1
Q
Describe the law of visual direction?
A
- 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
2
Q
The location of an object in space is determined by what two values?
A
Direction
Distance relative to ourselves
3
Q
Define visual direction?
A
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)
4
Q
Define Distance
A
- 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
5
Q
Visual Space
A
- 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)
6
Q
Define Local Sign
A
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
7
Q
The two main purposes of vision are to see..?
A
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)
8
Q
Define Egocentric Direction
A
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
9
Q
Egocentric localization
A
- 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
10
Q
Oculocentric Direction
A
- 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
11
Q
Oculocentric localization
A
- 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
12
Q
When gazing at an object located directly in front of the right eye, the oculocentric direction will be ______, but the egocentric direction is ________
A
The oculocentric direction will be straight ahead for each eye, but the egocentric direction is slightly to the right of center.
13
Q
True or False: The egocentric localization is always the same direction as the oculocentric direction in each eye
A
False, it is possible for an object to appear straight ahead by egocentric localization, but have a different oculocentric direction in each eye
14
Q
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)
A
Straight, left, right
15
Q
The cyclopean eye helps us to understand the ____ direction.
A
egocentric
16
Q
The brain combines two ____ directions from each eye and creates a new ____ sense of direction (that may be different from either of them)
A
oculocentric, egocentric
17
Q
binocular perception of direction is ____
A
egocentric
- Visual directions are all relative to point in our head, referred to as the egocenter
- Our brain computes the visual direction based on oculocentric direction in each eye and orientation of each eye
18
Q
True or False: Egocentric and oculocentric localization may or may not be the same, depending on the view situation
A
True
19
Q
What are the Hering’s Laws of Visual Direction?
A
- Visual Line or Line of sight
- Visual Axis-Primary visual Line
- Principal Visual Direction
- Secondary Visual Direction
- Hering’s Laws of Visual Direction
- Law of oculocentric visual direction
- Laws of Egocentric (cyclopean) visual direction
- Laws of identical visual directions and correspondence
20
Q
Define Visual line or Line of sight
A
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
21
Q
Define visual axis
A
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
22
Q
Laws of Oculocentric Visual Direction (Hering’s Law of Visual Direction)
A
- 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.
23
Q
Decribe the Laws of Egocentric Visual Direction (Cyclopean)
A
- Helps us understand how oculocentric visual direction contributes to egocentric direction
- The positions of all objects in space are judged as if seen by an imaginary cyclopean eye. It is as if data from each eye is transferred to the cylcopean eye
- The cyclopean eye also has its own visual directions. It has a primary visual line & other visual lines, each of which have different visual directions
- An object seen on the primary visual line of either eye will be transferred to the primary visual line of the cyclopean eye
- If a peripheral object and its visual line make some angle with the primary visual line in one eye, the cyclopean eye’s primary & peripheral visual lines will form the same angle and have same geometry
- The direction of primary visual line in the cyclopean eye is the mean of the directions of the primary visual line directions received from the two eyes
24
Q
Describe the Laws of Identical Visual directions & correspondence
A
- 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
25
Perecept of Motion
* Can be described as **change in the visual direction** of the object as function of time when near by (or one, then another) retinal locus is stimulated by its image
* Our finest judgements of visual direction -\> central vision (fixation).
* Because of **small receptive fields** at the fovea & magnified portion of cortical structure devoted to central vision.
* Can appreciate smaller displacement of object when neighboring receptive field is stimulated
* Judgements of visual direction falls off in -\> peripheral loci
* Because of **larger receptive fields**
* Takes a larger displacement for an object to be seen in a different direction
26
The fovea is sensitive to _____ and the periphery of our retina is sensitive to _____ ? (Visual Direction & Motion)
Small displacements or slow velocities of motion at the **fovea**
Large displacements or faster velocities of motion in the **periphery**
27
What are some clinical examples of visual direction disruption?
1. Central serous retinopathy
2. Pt wearing new spectacles or prisms or when they remove it
3. Binocular vision issues (**Paretic muscle, strab, eccentric fixation**)
4. Cerebellar diseases associated w/ abnormal eye mvmts.
28
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**
29
Define Past pointing
* Pt will make errors when attempting to point toward objects in VF
* Occurs in pt w/ **paretic muscle** (strabs) and **cerebellar disease**
* When a pt w/ paretic muscle is asked to fixate into the field of action of the affected muscle, the pt will reach past the object- **beyond actual location**
* **Reason** for this phenomenon happens due to **mismatch** between actual eye position and the amt of innervation required to move eye to its final position
* Interpreted by **Von graefe** as change in **egocentric localization**
* Occurs in **patients wearing new spectacles or prisms** when they remove it
* Moves the entire visual space several degrees to one side
* Initially pt will make errors when attempting to point towards objects in VF
* After a **trail period** (adaptation) the visual system will remap itself to associate the stimulation of new relational location w/ particular visual direction
30
**True or False:** Distance is a factor in visual direction
False. Distance is NOT a factor in visual direction. It is a 2D localization of objects taking into accountly ONLY the direction.
31
**True or False.** All objects falling along a line of sight appear to be in different directions
False. Same direction
32
**True or False.** Diseases that affect photoreceptor orientation disrupt local sign yielding metamorphopsia (spatial distortion of vision)
True
33
**Local sign of fovea**
monocular vision in oculocentric, w/ visual direction perceived relative to the principal visual direction
34
As the eye moves so does the ____ visual direction as do all ____ visual directions
Principal, Secondary
35
The law of oculocentric visual direction states that..?
superimposed retinal images will be interpreted as arising from objects in the same visual direction
36
**True or False.** Binocular vision is oculocentric
False, egocentric
Images formed on both foveas are seen in a single straight-head common subjective principal visual direction, as if their images actually had formed
37
**True or False.** Whenever the image of an object is formed on corresponding retinal points in each retina, the object will be seen lying in a single common subjective visual direction
True
38
The law of identical visual directions states that....?
Objects with the same direction in each eye will be perceived as being in **single visual direction** under binocular visual conditions
39
Oculocentric Direction Vs. Egocentric Direction
* **Oculocentric Direction**
* ****Monocular
* Eye-based
* **Fovea** is center of the coordinate center
* Objects are referenced with respect to fovea
* **Egocentric**
* ****Binocular
* Body-based
* Center of the trunk along midline is the center of the coordinate system in normals
* Objects are referenced with respect to the body
* Subjective straight-ahead is within -/+2 degs of objective zero in normals, so it is very accurate
40
How does the brain compute Egocentric Direction?
* MUST HAVE INPUT FROM BOTH **oculocentric & headcentric directionality** (position of eyes in the head) to compute an egocentric direction
* **Headcentric directionality** info is provided by the efferent motor signal to the EOM
* **Direction of gaze** proprioception within EOM & oculomotor neurons that drive the muscles
* **Law of Headcentric localization -** states that for a given position of the eye in the head, objects lying on the same line of sight are seen in the same **headcentric visual direction** (direction of gaze)
