How do we see?
The light reflected from an object makes an image on the retina which is a layer at the back of the eye
The retina is sensitive to light and sends nerve impulses to the brain.
This is possible because the retina is covered with thousands of cells that can detect light.
Rods: They are very sensitive to light and they also respond to movement
Cones: These only work in bright light and can detect different colours
Rods and cones are special neve cells.
When enough light falls on them, they respond by sending a nerve impulse. The nerve impulses travel along the optic nerve to the brain.
The optic nerve and the brain
At the point where the optic nerve leaves the eye, there is no space for any rods or cones.
This little area is therefore blind
This area is called a blind spot. There is a blind spot in each eye.
The information from the retina goes along the optic nerve to the brain.
Information from the left and the right eye crosses at a point called the optic chiasma.
From the optic chiasma, visual information is carried to the back of the brain.
Many visual processes happen in the area called the visual cortex.
Depth and size
When an object is close, our brain scales it down in size so that it looks normal rather than enormous.
When an object is far away it is scaled up sop that it looks normal size rather than tiny.
This is known as size consistancy
Distant= scaled up=bigger on retina
Nearby= scaled down= smaller on retina
Monocular depth cues 1
- This helps us to understand that closer objects appear bigger and objects in the distance appear smaller.
- When you are looking at something with texture nearby, you are able to see a lot of detail. From further away, the texture is less clear.
Height in plane:
- Above the horizon is further away and below the horizon is close. If things are close to the horizon they seem further away and if they are further away from the horizon they seem closer.
Monocular depth cues 2
- This basically means layering or overlapping. When one object is in front of the other, we assume that the one in front is closer and the one behind is further away.
- Parallel lines getting close means being further away and when they appear to be the same distance apart they appear to be closer.
- This is where parallel lines appear to meet on the horizon, they appear to be further away.
This a binocular depth cue.
It is when the brain compares the view from the left and the right eye.
If the view is the same, the object is in the distance.
If the view is different, the object is close up.
The brain uses one image as dominant and the other image is mapped onto it.
Figure and ground:
- A small complex symmetrical object is seen as separate from the background
- Figures sharing size, shape or colour are grouped together with other objects that look the same
- Objects which are close together are perceived to be related
- Straight lines, curves and shapes are perceived to carry on being the same
- Objects are perceived as complete figures even if parts are missing
Visual illusions occur when our perception conflicts with reality
We see an illusion when we misinterpret a stimulus; the physical reality and our perception disagree
There are three types of illusion: fictions, ambiguous figures and distortions
FICTIONS occur when you perceive a shape that is not actually there. Sometimes we perceive edges amd believe that a shape exists when there is no actual boundary.
DISTORTIONS occur when our perception is deceived by some aspect of the stimulus. A straight line may appear curved or a shape may look bigger than it actually is.
AMBIGUOUS FIGURES occur when a stimulus can be perceived in two ways. But, only one alternative can be seen at a time.
Gestalt theory of illusions
- When we see a figure which is incomplete, our perception makes a whole shape, using closure. We complete the edges to make a regular or familiar shape. this is the figure of the figure- ground relationship. In the case of the Kaniza triangle, the "pac- men" and the space around them become the figure. The space between them is the white triangle.
- When perceiving the figure as a whole we tend to add lines to the central lines. In the top image, the lines are pulling the central lines together, making it look smaller and vice versa.
- A stimulus with two possible interpretations in which it is possible to perceive only one of the alternatives at any time. You often swap between alternatives.
Evaluating the Gestalt theory
- It provides a good explanation for ambiguous figures
- It explains fictions well
- It can explain many illusions
- It can not explain any distortions other than the Muller-Lyer illusion
- If we used closure to explain the Kaniza triangle, we could see a six pointed star.
- The theory seems to use different explanations at different times.
Gregory's theory of illusions
- In the Hering illusion, the radiating lines look like a linear perspective cue. Linear perspective is a depth cue. It is the apparent convergence of parallel lines in the distance. This linear perspective seems to make the two horizontal, straight lines seem curved at the centre point.
- In the Ponzo illusion, the top line looks a lot bigger than the bottom line but in fact, they are the same size.' This illusion uses linear perspective. The closer the two vertical lines are to each other, the further away we perceive them to be.
- The Muller Lyer illusion can also be explained using the ideas of linear perspective and consistancy scaling. The depth cue of linear perspective is when lines appear to meet. In this illusion, the arrows on the top close the line whereas the ones of the bottom open the line, making it look bigger.
Evaluating Gregory's theory
- The Hering and the Ponzo illusions agree with the theory
- It is a good way to explain distortions
- The Necker cube requires superimposition
- It doesn't agree with the Muller-Lyer illusion circle illusions as circles cannot give cues to depth.
- On the Necker cube, without the cue of superimposition, it doesn't really work
Schemas and eyewitness memory
- A schema is a framework of knowledge about an object, event of group of people that can affect our perception and help us to organise information and recall what we have seen
- Schemas can affect eye witness accounts because if we witness a robbery, our schema may affect that way we remember it. If we have a certain scheme about black men committing robberies then we may think we saw a black man committing the robbery; even if it was not.