cannot animate layer mask on a video layer

I realize this is a job for aftereffects however I would like to use photoshop CS5 Extended's video capabilities to make it happen. I have two video layers, and I want to use a layer mask on the top layer. I must change the layer mask every few frames, and I cannot figure a way out to animate it. I do not need photoshop to create tween frames, in fact I do not mind tweaking each frame but I cannot seem to add keyframes for a layer mask. So anytime I change the layer mask, it becomes the one layer mask for the entire video layer. It seems like if I can apply a layer mask to video, I would be able to change that layer mask frame to frame. The only peramaters I can change using keyframes are layer mask position and layer mask enable.However I do not want to change its position. I want it to stay where it is and I want to adjust it using the brush tool/black and white paint, frame to frame. Any guidance would be hugely appreciated. Some more details: I imported the 1920X1080 QTs into photoshop by choosing LAYER>VIDEO LAYERS>NEW VIDEO LAYER FROM FILE. There are two video files. Thanks in advance!