Fruit flies have been studied very thoroughly in this manner. A lot of their brains are hardwired to sensory inputs and motor control, but the interesting thing is that their thoughts aren’t just made up of the connections between nervous tissue; the connective tissue itself is also used to store and modulate information.

What they don’t have is a highly developed cerebral cortex; so while they can feel pain and anxiety and probably anger and frustration, they don’t have much ability to reflect on any of it after the fact.

Animals that can do that include cats, whales, elephants, octopuses, parrots, ravens and jumping spiders.