Perception Lesson 2 Visual Cues and Depth Perception

Perception Lesson 2: Visual Cues and Depth Perception

Visual Cues and Depth Perception Learning outcomes • You must be able to: • Explain what is meant by depth perception • Describe key monocular depth cues, and give example situations where they are used • Explain the role of binocular depth cues

Visual Cues and Depth Perception Depth and distance

Visual Cues and Depth Perception Depth and distance • As the mind builds up a picture of the world by interpreting information from the senses, it needs to make sense of how close or far away objects are. • This is known as depth perception and is essential for survival in most species. For example, when a monkey or squirrel leaps for a tree branch, it must be able to calculate how close the branch is.

Visual Cues and Depth Perception Depth and distance • Depth cues allow the brain to make sense of the twodimensional image that hits the retina, converting it to a threedimensional mental image of the world. • In other words, depth cues are aspects of the visual scene that make it possible to work out how close or far a way things are.

Visual Cues and Depth Perception Monocular Depth Cues

Visual Cues and Depth Perception Monocular Depth Cues • The simplest cues to depth and distance can be perceived using only one eye if necessary. These are therefore called monocular depth cues. • Monocular depth cues can include the following:

Visual Cues and Depth Perception Relative size Occlusion More distant objects appear smaller than other similar objects. For example, the apparent size of a car on the road ahead allows a driver to perceive how close it is, given that most cars are roughly the same size. If one object partially obscures another from where someone observes them, this shows that it must be the closer of the two objects. Linear As things move further away, they appear closer together to the perspective observer. This results in the perspective skills used by artists, where lines on a painting are drawn towards an imagined ‘vanishing point’, making the painting look much more realistic. Texture When objects are more distant, the eye takes in less detail of

Visual Cues and Depth Perception Binocular Depth Cues

Visual Cues and Depth Perception Binocular Depth Cues • The mind can build on these basic depth cues by comparing the different visual sensations from the two eyes. This results in further cues known as binocular depth cues. • Most predators have two forward-facing eyes, indicating that these cues play an important role in precise judgements of distance.

Visual Cues and Depth Perception Depth and distance • The eyes are at two different positions on the head. Differences between the images from the two eyes (at least for people who have the use of both eyes) give a cue to distance know as retinal disparity; • If an object is closer, the discrepancy between the two eyes will be quite large. • If an object is further away, there will be a smaller difference between the two eyes.

Visual Cues and Depth Perception Depth and distance • A further binocular cue is convergence (or ‘eye convergence’). This cue comes primarily from the muscles that move the eyes rather than from the image itself. When an object is closer, both eyes must rotate inwards slightly in order to bring it into focus in the centre of the retina. For a more distant object, less rotation is required.

Try this… • Line up two fingers one in front of the other, close to your face. Look at them with your right eye, keeping the left eye closed. Now close your right eye and look at them with your left. Do the images that you see with each eye look the same? You should notice quite a large difference between how your fingers appear to each eye. • Now try moving your fingers further away from your face by stretching your arms out. Again, close one eye, and then the other. What can you see now? The difference between what is seen by each eye should be less. This is because things that they are seeing (your two fingers) are now further away, reducing the amount of retinal disparity

Visual Cues and Depth Perception Key Points • A range of monocular cues and a smaller number of binocular cues allow an individual to perceive distance. • Binocular cues rely on differences between images processed by the two eyes.

Test Your Understanding on Visual Cues and Depth Perception • Show what you have learnt with the following quick quiz. • Click on the link and follow the instructions