Songbirds, single-celled giants and even pea plants show signs of learning
Put a stuffed owl in a songbird's cage and the birds panic, then gradually calm down. Swap in a different owl and the alarm returns. That narrow, stimulus-specific fading is habituation, the simplest learning there is, and it turns up in nearly every animal species as well as a touchy plant and a single-celled giant.
Researchers split learning into two broad families. Non-associative learning changes how strongly an organism responds to one repeated stimulus. Habituation weakens the response; sensitisation amplifies it. Rub your arm for long enough and the warmth can become pain, as nerve signals are progressively boosted into a warning. Habituation happens faster when a stimulus repeats often, and it has been recorded in the touch-shy Mimosa pudica and in Stentor coeruleus, an unusually big single-celled organism. In humans it has been seen before birth, as early as 32 weeks into pregnancy.
Associative learning links two things together. In Ivan Pavlov's famous experiments, dogs naturally salivated at meat powder; after he repeatedly rang a bell first, the bell alone made them drool. Classical conditioning like this has been shown in honeybees, which extend their tongues on cue, and recently in garden peas. Operant conditioning instead shapes voluntary behaviour through consequences. Reinforcement makes a behaviour more likely, whether by adding something pleasant, such as a treat for a dog that sits, or removing something unpleasant. Punishment tends to teach avoidance of the punishment itself rather than of the unwanted behaviour, and timing matters: rewards work best immediately.
Why learn at all instead of relying on instinct? Evolutionary biologists argue it pays in environments that change, but neither too little nor too chaotically. In a perfectly stable world hard-wired behaviour suffices, and in a wholly random one past experience offers no guide.
Humans add further layers. Lev Vygotsky saw play as a child's first way of grasping rules, symbols and language. Active learning, where students check what they do and do not understand and control what they study, tends to produce stronger results than passive listening. Not all lessons help, though: discovering that an unpleasant event cannot be escaped can produce learned helplessness.
Source: Learning
