Abstract
How are three-dimensional surface orientations represented in the visual cortex? We have examined an idea that these surface orientations are encoded by neurons with a variety of tilts in their binocular receptive field (RF) structure. To examine if neurons in the early visual areas are capable of encoding surface orientations, we have recorded from single neurons extracellularly in areas 17 and 18 of the cat using standard electrophysiological methods. Binocular RF structures are obtained using a binocular version of the reverse correlation technique. About 30% of binocularly-responsive neurons have RFs with statistically significant tilts from the frontoparallel plane. The degree of tilts is sufficient for representing the range of surface slants found in typical visual environments.We also investigated how tilted binocular receptive fields may be constructed. We proposed two models, an interocular spatial frequency difference model (dif-frequency model) and a pooling model. To test the dif-frequency model, we compared the actual slant of the receptive field with the tilt predicted from the spatial frequency tuning of the neuron in the left and right eyes. A subset of neurons exhibited responses consistent with the dif-frequency model. The pooling model was also partially consistent with some neurons, but inconsistent with others. Our results indicate that encoding of 3D surface orientation begins with binocular neurons in early visual cortex, and this selectivity has contributions from both spatial frequency differences and pooling. [J Physiol Sci. 2007;57 Suppl:S47]
