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Summer School - resources 2011

School of Computing & Mathematics


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Intelligent Computing Technology & Perception

Is seeing (more accurately, perceiving) believing? Surprising effects with human and machine perception.
Some background info (in PDF format).


Perceiving straightness

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A sample RGB colour mix

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Perceiving length

Use the 'Step' button to update the diagram below with lines that have arrow-heads of different degrees of pointedness. What is the angle of the arrow-heads when the black horizontal line is roughly 10% shorter than the blue horizontal line?

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What's being demonstrated when you click the 'Step' button is a very famous perceptual illusion called the Muller-Lyer illusion.

  • The illusion is thought to arise because the human perceptual system has learnt, more often than not, that objects with lines that predominantly converge-inwards are usually closer to the viewer; whereas objects with lines that converge-outwards are usually further away from the viewer.


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Perceiving motion?

For each of the next five diagrams do the following:

  • Use you scroll-bar to place the cross-hairs of the diagram below in the centre of your screen. Then with your eyes firmly fixed on the cross-hairs gently rock backwards and forwards in your chair.

Diagram 1:

Apparent rotational motion - where there is none!

Diagram 2:

Apparent rotational motion - where there is none!

Diagram 3:

Apparent rotational motion - where there is none!

Diagram 4:

Apparent rotational motion - where there is none!

Diagram 5:

Apparent rotational motion - where there is none!

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Perceiving colour (well, pink dots)

Use your scroll-bar to place the cross-hairs in the centre of the pink-dots below, somewhere near the middle of your screen. Then, without blinking or moving your eye, stare intently at the cross-hairs for as long as you can.

An animated RGB illusion using pink dots

What you should see if you're able to observe this illusion:

  • After a few seconds you should start to perceive a ghostly green-dot moving around the circle.
  • After a considerably longer time, you should start to see the pink-dots begin to fade away - eventually melting into the grey background. This can be tricky to achieve if you allow your eyes to drift away from the cross-hairs for even an instant!

What's going on with the pink dots illusion?.

  • The are many millions of special colour sensitive cells in the retina, at the back of the human eye.
  • These cells are called cone-cells and different cone-cells respond either to red, green or blue light.
  • All of our colour perception depends on the rate at which these red, green and blue cone-cells send nerve impulses to the brain, as the light hits them.
  • By keeping your eye fixed on the cross-hairs the pink light from the dots is always landing on the same little groups of cone-cells on your retina - as a result those cone-cells become temporarily "exhausted" and stop sending signals to the brain: that's why the dots gradually fade.
  • Luckily, the cone-cell "exhaustion" is completely reversible in less than a fraction of a second! That's why if your eye moves, even a little, the cone-cells are replenished and are able to start firing once again
  • That's why you have to stare intently at the cross-hairs without blinking or moving the eyes!

By the way, there are never any green-dots on the computer screen - at any time!! The 'green dot' is a pure illusion created by the light from the grey-background falling on a population of cone-cells where the red-sensitve cones are more exhausted than the green-sensitive cones.


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Perceiving text

Working from top-left to bottom-right, and as quickly as you can, tell the person sitting next to you the colour of each word as it is displayed in the diagram below:

Stroop effect example

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Perceiving Speech

The three links below will each play a short (sadly not very interesting) video clip. In each video-clip the speaker is repeating the same syllable over and over again. For example "baa, baa, baa ..." or "gaa, gaa, gaa ..." and so on.

View each of these video-clips in turn. The lab could get very noisy at this point, so you may want to use your headphones!

(Some of the above video-clips require a media-player that can accept files in Windows MovieMaker's .wmv format. You may experience difficulties viewing these clips if your web-browser does not have access to such a media-player.)


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Summer-time photo of Keele Hall

Sources of further information

The pink-dots illusion (also known as 'Lilac Chaser') was first demonstrated by Jeremy Hinton sometime before 2005. More refined demonstrations of this effect have been provided by Hinton and Michael Bach.

The rotational illusion featuring the concentric rings of 'S' shaped line-segments is based on the the work of Pinna and Brelstaff (Vision Research, 40, 2091-2096, 2000) and the rendering of Akiyoshi Kitaoka.

The confounding influence of the written meaning of textual stimuli over the percept of the 'ink' colour is known as the Stroop Effect. John Ridley Stroop first reported this effect in his Ph.D. thesis published way back in 1935.

The McGurk Effect is the name given to the ability of visual lip-reading input to influence the brain's perception of the the speech-sound falling on the ear. The McGurk effect is sometimes called the McGurk-MacDonald effect. It was first described in a paper by McGurk and McDonald (1976).


Page loading problems:
The Java applets we looked at as part of this workshop are compatible with common Java-enabled web-browsers such as Internet Explorer and Mozilla etc. and are available for wider educational use beyond the university.
However, for these Java applets to load and run properly on your computer system you must have the software for the Java Runtime Environment (JRE) installed. This software can be freely downloaded from the following website Java SE Downloads. The download is a simple process but you should probably seek the permission/assistance of your system administrator before doing this.

Also, please note that some of the above video-clips, used to to demonstrate the McGurk Effect, require a media-player that can accept files in Windows MovieMaker's .wmv format. You may experience difficulties viewing these clips if your web-browser does not have access to such a media-player.

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