On the left you see the Fraser spiral. The overlapping black arc segments appear to form a spiral; however, the arcs are really a series of overlapping concentric circles. On the right, we have connected the overlapping segments, so that the illusion is broken. If you cover half the figure (the one on the left), this will also cause the illusion to be reduced. This means that a global perception is needed to cause the overall effect.
 
 

What's Going On?

    The Fraser spiral is related to the twisted cord, Münsterberg, and café wall illusions. The Fraser spiral, however, is the most complex of the twisted cord illusions, and some additional factors may be involved in the visual processing of this illusion. In this case, even though the twisted cords themselves trace out circular paths, each of the directional units gives rise to a perception of a spiral.

   The illusion is augmented because there are spiral components in the checkered background. On the left you can see the illusion is greatly reduced when one removes the checkered background, but it is still present. If the directional units are grouped with those in neighboring cords, these groupings would also follow an underlying spiral path.

     All three illusions demonstrate the effect of simple image processing occurring at the retina combined with some complex processing in the cortical cells of the striate cortex.. The spiral effect is likely due to orientation-sensitive simple cells found in this area. For example,  the contextual effect that is a result of horizontal connections between "like" cells in V1.  Cross-correlation patterns between pairs of cells are not completely static but change somewhat depending on context. The cells interact with one another to interpret the diagonal bands produced by the retina as a single continuous line, tilted in the direction of the directional units.