5 - BINOCULARITY AND CONVERGENCE

Question 1

binocular disparities

Answer 1

‘differences in the images of the same things as they appear in the two eyes’

Question 2

converge

Answer 2

the eyes turn inwards to look at something nearby with both eyes

• useful for close objects

Question 3

diverge

Answer 3

the action of the eyes turning outwards as to transfer gaze to things further away

Question 4

vergence angle

Answer 4

the angle between the two lines of sight from the centre of the foveola and through the middle of the pupil for each eye

• ‘not an image cue as it doesn’t involve stimulation of the retina’

small vergence angle = far object
large vergence angle = nearby object

Question 5

disconjugate eye movements when focusing on something close

Answer 5

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Question 6

symmetrical convergence

Answer 6

to fixate on an object directly in from of you, the eyes are required to turn by the same amount so that they’re in corresponding positions in the socket

Question 7

asymmetrical convergence

Answer 7

the object that is being fixated is off to one side so the eyes turn in different positions that are not corresponding

perhaps only one eye turns

Question 8

using convergence to calculate object distance from the head

when the object is straight ahead of one eye

Answer 8

forms a right angle triangle:

• the interpupillary distance forms the BASE of the triangle
• the two lines of sight from the eye form the other two lines of the triangle
• the straight line (eye in which the object is directly in front of) forms D (distance of fixated object from eye)
• D can be calculated from the vergence angle (y) and the interpupillary distance using trigonometry

distance (D) = interpupillary distance / tan(y)

• ‘if you can measure (sense) your state of convergence then you have information about the distance of the fixated object in units of interpupillary distance
• can convert into body-scaled units

Question 9

absolute distance cue

Answer 9

one that provides information about distance in actual known units

eg the vergence angle in eye lengths or interpupillary distance

Question 10

extraocular cue to distance

Answer 10

• the extraocular muscles and their tendons get information about the convergence through their sensors

Question 11

how prisms work

Answer 11

• ‘made so that the differential refraction effects are kept as small as possible’
• ‘misalignment in your eyes, your optometrist will prescribe prismatic spectacle lenses to correct for it’
• light enters the prism and leaves the prism in another direction
• ‘the size of the change in direction depends upon the thickness of the fat end (base) of the prism’ and whether the direction change is to the left or the right depends upon whether the base is on the left or right’
• prism based left = moves left
• prism based right = moves right
• changes the direction that the objected appears to be located
• the eye then needs to turn to fixate on the object in the fovea

Question 12

when both eyes are open but only one prism

Answer 12

• the convergence is different to normal but the perceived direction of the object is not altered

Question 13

usually needs lots of cues together to figure it out

• having only small amounts of info about the actual distance means you have to guess more
• conservative
• brain combines them together, resulting in a perception of the three dimensional world with objects appearing to be laid out in depth
• more cues = better accuracy and better at determining distance and size of objects

Answer 13

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Question 14

‘perceived distance is greater when the vergence specified distance is larger’

Answer 14

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Question 15

placing prism on nasal size = apparent location comes further away

Answer 15

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Question 16

familiar size cue and vergence cue

Answer 16

only cues that give information about absolute distance

- focus on just one object rather than a whole scene

Question 17

how does the vergence cue differ from the others?

Answer 17

it’s extraocular rather than being image based

• ‘provides a scale factor for the relative distance cues, which allows you to convert the relative distance information you get from these cues into absolute distances (eg distances in a specific unit of distance)

Question 18

telestroscopes

Answer 18

• optical device invented by Hermann von Helmholtz
• system of mirrors which can increase the distance between the eyes
• scene looks smaller and closer
• like a small scale model of the scene
• due to EMMERTS LAW and VERGENCE CUE
• looking through the telestroscope causes the eyes to basically be positioned at the outermost mirrors, increasing the interpupillary distance (interocular separation)
• this larger interpupillary distance increases the convergence required to fixate on the object
• this then also increases the vergence angle
• so this makes the objects (the whole scene as it’s the interpupillary distance) seem closer
• ‘the whole scene is scaled to the fixation distance’
• in a way, the fixated object acts a scale factor for the whole scene

Question 19

why does everything look smaller and closer through a telestroscope?

Answer 19

• due to emmert’s law
• it increases the interpupillary distance but it does not change the sizes of the image
• ‘image size remains approximately the same’
• according to emmert’s law, if something is perceived to be closer then it would also be perceived as appearing to be smaller as well

Question 20

at a distance over 100m, vergence and binocular disparity don’t work

Answer 20

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Question 21

advantages of 2 eyes

Answer 21

• spare eye
• greater field of view (eg rabbits, horses have almost a 360 view - only can’t see under itself) due to binocular visual field (both eyes can see)
• binocular summation - ‘detect things better with two eyes (in low light levels etc due to two versions of similar thing)’
• ‘binocular threshold intensity for seeing a faint stimulus is lower by a factor of (square root 2 /2) than monocular threshold’
• this ‘contributed to evolution of forward facing eyes in nocturnal animals’
• binocular stereopsis (3D) - ‘slightly different perspective from both eyes - binocular disparities’
• ‘allows seeing more detailed images (depth) in 3D images’
• convergence as a depth cue
• angle of convergence - scale relative cues in the image

Question 22

how do we perceive information about the location of a light in a dark and empty room?

Answer 22

• ‘the light is seen somewhere relative to you’
• ‘no retinal image about its distance or direction’
• ‘use kinesthetic information about the position of the eyes in the head and position of head on shoulders’
• shows that ‘visual perception is not based exclusively on retinal stimulation’
• ‘involves proprioceptive afference and efference as well
• ‘distance information of (fixated) spot is provided by proprioceptive info about convergence angle of the eyes’

Question 23

proprioceptive afferents and efferent

Answer 23

• muscular tissues and tendons
• muscular afferents = ‘represented by axons of sensory neurons that reside within the spinal dorsal root ganglion. These pseudo-unipolar neurons transfer sensory information from muscles to the spinal cord via spinal nerves’
• efferent = ‘the efferent fiber is a long process projecting far from the neuron’s body that carries nerve impulses away from the central nervous system toward the peripheral effector organs (mainly muscles and glands). A bundle of these fibers is called an efferent nerve (if it connects to muscles, then it is a motor nerve)’