Experimental psychology: event timing turns punishment to reward.

Can relief from pain be a pleasure? If so, noxious events should--despite their typically aversive effects--also have a 'rewarding' after-effect. Through training fruitflies by using an electric shock paired with an odour, we show here that the shock can condition either avoidance of this odour or approach to it. These opposing behaviours depend on the relative timing of the shock and odour presentations during training, and indicate that a shock can act as either an aversive reinforcer or an appetitive one.