Accommodation and Extrinsic Eye Muscles

What is accommodation

the ability of the lens to FOCUS objects at different distances

What does accommodation require?

requires changing the shape of the lens achieved by the ciliary muscle. When they contract then lens rounds for closer vision (tension on suspensory ligaments is reduced) . When they relax the lens flattens for distant vision

Ex) When object gets closer

its image moves behind the eyeball, the lens accommodates by thickening its shape to reflect the image on the retina, specifically the fovea centralis

Accommodation procedure

wire screen is held 10in from face → focus on distant object → quickly focus on screen

• demonstrates their eyes’ ability to focus on objects at different distances

• When opening + closing eyes: determines what the lens defaults to, either near (convex) or distant vision (concave)

what is the near point

the closest distance at which the human eye can focus clearly on an object through maximum accommodation

Near point procedure

• Near point determines?

hold meter stick at eye level, move small object toward one eye until it first begins to blur, this distance is then recorded in centimeters (cm)

• the near point determines visual age

How is accommodation measured?

measured (cm) using the near point of vision, the closets distance in which an object remains in focus

How did we use the near point to determine visual age?

by comparing measured near point (cm) to a standard table you can determine your visual age W

Why does the near point suddenly jump between the ages 40-50

As you age, the lens loses elasticity (presbyopia), causing the near point to increase. A major jump occurs between ages 40(18cm) and 50 (53cm) because of presbyopia

What is presbyopia

“old eyes”

the ability to focus on close objects decreases because as a person ages due to a decline in the elasticity (pliability) of the lens

Chronological age vs. Visual age

how old you are vs. how old your eyes are

What is Nsytagmus?

Rapid, involuntary eye movement. If present when tracking eye movement it may indicate weak muscle tone or damage to the extrinsic muscles or the nerve innervating them (III, IV, VI)

Extrinsic eye muscles procedure

track pencil moved in different direction

convergence: move the pencil slowly towards the subjects nose

What do you see in Nystagmus?

antagonistic muscle is trying to keep the eye in the middle. damaged eye will drift slowly in the direction of the stronger muscle and then rapid movement back to where you are focusing on

EEM: What is retinal disparity, and why is it important for depth perception?

retinal disparity is the slight difference between the visual fields of the right and left eye, because they are positioned differently they each receive a slight different image of the same object. The disparity is essential for depth perception because it allows the brain to process the overlapping information and determine how far an object is

EEM: What is convergence?

convergence is an eye movement where the extrinsic eye muscles adjust the angle of they eyes so they both point inward towards a close object. This movement is important to maintain the overlap between the visual field of they eyes and the proper retinal disparity as an object moves closer to the face.