"A group of 12 monkeys was trained to discriminate between visual patterns under conditions of substantial separation of the stimuli and the responses. The treatment under study was insertion or removal of a panel which, when present, visually isolated the manipulanda from the corresponding stimuli. Of the six that showed evidence of learning, all learned better with the panel present. This supports the view that sampling gradients produce the spatial contiguity effect and has implications for design of primate test equipment." (PsycINFO Database Record (c) 2006 APA, all rights reserved)