The sixth season of Criminal Minds premiered on CBS on September 22, 2010 and ended May 18, 2011. A.J. Cook appeared as a series regular for two episodes, and as a special guest star in two episodes, due to contract negotiations. Paget Brewster was a regular for eighteen episodes, but returned in the seventh season premiere. Rachel Nichols appeared as a guest star for two episodes and was then promoted as a regular, but departed after the season finale. The opening sequence was changed as of this season, more sound and visual effects were added as well as the theme song itself being amplified, an electric guitar playing being added.