I placed a Block of Redstone on top of a block of Stone. Then I placed a piston beneath the stone block and it was activated. When the Redstone Block is placed last, the piston is not activated. This even occurs with a block of air between them. It only activates the piston below it, but not pistons 1 block away on any other side. In the attachment is an activated piston with a Block of Redstone floating above.