined. When we compared the activated regions by stimuli having different spatial frequency and moving at different velocities we observed that neurons were clustered also in these respects. We also investigated the functional architecture at the area 17/18 border and found that orientation maps at both sides of the border were not independent of each other. The map of area 17 smoothly blended into that of area 18. Similarly, the preferred spatial frequency of the neurons changed gradually over a distance of approximately 0.8 mm at the region of the area 17/18 border.