Reflexes are automatic responses processed unconsciously in the central nervous system. Classical conditioning occurs when a previously neutral stimulus (the conditioned stimulus or CS) is paired with an unconditioned stimulus (UCS) that elicits an innate response, causing the CS to elicit a conditioned response over time through repeated associations formed in the brain between the CS and UCS. For optimal classical conditioning, there should be a short, half-second interval between the CS and UCS.