Müller-Lyer illusion

Around the turn of the 20th century, W. H. R. Rivers noted that indigenous people of the Australian Murray Island were less susceptible to the Müller-Lyer illusion than were Europeans.

[6] In 1963, Segall, Campbell and Herskovitz compared susceptibility to four different visual illusions in three population samples of Caucasians, twelve of Africans, and one from the Philippines.

[7] In 1965, following a debate between Donald T. Campbell and Melville J. Herskovits on whether culture can influence such basic aspects of perception such as the length of a line, they suggested that their student Marshall Segall investigate the problem.

They wrote that "European and American city dwellers have a much higher percentage of rectangularity in their environments than non-Europeans and so are more susceptible to that illusion.

Subsequent work by Jahoda suggested that retinal pigmentation may have a role in the differing perceptions on this illusion,[9] and this was verified later by Pollack (1970).

However, in a recent report[15] Catherine Howe and Dale Purves contradicted Gregory's explanation: Although Gregory's intuition about the empirical significance of the Müller-Lyer stimulus points in the right general direction (i.e., an explanation based on past experience with the sources of such stimuli), convex and concave corners contribute little if anything to the Müller-Lyer effect.Neural nets in the visual system of human beings learn how to make a very efficient interpretation of 3D scenes.

In the Müller-Lyer illusion, the visual system would in this explanation detect the depth cues, which are usually associated with 3D scenes, and incorrectly decide it is a 3D drawing.

[16] Morgan et al., suggest that the visual procedure of centroid extraction is causally related to a spatial pooling of the positional signals evoked by the neighboring object parts.

[17] Though the integration coarsens the positional acuity, such pooling seems to be quite biologically substantiated since it allows fast and reliable assessment of the location of the visual object as whole, irrespective of its size, the shape complexity, and illumination conditions.

Two sets of arrows that exhibit the Müller-Lyer optical illusion. The set on the bottom represents the classic Müller-Lyer stimulus, while on the top is its Brentano modification. All the shafts of the arrows are of the same physical length.
The Müller-Lyer effect in a non-illusion
The classic Müller-Lyer figures (A) and three modifications (having no shaft line) of the Brentano versions of illusory figures comprising different contextual flanks: separate dots (B), the Müller-Lyer wings (C), and arcs of a circle (D, distances between the points seem to be different)
A dynamic visual demonstration by Italian researcher Gianni A. Sarcone : the blue and black segments of the star are equal in length and always the same length, though they appear to alternately stretch and shrink.
This variant of dynamic Müller-Lyer illusion by Italian researcher Gianni A. Sarcone shows that though the collinear blue and red segments seem to oscillate up and down, they are always the same length. Nothing moves except the arrows at the endpoints of each color segments. This visual illusion also involves a dynamic " neon color spreading " effect.
Brentano figure with the rotating Müller-Lyer wings (distractors); actually, apices of the wings (stimulus terminators) are aligned and spaced equidistantly