Vision vs Perception
- Vision and perception are different – vision is the biological process of seeing and perception is the psychological process of making sense of the image.
- The light reflected from an object enters the eye and makes an image on the retina (at the back of the eye). It is here that nerve cells called rods (sensitive to bright light) and cones (detect colours) help us to perceive objects.
- The optic nerve carries the nerve impulses from the rods and cones to the brain.
- The blind spot is found in each eye. It is the area in the retina where there is no space for rods and cones therefore the area is ‘blind’ as there are no light-sensitive cells. We often don’t notice our blind spot because our two blind spots don’t overlap so if one eye can’t see something, the other one can.
- The optic chiasma is needed because information from each eye goes to both sides of the brain; some from the left eye goes to the left side of the brain and some to the right.
- The visual cortex allows us to understand shapes and distances and fills in the gap left by the blind spot in each eye.
Gestalt Laws Explained
Gestalt laws – perceptual rules that organise stimuli.
- Figure-ground – a small, complex, symmetrical object (the figure) is seen as separate from a background (the ground).
- Similarity – figures sharing shape, size or colour are grouped together with other things that look the same.
- Proximity – objects which are close together are perceived to be related.
- Continuity – straight lines, curves and shapes are perceived to carry on being the same.
- Closure – lines or shapes are perceived as complete figures even if parts are missing.
Gestalt Laws Evaluated
For fictions such as the Kanizsa triangle when we see a figure as incomplete, our perception makes a ‘whole’ shape. This is the figure of the figure-ground relationship.
When explaining distortions – the Muller-Lyer illusion - in perceiving the figure as a whole we tend to ‘add’ fins or circles to the central lines. When pointing out the fins drag out the line and make it look longer.
Gestalt theory explains ambiguous figures by saying that we normally identify the figure or ground but in ambiguous figures it could be either figure or ground because we cannot tell whether the black or white area is the figure.
Gestalt theory provides a good explanation for ambiguous figures however it cannot explain any illusions other than the Muller-Lyer illusion.
Gestalt theory explains fictions well but in the case of the Kanizsa triangle Gestalt explanation says we would use closure to organise this figure which means we should see a six pointed star, but we don’t, we see two triangles.
Gregory's Theory Explained
In the Hering illusion the radiating lines look like a linear perceptive cue so we use constancy scaling as if the scene really had depth. The person who appears furthest away would be scaled up so they look bigger and the person who appears closer would be scaled down, and look smaller.
The Ponzo illusion (the top bar looks bigger than the bottom bar) – if the railway tracks were used as cues to linear perspective, the top bar would seem further away. As it is perceived to be more distant, it is scaled up so it seems bigger than the bottom bar.
The Muller-Lyer illusion can be explained using the ideas of linear perspective and constancy scaling. On the left hand photo the front of the shop is closer than the back. We scale things down that are near us. In the picture on the right the middle vertical line looks further away as it is scaled up.
Gregory's Theory Evaluated
It is a good explanation of distortions. If angled lines are used as depth cues, this explains many illusions.
However, Gregory’s theory cannot explain some versions of the Muller-Lyer illusion.
Gregory’s theory can explain some ambiguous figures when the two alternative figures are perceived using depth cues. e.g. On Leeper’s Lady the nose of the young woman looks further away than the wart on the old woman’s nose.
Depth cues can also explain some fictions as the background lines appear closer to the horizon and so further away.
Ethics and Experiments
Wondering about what will happen in your experiment, being concerned about how good your answers were, making sure your participants are not harmed are all examples of ethical issues.
One problem for experimenters is that ethics sometimes conflict with the need for controls.
When conducting experiments in psychology you need to ensure you meet all ethical guidelines before you begin your research. Participants must understand the nature of the study and agree to participate (this is fully-informed consent) and if they want to leave the study they can at any time and have the right to take their data with them (the right to withdraw).
The BPS (British Psychological Society) has a ‘Code of Conduct’ to help psychologists conduct their research in a way that will meet ethical guidelines. Psychologists often give participants a summary about what will happen in a study although this is difficult in public places.
Strengths of experiments:
- In laboratory settings it is easy to gain consent because you can tell participants exactly what is happening and they can give their fully-informed consent.
- When the participants come to the laboratory their right to withdraw can be explained.
- The experimenter can control other factors that could change the DV. By controlling other variables, the experimenter can be certain that differences in the DV have been caused by the different conditions.
- The DV can be measured accurately.
Weaknesses of experiments:
- Not knowing the aims of the study may upset participants but sometimes researchers need to deliberately deceive participants. When deception is used psychologists minimise harm by: avoiding deception unless it is absolutely necessary, avoiding other ethical problems such as embarrassment, explaining the real purpose as soon as possible and allowing participants to withdraw their results at the end.
- Experiments should try to represent real life as much as possible.
Disortions: Our perception is decieved by some aspect of the stimulus.
Ambigious Figures: You can understand the image in two different ways, and can switch between them.
Fictions: You perceive a shape that isn't there.
After - Effect: When we look at a stimulus for a long time then look away, the after effect is of the opposite of the stimulus.
Linear Perspective: Lines that are parrallel appear to get closer in the distance.
Stereopsis: The bigger the difference between the view from the left eye and the right eye, the closer the object we are looking at.
Height In The Plane: Objects lower in your field of view are closer (or those furthest away)
Texture Gradient: If greater detail can be seen in a texture, it is perceived as being closer.
Relative Size: Objects making bigger images on the retina are perceived as being close.
Superimposition: If something is partially covered by something else, we know it is furthest away.