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

• 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 are those which involve the use of both eyes (‘bi’ = ‘two’)
• Each human eye takes in a slightly different view when both are used together and this is what enables human beings to use depth perception (see the image below):

Binocular Depth Cue Example

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

• Each eye takes in a view that is processed by the brain to create a 3-D image: the fact that each eye takes in a slightly different view is due to retinal disparity (if human eyes took in the same view they would have to be stacked on top of each other which would be of no use whatsoever!)
• Objects/people which are further away do not produce as much retinal disparity as those which are closer to the viewer - this is due to convergence i.e. the eyes coming together to take in the view
• Both retinal disparity and convergence allow human beings to process and understand depth
  • Animals such as birds have poor depth perception as they are only able to use monocular depth cues due to the placement of their eyes