Simultaneous sampling and length contrast.

The illusion of perceived line length produced by the parallel lines configuration reverses from assimilation to contrast with large spatial separation of the contextual and test lines (Jordan & Schiano, 1986). In the present experiment, we tested whether length contrast is produced by the sequential sampling of contextual and test lines or by the simultaneous sampling of the stimuli from a spatially distributed attentive field. Scanning instructions were manipulated such that one group of observers was encouraged to sample the lines sequentially, and a third group was given no specific scanning instructions. The data indicated that the reversal to length contrast is produced by simultaneous sampling of contextual and test lines. Implications for attentional descriptions of visual illusions are discussed.