Visual Direction Flashcards
Describe the law of visual direction?
- Perceiving an object single means that, our brain assign it as a single sense of direction even though the image in each eye may have different visual direction
- Diplopia - seeing 2 objects, each w/ a different visual direction
- Single vision = single direction
- Double vision = two different directions
The location of an object in space is determined by what two values?
Direction
Distance relative to ourselves
Define visual direction?
It is a 2-D localization of an object
Taking into acount only the lateral & vertical position (direction) of the object, regardless of the distance (distance does NOT take into account)
Define Distance
- Measure far the object is from ourselves - the third dimension
- Perception of distance is dependent on the processing of visual direction
- The perception of distance and the perception of object size are interrelated
Visual Space
- The visual system uses directionality, along with other cues to build its interpretation of 3-D world
- NOT always the same as real space
- Visual system assume its interpretation of the world is correct - this could incorrectly causing optical illusions (mismatch between optical space & physical space)
Define Local Sign
The capacity of visual neurons to process direction
Each visual neuron has a local sign, a unique line of sight from a given point on the retina passing through the nodal point of the eye & out into visual space
The two main purposes of vision are to see..?
What they are & Where they are
- Any system that specifies location or direction must define these with respect to some reference point (Ex. Rm 103 is next to Rm 102)
- For an individual, combined sensory input provides information about where objects are located relative to himself (The computer is in front of me)
Define Egocentric Direction
Under binocular conditions, we see directions not relative to each eye alone but relative to a single reference point or origin, point in the head known as the egocenter
Egocentric localization
- Egocentric direction form of directionality is called egocentric localization
- Egocentric localization specifies the position of objects relative to the egocenter (where things are located w/ respect to you)
- The brain receives directional information form the two eyes & combines them into one unified sense of direction
- It is almost as if we see things from a single eye that is located at the egocenter
- Aka cyclopean eye representing a single imaginary eye midway between the 2 eyes
- When asking a 2-3 year old child to view an object through a tube, he may bring it to a point between the eyes & then shift to either eye to see
Oculocentric Direction
- The brain recieves input from the two eyes, and computes the egocentric direction of an object based on 2 critical information
- The retinal location of the objects image in each eye. (Oculocentric direction)
- Each eye’s orientation, of direction of gaze
- The brain receives direction of gaze information from proprioception within the EOM and/or from the oculomotor neurons that drive the muscles
Oculocentric localization
- Each retina has its own oculocentric direction
- the retinal position of the eye can be specified w/ reference to the fovea
- The fovea represents the origin in the field of vision for one eye, since it corresponds to the fixation point, or straight ahead
- Direction relative to the fovea of one eye is called oculocentric localization
- If eye is point straight ahead & image falls on the fovea, binocular will interpret the object’s direction as straight ahead
- If the object is located on the right & foveally fixate it (assuming your head does not move), the object direction is on the right (eyes will rotate right to foveally fixate on the object)
- To correctly compute visual direction, the brain must also take into account the eye’s orientation
- It is important to make a distinction between egocentric & oculocentric localization, they may/may not be the same, depending on the viewing situation
When gazing at an object located directly in front of the right eye, the oculocentric direction will be ______, but the egocentric direction is ________
The oculocentric direction will be straight ahead for each eye, but the egocentric direction is slightly to the right of center.
True or False: The egocentric localization is always the same direction as the oculocentric direction in each eye
False, it is possible for an object to appear straight ahead by egocentric localization, but have a different oculocentric direction in each eye
While the eyes are looking straight ahead at a distant object, a nearer object (A), is percieved as _______ (egocentric localization), even though the oculocentric direction for the right eye is (temporal retina) _____ and for the left eye is _____ (temporal retina)
Straight, left, right
The cyclopean eye helps us to understand the ____ direction.
egocentric
The brain combines two ____ directions from each eye and creates a new ____ sense of direction (that may be different from either of them)
oculocentric, egocentric
Define Visual line or Line of sight
line of passing from an object in the visual field of one eye, through the nodal point, to the retina
- Visual lines may be used for objects located anywhere in the VF including the fixation point as well as peripheral object
Define visual axis
primary visual line
since it passes from the fixated object to the fovea
- other visual lines point to objects located off visual axis and these are associated w/ points on the retina other than the fovea
- All points in space falling along this visual line, at any distance from the eye, appear to lie in the same direction of the VF
Laws of Oculocentric Visual Direction (Hering’s Law of Visual Direction)
- Each point on the retina has one unique visual direction associated with it (oculocentric direction)
- One can draw a visual line (retinal point > Nodal point > . object in space)
- Different visual line = different visual direction (different directions)
- Same visual line = same visual direction
- Each retinal neuron has a particular visual direction
- local sign is a direction associated with a particular neuron (neuron stimulated by image/light elicit same sense of oculocentric direction)
- High-order neurons that receive information from the retina (LGN, visual cortex) also have local signs associated w/ them.
Describe the Laws of Identical Visual directions & correspondence
- Every visual line in the VF of one eye has a corresponding visual line in the other eye, & the corresponding visual lines have identical perceived visual directions
- If an object is seen by the right eye, and the same object seen by the left eye have the same visual directions from two eyes, they will easily fuse into a single visual direction in the cyclopean eye
- Having the same visual direction in 2 eyes & fusing them into one is a fundamental principle of binocular vision (fusing 2 images from 2 eyes into one)
- The visual direction of fused images that fall slightly on disparate retinal points is the average of the two visual directions
- If the visual directions to an object seen by two eyes are not exactly the same. Then there are three possibilities
- Reconciliation: The brain will fuse the 2 images & assign it as a cyclopean visual direction.
- Suppression: The brain will ignore one eye
- Diplopia: The object will be seen in 2 different directions
Directionality (visual direction) can be disrupted by retinal disorders.
What are some examples?
What does this result in?
How can it be tested?
Examples: Central serous retinopathy
Fluid beneath retinal layers disrupt the photoreceptors & its orientation
Results in metamorphopsia (a spatial distortion of vision)
Can be tested using Amsler Grid
