Evidence for spatial frequency and orientation labelled detectors in second-order visual processing

We compared the number of spatial frequency and orientation mechanisms (or labelled detectors) (Watson & Robson, 1981, VR, 21) underlying first- versus second-order processing. We measured discrimination at detection threshold for first- and second-order gratings in order to determine the smallest difference in spatial frequency (Exp. 1) and orientation (Exp. 2) that permits accurate discrimination. When the psychometric functions for detection and discrimination are statistically equivalent, the underlying mechanisms are considered to be different. In a 2 × 2 AFC paradigm, observers indicated whether the grating was presented in the first or second interval, and identified the spatial frequency or orientation of the grating. One of the authors and two inexperienced observers participated in the study. In agreement with previous findings, for first-order gratings between 0.1 and 22 cpd, we found evidence for seven labelled channels for spatial frequency discrimination at detection threshold. For second-order gratings, the number of channels is the same as for first-order gratings for spatial frequencies up to about 2cpd; however, there are fewer second-order channels at higher spatial frequencies. In contrast, the number of labelled channels for orientation is the same for first- and second-order gratings. In conclusion, our findings provide evidence for distinct spatial frequency and orientation labelled detectors in second-order visual processing. Second, relative to first-order, there are fewer second-order channels processing higher spatial frequencies. This is consistent with a filter-rectify-filter scheme for second-order in which the second stage of filtering is at lower spatial frequencies.