ADAPTATION , AFTEREFFECT AND CONTRAST IN THE PERCEPTION OF TILTED LINES

An essential element in visual perception is one indicated by the terms edge, boundary, contour or line. Things are seen because they are delimited from the rest of the visual field and this delimitation depends on, or consists of, the formation of a visual line. A line has the function of enclosing, in addition to its function of simply delimiting, but we are here concerned with it in the latter function only. Conceived in this way a visual line, or any designated portion of a line, may be said to have two characteristics: its shape, straight, curved, or 'bent,' and its direction, vertical, horizontal, or oblique. It has already been demonstrated that the shape of a line is subject to what has been termed adaptation and negative after-effect with respect to rectilinearity. That is, a curved or bent line-segment changes during continuous perception in the direction of becoming straight, and thereafter an objectively straight line appears curved the opposite way. By analogy, the direction of a line might be expected to behave in the same manner with respect to the vertical and the horizontal axes. Experimental test of this possibility bears out the expectation; that is, a line seen as tilted somewhat from the vertical or the horizontal axis appears progressively less tilted during the course of perception, and a line objectively vertical or horizontal appears thereafter as tilted in the opposite direction. In short,