He says that we perceive depth using monocular and binocular depth cues. Depth cues help us understand cues to depth. Shapes in our environment affect the way we learn to perceive depth. Monocular depth cues are depth cues that only need one eye. Binocular depth cues are depth cues that need both eyes. He suggests that illusions happen when we interpret patterns as if they were depth cues.
We can judge depth in the real world in 3D ways. The ‘clues’ we use to do this are called depth cues. They are pieces of visual information that trigger or ‘cue’ our understanding of distances.
Monocular depth cues: They are clues to distance that only need one eye.There are 5 monocular depth cues that you need to learn in this syllabus:
Superimposition: a partly hidden object must be further away than the object covering it.
In the photo on the right, you can see an example of superimposition. We know that the gate is in front of the house because the gate is covering part of the house.
Relative size: smaller objects are perceived as further away than larger ones. In the photo on the right, the smaller fisherman is seen as further away than the bigger fisherman. However we know that the fishermen are the same size because of size constancy, we scale up images to fit our schema.
Texture gradient: an area with a detailed pattern is perceived to be nearer than the one with less detail. In the photo on the right, we can tell that the lily pads are further away because they are unclear. ok
Linear perspective: parallel lines appear to converge (meet) in the distance.
We can tell that the railroad is getting further away from us because the lines seem to be meeting in the distance. However we know that the railroad is not bending in.
Height in the plane: objects closer to the horizon are perceived to be more distant than ones below or above the horizon.In the photo on the right, the ship seems to be above the other ship. However we know that the ship is not actually above the other ship, it is just further away.
Binocular depth cues: Binocular depth cues are clues to distance that need both eyes.
For example: When we are looking at things in the distance our eyes point straight out. As we look at closer objects our eyes point inwards.
Stereopsis is an example of a binocular depth cue. The brain compares the views from the left eye and the right eye. The bigger the difference between them, the closer the object is to you.
How does Gregory's theory explain illusions?[]
A 2D shape is viewed as 3D because of depth cues, we think that we see depth in it, creating a visual illusion.
Strengths[]
Explains distortions well, such as Muller Lyer illusion and Ponzo illusion. In the Ponzo illusion, the depth cue linear perspective says that it is perceived to be more distant, it is scaled up, so seems bigger than the bottom bar.
Explains ambiguous figures well, such as Necker's cube and Leeper's Lady. In Leeper's Lady the wart on the nose of the young woman looks further away than the wart on the old woman's nose.
Weaknesses[]
Problem with Muller Lyer illusion with CIRCLES, because Gregory's theory suggests that circles cannot provide cues to depth. It should not work but it does.
