Human visual system: L7

Question 1

electromagnetic spectrum

1. we are sensitive to
2. colour of light is determined by 3 dimensions

Answer 1

1. narrow range of wavelengths known as the visible spectrum. ranges - 380 nanometres to 760nm
2. (1) hue - wavelength of electromagnetic radiation
   (2) brightness - intensity of electromagnetic radiation
   (3) saturation - purity of electromagnetic radiation

Question 2

The human eye

1. contains
2. how it happens
3. eyeball filled with
4. light sensitive cells in the retina
5. optic disk
6. axons bundled at the optic disk

Answer 2

1. peripheral apparatus for transducing (transferring) light into a neural signal
2. light enters through cornea. behind the cornea is the lense which alters its shape to focus the image onto the back of the eye. This is lined by a light sensitive structure called the retina
3. fluid called vitreous humour
4. rods and cones send their axons out of the eye from a common point - optic disk
5. no photoreceptors = blindspot
6. optic nerve

Question 3

Cells of the retina

1. rods contain
2. number of cones and rods
3. cones are important for
4. cones are concentrated in a region called
5. rods are much more sensitive to
6. retina divided into 3 layers

Answer 3

1. photopigments which break down when exposed to light -> triggers stages leading to neural impulses -> conveyed to brain via optic nerve
2. 120 million rods, 6 million cones
3. fine details (most active in daylight)
4. fovea = responsible for central dew degrees of visual field
5. light than cones are = used in dimly illuminated environments
6. (1) photoreceptor: (photoreceptors, cones, rods)
   (2) bipolar cell layer: (bipolar cells, amacrine cells & horizontal cells)
   (2) ganglion cell layer: (ganglion cells)

• > rods & cones form synapses with bipolar cells -> ganglion cells -> send their axons through the optic nerve
• > horizontal & amacrine cells combine messages from several photoreceptors
• > photoreceptors and bipolar cells DONT produce APs. They release neurotransmitters that increase/decrease rate of firing of APS generated by ganglion cells

Question 4

Three cone types

Answer 4

1. short-wavelength cones (S): peak sensitivity - 440nm, blue light
2. medium-wavelength cones (M) peak sensitivity - 530nm, green light
3. long-wavelength cones (L) - 560 nm, red light

Question 5

Ishihara colour plates

1. colour blindness
2. responsible gene
3. deficient in
4. most common
5. Ishihara used to test?

Answer 5

1. genetic condition, abnormalities in the photo pigments of 1+ cone types in retina (S, M or L)
2. located on the X chromosome (more common in males who have 1)
3. discriminating certain hues
4. red-green deficient, 10% m, 1% f
5. abnormalities of colour perception

Question 6

Visual pathways to the brain

1. pathway
2. visual field + hemisphere

Answer 6

1. optic nerve -> optic chiasm -> lateral geniculate nucleus (one in each hemisphere) -> primary visual cortex
   - 90% of LGN axons terminate in the primary visual cortex, 10% project to other areas e.g. superior colliculus (midbrain) and pulvinar nucleus of the thalamus
2. temporal half of retina (outer side of eye) travel to the same hemisphere, but the nasal half crosses over to the opposite hemisphere
   = visual info from right visual field -> left hemisphere (& the opposite)

(primary visual cortex in each hemisphere represents visual information from the contralateral half of the visual field NOT the contralateral eye)

Question 7

Interactions in the retina

1. each ganglion cell responds to

Answer 7

1. signals from one small cluster of photoreceptors