title: Human primary visual cortex (V1) is selective for second-order spatial frequency.
abstract: A variety of cues can differentiate objects from their surrounds. These include "first-order" cues such as luminance modulations and "second-order" cues involving modulations of orientation and contrast. Human sensitivity to first-order modulations is well described by a computational model involving spatially localized filters that are selective for orientation and spatial frequency (SF). It is widely held that first-order modulations are represented by the firing rates of simple and complex cells ("first-order" neurons) in primary visual cortex (V1) that, likewise, have spatially localized receptive fields that are selective for orientation- and SF. Human sensitivity to second-order modulations is well described by a filter-rectify-filter (FRF) model, with first- and second-order filters selective for orientation and SF. However, little is known about how neuronal activity in visual cortex represents second-order modulations. We tested the FRF model by using an functional (f)MRI-adaptation protocol to characterize the selectivity of activity in visual cortex to second-order, orientation-defined gratings of two different SFs. fMRI responses throughout early visual cortex exhibited selective adaptation to these stimuli. The low-SF grating was a more effective adapter than the high-SF grating, incompatible with the FRF model. To explain the results, we extended the FRF model by incorporating normalization, yielding a filter-rectify-normalize-filter model, in which normalization enhances selectivity for second-order SF but only for low spatial frequencies. We conclude that neurons in human visual cortex are selective for second-order SF, that normalization (surround suppression) contributes to this selectivity, and that the selectivity in higher visual areas is simply fed forward from V1.
authors.affiliation: Department of Psychology and Center for Neural Science, New York University, 6 Washington Place, New York, NY 10003, USA. hallum@cns.nyu.edu
authors.forename: Luke E
authors.initials: LE
authors.lastname: Hallum
authors.forename: Michael S
authors.initials: MS
authors.lastname: Landy
authors.forename: David J
authors.initials: DJ
authors.lastname: Heeger
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology; D000328:Adult; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008875:Middle Aged; D009949:Orientation / Q000502:physiology*; D010775:Photic Stimulation / Q000379:methods*; D011930:Reaction Time / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D014793:Visual Cortex / Q000502:physiology*; D055815:Young Adult