title: What is second-order vision for? Discriminating illumination versus material changes.
abstract: The human visual system is sensitive to second-order modulations of the local contrast (CM) or amplitude (AM) of a carrier signal. Second-order cues are detected independently of first-order luminance signals; however, it is not clear why vision should benefit from second-order sensitivity. Analysis of the first- and second-order contents of natural images suggests that these cues tend to occur together, but their phase relationship varies. We have shown that in-phase combinations of LM and AM are perceived as a shaded corrugated surface whereas the anti-phase combination can be seen as corrugated when presented alone or as a flat material change when presented in a plaid containing the in-phase cue. We now extend these findings using new stimulus types and a novel haptic matching task. We also introduce a computational model based on initially separate first- and second-order channels that are combined within orientation and subsequently across orientation to produce a shading signal. Contrast gain control allows the LM + AM cue to suppress responses to the LM - AM when presented in a plaid. Thus, the model sees LM - AM as flat in these circumstances. We conclude that second-order vision plays a key role in disambiguating the origin of luminance changes within an image.
authors.affiliation: School of Psychology, University of Birmingham, Birmingham, UK. a.j.schofield@bham.ac.uk
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Schofield
authors.forename: Paul B
authors.initials: PB
authors.lastname: Rock
authors.forename: Peng
authors.initials: P
authors.lastname: Sun
authors.forename: Xiaoyue
authors.initials: X
authors.lastname: Jiang
authors.forename: Mark A
authors.initials: MA
authors.lastname: Georgeson
mesh_terms: D015350:Contrast Sensitivity / Q000502:physiology*; D003463:Cues; D003867:Depth Perception / Q000502:physiology*; D004192:Discrimination, Psychological / Q000502:physiology; D006801:Humans; D008029:Lighting; D008959:Models, Neurological; D009949:Orientation / Q000502:physiology; D010775:Photic Stimulation / Q000379:methods; D013499:Surface Properties