Neural correlates of boundary extension during visual imagination

Participants typically remember seeing a greater expanse of a scene than was visible in a studied close-up (boundary extension, BE). Multivoxel pattern analysis (MVPA) was used to test the neural correlates of BE. For each participant, a classifier was trained using a whole-brain searchlight method to discriminate between close-up and wider-angle versions of 16 scenes during repeated perceptual exposures. Earlier in the experiment, each participant had studied either the close or the wide version of each scene and then, on cue, visually imagined it from memory. If a brain area reflects BE, then unlike classification during perception, visual images of close views should now be misclassified as wide (capturing false memory beyond the view) whereas visual images of wide views should be correctly classified. The classifier indeed revealed BE-consistent patterns during imagery in several high-level visual regions, especially in the posterior parietal cortex (cluster TFCE corrected for multiple comparisons). Importantly, this BE-consistent pattern did not reflect a brain-wide bias toward better classification of wider-angle views: (1) the pattern was constrained to visually-responsive regions (occipital, parietal, and inferior temporal); and (2) the pattern reversed (better classification of close views) in early visual cortex, suggesting a bias toward the object in these regions. Following the visual imagery task, participants were again shown their originally presented views and rated each one as closer or farther away than before (4-pt scale); rating analysis revealed the typically pattern: robust BE for close views and no directional error for the wide views, thus verifying BE with a common behavioral measure. We propose that our new method reflects active maintenance of boundary-extended scene representations in memory, and that it holds promise, not only for further exploration of BE, but as a general purpose tool for decoding false memory in the brain.