49355
document
Human primary visual cortex ( V1 ) is selective for second-order spatial frequency . 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 .
49356
Intervention_Other
FRF model
49357
Intervention_Other
functional ( f ) MRI-adaptation protocol
49358
Outcome_Other
second-order modulations