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Many models of spatial vision assume that early visual mechanisms analyze images with a bank of independent linear spatial filters that are tuned specifically to different ranges of spatial frequencies (sizes) and orientations. Individual filters sample the image locally at different scales [16,40,42] and the outputs of these filters are pooled in parallel.
Although the definition of a channel involves mutual independence in some respect, psychophysically defined channels show interactions across space, spatial frequency or orientation [7,12,37]. Orientation and spatial frequency domain interactions have been assumed to be restricted to channels having spatially overlapping receptive fields.
Recent work in physiology has shown that stimuli placed outside of the classically defined receptive field can modulate the firing patterns of isolated single-units [3,11,17,18,21]. Psychophysical channels are often associated with physiological receptive fields . Psychophysically, long-range interactions in the spatial domain have been found by several studies. The apparent contrast of small central regions can be reduced by remote lateral annuli containing random texture  or gratings [4,5,35,36].
In this chapter we will describe more recently discovered long range excitatory and inhibitory interactions that have been found psychophysically [30,31,32,33] and neurophysiologically . Possible mechanisms for these effects are reviewed and their possible role in establishing Gestalt effects outlined.