Operant learning is a process by which a specific behavioural response (such as a lever press) is associated with a biologically salient stimulus known as a reinforcer (such as food). Key neurons in ...

Millions have seen the heart-wrenching video of a polar bear clinging to life, its white hair limply covering its thin, bony frame. Shot by Paul Nicklen and Cristina Mittermeier of the nonprofit group ...