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PPL » Human Factors » Vision and Visual Perception

10.18 Vision and Visual Perception

10.18.2 Identify the following eye structure components:

(a) lens

(b) cornea

(c) retina

(d) fovea

(e) optic nerve disc

(f) cone cells

(g) rod cells.

 

Vision

One of the most important medical requirements for a pilot is to be able to see, and see well. 

The Eye

The eye detects light energy of different colours.

When light energy enters the eye, it passes through the tough protective lining of the eye, the cornea, and through the lens of the eye which together focus the incoming rays against the back of the eye (the retina).

If the light energy is strong enough, it will cause a light chemical reaction within the retina and this generates a nerve signal which passes from the retina to the vision centre located at the back of the brain.

The overall light signal or visual image is then interpreted by the brain - interestingly we can see a single candle out to 30 kilometres. 

 

10.18.4 Distinguish between rod and cone cell functions and distribution in the retina. 

Rods

  • In dim light, we use our rods, which cannot work in bright light.
  • Rods outnumber cones (120 million rods and about 6 million cones in each retina) and they amplify a light signal much more than cones.  
  • Another retinal mechanism that helps us to see in dim light or to see a tiny amount of light in the dark is the convergence of rod cell signals onto other retinal neurons.
  • Many rods, up to 150, contact onto one another, where the signals are pooled and reinforced, which increase’s the ability of the brain to detect a small amount of light.
  • This convergence amplifies weak signals, but spatial resolution is lost because rod responses are averaged. That is, we cannot see fine detail using rods.
  • In order for our eyes to make the transition to dim light, rods must adapt after being saturated with light in brighter conditions.
  • The rods take about ten minutes to half adapt and 30 minutes to fully adapt to the dark.
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