High-speed VSD imaging of visually evoked cortical waves: decomposition into intra- and intercortical wave motions.

In the pond turtle, Pseudemys scripta elegans, visually evoked cortical waves propagate at different velocities within the primary visual area compared with waves that pass into the secondary visual area. In an effort to separate intra- and intercortical wave motions, movies of visually evoked cortical waves recorded by high-speed voltage-sensitive dye (VSD) imaging were subjected to Karhunen-Loéve (KL) decomposition. This procedure decomposes the VSD movies into a series of basis images that capture different spatial patterns of coherent activity. Most of the energy of the compound wave motion (>95%) was captured by the three largest basis images, M(1,1), M(1,2), and M(2,1). Based on visual comparison with maps of wave front latency, KL basis image M(1,2) appears to capture the spread of depolarization within the primary visual area, whereas KL basis image M(2,1) appears to capture the spread of depolarization from the primary into the secondary visual area. The contribution of different basis images to the intra- and intercortical wave motions was tested by reconstructing the response using different combinations of KL basis images. Only KL basis images M(1,1) and M(1,2) were needed to reconstruct intracortical wave motion, while basis images M(1,1) and M(2,1) were needed to reconstruct intercortical wave motion. It was also found that the direction and speed of wave propagation could be deduced by visual inspection of the basis image projections on to the original data set. The relative advantage of KL decomposition for the analysis of complex wave motions captured by VSD imaging is discussed.