Real-world images are highly complex, and it is not yet understood how the human visual system processes this information in recognition tasks. Most current psychophysical models of discrimination assume that decisions are made on the basis of information directly accessible from the spatially-tuned mechanisms that mediate detection. Detection mechanisms are localized with respect to orientation and spatial frequency in the Fourier domain, and have been shown to process disparate components along each dimension independently. We demonstrate that in discrimination tasks with complex stimuli, disparate Fourier components are not generally processed independently. Both masking and configuration-dependent effects are found. The pattern of results suggests that mediating pathways are not always localized in Fourier space, but in some case integrate information across wide regions of the domain. The integrating mechanisms appear specialized to signal particular differences or transformations that apply to rigid objects. We present a quantitative model based both on primary Fourier components and on non-arbitrary combinations of these components, and we show how this model accounts for our current complex-discrimination results. Finally, we suggest how a class of concurrent-rating experiments can be used to further test this model and identify the nature of the integrating mechanisms.
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