Visual Cues & Constancies: Depth Cues (AQA GCSE Psychology): Revision Note

Monocular depth cues

• Depth cues are made up of information taken from the surrounding environment which help humans to understand depth i.e. that the world is 3-D rather than flat and depthless

• A visual cue is any environmental stimulus which provides humans with information about spatial dimensions, movement, distance and proximity of objects and people within the visual field

• Visual constancy is the ability of humans to understand that objects remain the same size even though they may appear smaller or larger depending on where they are located in the visual field (e.g. a cow which is far away in a field appears smaller than cows standing near the fence)

• Humans use a perceptual system of visual cues and constancies to enable them to understand depth
  
  

• Monocular depth cues are those which allow someone to judge distance and depth using only one eye (‘mono’ = ‘one’) i.e. if one eye is covered it is still possible to understand that an object is not smaller simply because it is seen in the distance

• Monocular depth cues (which interpret the world as being in 2-D) include:

  • Linear perspective: this occurs when parallel lines (such as those on a railway track) appear to converge in the distance 

  • Relative size: the further away an object/person is from the viewer, the smaller they appear in the visual field

  • Height in plane: objects/people who are higher up in the visual field appear to be further away from the viewer

  • Occlusion: when one object/person is obscured by another object/person they appear further away in the visual field

Monocular Depth Cue Example

Linear perspective is a monocular depth cue.

Binocular depth cues

• Binocular depth cues are those which require both eyes working together ('bi' = two)

• Because our eyes are around 6 cm apart, each eye receives a slightly different view of the world; the brain combines these two images into a single 3-D image and uses the differences between them to judge depth and distance

• There are two binocular depth cues: retinal disparity and convergence

Retinal disparity

• Retinal disparity is the difference between the images received by the left and right retina ('disparity' = difference)

• The brain compares the two images: the bigger the difference, the closer the object; the smaller the difference, the further away it is

  • Hold a finger close to your face and close one eye, then the other – the finger appears to jump position. Do the same with a distant object and it barely moves

Retinal disparity: each eye takes in a slightly different view from the other. 

Convergence

• Convergence is the inward movement of the eyes as they focus on an object that is close to us ('converge' = come together)

• The muscles controlling the eyes have to work harder to turn both eyes inward when looking at something nearby; when looking at a distant object the muscles relax and the eyes look straight ahead

• The brain uses this muscle feedback as information about distance: more muscle tension = closer object

  • Watch someone follow their own finger as it moves towards their nose – their pupils move towards the middle (they go 'cross-eyed')