Depth Perception vs. Stereopsis
As described in this post last week, I recently found myself in a position of trying to explain the difference between depth perception and stereopsis to a COVT candidate. In working to clearly understand it myself, assistance was sought out, and subsequently offered by four great doctors who were named in this post, and indirectly by others whose information was shared with me in subsequent days and whom I hope to properly give credit as I work to explain it here. Interestingly, I discovered through my research that although these terms are not always mutually exclusive, they are not interchangeable either; however, we at times consider them to be. There is, however, a degree of dependence at the highest levels.
Depth perception seems to live up to its name, it is simply the ability to perceive depth, but not so simple is understanding how it’s achieved. In researching a list of monocular cues for understanding depth, I located two lists. The first list provided by WikiAnswers – which my doctor qualified as the more traditional of the two – contains 6 cues which include relative size, texture gradient, interposition, linear perspective, height in plane and light in shadow. The second list which seems to contain slight redundancies and is provided by Wikipedia, offers 14 depth cues which seem to contain small divisions of items on the traditional list. The 14 cues on the second list are motion parallax, depth from motion, kinetic depth, perspective, relative size, familiar size, aerial perspective, accommodation, occlusion, curvilinear perspective, texture gradient, lighting and shading, defocus blur, and elevation relative to the horizon. Definitions for each come compliments of Wikipedia and can be found here. The take-away message being depth perception does not require binocularity – a common misconception – and is available under monocular or binocular conditions. Noteworthy also is stereopsis is not a requirement for achieving depth perception, but depth perception must be present to achieve stereopsis.
Binocular cues only serve to enhance depth perception and are listed by Wikipedia as stereopsis, convergence and shadow stereopsis . These cues provide further depth awareness by offering a slightly different perspective when viewing the same object.
Stereopsis is simply a quantifier of binocular depth perception. It is a type of depth perception requiring a high degree of accuracy, that occurs when two eyes in slightly different locations view the same object in space and offer the brain an opportunity to create a three-dimensional image using the overlapping pictures taken from slightly different angles. This explains why some patients with strabismus cannot achieve stereopsis, as they have difficulty pointing both eyes at the same target simultaneously; they may however achieve depth awareness through monocular cues.
As part of a larger write-up, Ken Koslowe OD, MS, FCOVD-A offered this explanation of stereopsis:
Because the eyes are separated by about 6.5 cm in the horizontal plane, each eye has a slightly horizontally disparate view of the world. Stereopsis is the ability to use horizontally disparate cues to construct a percept of solid depth. Essentially the observer is constructing a three-dimensional percept from two two-dimensional retinal images.
Again I would like to thank everyone that assisted, either directly or indirectly, with my learning process. Aside from those mentioned here and in this post, emails were forwarded and documents shared that were truly helpful in growing my knowledge base. If anyone has a different perspective on this – pun fully intended – please feel free to comment!
Again, many thanks to all.