title: The temporal evolution of coarse location coding of objects: evidence for feedback.
abstract: Objects occupy space. How does the brain represent the spatial location of objects? Retinotopic early visual cortex has precise location information but can only segment simple objects. On the other hand, higher visual areas can resolve complex objects but only have coarse location information. Thus coarse location of complex objects might be represented by either (a) feedback from higher areas to early retinotopic areas or (b) coarse position encoding in higher areas. We tested these alternatives by presenting various kinds of first- (edge-defined) and second-order (texture) objects. We applied multivariate classifiers to the pattern of EEG amplitudes across the scalp at a range of time points to trace the temporal dynamics of coarse location representation. For edge-defined objects, peak classification performance was high and early and thus attributable to the retinotopic layout of early visual cortex. For texture objects, it was low and late. Crucially, despite these differences in peak performance and timing, training a classifier on one object and testing it on others revealed that the topography at peak performance was the same for both first- and second-order objects. That is, the same location information, encoded by early visual areas, was available for both edge-defined and texture objects at different time points. These results indicate that locations of complex objects such as textures, although not represented in the bottom-up sweep, are encoded later by neural patterns resembling the bottom-up ones. We conclude that feedback mechanisms play an important role in coarse location representation of complex objects.
authors.affiliation: University of Aberdeen.
authors.forename: Ramakrishna
authors.initials: R
authors.lastname: Chakravarthi
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Carlson
authors.forename: Julie
authors.initials: J
authors.lastname: Chaffin
authors.forename: Jeremy
authors.initials: J
authors.lastname: Turret
authors.forename: Rufin
authors.initials: R
authors.lastname: VanRullen
mesh_terms: D000328:Adult; D000465:Algorithms; D001931:Brain Mapping; D004569:Electroencephalography; D005074:Evoked Potentials, Visual / Q000502:physiology*; D025461:Feedback, Physiological / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010775:Photic Stimulation; D012160:Retina / Q000502:physiology*; D013028:Space Perception / Q000502:physiology; D060388:Support Vector Machine; D013997:Time Factors; D014793:Visual Cortex / Q000502:physiology*; D055815:Young Adult