Outlined objects in a Mach pattern create illusions: further evidence of a frequency code for size

In two experiments, new illusions of size were created using stimuli composed of nine trees, otoscopes, pens, hourglasses, hands, or mailboxes. The size of the nine objects varied across space in accordance with a Mach pattern. At the small side of the stimuli where there were three objects of identical size, the object at the inflection point was seen as smallest; at the large side of the stimuli where there were also three objects of equal size, the object at the inflection point was seen as largest. In Exp. 3, isolated objects and objects at the inflection points of the Mach stimuli were compared. When the stimuli were oriented to place the judged objects at the inflection points near the isolated object, figures with both pens and otoscopes at the small side of the pattern were underestimated and the figures of pens and otoscopes at the large side of the pattern were overestimated. These illusions resemble Brigner and Kauffman's Mach lines illusion and appear to result from interaction among visual neurons which are encoding the size of complex objects by frequency of their response.