Vertical-horizontal illusion

The vertical–horizontal illusion is the tendency for observers to overestimate the length of a vertical line relative to a horizontal line of the same length. This involves a bisecting component that causes the bisecting line to appear longer than the line that is bisected. People often overestimate or underestimate the length of the bisecting line relative to the bisected line of the same length. This even happens if people are aware that the lines are of the same length.

Cross-cultural differences in susceptibility to the vertical–horizontal illusion have been noted. People from Western cultures and people living in urban landscapes show more susceptibility than those living in eastern or open landscapes.

There are several different configurations of the vertical–horizontal illusion. The three configurations which seem to produce the highest illusion magnitude are the L configuration, the plus (+) configuration, and the inverted-T configuration. Of these three, the inverted-T configuration produces the highest illusion magnitude. When the bisecting line of the T illusion is configured horizontally, the illusion magnitude is lowered. However, when the bisecting line of the T illusion is configured vertically, the illusion magnitude is higher.

A gradual decrease in error of vertical–horizontal illusions occur as participants age increases from eight to fourteen years. Gough and Meschieri attribute this decrease in error to the child's improved ability to detect and de-center their attention in a visual display, i.e. position their body differently to gain other pererspectives. Children who showed greater personal independence, verbal articulation, and visual scanning ability were more effective and resourceful in their ability to gauge vertical–horizontal illusions.

Cross-cultural differences in susceptibility to the vertical–horizontal illusion have been noted in several studies. People living in developed urban cities show greater susceptibility than people living in rural areas. An explanation could be that those in rural areas are more accustomed to living in round houses on flat plains, or scrubland. Rural inhabitants have more exposure to distance and living on plains than people living in highly developed, commercialized cultures.

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