A visual illusion

A brainteaser for you this week: take a look at the two shapes below. Which looks bigger, A or B?



It’s obvious isn’t it, B is clearly the longer shape.

What if I were to tell you that both of these shapes are exactly the same size? This illusion is know as the Wundt Jastrow illusion, in which two identical segments of a circle appear to be different lengths when aligned by their shortest edge.


How does this illusion occur?

In short – nobody knows! Although the exact causes are unknown, one explanation is that our brain is fooled as it automatically compares the lower line of Shape A with the upper line of Shape B, therefore judges Shape B to be larger. This, along with other illusions, shows the shortcuts our brain makes while processing our visual environment.


So what?

This illusion has also been used be researchers to examine a condition known as hemi-spatial neglect – when individuals fail to pay attention to one half of their visual field, occurring after head injury or stroke. Massironi et al (1988) showed this illusion to patients with left or right side brain damage, and asked them to identify which shape was larger. Interestingly, they found that when this image was presented with the shapes aligned to the opposite side of their visual field to their brain damage, participants were unable to see this illusion. Therefore, this illusion could be used to test for hemi-spatial neglect in future patients after brain injury.



Massironi, M., Antonucci, G., Pizzamiglio, L., Vitale, M.V. and Zoccolotti, P., 1988. The Wundt-Jastrow illusion in the study of spatial hemi-inattention. Neuropsychologia26(1), pp.161-166.