Object constancy

Perceptual constancy

From 2D images, the brain manages to generate the rich visual experience we have of objects in a 3D world. Objects do not appear to change in size when we view them from different distances, even though their 2D images change in size
We perceive the constant unchanging properties of external objects (their objective properties) rather than the transient properties of the images they cast at the backs of our eyes (our subjective impressions of the objects). This is perceptual constancy
Objects of the same size at different distances project different sized images. Objects of different sizes at different distances can project the same sized images. Perceiving object sizes relies on assumptions about depth
When arriving at assumptions about depth there are multiple information types: ocular or optical, binocular or monocular, static or dynamic, relative or absolute, qualitative or quantitative
Two eyes have different views of the world. Stereoscopic depth perception: when the brain combines information from both eyes with assumptions about how relative depth is reflected in the different views of the two eyes
Shape constancy: when people's perception of the shape of an object doesn't change regardless of changes to the object's orientation

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In the natural world, different sources of depth information complement each other and support a single depth interpretation
Using unnatural stimuli, different sources can be made to disagree (i.e Ames' room)
In these circumstances, we discover that information from different sources is combined additively and interactively, although certain information can be weighted more strongly than others in certain contexts.
Size constancy: perceiving an object as having a constant size across different viewing distances and ignoring changes in image size across different viewing distances by using changes in distance to discount changes in image size.
When objects at different depths are viewed side by side, with poor perception of depth difference (e.g with one eye), and with attention to image size, you perceive image size, not object size (i.e Ames' room).
When objects at different depths are viewed widely separated, with good perception of depth difference and with attention to object size, you perceive object size and much of the information about image size is lost.
The same sized image can be projected by large objects at far distances and small objects at near distances

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Emmert's law states that an afterimage appears to increase in size when it is projected to a greater distance. A modern version of the law states that objects that generate retinal images of the same size will look different in physical size if they appear to be located at different distances.
Specifically, the perceived linear size of an object increases as its perceived distance from the observer increases
An object of constant size will project progressively smaller retinal images as its distance from the observer increases. Similarly, if the retinal images of 2 different objects at different distances are the same, the physical size of the object that is farther away must be larger than the one that is closer.
Both Emmert's law and the blindspot are strong indications that we perceive an assumed model of an object, rather than what we actually register of the object at the back of our eyes

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