Review: Christina Aguilera, ‘Bionic’: Aidin Vaziri | What has Christina Aguilera been doing since she released 2006’s retro-themed “Back to Basics”? Apart from raising her son, making a cameo in the Rolling Stones documentary “Shine a Light” and letting Lady Gaga steal away all her fans, it appears the 29-year-old has spent most of her time reading Pitchfork.com. Her fourth studio album is stacked with left-field indie collaborators. Shouty feminist punk-funk trio Le Tigre and raunchy rapper Peaches join her on the spastic “My Girls”; M.I.A. brings gritty digital funk oomph to “Elastic Love”; and Sia leads Aguilera on a breezy down-tempo path on a handful of songs, including “I Am” and “All I Need.” The odd thing is that Xtina, who if nothing else has always been a forceful vocalist, sounds totally lost in the mix. But she has been away too long to alienate her fans completely, and the rest of the album oddly hedges to familiar terrain, moving from glossy slow jams such as “Sex for Breakfast” (sample lyric: “Taste me/ And I’ll taste you”) to synth-heavy club bangers such as “Desnudate” (sample lyric: “Get naked/ Oh, oh, oh, oh”) to songs that would have sounded dated even if Madonna did them in 1991 (“Glam”). Ms. Gaga can sleep easy.