"an alternative hypothesis within your framework could be that depression is a disorder where internal thoughts are overrepresented rather than external sensory experiences underrepresented."

I think both are happening. Basically internal thought processes increase to an unhealthy degree as a way of compensating for the decrease in sensory perception, which is the same thing as happens in a sensory deprivation tank.

I think the initial cause of the depression can be either not enough sensory stimulation or too much thinking, as happens when there is a traumatic or very stressful event. If the latter is the root cause then you may need CBT or meditation or something like that. But in both cases using my sensory jumpstart method would work to fix a bout of depression, although it will keep recurring until you change your lifestyle (sensory as root cause case) or come to terms with whatever is stressing you, or at least learn to avoid the negative thought triggers.