Calculating direction maps from intrinsic signals revealed by optical imaging

Previous optical imaging studies used the vector-summation (VS) method for calculating direction and orientation preference maps. However, for direction maps it often resulted in direction vectors which showed a steep angle to that of orientation vectors violating the 'aperture rule'. The present report provides a simple procedure for calculating direction preference maps using the 'electro- physiologist's ear' approach. This approach takes into account the strongest directional response component (vector-maximum, VM) in each pixel of the optical image, reminiscent of how electro- physiologists determine direction preference by audio-monitoring of the firing rate of neurons. The major advantage of this method is that the orthogonal relationship between orientation and direction preference vectors is preserved and that for most image pixels direction preference can be faithfully described by a single vector parameter. Here we used the VM method for calculating direction and the VS method for calculating orientation preference maps and quantified their spatial relationship. The results showed that, typically, an iso-orientation domain contained a pair of patches that preferred opposite directions orthogonal to the orientation. Rate-of-change maps for direction revealed that virtually all direction discontinuity lines linked orientation centres. Close to orientation centres, direction discontinuity lines ran chiefly parallel with iso-orientation lines, whereas more remotely they had either parallel or perpendicular courses.