the visual system - introduction and spatial location Flashcards
what is the environmental stimulus that allows us to see?
light
what is a photon?
one unit/packet of energy
wavelength and frequency are the 2 properties of waves. what do each represent?
wavelength (nanometres) - the length of each wave from each point at the bottom.
frequency (Hz) - the number of times an event repeats within a given amount of time.
what does it mean by saying that wave properties are inversely proportioned?
high frequency means low wave length. low wave length means high frequency.
what is the first stage of visual information processing?
the retina
how is the retina mapped?
3 vertical layers of cells
- photoreceptors (rods/cones)
- bipolar
- ganglion
what are photoreceptors and what do they do?
they are the input cells. they release neurotransmitters which control the activity of the bipolar cells that synapse with them,
how does the ganglion cell pass on information?
the axons of ganglion cells form the optic nerve, which carries information from the eye to the brain.
what can be said about rods?
- about 120 million of them
- primarily black and white receptors
- used for night vision
- gives information about brightness
what can be said about cones?
- about 4 million
- colour receptors
- less sensitive, used for day vision
- densely packed in the fovea
the amount of neurotransmitter released from the axon terminal depends on…
the level of light produced.
name some characteristics of the photopic system?
- cones
- 4 million per eye
- rapid temporal responses
- concentrated in the fovea
- the basis of colour vision
- low sensitivity
name some characteristics of the scotopic system?
- rods
- 120 million in each eye
- slow temporal responses
- larger, less acuity field
- located outside the fovea
- used for night vision - high sensitivity
what is transduction?
the process of rods and cones detecting electromagnetic energy - evironmental stimuli are transformed into a neural impulse
what do photopigments (found in photoreceptors) do?
they capture the photon of light that strike the cell.
what is meant by the term that photoreceptors use a secondary message system?
the neurotransmitter binds to the gprotein receptors which break off and bind to a channel.
light ………. the photoreceptor
hyperpolarises (further from 0, more negative). this reduces neurotransmitter release.
when do photoreceptors release neurotransmitters?
in resting state.
which cells in the retina produce action potentials?
ganglion cells.
which are the 2 classes of ganglion cells?
magnocellular (M)
parvocellular (P)
what are the characteristics of magnocellular cells
- think RODS
- have large receptive fields
- respond to large objects
- the analysis of MOVEMENT
- think M for movement
what are the characteristics of parvocellular cells?
- think CONES
- have small receptive fields
- involved in colour vision
- the analysis of fine detail
what is the optic disc?
the area where ganglion cells and blood vessels exit the eyeball. located on the nasal side of the fovea.
what are saccades?
fast movements of the eyes that present various parts of the visual system in the fovea.
what is the optic chaism?
the point at which the two optic nerves meet.
where do temporal retinal cells send their exons? (which side of head)
to the same side of the brain - eg do not pass through the optic chaism
where do nasal retinal cells send their axons? (which side of head)
to the opposite side - across the optic chaism.
where does information go once it has travelled along the optic nerve?
the Lateral Geniculate Nucleus (LGN) - located in the thalamus
what function does the LGN serve?
- it receives information via optic tract and sends it to visual areas in the occipital lobe
- information is kept segregated - Marvocellular and parvocellular
how is the LGN mapped and which cells are in which layers?
retinotopic mapping - 6 layers.
layers 1,4 + 6 receive input from the eye on the opposite side
layer 2, 3 + 5 receive input from the eye on the same side.
spatial location is therefore mapped onto the LGN
what do we know about area V1?
information is passed here via optic radiations from the LGN.
spatial location is mapped onto the visual cortex.
