Outline: Spatial Frequency Channels



Fourier (spatial frequency) analysis of images
Image:  intensity as a function of position
Spatial frequency representation
Sine wave gratings, defined by:
spatial frequency (cycles/degree)
orientation
phase
amplitude = contrast
Weber contrast:  ΔI / I
Michelson contrast:  (Imax-Imin)/(Imax+Imin) = (Imax-Imin)/(2 Imean)

Evidence for the visual system as a Fourier analyzer
Visual contrast sensitivity function (CSF) as a behavioral analogue of the modulation transfer function (MTF)
Sweep frequency grating as a quick demonstration of the CSF
Acuity (20/20 = 1 minute at 20 feet) is a test of the high frequency cutoff only
Other eye charts (Pelli-Robson, etc.)
Multiple channels hypothesis: early vision as a Fourier analyzer
Summation
Masking (Lincoln portrait)
Adaptation
Notch in CSF post-adaptation
Size aftereffect
Tilt aftereffect
Channels as cells: low frequency loss = lateral inhibition

However:
Visual system is not shift-invariant (changes in the periphery)
Visual system is not isotropic (oblique effect, hence CSF is a poor summary)
Visual system's contrast sensitivity is not constant with light level (and hence is nonlinear)
Bandpass at photopic levels
Lowpass at low light levels