When Your Eyes Deceive You

We assume that everything we see is “real”, yet our visual systems often fail us. What we see is sometimes subject to distortions and illusions. Psychologists are interested in such illusions because they provide a valuable insight into how normal visual processing works. Below are a few examples that I’ve stumbled upon over the past year, along with some brief explanations…

1. The Attentional Blink

In the video below, you’ll be asked to identify the letter “R” followed by the letter “C”:

When targets are presented 180–450 milliseconds apart, it is difficult to identify the second target. Thus most people can spot the “R”, but not the “C”.

It’s possible that information is processed in two stages: first we notice the target, then we process it. If the first target is still being processed, it’s possible that the second target will not be identified.

But the attentional blink only occurs when there is a slight delay between targets; when they are presented in rapid succession, there is no impairment in identification. Weird.

It’s been suggested the neurotransmitters which aid detection of the stimulus are still present for 100 milliseconds after the first target, and therefore they benefit detection of the second target as well. After this 100 milliseconds, there is a refractory period and the neurons can no longer release the neurotransmitter. This means detection is impaired. This is only speculation and there is currently no conclusive evidence on why this phenomenon occurs.

2. The Muller-Lyer Illusion

The Muller-Lyer Illusion — Image from: http://museumofconsciousness.com/rm10%20images/muller.gif

A particularly famous illusion is the Muller-Lyer Illusion. When looking at the lines above, our visual system intuitively tells us that the top line is longer than the bottom line. However, both lines are exactly the same length (cue people tracing their fingers down the screen to check I’m not lying).

There’s been a lot of debate over why the illusion occurs. One of the most predominant theories is the Perspective Explanation. This proposes that we process “angles out” (the top line) as an object that is far away, and “angles in” (the bottom line) as one that is close up. In other words, we incorrectly perceive the lines as a 3D scene. Size constancy (the same mechanism that allows us to know someone isn’t reeeeally tiny just because they’re far away) then tells us that the top line *must* be bigger because it’s further away, yet still the same size as the smaller line.

This seems plausible, but the same results were found when circles were used instead of angles. And circles don’t really tell us how far away something is in a 3D drawing.

Back to the drawing board…

3. The Motion Aftereffect

Watch the video below (apologies in advance for any headaches caused by the black and white lines):

When the stationary image popped up, you should have been able to see it “moving” in the opposite direction to the original moving stimulus. This is because of a phenomenon known as the motion aftereffect or the waterfall illusion.

It is believed the illusion occurs because of motion adaption; when we are exposed to a moving stimulus for an extended period of time, the neurons responsible for perceiving that direction of movement become less responsive. Even when the movement stops, these neurons fire at a reduced rate for a short period of time.

In the video above, “upwards firing” reduces while watching the moving image. When the stationary image is shown, this firing remains lowered, but “downward firing” is business as usual. Because “down” is now greater than “up”, the waterfall seems to be moving downwards.

4. The Thatcher Illusion

This illusion got its name after the beloved PM whose photo was first used for the illusion. When the eyes and the mouth on a photo are inverted on an upside-down photo, the effects go largely unnoticed. This is probably the most disturbing thing you’ll see all day:

The Thatcher Illusion — Image from: http://scienceblogs.com/mixingmemory/wp-content/blogs.dir/455/files/2012/04/i-04c396b4410d5d3ae1d62505598c1f44-ThatcherIllusion3.JPG

The prevailing theory suggest that when we process faces normally, we look at the arrangement of the features. We then go on to inspect the local features of the face (the eyes, nose and mouth) in more detail. We don’t have much experience with the arrangement of features in upside-down faces. This means we focus on the local features, which all look a-okay, even when they’re upside-down. It’s only when the image is rotated that we notice the grotesque features *shudders*

Thank you for taking some time out of your day to read this. Here are some references for all you keen beans.

  • The Attentional Blink/Muller-Lyer Illusion:
    Braisby, N., and Gellatly, A., (2005). Cognitive Psychology. 2nd ed. Oxford: Oxford University Press.
  • The Motion Aftereffect:
    Anstis, S., Verstraten, F. A. J., and Mather, G., (1998). The Motion Aftereffect. Trends in Cognitive Sciences. 2(3), pp.111–117
  • The Thatcher Illusion:
    (2006) Cool Visual Illusions: The Margaret Thatcher Illusion. Retrieved from http://scienceblogs.com/mixingmemory/2006/09/23/cool-visual-illusions-the-marg/