vision 2 Flashcards
what is a receptive field
the receptive field is the region in space in which the presence of a stimulus will affect the electrical activity of a neurone
how is size of visual receptive field measured
size of visual receptive fields in retina and in the brain are measured in degrees
theta= 2arctan(h/2D)
theta is the width of the receptive field at a given distance
D is distance of h away from retina and h is width of a receptive field
describe the structure of ganglion cells
on centre ganglion cells have excitatory regions in the centre and inhibitory regions on the outside, off centre ganglion cells have ann off centre and excitatory outside
light in the centre of on ganglion cells causes increase in AP firing rate, light in centre of off centre ganglion cell causes inhibition of APs, the opposite is the case if light hits the outer parts of the cell
what does uniform light exposure to ganglion cell cause
uniform light in centre and surround causes firing at a slow rate
“surround” illumination antagonises the response to light in the centre, ganglion cell receptive fields are therefore referred to as centre surround antagonistic receptive fields (CSARFs)
what is the benefit of ganglion cell structure, how is this enabled
centre surround receptive fields detect and emphasize edges
formation of centre-surround receptive fields is formed by lateral inhibition from horizontal cells
photoreceptors form inhibitory glutamatergic synapses with horizontal cells, horizontal cells synapse onto GABAergic synapses to other photoreceptors, enhancing the response of the postsynaptic bipolar cells to lightm GABA release is max in the dark
how are photoreceptors indirectly connected to bipolar cells
via lateral inhibitory inputs from horizontal cells
what are main types of ganglion cells
parvocellular/midget, and magnocellular/parasol
different size of receptive field enables different range of levels of detail and contrast
parvocellular/midget: can either be on or off centre, have sustained responses to light, small physical size and small receptive fields, occupy mostly the centre of visual field (fovea), carry information about colour, centre and surround sum linearly
magnocellular/parasol: can either be on or off centre, have transient responses to light, large physical size and large receptive field, are distributed across the whole retina, center and surround do not always sum linearly, receives info from many photoreceptors via BPs
describe colour opponency in ganglion cells
parvocellular have colour-opponency: in that the centre and surroudn regions are responsive to different wavelengths of light, can be red/green with either red or green on outside and other in the middle, outside and inside can still be both on or off, called M-L opponency
parvocellular cells also have yellow blue opponency, but only with blue in the middle as excitatory and yellow on the outside as inhibitory, termed S-(L+M) opponency since the yellow is made from L+M
magnocellular cells have no colour opponency: M L and rare S are mixed, only detect differences in luminance levels
what is pattern of ganglion cells in retina
mosaic pattern between different types
what do axons of retinal ganglion cells synapse onto
superior colliculus which controls eye movements
pretectum which controls pupillary reflex
lateral geniculate nucleus which is main route to visual cortex for visual perception, 90% of retinal ganglion cell axons go here
how are parallel pathways organised
all functionally parallel pathways are kept seperate on their path to visual cortex and are anatomically organised by right and left eye, by ganglion cell type and their location on the retina
describe the receptive field property of neurones in the lateral geniculate nucleus
lateral geniculate nucleus neurones inherit their receptive field properties directly from their ganglion cell inputs (have same centre/surround structure as their inputs from ganglion cells)
where is the visual cortex
located at the back of the brain in the occipital lobe
how do ganglion cells connect to the visual cortex
parvocellular information is relayed from regions V1 to V2 to V4 to the temporal lobe, the parvocellular pathway,
the magnocellular pathways runs from V1 to V2 to MT to the parietal lobe
describe structure of visual cortex
the visual cortex (V1) is a layered structure, with 6 layers, layer 1 on top and 6 on the bottom, layer 2 transmits to V2-5 and MT, layer 5 to subcortical layers, information is transferred between layers
axons from thalamic relay cell innervate layer 4 in patches that are segregated according to eye origin, right and left eye inputs alternate, these are called ocular dominance columns, mixing of input from 2 eyes occurs in upper and lower cortical layers via intracortical circuitry
