visual perception Flashcards
stages of vision?
3
object seen against background
seen by retina upside down in 2D form
seen in upright, 3D colourful form
what are the 4 classes of photoreceptors?
3 cone types
1 rod
how many classes of ganglion cells are there?
20-30
what are the first 3 stages in visual processing?
edge detection
edge enhancement
filtering of spatial, wavelength, movement and directional info
when does lateral inhibition occur?
and what is it?
what is the result on edges?
neurons in same layer of retina are interconnected (either by axons or interneurons) so each neuron inhibits its neighbours mutually
makes edges stand out more as inhibited photoreceptors report seeing less light than they actually do (looks darker)
when do lateral inhibitions cancel each other out?
when light falling on group of retinal neurons is uniform
so reciprocal inhibitions cancel each other out
examples of horizontal connection cells?
horizontal cells
amacrine cells
examples of vertical connections?
fovea - 1 cone linked to 1 bipolar
periphery - many cones to 1 bipolar and many bipolars to 1 ganglion
what forms a receptive field of a bipolar and ganglion cell?
and what actually is a receptive field?
cones or rods converging on a bipolar cell form its receptive field
converging bipolar cells on a ganglion cell forms its receptive field
receptive field - region in which stimulus will modify firing of that cell
what do bipolar and ganglion cells have?
centre surrounded (classical) receptive fields
how do horizontal cells influence bipolar cells?
either directly or by feeding back info to the cones (indirectly)
role of bipolar cells in defining activity of ganglion cells?
bipolar cell integrates inhibitory and excitatory post-synaptic potentials
signals from several of these cells define the activity of the ganglion cell
what do retina photoreceptors relase when not sitmulated?
glutamate
decreases when exposure to light
what happens when photoreceptors and bipolar cells exposed to light?
photoreceptors - exposure hyperpolarises and decreases rate of glutamate
bipolar cells -invert the receptor signal to the standard:
depolarisation when light intensity increases and hyperpolarisation when intensity decreases
(usually the opposite)
describe On-centre bipolar cells?
what is the sombrero shaped response?
inhibitory synapse in presence of glutamate in the centre
when light spot is in the off-surround, more inhibitory potentials than excitatory as more cones stimulated in off-surround than in on-centre
when light spot reaches ON-centre, more excitatory potentials than inhibitory as not inhibited by glutamate and more cones in the ON-centre are stimulated than in the OFF-surround
describe OFF-centre bipolar cells?
have excitatory synapse in presence of glutamate in the centre
turning off light in centre of off-centre excites as receives more glutamate which depolarises the bipolar cells
(opposite to on-centre bipolar cells)
response of retinal ganglion cells to light exposure?
centre-surrounded receptive fields show sombrero response similar to bipolar cells
how does On-centre ganglion cell respond to light?
when is it at maximum and minimum?
increases firing (spike) rate
at maximum when light spot covers whole centre as all cones stimulated
at minimum when all of surround but none of ON-centre covered
how does OFF-centre ganglion cell respond to light?
decreases firing (spike) rate
spike reduced if spot illuminates centre
spike increased if spot illuminates surround
(opposite of ON-centre ganglion cells)
what happens to the ganglion cell when the whole receptive field is equally stimulated (uniform illumination)?
ON and OFF centre ganglion cell is at rest and fires with a spontaneous frequency so doesn’t respond to uniform illumination
characteristics of P-ganglion cells?
small receptive fields
slow conduction field
high acuity
colour sensitive
mainly located in fovea
characteristics of M-ganglion cells?
large receptive field high conduction speed low acuity sensitive to motion no colour discrimination
difference between P and M pathways?
P - parvocellular small soma small cell
M- magnocellular large soma
project to different layers in V1 so remain segregated
what does LGN stand for?
what does it segregate?
lateral geniculate nucleus
segregates left and right eye projections
what is V1 and what projects to it?
V1 = first layer of the visual cortex (primary visual cortex) and is important for perceptual processing
m and p pathway neurons project to different layers in the V1
what structure does the V1 have?
columnar structure
what do neurons in the orientation column of the V1 share?
how is this shown?
who found this out?
same preference for a particular orietation of a bar stimulus in their receptive field
shown by maximal spike rate at that orientation
Hubel and Wiesel (nobel prize in 1981)
what have microelectrode recordings revealed about cells?
examples of this?
differ greatly in their receptive fields
visual cells in LGN have concentric receptive fields (like ganglion and bipolar cells) so respond when light in certain part of visual field
visual cells in cerebral cortex more responsive to bars of light and preference for orientation
what did Hubel and Wiesel propose about simple cells in orientation columns of V1?
that they receive input from several neighbouring retinal ganglion cells
difference between simple and complex cortical cells?
simple: respond best to edge or bar of particular location and orientation in visual field
complex: respond best anywhere within particular area of the visual field
what do the dorsal and ventral stream do?
dorsal - interacting with the world - where system
ventral - making sense of the world - what system
what is meant by perceptual shape constancy?
We recognise the same shape from different view points and directions despite the distortions in the retinal projections
how do receptive fields differ in the fovea and periphery?
receptive fields are large in the periphery (low acuity)
receptive fields are small in the fovea (high acuity)
