The layout of iso-orientation domains in area 18 of cat visual cortex: optical imaging reveals a pinwheel-like organization.

In this study we used optical imaging based on activity-dependent intrinsic signals to determine the distribution of cells responding to gratings of various orientations moving in different directions in area 18 of cat visual cortex. To test directional-selective clustering of neurons, we compared cortical activity maps obtained by stimulation with two gratings of identical orientation but moving in opposite directions. We found those maps to be almost identical, suggesting that neurons are not notably clustered into directionality columns. We also compared activity maps obtained with gratings of different orientations. Each of the orientation maps was similar to the 2-deoxyglucose maps previously reported. Having compiled the information obtained from the different orientations into one "orientation preference map," we found, in contrast to earlier reports, that iso-orientation domains are not elongated parallel bands but are small patches organized in "pinwheels" around points that we refer to as "orientation centers." We furthermore show that the only locations at which orientation preference changes rapidly are these orientation centers and not lines or loops. In addition, this report clarifies that our observations on the functional architecture of cat area 18, although at first sight at variance with earlier observations, are actually fully consistent with them. We therefore propose that in cat visual cortex pinwheel-like patterns of orientation preference form an irregular mosaic of modular units with an average density of 1.2 pinwheels per square millimeter.