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Abtine Tavassoli, Ian van der Linde, Alan C. Bovik, Lawrence K. Cormack; Noise unveils spatial frequency and orientation selectivity during visual search. Journal of Vision 2006;6(6):450. doi: 10.1167/6.6.450.
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© ARVO (1962-2015); The Authors (2016-present)
Spatial frequency and orientation are features whose significance in visual selectivity is supported by physiological and psychophysical evidence. In this study, a fast classification image framework (Tavassoli et al., in press) distinguishing foveal and non-foveal search processes was employed to examine human observer strategies in 8 separate visual search experiments using Gabor targets (2 and 8cpd spatial frequencies at 0, 20, 70, and 90deg orientations, with 0deg corresponding to vertical bars). Eye movements were recorded during every trial as observers searched for one target randomly embedded in one tile of a grid of 1/f noise tiles. A variant of signal detection theory was then used to classify noise tiles. Estimates of spatial frequency and orientation tuning were obtained by spectral domain Gabor fitting using the noise spectra. Results showed that observers' saccade destination selection and final tile choice were based on available phase information for low but not high spatial frequency targets. Center/surround frequency responses were observed in which the frequency of the sought target was excited and nearby frequencies inhibited, suggesting a strategy different from an ideal observer. Discrepancies between the foveal and non-foveal orientation tuning revealed lower accuracy for peripheral vs. foveal targets. In at least one case, confusion between orthogonal orientations (only) occurred. Our data were consistent with earlier parafoveal studies and provided additional insight into observers' dynamic decision-making, highlighting different search strategies that predominate at different target frequencies.
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