1
Q
Visual constancies
A
– a group of perception principles that allow us to view objects as unchanging in terms of their actual size, brightness and orientation regardless of changes to the image the object casts on the retina
2
Q
Size constancy
A
- perceived size of an object remains the same, despite changes in size of object cast on retina
- there is a perfect linear relationship between the size of an actual object and the size of its retinal image (size-distance invariance)
- retinal image also measured by visual angle (person further away casts a smaller visual angle, further away = larger
- if two objects at different distances make retinal images of the same size, the one further away must be larger
- can be interrupted if depth cues are absent or misapplied
- depth perception can be affected if sizes of object are altered
3
Q
Size constancy failure
A
- perception of size based on prior experience
- e.g. Colin Turnbull (1961)
- took a Pygmy ( member of the tribes of equatorial Africa) from dense rainforest to plains
- Pygmy had never seen objects at great distance
- when seeing herd of buffalo for the first time, thought it was a swarm of insects
- concluded that it was witchcraft
4
Q
Shape constancy
A
- perceived shape remains unchanged regardless of shape of image on retina
- e.g. looking at a book from the top, you still know it’s rectangular
- difficult to preserve in the front or side rows in theatres
- we can tolerate fair amount of shape distortion, as long as all objects are similarly deformed
- alcohol intoxication impairs size and shape constancy
5
Q
Brightness constancy
A
- object’s perceived level of brightness relative to its surroundings stays the same under changing light conditions
6
Q
Orientation constancy
A
- object’s true orientation is perceived as unchanged despite changes in the orientation of the object’s image cast on the retina
- e.g. lying horizontally in front of TV
- depends on feedback from vestibular system (located in inner ear)
- info from retina, vestibular system and environmental cues (e.g. vertical aspect of walls, buildings, trees) combines to make orientation constant
