The brain can then interpret this offset as a binocular depth cue. This slight offset is termed retinal disparity. Due to the front-facing location of our two eyes, each eye sees a very similar, yet slightly offset image when looking at an object in space. The eyes must have a relatively similar image (this is why patients with lazy eye or strabismus have difficulty with stereopsis - the brain is working with dissimilar quality of images!). Stereopsis is the highest (most difficult) level of extracting depth information from the visual world. Flat fusion has no cues to depth - these are simple objects that look flat on a surface. Fusionįusion is the ability of the visual system to take two similar objects and combine them into a single object. The eyes must both move in the correct direction so that the visual axis of each eye is pointing at the same object of interest, and the images must be able to be fused. Binocular Depth Cuesīinocular depth cues are more complicated and required a coordinated effort from both eyes. Objects that are closer move across the retina at a faster velocity than distant objects when moving. Do you see the nearer finger moving faster than before? This is the parallax effect. Focus on your far away finger and rock your head left and right. What happened? Did you notice your finger moving slightly in the opposite direction of your head movement? Now keep your finger out at arms length and use your other hand to do the same motion, but at a distance closer to your nose. Look past your finger or thumb at a distant object and shake your head gently from side-to-side as if saying "no". Now hold your finger or thumb up at arms length away. Our visual system assumes light comes from the top-down of an object - changing how light and shadows appear on an object can change how the depth is interpreted. These are grouped together as light or shadowing of an object becomes a depth cue. The blue outline of an object against the sky gives a cue that the object is farther away. The "blueness" of the sky is due to scattering of shorter wavelength light (Rayleigh scattering). On a broader scale, we learn from young age that the earth is brown or green and the sky is blue. As you look into the distance, the lines appear to get closer and closer, which gets interpreted as the lines being farther away. Think of two lines that are parallel (lines on a road or railroad tracks are a good example). This is a fun drawing trick usually learned at an early age.
Overlap (also called interposition) is a cue that tells the visual system that the front-most object is likely closer because it blocks the view of the back-most object.
When a big object (say a car) appears small, you have a good idea that this isn't really a small car, but rather a car that is far away. You learned when looking around in the world that some objects are big and some are small. This is probably the easiest one to understand. Are you able to tell what items in the room are closer to you? Farther away? The answer should be yes! Even someone with one eye is able to use single-eye cues to depth. Cover one eye with your hand and look around. Take a moment to perform a quick experiment. What's very interesting about vision is that crude depth perception does not require the use of both eyes. You could also use the word "clues" for cues as these are the "clues" that tell the visual system about the 3D components of an object or space. These cues may be monocular (single-eye) or binocular (two-eye) cues to depth. Stereopsis (depth perception) is the visual ability to perceive the world in three dimensions (3D) - length, width, and depth - which then allows a person to judge where an object is relative to him or her.ĭepth perception arises from a variety of visual stimuli referred to as depth cues.
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