Neri P & Heeger DJ, Spatiotemporal mechanisms for detecting and identifying image features in human vision, Nature Neuroscience, 5:812-816, 2002.
Abstract: The human visual system constantly selects salient features in the environment for further attention, processing and identification. Models of feature detection often assume that salient features are selected on the basis of contrast energy (local variance in intensity in the visual stimulus. This hypothesis, however, has not been tested directly. We used psychophysical reverse correlation to study how humans detect and identify basic image features (bars and short line segments). Subjects detected a briefly-flashed 'target bar' that was embedded in 'noise bars' that randomly changed in intensity over space and time. By studying how the intensity of the noise bars affected performance, we were able to dissociate two processing stages: an early 'detection' stage, whereby only locations of high contrast energy in the image were selected, and an identification stage (~100 ms later) during which subjects used image intensity at selected locations to determine whether the target was bright or dark.