Are the lines parallel... or not?

What about this one?

Is the black line a continuation of the red line, or the blue line?

Or if it's misaligned, which direction does it have to move in to be in line with the red one?

These are examples of geometrical-optical Illusions - namely the Z öllner and Poggendorff illusions.

The Z öllner illusion is made up of diagonal lines - they are parallel - but they're crossed with shorter lines that alternate between horizontal and vertical.

The angles of these shorter lines give the impression that the diagonal lines are not parallel.

In the Poggendorff illusion, we have a tendency to misestimate the unknown angles hidden by the rectangle - the acute angles have an effect of us over-predicting the position of the line.

Many of us are tricked by looking at these images, and for centuries we thought that perceptual illusions just involved the visual system - but people who have been blind since birth can be tricked by these illusions too.

In a classic 1938 study, an experimental psychologist mounted raised versions of the Poggendorff, Z öllner and M üller-Lyer illusions on veneer-ply board.

When participants who were blind since birth touched the illusions and explored them haptically, almost all experienced the Poggendorff and M üller-Lyer illusions - and about half were tricked by the Z öllner illusion.

When sighted participants only touched the illusions, there was no effect - But when the sighted participants looked at the illusions, they were tricked by them too.

For people who are blind, they could feel the illusions that we see - and still be fooled.

And over time, with more participants and more accurate ways of running these experiments, researchers found that many visual illusions exist in a tactile mode.

Take the Muller-Lyer illusion.

Do you think the two lines the same length, or are they different?

For sighted people, some suggest this illusion prompts perceptual differences in depth, and in people who are blind, many have difficulty in separating where the straight line ends and where the arrows begin, leading to an overestimation or underestimation of the straight line.

In many studies, this illusion has tricked participants who are both blind and sighted - There are perceptual processes common in both vision and touch.

But there are some differences too.

Take a look at this image - are the yellow lines the same length, or different?

Mario Ponzo, an Italian psychologist, suggested that our minds judge an object's size based on its background.

When you can see an object, that is.

In numerous studies, the Ponzo illusion hasn't been found in touch in those who are blind, which means a different perceptual process is underlying the illusion.

It's thought the Ponzo illusion is dependent on our impression of linear perspective, which isn't universally present in those blind since birth.

Of course people who are blind can learn about these principles of perspective, just as we do, but research suggests they may not spontaneously use these cues in interpreting raised line drawings.

Studying haptic perception in blind, vision impaired and sighted people can help us better understand the sense of touch, and improve the design of things like raised maps, accessible money and even haptic feedback on our phones.

And in addition to that - now you can see that there's more to the humble illusion than first meets the eye