Week 6 - Space and Motion Perception
We perceive three-dimensional space and objects, meaning we can perceive depth and distance
However, our retinal image is two-dimensional
Depth cues are made up of various components and factors that give us insight into Depth, such as Pictorial (static) cues
Monocular pictorial cues include factors such as
Occlusion (process where something is obscured from view)
Linear Perspective (parallel lines converging at point in distance)
Size
Texture gradient (objects having finder detail when they are closer than when they are further away)
Atmospheric perspective (how atmosphere affects appearance of distant objects, IE fog, mist)
Shading
Height in the visual field
Movement cues include factors such as
Motion parallax: objects closer to you move more as compared to objects further away, IE cows out of a window of moving car, close ones fly by, further ones seem more stationary. Direction of movement also changes based on object distance (once they pass straight in front of you)
Kinetic depth effect: how 2D pattern can be perceived as providing depth information when moving
Binocular Disparity:
Retinal images are slightly different between two eyes, and the greater this disparity, the farther the object from one eye compared to other (unless equal distance from both eyes). Direction of disparity indicates whether objects are closer of farther relative to the fixation point. Stereopsis describes the process through which depth information is extracted from binocular disparity
This disparity is used to create visual illusions such as stereograms, and autostereograms, as well as old-fashioned 3D movies
non-retinal cues:
Accomodation: muscles that change the thickness of the lens provide some information about distance to a focused object
Convergence: Inward/Outward rotations of the eyeballs correspond to the distance to the focused object
These are generally weak cues, but they are among the few that can inform about absolute distance to the object
Cues can be ambiguous/insufficient
Examples:
Necker cube
Schroeder stais
These 2D patterns cause bistable (or multistable) perception
Motion Perception:
Objects in the environment are constantly moving
Even if they are not in motion, you are rarely perfectly still
Thus, our visual perception is based on dynamic retinal images
Observer movement factors:
Optic flow: as we move through 3D environment, different parts of a retinal image move in different directions, depending on how we move and where our eyes are focused
We are very sensitive to optic flow patterns, eg we can estimate the direction of self-motion within a 1-2 degree based on sparse optic flows alone
Object movement factors:
Looming: A pattern on a retinal image gradually increases its size as it approaches an observer. Non-visual self-motion information and optic flow inform how we know whether an object is moving toward us or us toward it
Biological movement: structured patterns of motion unique to animate objects. Point-light displays that only indicate joint movements are sufficient for us to perceive human and animal motion. Some of this ability can be innate, with 2-day old infants showing sensitivity to biological motion
Illusory movement: Perception of movement from a static series of images. May show one way our visual system extracts information about motion.
Can also be in still images from famous illusions, where other cues like colour and shading create effect of movement by giving brain cues, when no movement is actually present
Stroboscopic effect: Illusion of motion that occurs when a series of still images are presented in rapid succession