Mode shapes are similar on a glass substrate. All other in vitro data used a plastic substrate. To confirm that the higher level of image quality available using a glass substrate (e.g., greater detail regarding filopodia) did not affect our overall conclusions, we also performed time-lapse imaging of rat SCG axons growing in glass-bottomed dishes (no gradient, ten movies, 2,325 frames). (a) In this case there were seven significant mode shapes, but their form was similar to those observed on a plastic substrate (Figure 1). (b) Eigenshapes from the same dataset with artificially degraded image quality are very similar (in this case there were five significant modes). This is because all fine details (for instance relating to filopodia) occur with a fairly random distribution around the growth cone, and are thus smoothed out once the dataset of images is appropriately large. SCG, superior cervical ganglion.