Sometimes we make visual perceptual mistakes. These lapses of judgement provide insight into how our perceptual processes function and which factors influence our interpretation of visual stimuli.
A false visual perception is called an optical illusion. One of the most thoroughly studied illusions is called the Müller-Lyer illusion, in which two lines of equal length appear unequal.
The differing line ends, for reasons that have been debated for decades, seem to trick the perceiver into perceiving one line as longer than it actually is and/or the other as shorter. Here is how the lines look:
The handout that corresponds to this class activity can be downloaded at this link:
Class research activity on the Müller-Lyer Illusion
One explanation of this illusion is that it is at least partially based on our experience with the angular lines created by the inside and outside corners of buildings. These angular lines suggest either a further away line or a closer one; if we interpret a line of the same length as being further away, we shall naturally perceive it as longer. The picture below illustrates this idea:
The lines in this photo are similar to the line with the arrowheads. According to one theory, we perceive this building corner as pointing towards us, so we interpret the vertical line as shorter than when it represents another corner that is receding into the distance.
If the corner we are viewing is receding from us or pointing away from us, as in the photo below, the lines created by the angles of the corner are like the V-shaped line. We interpret this line as being further away and therefore longer, according to this theory of why the illusion occurs.
Evidence that supports this theory includes studies of people in non-angular environments such as the Kalahari Desert. These desert dwellers displayed greatly reduced susceptibility to the Müller-Lyer illusion. In contrast to the people of “carpentered” societies, they simply were not deceived by the illusion. This suggests that cultural experiences in childhood influence the way in which we interpret certain visual stimuli and apply depth cues.