Analyses of rotational singularities in ferret visual cortex

Activity maps of spatial orientation, obtained by intrinsic optical imaging of the mammalian visual cortex, show the formation of pinwheel-like structures that rotate either clockwise or counterclockwise around zero dimensional points called singularities. Any research that is oriented towards exploring the formation and physiological role of these singularities during an experiment requires an automated tool that can rapidly identify the location of these singularities. In this work we have developed such a tool that looks for the existence of singularities for a certain radius at every pixel location in the angle map. Using data from eleven ferrets, the number of singularities identified was a function of the search radius (0.03325, 0.04665, 0.05985, 0.07315, 0.08645, 0.09975, 0.11305, and 0.12635 mm) and level of image smoothing used. For a given smoothing value, the number of singularities decreased with increasing radius. But the rate of decrease was greater with less smoothing. The more the original image was smoothed, the fewer singularities were identified at a given radius. The trade off between search radius and image smoothing can be partially attributed to spatial sampling resolution.