vision Flashcards
DEFINE: orientation invariance
recognising objects in their natural orientation enables the brain to recognise objects in any orientation
DEFINE: scale invariance
recognising objects in their natural size enables the brain to recognise objects at any size
what do parvocellular ganglion cells process information about?
shape and colour
what are blobs in the cortex?
parts of the brain processing colour
what are the three types of columns in the cortex?
- ocular dominance
- orientation
- blobs
what are the simple cells?
- elongated neurons in layers 4 and 6 of v1 cortex responding to a bar in a certain orientation in middle of receptive field
what are complex cells?
- neurons in layers 2, 3 and 5 of v1 cortex responding to a bar in a certain orientation anywhere in the receptive field
what is the grandmother cell/jennifer aniston neuron?
- neuron responding to very specific objects. at the top of the hierarchal model.
ventral stream pathway?
parvocellular GCs –> LGN layers 3, 4, 5, 6 —-> v1 —-> v2 —> v4 –> inferior temporal cortex
which part of the eye has the highest visual acuity?
fovea
DEFINE: retinotopic map
neighbouring cells in the retina feed information to neighbouring cells in their targets
DEFINE: foveation hypothesis
retinatopic map and topographic map matches up
direction-selective neuron
neuron responds to motion in a particular direction
what is the morphology of a direction sensitive cell
dendrites located in one particular direction
when a stimulus moves in a null direction, what happens to the membrane potential of a direction selective ganglion cell?
excitatory input from bipolar cells = smaller and delayed
inhibitory input from amacrine cells = larger
neuron is still depolarised but does not reach threshold –> action potential does not fire
what kind of photoreceptors does the fovea mostly contain?
cones
DEFINE: visual acuity
the ability of the eye to distinguish 2 nearby points
amacrine cells
receive input from bipolar cells
project laterally to inhibit ganglion cells
horizontal cells
receive input from photoreceptors
project laterally to inhibit bipolar cells
outer plexiform layer
where photoreceptors synapse with bipolar cells and horizontal cells
inner plexiform layer
contains synapses between ganglion cells, bipolar cells and amacrine cells
what are the layers of the retina (innermost to outermost)
- ganglion cell layer
- inner plexiform layer
- inner nuclear layer - contains cell bodies of bipolar cells, ganglion cells and amacrine cells
- outer plexiform layer
- outer nuclear layer - contains cell bodies of photoreceptors
- layer of photoreceptor outer segments embedded in pigmented epithelium
when are rods active?
at dim light
when are cones active?
at bright light
how do photoreceptors respond in the dark
- channels permeable to Na+ and Ca2+ open
- depolarisation
large concentration of cGMP in the cytoplasm
how do photoreceptors respond to light
- photoreceptor activated by light
- g proteins activated
- g proteins activate phosphodiesterase
- phosphodiesterase converts cGMP to GMP
- GMP causes ion channels to close
- hyperpolarisation
OFF bipolar cells
- hyperpolarise in response to light
spot of light in centre of receptive field -> photoreceptor synapses directly onto bipolar cell -> hyperpolarise
annulus of light -> horizontal cells synapse onto bipolar cell -> depolarise
ON bipolar cells
- depolarise in response to light
spot of light in centre of receptive field -> photoreceptor synapses directly onto bipolar cell -> depolarise
annulus of light -> horizontal cells synapse onto bipolar cell -> hyperpolarise
what would be observed in the ganglion cells if the whole receptive field is illuminated?
no change in spiking
GCs respond to differences in illumination within receptive field
what % of ganglion cells do parvocellular GCs make up?
80%
