Human vision and function/Part 1: How the eye works/1.3 Light stimulus and the eye

1.3 Light stimulus and the eye
This topic will consider what happens when light strikes a photoreceptor (rod or cone) and how the retina processes this light in order to create a visual image.

Task 1
Task 1 requires you to do a short activity in order to help you understand the concepts of light sense and vision.

Light sense and vision
The concept of vision involves a process that begins with a stimulus. Generally speaking, that stimulus is light. Light enters the eye and undergoes optical refraction (bending) as it passes through the structures of the eyeball that are transparent. It is transmitted to photoreceptor cells within the retina and evokes a chemical reaction. This then evokes an electrical stimulus which travels along the optic nerve and the visual pathways: optic chiasm (crossing), optic tract, lateral geniculate nucleus (LGN), optic radiations to the primary visual cortex in the brain. The light rays form an image on the retina and the amount of electrical impulses transmitted to the brain are interpreted, and we 'see'. We 'see' with our brain. Different parts of the brain, for example memory, are then involved in the process of perception.



Visual awareness of the environment can be studied in separate categories, although all functions are simultaneous.
 * Light sense - differentiating between light and dark
 * Colour sense - discrimination of different wavelengths of light
 * Form sense - discrimination of the parts of an image of the retina

Activity
Let's observe how the sensory cells within the eye perceive vision and how sensitive they are.

What you need:
 * 1) 3 different coloured pens or texta lids (eg: red, blue, green)
 * 2) A volunteer (friend, family member, neighbour etc)

With your volunteer, sit opposite each other (examiner & observer) about 0.5 m apart. Use palm of hand to cover one eye. Observer stares at the examiner’s nose, without moving their eye. Examiner slowly brings a vertically held coloured pen lid (colour unknown to subject) towards the centre: along the horizontal midline, then the vertical midline (holding horizontally), first from one side & then the other. This video about Confrontation Visual Field testing will show you the technique.

Things to consider

 * 1) What conclusions can you draw from your observations?
 * 2) Is your observer sensitive to changes for different coloured pen lids? Do they see one particular colour better in a different position or are they all the same?
 * 3) What could this mean about the eye receptors?

Task 2
Watch the lecture Eye Works 2: Image, Retina, Optic Nerve, Brain.

Supplementary reading
Neuroscience Online is a great reading resource to gain a deeper understanding of retinal processing and generally how the eye works. The sections that specifically relate to this topic are 14.3, 14.4 and 14.5.

Revision questions
Discuss the answers to these questions on your group LMS forum.


 * 1) What happens when a photoreceptor absorbs a photon of light?
 * 2) Apart from rods and cones, what other cells are found in the retina?
 * 3) Which photoreceptor is most sensitive to light?
 * 4) What three wavelengths of light are the cone photoreceptors sensitive to?
 * 5) In what section of the retina are the cones mostly found and what is the impact of this distribution?
 * 6) What is the blind spot?
 * 7) What happens to the fibres of the optic nerve at the chiasm?
 * 8) Vision is decoded in the occipital cortex. Where are movement and sensation decoded?
 * 9) How is the receptive field of the retina organised?
 * 10) Where in the brain does decoding of lines and edges occur?

/Answers/