In a recent study, NYU researchers flashed a low-contrast image of striped circles rotated to different angles in front of participants.  The duration each participant was exposed to the image was a mere 17 milliseconds, which is around 1/60 of a second.  Despite participants in the study generally reported not seeing the image at all, they were able to effectively compare it to later images of other circles.  This suggests that even apparently invisible information may briefly be maintained within higher processing stages of visual perception.

See  Neuron journal and CBC.