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Objective Contrast sensitivity to visual stimulus shows an evident dependency on stimulus spatial frequency in many species of vertebrates.The neural basis remains unclear.Here, we investigated the role of primary visual cortex (V1: area 17) in spatial frequency-dependent contrast detection.Methods The perceptual contrast sensitivity functions (CSF) of three cats were measured using a two-alternative forced choice (2AFC) method.The responses of V1 neurons to their optimal visual stimuli in a range of luminance contrast levels (from 0 to 1) were recorded subsequently using in vivo extracelIular single-unit recording techniques.The contrast sensitivity of each neuron was determined.The neuronal CSF in each cat was constructed from the mean contrast sensitivity of neurons at different preferred spatial frequencies.Results (1) The perceptual and neuronal CSFs in all three cats exhibited a similar shape with a peak near 0.4 c/deg.(2) The neuronal CSF of each cat was highly correlated with its perceptual CSF.(3) V1 neurons responding preferentially to stimuli at different spatial frequencies had different contrast-gain.Conclusion (1) Visual contrast detection at different spatial frequencies may likely involve population coding of V1 neurons with different preferred spatial frequencies.(2) Differences in contrast-gain may underlie the observed contrast sensitivity variation of V1 neurons with different preferred spatial frequencies, resulting from long-term biological evolution or postnatal visual experience.