The effect of fixation point on the appearance of rectilinearity

Data from two experiments indicate that (a) within 3.5 deg visual angle of the fixation point, an eccentrically fixated line and dot appear rectilinear when they are objectively convex toward the fixation point; (b) beyond 3.5 deg, they appear rectilinear when they are objectively concave; (c) the latter effect is not an artifact of the use of stationary fixation points; and (d) the addition of an extraneous line to the stimulus line so as to form a 90-deg angle acts as if to reduce the apparent size of the angle, but does not alter fixation point effects. The data were interpreted as supporting (a) the hypothesis that the stimulus line was more sensitive to illusory distortion than the dot, and (b) the ID (increasing-decreasing) model of the visual field—an inner area of increasing concentric contraction (stimuli appear to lie nearer the fixation point to a degree which increases with distance from the fixation point) is bounded by an outer area of decreasing concentric contraction.