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An intriguing mystery concerning spatial vision is that the perceived spatial property information of the same stimulus varies in different contexts. While previous findings are suggestive of an adaptable mental ruler by which the raw retinal signals are flexibly scaled according to the prevailing context, whether and how such a putative mental ruler is utilized by the visual brain remain largely unknown. We hypothesized that the putative mental ruler was represented by multiple differently-tuned spatial frequency (SF) channels. A finer division of the ruler, signaled by dominance of the high-SF channels, is translated to a wider-spread topography of neural activation via concentration of neuronal receptive fields, causing perceptual inflation. Combining psychophysics and fMRI, we found that modulation of SF channels caused a systematic distortion in perceived separation, a representative fundamental spatial property, and a global displacement of population receptive fields (pRF) in primary visual cortex. Computational modeling further showed that both the perceptual distortion and the global pRF displacement were functionally coupled with the magnitude of SF channel modulation. Our findings reveal, for the first time, an adjustable mental ruler that commonly governs the perception of spatial property information in different contexts, and suggest a cognitive scaling system based on SF channel reweighting and displacement of neuronal RFs.