Abstract Given a pattern consisting of x 1, x 2,…, x n similar elements and y 1, which is perceived as different from the former, it is more plausible to assert that y is not x rather than to assert that x is not y ( Wason, J. verb. Learn verb. Behav. 4, 7–11, 1965). In order to appreciate such a difference, the entire set has to be considered. Right-hemisphere brain-damaged patients were submitted to a series of visually presented patterns, each pattern consisting of seven similar items and one dissimilar item. Their task was to complete a statement referring to a single element of each pattern. Statements were either simple affirmative or negative sentences. Errors and reaction times were recorded. Patients with a right-hemisphere injury were found to be insensitive to the plausible-implausible dimension in completing negative sentences. It is hypothesized that right-hemisphere brain-damaged patients are less adequate in this task because they are less capable in putting each element into the visual context.