- Duplicate your Background layer, name it "Contrast mask", this new layer should be above any adjustment layers.
- With the Contrast Mask layer selected go to, Image / Adjust / Desaturate. The layer will now become monochrome, as will everything below it (for the moment).
- Go to, Image / Adjust / Invert. This turns the Contrast Mask later into a negative.
- Double-Click on the Contrast Mask layer on the Layers Palette and select Layer Options. Select Overlay blend mode.
- Go to, Filter / Blur / Gaussian Blur and set a value of between 10 and 250. This will enable you to fine tune the effect on your final image.
Before contrast mask
After contrast mask
See finished image here.