Biological Vision

 Vision is the biological process of seeing.

  • 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.
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Depth Cues

Depth Cues – The visual ‘clues’ that we use to understand depth or distance.

Monocular Depth Cues – Information about distance that comes from one eye, such as superimposition, relative size, texture gradient, linear perspective and height in the plane.

Binocular Depth Cues – Information about distance that needs two eyes, such as stereopsis.

  • Size Constancy – We perceive an object as the same size even when its distance from us changes.
  • Relative Size – Smaller objects are perceived as further away than larger ones.
  • Texture Gradient - An area with a detailed pattern perceived to be nearer than one with less detail.
  • Height in The Plane – Objects closer to the horizon are perceived to be more distant than ones below or above the horizon.
  • Superimposition– A partly hidden object must be further away than the object covering it.
  • Linear Perspective – Parallel lines appear to converge (meet) in the distance.
  • Stereopsis – A binocular depth cue.  The greater the difference between the view seen by the left eye and the right eye, the closer the viewer is looking.
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Gestalt Laws

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.
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Visual illusion – A conflict between reality and what we perceive.

  • Fiction – An illusion caused when a figure is perceived even though it is not present in the stimulus.
  • Illusory Contour – A boundary (edge) that is perceived in a figure but is not present in the stimulus.
  • Motion After-Effect – An illusion caused by paying more attention to movement in one direction and perceiving movement in the opposite direction immediately afterwards.
  • Colour After-Effects – An illusion caused by focusing on a coloured stimulus and perceiving opposite colours immediately afterward.
  • Ambiguous Figure- a stimulus with two possible interpretations, in which it is possible to perceive only one of the alternatives at a time.
  • Distortion Illusion – where our perception is deceived by some aspect of the stimulus.  This can affect the shape or size of an object.
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Gregory's Theory of Illusion's

  • Hering - In the Hering illusion, the radiating lines look like a linear perspective (depth cue). It is the apparent connegence of parrael lines in the distance. This linear perspective seems to make the horizontal straight lines seem curved at the centre point.
  • Ponzo - In the Ponzo illusion, the top line looks a lot bigger than the bottom line but in fact they are the same size. This ilusion uses linear pespective. The closer the two vertical lines are to each other, the further away we perceives them to be. 
  • Muller-Lyer - The muller lyerillusion can also be explained using the ideas of linear perspective. The depth cue of linear peerspective is when parallel lines apparer to meet. In this illusion the arrows on the top close the line whereas the ones of the bottom open the line, making it better.
  • Strengths - 
    • The Hering + ponzo illusions agree with the theory
    • It is a good way to explain disortions
    • The Necker cube requires superimposition
  • Weaknesses -
    • It doesnt agree with the Muller-Lyer illusion as circles cant give cues to depth.
    • On the Necker cube withot the cue of superimposition it doesnt really work
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Experiments and Ethics

  • An experiment is a way to find out whether one factor affects another. 
  • Sometimes participants need to participate in all conditions of the experiment, other times they only participate in one.
  • Hypotheses are written to say what an experimenter expects will happen in an experiment. They always operationalise the IV and DV and say how the IV will affect the DV.
  • The controls are what the experimenter does to keep variables the same in all conditions.
  • 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.
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Pros and Cons of Experiments

  • Strengths of experiments: 
    • In laboratory settings is easy to gain 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. 
  • Weaknesses of experiments:
    • Sometimes we need to avoid giving participants full information about a study.  This is because knowing the aims of the study might alter the way they behave.  This is called deception.  Not knowing the aims of the study may upset participants but sometimes researchers need to deliberately deceive participants.
    • Experiments should try to represent real life as much as possible.
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Schemas are useful because they help us to predict what might happen but in the case of eyewitness memory we might think we see something based on our perception. e.g. stereotypes such as black people are violent and are likely to have committed the crime (Allport and Postman, 1945, the black man in the suit and the white man with the razor – people remembered the black man holding the razor in a threatening way.

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