Perception of orientation of short lines: its dependence on line's length and intensity.

The threshold intensity necessary for correct identification of orientation of a briefly presented line was measured as a function of line's lenght and number of possible orientations (2 orientations differing by 90 degrees, 4 differing by 45 degrees, and 8 differing by 22.5 degrees). The thresholds were the same for 2, 4 and 8 orientations when the lines were longer than 20 min of arc. On the contrary, in the case of short lines, the correct report of 8 orientations required much more intensive stimulation than the report of 2 orientations. The discrimination of small differences in orientation of short lines involves additional processes to those for their detection and coarse perception of their orientation.