A top line decreased the visibility of a simultaneous, identical, distant bottom line. This context-produced decrease in visibility (DV) occurred when the bottom line was masked by flanking lines and hence was less visible than the top line. It continued when the top line was three times as far from the bottom line. It disappeared without the mask. There was a hint of an opposing contextproduced increase in visibility (IV) when the lines were close together. The DVis not accounted for by numerous extant phenomena and theories. It means that the top line decreased the similarity in visibility between it and the bottom line, a contrast effect for visibility, rather than for a typical attribute. Contrast does not occur between two attributes that are perceptually equal. Therefore, the reason why two distant equally visible objects fail to result in a DVmay be that their equal visibility precludes the occurrence of contrast. This Dv-as-contrast theory is consistent with evidence that two groups (phenomenal wholes) are associated with both contrast and DVs, and thus also with evidence that one group is associated with both assimilation and IVs.393

Subjects performed a series of forced-choice discriminations to determine whether both group-assimilation and group-visibility associations could be obtained from nearly identical strong and weak group patterns. The discrimination between the context+target square and the context [symbol: see text] was better than between the target- and background, as was the case for --, whose context and target components were its left and right halves, but not for [symbol: see text]. Square and -- produced a better performance when their lines (halves) were the same in color, and a poorer performance when their lines were different in color, but [symbol: see text] produced the reverse. Likewise, only square and -- produced a better performance when closed, and a poorer performance when open. These context+target etc., same-different, and closure results argue that square and -- produced a greater increase in visibility of their component --, more assimilation among their parts, and a stronger group than did [symbol: see text]. This evidence of a group-assimilation-visibility association cannot be attributed to the fortuitous occurrence of an increase in visibility with one object, assimilation with a second, and closure with a third, unlike previous evidence. This association cannot be explained by feature-based theories. Therefore, a superordinate unit is the cause of this association.