Lecture 7 Convergence and Accommodation

Question 1

The Near Reflex

Answer 1

When you look up close, three things need to happen:

1. You need to converge to keep the object of fixation on the fovea (stimulated by disparity)
2. You need to accommodate to refocus the light on the retina (stimulated by blur)
3. Your pupils need to constrict

This is called the near reflex. It is a synkinesis, each response is elicited by the nearness of the object and occurs together. However, they can also each be elicited separately.

Question 2

The Near Reflex - Pathway

Answer 2

• The pathway by which the near reflex is exerted starts with information about blur and disparity being sent via the visual pathway to the visual cortex
• The information is transmitted to the midbrain of the brainstem
• It transverses through a nucleus called the Edinger-Westphal nucleus and then goes to the 3rd cranial nerve (occulomotor)
• The 3rd CN innervates:
  
  • The ciliary muscle to produce accommodation
  • The pupillary sphincter to produce pupil constriction and
  • The medial recti (R and L MR) to produce convergence
• The near respose then, can be impacted by ocular problems as well as midbrain and 3rd CN

Question 3

What is motor fusion, and its different types and what is the stimulus for this response?

Answer 3

Disparity, ability to fuse 2 dissimilar images because they have disparity for motor fusion. The different types include positive and negative (convergence and divergence motor fusion). You also have positive relative fusion and negative motor fusion. Like converging without accommodating (leaving it steady - positive)

Question 4

Convergence

Answer 4

• Occurs when an object is moved closer due to disparity
• The image of a close object when the eyes are fixated at distance will fall in front of the horopter e.g. on temporal retina
• The eyes will have to move in (converge) to re-fixate the object on the fovea (maintain BSV)
• Convergence can also be elicited voluntarily.
  *