Errors or the anticipation of a reward and then no reward increase attention and the ability to focus on the next trial. Errors cue up the forebrain and increase activity in the prefrontal cortex, leading to learning. The optimal ratio of errors to successful trials for optimal learning is the 85/15 rule: 85% of the time performing correctly and 15% of the time performing incorrectly. This level of difficulty optimizes motor skill learning, cognitive learning, and attentional systems. More than 15% demand excessive attention, and less than 15% demand insufficient attention.