The role of gist in scene recognition.

Studies of change blindness suggest that we bring only a few attended features of a scene, plus a gist, from one visual fixation to the next. We examine the role of gist by substituting an original image with a second image in which a substitution of one object changes the gist, compared with a third image in which a substitution of that object does not change the gist. Small perceptual changes that affect gist were more rapidly detected than perceptual changes that do not affect gist. When the images were scrambled to remove meaning, this difference disappeared for seven of the nine sets, indicating that gist and not image features dominated the result. In a final experiment a natural image was masked with an 8x8 checker pattern, and progressively substituted by squares of a new natural image of the same gist. Spatial jitter prevented fixation on the same square for the sequence of 12 changes. Observers detected a change in an average of 2.1 out of 7 sequences, indicating strong change blindness for images of the same gist but completely different local features. We conclude that gist is automatically encoded, separately from specific features.

The induced Roelofs effect: two visual systems or the shift of a single reference frame?

Cognitive judgments about an object's location are distorted by the presence of a large frame offset left or right of an observer's midline. Sensorimotor responses, however, seem immune to this induced Roelofs illusion, with observers able to accurately point to the target's location. These findings have traditionally been used as evidence for a dissociation of the visual processing required for cognitive judgments and sensorimotor responses. However, a recent alternative hypothesis suggests that the behavioral dissociation is expected if the visual system uses a single frame of reference whose origin (the apparent midline) is biased toward the offset frame. The two theories make qualitatively distinct predictions in a paradigm in which observers are asked to indicate the direction symmetrically opposite the target's position. The collaborative findings of two laboratories clearly support the biased-midline hypothesis.