Visual Illusions

Don’t believe what you see Visual perception directs our interactions with the environment Before the brain can transform sensory information into meaningful perceptions, it must organise it into something that the brain can recognise The brain must perceive things as distinct from their surroundings, see them as having a meaningful and constant form, and estimate how far they are away This allows us to make sense of the world BUT sometimes errors do occur…….

Is it all an illusion? https://www.youtube.com/watch?v=ynx6pb8 GVTA https://www.youtube.com/watch?v=tVgOLWV YytM https://www.youtube.com/watch?v=4u64HD XoKVM

Don’t believe what you see….. Visual Illusion: A constant perceptual error in interpreting a real external stimulus When we experience a visual illusion, the stimulus provides us with cues that mislead our perception As a result, a mismatch between our perception and the reality of the actual stimulus occurs Although perception of common stimuli is often described as unique, this is not the case with a visual illusion

With Visual Illusions….. All individuals tend to be mislead in the same way The stimulus can be misjudged by the: Direction Motion Position Length Curvature

In contrast……… In hallucinations, people perceive (see, hear, taste smell) objects or events that have no external reality That is hallucinations create stimuli that do not have a separate material existence For example: A visual hallucination can generally be disproved by trying to experience the stimuli with another sense (trying to touch the wings of a giant hallucination of a butterfly)

Some things to think about….. We know that illusions intentionally manipulate the cues we experienced in using to create a perception But not all illusions can be explained to everyone’s satisfaction What produces visual illusions include: Size Shape Constancy Habitual eye movements Continuity Perceptual habits

Which line is bigger?

Muller-Lyer Illusion A visual illusion in which two lines of equal length- one capped with inward-pointing arrowheads, the other capped with outward- pointing arrowheads are perceived as being of different lengths

Muller-Lyer Illusion How is it created? The illusion of differing lengths is created by the different figures at the end of each line. How does it work? It is based on our past experience of living in a three dimensional world where our buildings are basically square or rectangular shaped so that we have constantly been exposed to the edges and corners of buildings and rooms

Muller Lyer Illusion It is believed that we see horizontal lines with the inward pointing arrowheads as being inside the corner of a room. Compared to the outward pointing arrowheads representing the corner of the outer wall of a building Because of the lines cast images of the same length the more “distant” line is perceived as being longer This is also known as apparent distance hypothesis(Gregory & Wallace 1963), whereby an apparently more distant object that has the same size retinal image as an apparently nearer image will be perceived as the larger

Re-create the Muller Lyer Illusion Students are to complete 2.12 of the activity manual You are to complete all sections of this activity. Note: that if you have made the apparatus before the end of the class ask your teacher if you can be excused to test this illusion on someone form another class. Students should only ask for permission in other classes if students are working individually

As a Class describe what you see and how this is an optical illusion

Ames Room Illusion Explained Watch the following clip Write down some key points Go through key notes at the end of the clip as a class https://www.youtube.com/watch?v=gJhyu6nlG t8

Ames Room Illusion An illusion caused by an intentionally distorted room, viewed through a peephole, disrupting perceptual constancies and misleading the viewer to perceive people changing size (shrinking or growing) as they cross the floor from one corner of the room to the corner directly opposite.

Ames Room Illusion The illusion was designed by Adelbert Ames (1934) Besides now realising that the Ames Room is created by distorting the shape of regular sized room and using windows and flooring to match this shape People who view the Ames room are asked to look through a peephole This stops our binocular depth cues providing us with information about the depth and the distance of objects in the environment.

Ames Room Illusion This explanation can be linked back to the “apparent distance hypothesis (Gregory & Wallace 1963). If two objects (people) of the same size are introduced into the room, the object at the left hand corner comes in further away from the viewer then the object on the right hand side. Yet because of the rooms distortion, both objects seem to be the same distance from the viewer

Which green line is longer in length?

Ponzo Illusion An illusion where two horizontal lines of equal length are drawn inside two converging lines, but the line in the narrow section of the converging lines in perceived to be as longer https://www.youtube.com/watch?v=MvlaNYPkI dg https://www.youtube.com/watch?v=MtztgRI6Q 08

Ponzo Illusion Explained This illusion is created by again by the apparent- distance hypothesis In the image of the converging lines remind us of railway tracks we would see in our real 3D world. The Ponzo illusion is a man made 2D drawing, but we use the depth perception principles we use in our 3D world to try and estimate the length of the horizontal lines Our experience of using depth cues influences our perception of the top line as being further away because that is what we know from our “real life” experiences

Student Activities Activity 2.11- Build your own Ames Room Check your understanding 3.9 questions 1-5 finish for homework