To compare the space-filling and complexity of control and mutant SAC arbors, we computed fractal dimensions, Df, which provide a measure of how completely dendrites fill its area2,27,47,48. To calculate Df, we applied the box-counting method as implemented in the FracLac 2.5 plug-in for ImageJ software (http://rsb.info.nih.gov/ij/plugins/fraclac/FLHelp/Introduction.htm.; NIH). Confocal images of cultured mutant and control SACs were obtained at equivalent laser scanning parameters with a 60X oil immersion lens, and maximum projections and thresholded, binary images were processed using Image J. Box counts using a series of progressively smaller box sizes (d) were scanned in a region of interest covering the SAC arbor, and the number of boxes intersected by pixels [k(d)] were analyzed; this computes Df, which represents an inverse linear regression between log[k(d)] and log(d). Df ranges from 1.0 (straight line with a dimension of 1) to 2.0 (plane with a dimension of 2); a difference of 0.1 represents a doubling of complexity27.
