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Optical Illusions

Introduction
Public summary: 

Confusing our brains and eyes.

Display of several optical illusions, to illustrate the way the brain perceives certain visual stimuli
Useful information
Kit List: 

Various optical illusions (laminated sheets)
Optical Illusion Book

Packing Away: 

Into Eye Model box

Explanation
Explanation: 

IN GENERAL:

These optical illusions are best used with the eye model/blind spot test. Some explanations may be a little difficult for younger children.

STROOP EFFECT

To do:

•Get the kids to read out the colour of the words (tell them specifically that they should not read the actual words) – first with the sheet where the colours and words match, and then on the sheet where they don’t – and time them (or get their friend/sibling to time them if you’re working with more than one child)
•Is there a difference? (Hopefully it will take them longer to say the one where the names and colours don’t match!)
•Ask them why this might be – basically, the words have a strong influence over your ability to say the colour – the interference between the different information (what the words say and the colour of the words) your brain receives makes it more difficult
•Couple of different theories – words might be read faster than colours are named (speed of processing theory) or possibly naming colours requires more attention than reading words (selective attention theory)
•Might be quite nice to do this comparing very young children with older ones if possible – might expect that younger children, who are less confident at reading, might experience less interference and therefore see less difference in their times

Extra stuff:

•There is some evidence that the anterior cingulate area/dorsolateral prefrontal area might be active while performing the Stroop test – thought to be important for conflict monitoring/resolution
•You can see similar effects if you’re scared of spiders and the words are related to spiders e.g. “hairy” and “web” – it takes you longer to read the colours of these words than a set of neutral words. Again, this is thought to be because the scary words grab your attention too much and slow you down.

CHECKERBOARD SHADOW ILLUSION

To do:

•Very simple – ask the child if squares A and B are the same colour (probably will say they aren’t)
•Then show them second picture – they are!
•If still not convinced – might be worth making a mask with holes in the appropriate places so you can see that they are in fact the same

Explanation:

•Visual system needs to determine the colour of objects in the world
•Has to take into account shadows – a white surface in shadow might be reflecting less light than a black surface in full light
•Visual system uses several tricks to determine where the shadows are and how to compensate for them
•First trick – check that is lighter than neighbouring checks is probably lighter than average, and vice versa
•In figure – light check in shadow is surrounded by darker checks – even though check is physically dark, it is light when compared to its neighbours
•Dark checks outside shadow – surrounded by lighter checks – look dark by comparison
•Second trick – shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges
•Visual system tends to ignore gradual changes in light level so that it can determine the colour of the surfaces without being misled by shadows
•'Paintness' of checks – aided by form of “X junctions” formed by 4 abutting checks – this type of junction is usually a signal that all the edges should be interpreted as changes in surface colour rather than in terms of shadows or lighting
•Effect really demonstrates successes rather than failure of visual system – visual system not very good at being a physical light meter – but that is not its purpose – important task is to break image information down into meaningful components and thereby perceive the nature of objects in view

HERMANN GRID ILLUSION

To do:

•Stare at the central intersection of white lines
•Black dots appear to flash in the peripheral intersections of white lines but not in the one we’re staring at

Explanation:

•Complicated – probably more for older children/adults
•Lateral inhibition between centre and surround of receptive field (analogy of positive and negative numbers – if you have the same amount of both, they cancel each other out. Light hits the receptive fields – the particular pattern of activity determines what information is transmitted to the rest of the brain)
•Receptive field that lies at the intersection of the white cross has more light falling on its inhibitory surround than does receptive field that lies between 2 black squares (see lower bit of diagram) – excitatory centre of receptive field between squares yields a stronger response than that which lies at intersection of white cross
•Upper part of diagram – receptive fields in central fovea – much smaller than rest of retina (as we have lots and lots of receptors here)– so in all cases, both the centre and surround are completely in the white sections, and you don’t see the illusion at the intersection where you stare

YOUNG GIRL AND OLD WOMAN (or other similar illusions)

To do:
• Duck or rabbit?
• Native American Indian or Eskimo?
• Girl or woman?

Explanation:

A lot of optical illusions are caused by ambiguities in 'perception'. Our eyes contain receptors that send signals to our brain about the light that we see. 'Perception' is how our brain interprets these signals and tries to make sense of them. Where there are ambiguities in perception there can be more than one interpretation of what we see, this is what’s happening here. The lines and shapes of an image have two different meanings depending on which perception you have at that particular moment. E.g. what is the young woman's chin is the tip of the old woman's nose. Once you see both interpretations, you cannot see only one of them. The other will keep "popping" into your vision from time to time.

NECKER CUBE (and similar)

To do:

•Is the red dot inside or outside the cube?
•Cube appears to flip – sometimes appears to be inside, sometimes appears to be outside
•(The other one is similar – perceptual flipping – get them to try with this one when they have seen the “flipping” with the Necker cube)

Explanation:

•The drawing is obviously 2D – but our brains automatically perceive it as a 3D cube
•However, the drawing does not give enough information for the visual system to know exactly which face of the cube is at the front
•Makes the point that what we see is merely a “best guess” made by our visual system (the 2 different orientations can be thought of as 2 different hypotheses that the brain selects between)

If you can get hold of a computer – there is a really nice spinning female silhouette illusion as well which makes this point even more nicely (http://www.flavor8.com/images/spinningwoman.gif) You have to stare for a while – but she should suddenly start rotating in the opposite direction! If you have a group, you could also ask which way they start seeing the figure rotating and see if any way (clockwise or counterclockwise) is more popular than any other.

Risk Assessment
Date risk assesment last checked: 
Wed, 30/01/2019
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Tue, 05/02/2019
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 

Various displays - could be attached to display boards.

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Display boards It it possible that the display boards could fall on someone. 3 2 6 Use light professional display boards so they are not too heavy
Make sure the display boards are stable. Do not use them flat without taping them to something solid.
Mark boards with highly visible stuff (e.g. hazard tape, white paper) if they are being used in the dark room.
Call first aider in event of injury.
2 1 2
Optical Illusions Children might get dizzy / feel weird after staring at some illusions.
Could result in feeling ill, or wondering into walls / hitting things.
2 2 4 Monitor everyone, and stop if someone seriously expresses discomfort. 1 1 1
0
This experiment is sometimes run in a darkroom, see separate risk assessment.
Images