Light Perception & Adaptation

Light Perception

We are going to discuss how the eye can distinguish between light and dark at pigment level.

• photopigments
• rhodopsin
• rhodopsin vs. iodopsin

Adaptation

A discussion on what happens in the eye under different light intensities.

• Dark adaptation
• Light adaptation

Light Perception

a) Photopigments

Rods and cones, the photoreceptors in our retina, are responsible for detecting light. How? With the use of light sensitive pigments.

Each pigment is made of a protein called opsin and a chromophore called retinene, (a variant of Vitamin A) .

Photoreceptor	Rod	Cone
Photopigment =opsin + chromophore	rhodopsin	iodopsin
opsin	scotopsin	?
chromophore	retinene	retinene

b) Rhodopsin

The following cycle of events takes place in rhodopsin when light strikes on rod cells in retina:

1. In the dark, opsin is bound tightly to retinene.
2. When light intensity is increased, retinene changes shape. This is a structural change from cis- to trans- form.
3. Opsin cannot hold onto retinene. Retinene comes off (this process is called bleaching).
4. Generator potential is produced when the membrane of rod is depolarized.
5. Generator potentials summate (add together) to form an action potential.
6. A nerve impulse is fired off to brain via the optic nerve.
7. Rhodopsin is reformed when retinene resumes its original shape using the energy in mitochondria (the power-house of every cell). This is light independent, i.e. does not need light.
8. Rhodopsin is ready to be bleached again.

Micrograph of many rods and 2 cones (x 10,000)

c) Rhodopsin vs. Iodopsin

Bleaching of iodopsin in cones is similar to rhodopsin in rods. But more light is needed to cause an action potential to be fired in cones. (i.e. Threshold intensity for cones is higher than rods.)

In other words, rods are mainly used for dim light vision, cones for bright light vision.

The use of both rods and cones to see is called the Duplicity Theory.

Vitamin A myth: You must have heard that if we do not have enough Vit. A in our body, we would have night blindness. Take note: 1) Excess Vitamin intake can become toxic and harm our body. 2) Lack of Vit A is not the only cause of poor vision. Vit B is also essential for normal functioning of the retina.

Adaptation

We need a period of time before our eye can adapt to new light conditions and see properly. This is called Adaptation.

1. Dark Adaptation

When we move from a lit room to a dark room, we cannot see clearly, because:
• not enough stimulated rhodopsin:
  rhodopsin is bleached faster than it is reformed in strong light, insufficient rhodopsin reformed instantaneously
• cones are not stimulated:
  light intensity too low (remember threshold intensity?)

It takes about 20 minutes for enough rhodopsin to reform for us to see properly.

2. Light Adaptation

When we move from a dark room to a brightly lit room, we feel uncomfortable from the glare. But after some time, the visual threshold in cones increases relative to the generator potential. Cones is less stimulated, and we will see better. This takes about 5 minutes.

Pilots wear red goggles in strong light to be less influenced by changes in light intensity.
Reason: red light stimulates rhodopsin (in rods) less, i.e. less rhodopsin is bleached; less time is needed for the eye to become dark-adapted.

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