Hallum LE, Landy MS, Heeger DJ. Human primary visual cortex (V1) is selective for second-order spatial frequency. J Neurophysiol 105: 2121-2131, 2011. First published February 23, 2011 doi:10.1152/jn.01007.2010.-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 firstorder 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 secondorder 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)MRIadaptation 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. functional magnetic resonance imaging; second-order vision; adaptation; surround suppression MODELS OF TEXTURE PERCEPTION typically consist of a linear spatial filter, rectification of filter outputs, and a second linear spatial filter [called filter-rectify-filter (FRF) models]. The term "1st-order" refers to modulations of luminance, such as the edge illustrated in Fig. 1A. The texture-defined boundary in Fig. 1B is an example "2nd-order" image modulation. The average luminance is identical on either side of the boundary, as is the average luminance contrast; the boundary is defined by a change in a local texture property (here, orientation). A linear filter will respond to differences in luminance across a boundary (Fig. 1A). However, a linear filter cannot respond to the texture-defined edge of Fig. 1B because there is no change in average luminance across the edge. A FRF model is commonly used to explain human sensitivity to such texture boundaries (Fig. 1B). Each 1st-stage filter is selective for one of the constituent ...