This is an element of Dick's novel that suggests that Deckard is more machine than human: in the first paragraph, the "merry" surge of electricity brings Deckard to life, perhaps alluding to the electricity that animates Frankenstein's monster. The machine bring life to the human -- a great irony begins the novel.
The mood organ really only appears in the first and last chapters, bracketing Deckard's experiences in the novel, and it seems to provided an integral mediator between Deckard and his wife.
As a symbol, then, the Penfield Mood Organ stands for that which takes the edge off of life, in a positive way to allow Deckard to function, but in a negative way that deadens his humanity.
No matter how many times I read Androids, I'm always tickled and disturbed at the relevance and poignancy of this device. In lives that are increasingly mediated by our technologies (including pharmaceuticals), are we becoming less lively than our own machines? What are the implications?