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 intr insic signals to determine the distribution of cells responding to gra tings of various orientations moving in different directions in area 1 8 of cat visual cortex. To test directional-selective clustering of ne urons, we compared cortical activity maps obtained by stimulation with two gratings of identical orientation but moving in opposite directio ns. We found those maps to be almost identical, suggesting that neuron s are not notably clustered into directionality columns. We also compa red activity maps obtained with gratings of different orientations. Ea ch of the orientation maps was similar to the 2-deoxyglucose maps prev iously reported. Having compiled the information obtained from the dif ferent 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 ''pinwheel s'' around points that we refer to as ''orientation centers.'' We furt hermore show that the only locations at which orientation preference c hanges rapidly are these orientation centers and not lines or loops. I n addition, this report clarifies that our observations on the functio nal architecture of cat area 18, although at first sight at variance w ith 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.