QuickSpot's gameplay is sweet and simple: you must quickly spot differences between two similar pictures displayed on the top and bottom of the DS. Think back to your days of perusing the "Hidden Pictures" in Highlights for Children as a kid while waiting at the doctor's office. It's like that, only way, way more fun.
There are two single player modes, Rapid Play and Focus Play. Rapid Play involves a series of pictures in which you must spot a single difference as quickly as you can to move on to the next picture. As you progress, levels get harder and the differences between pictures are more subtle. Different objectives are also thrown in as you progress, like having to rub the screen with the stylus to reveal the bottom image, or having to select the correct puzzle piece from the bottom that completes an image on the top screen.
Since a DS game wouldn't be complete without it, there's also a breakdown of your current "brain activity" levels at the end of each level, which seems like an unnecessary and hasty add-on. Fortunately, these can be skipped through and ignored easily enough.