The relations between visual brightness discrimination and (a) stimulus size and (b) background brightness were measured with each of 2 variants of the psycho-physical method of constant stimuli. The 2 methods, phenomenal report and forced choice, differed only with respect to the response used to indicate discrimination. It was found that the functional relations studied depended significantly upon the psychophysical procedure employed. Evidence is presented indicating that measurements made with phenomenal report were invalidated by a response tendency designated response channelization. Since measurements with phenomenal report have been advocated by proponents of the neural quantum theory, the present data throw some doubt upon the validity of the evidence purported to support this theory.