Neural modulation transfer function of the human visual system at various eccentricities.

We measured r.m.s. contrast sensitivity as a function of retinal illuminance at various spatial frequencies within 3-37 deg of eccentricity in the nasal visual field. In dim light contrast sensitivity increased in proportion to the square root of retinal illuminance obeying the DeVries-Rose law but in bright light contrast sensitivity was independent of luminance following Weber's law. Critical retinal illuminance (Ic) marking the transition between the laws was found to be independent of grating area but proportional to the spatial frequency squared at all eccentricities, in agreement with the Van Nes-Bouman law of foveal vision. In addition, the proportionality constant was found to be independent of eccentricity and similar to that of the fovea. According to our contrast detection model of human vision the modulation transfer function (PMTF) of the neural visual pathways squared is directly proportional to the critical retinal illuminance. On this basis our result means that PMTF is similar, i.e. equal to spatial frequency across the visual field, thus attenuating low spatial frequencies relatively more than high spatial frequencies. Hence, up to the spatial cut-off frequency determined by the lowest neural sampling density of each retinal location the neural modulation transfer function is independent of visual location.