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Topic 5 visual system map 1: principles of perception (1-19), map 4:…
Topic 5 visual system map 1: principles of perception (1-19)
sensitive specialized receptors
convert energy into electrochemical patterns
law of specific nerve energies
a particular nerve's activity conveys the same type of info
influence perception
which neurons respond
amount of response
timing of respond
the eye & its connections to the brain
pupil
opening in the center of the iris
light enters the eye
light is focused by the lens and cornea
retina
light is projected
rear surface of the eye
lined with visual receptors
light from the left strikes the right side of retina
vision route
bipolar cells
messages from receptors sent to neuron
located closer to the center of eye
ganglion cells
messages from bipolar cells to neuron
located even closer to the center of eye
axons join together to form the optic nerve
amacrine cells
send info from bipolar cells to other bipolar, ganglion and amacrine cells
control the ability of ganglion cells
to respond to shapes, movements or other specific aspects of visual stimuli
blind spot
the point at which optic nerve leaves the back of the eye
contains no receptors
fovea & periphery of the retina
fovea
central portion of retina
acute & detailed vision
packed tight with receptors
nearly free of ganglion axons & blood vessels
midget ganglion cell
each receptor in the fovea attaches to a single bipolar cell & a single ganglion cell
responds to just a single cone
each cone in the fovea has a direct line to the brain
allow registering of the exact location of input
our vision is dominated by what we see in the fovea
periphery
greater number of receptors converge into ganglion & bipolar cells
less detailed vision
greater perception of fainter lights
adaptive arrangement of visual receptors
predatory birds
greater density of receptors on the top of the eye
rats
greater density of receptors on the bottom of the eye
map 4: visual cortex (35-50)
primary visual cortex (area V1)
receives info from lateral geniculate nucleus
responsible for the first stage of visual processing
damage to V1
blindsight
ability to respond to visual stimuli that they report not seeing
simple & complex receptive fields
simple cells
fixed excitatory and inhibitory zones
more light in excitatory zone, more cells respond
more light in inhibitory zone, less cells respond
receptive fields
respond to vertical and horizontal orientations outnumbering diagonal ones
bar-shaped or edge-shaped
complex cells
located in areas V1 & V2
have large receptive fields that can not be mapped into fixed excitatory or inhibitory zones
responds to a pattern of light in a particular orientation
responds most strongly to a moving stimulus
end-stopped or hypercomplex cells
similar with complex cells
has a strong inhibitory area at one end of its bar-shaped receptive field
respond to a bar-shaped pattern of light anywhere in its large receptive field
provided the bar does not extend beyond a certain point
feature detectors
neurons whose response indicate the presence of a particular feature/ stimuli
prolonged exposure to a given visual feature decreases sensitivity to that feature
secondary visual cortex (area V2)
receives info from area V1, processes info further & sends it to other areas
info is transferred in a reciprocal nature
columnar organization
cells are grouped together in columns perpendicular to the surface
cells within a given column process similar info
respond either mostly to the right or left eye, or respond to both eyes equally
do not consistently fire at the same time
map 2: visual receptors - rods and cones (20-28)
retina
rods
abundant in periphery
respond to faint light
cones
abundant in and around the fovea
more useful in bright light
essential for colour vision
outnumbered
provide about 90% of the brain's input
average number of axons in the optic nerve is 1 million
some may have 2 or 3 times as many
heightened visual responses
faster brain responses to visual stimuli
photopigments
contained by both rods & cones
release energy when struck by light
11-cis-retinal bound to proteins called opsins
light converts 11-cis-retinal to all-trans-retinal
release energy that activates second messengers within the cell
mammalian visual system
horizontal cells
rods & cones make synapses with horizontal cells & bipolar cells
make inhibitory contact onto bipolar cells
bipolar cells make synapses onto
amacrine cells and ganglion cells
optic chiasm
where the 2 optic nerves leaving the eye meet
ganglion cell axons
half of the axons from each eye cross to the other side of the brain
most ganglion cell axons go to the lateral geniculate nucleus
smaller amount to the superior colliculus & fewer to other areas
lateral geniculate nucleus
part of thalamus
specialized for visual perception
destination for most ganglion cell axons
sends axon to other parts of thalamus & visual areas of occipital cortex
Cortex and thalamus feed info
back and forth to each other
map 3: processing in the retina (28-35)
lateral inhibition
reduction of activity in 1 neuron by activity in neighboring neurons
cells response's depends upon the net result of excitatory and inhibitory messages it receives
retina's way of sharpening contrasts
to emphasize the borders of objects
receptive field
the part of the visual field that either excites or inhibits a cell in the visual system
receptor
receptive field is the point in space from which light strikes it
other visual cells
receptive fields are derived from the visual field of cells that either excite or inhibit
ganglion cells converge to form the receptive field of the next level of cells
lateral geniculate cells
receptive field that resemble ganglion cells
an excitatory or inhibitory central portion & a surrounding ring with opposite effect
primate ganglion cells
parvocellular neurons
located in or near fovea
smaller cell bodies & receptive fields
sensitive to detect colour and visual detail
magnocellular neurons
distributed evenly throughout the retina
larger cell bodies & receptive fields
sensitive to large overall pattern & moving stimuli
koniocellular neurons
found throughout the retina
small cell bodies
have several functions
their axons terminate in different places
map 5: self-study
development of visual cortex