When you get into a deep depression, your body shuts down some. Your senses become weaker. For me, I notice everything sounding at a lower volume when I get depressed. It also happens when I dissociate. The strange visuals about your curtains can also be from this. Not only do your senses get dulled some, other parts of your brain get dulled too, like the part that processes the senses. Let me show you a picture. This is a PET scan of someone's brain when they are depressed, compared to someone who does not have depression