Whoa, that just started me thinking! So, squid use several times more energy than rats to propagate an equivalent signal. It's possible that if their neurons were more efficient, they'd be smarter--not because of greater neuronal efficiency per se, but as an indirect result of a longer lifespan. Sounds crazy, but follow me through here: a nervous system that requires so much energy must be part of what keeps their metabolism so high, and the higher your metabolism, the shorter your lifespan (sweeping generality, of course). So if their neurons were more efficient, they could cut down their metabolism, therefore live longer, therefore have more time to take advantage of all that brainpower by learning, and therefore be smarter. A perennial question of cephalopod biology is why such smart animals have such short lifespans--why have the capability to learn without the time to apply it? It would be pretty funny if the answer turns out to be circular--they shot themselves in the foot, essentially, by developing brains that cut down their lifespan.