week 2: vision Flashcards
for an object to be visible it must:
emit or reflect light
what is light
one part of a continuum called electromagnetic radiation (EMR)
when is an emr generated
when an atom emits a particle called photon
what determines the emr wavelength
the energy of the wave length
how much light does the human eye detect
only able to detect light within a narrow range of wavelengths
what gives rise to the perception of different colours
different wavelengths within the range of visible wavelengths
what is saccades
the constant movement of the eyes
what happens if an image stays stabilised on the retina
it disappears as the photoreceptors stop firing and the brain fills in the missing information
what are ciliary muscles
muscles in the eye that control the shape of the lens to accomodate near or far targets
if the ciliary muscle relaxes what happens
the lens bulges and therefore focuses on the far and blurs the near
if the lens is round what is the focal point
the far is blurred and the near is focused
what does it mean to be far or short sighted
the focal point doesnt land right at the retina where it is meant to land
at what age does your eyes stop focusing
40
why do we have 2 eyes
so we have a back up and to derive more information from the image/environment
what is stereovision good for
depth perception
what is motion paralex
the more an object moves when you move your head the closer the object is to you
what part of the eye contains the most photoreceptors
the fovea
types of photoreceptors
cones and rods
what are cones good for
colour and detail
when is cones most active
during daytime/in good light
why are photoreceptors facing away from the light is travelling
Human vision was designed for use in poor lighting (scotopic/night vision). Therefore photoreceptors are extremely sensitive to light that light is toxic to photorecptors.
what kills photorecepetors
too much light causes the photoreceptors to oversaturate and remove colour which bleaches the photoreceptors and causes them to be inaffective
How does the choroid layer work to protect photoreceptors
the choroid layer absorbs most of the light and reflects a small amount of light back to the photoreceptors
where are cones located
fovea and periphery
how many cone cells are there
6 million
what are rod cells
low detail and low light, good for scotopic/ night time vision
where are rods located
periphery
how many rods are there
120 mil
what happens to the cone cells in the dark
the cones go to sleep and the rods come into focus which allow us to see in the dark
what are the 2 visual systems
photopic
scotopic
what is photopic vision
bright light vision via cones
what is scotopic vision
dim light vision via rods
black and white or slighty green
what did von Kries observe
that individuals without rods were night blind
how are peoples vision who dont have cones
they are blind only during the day/ bright light
what do receptor cells synapse with
bipolar cells
what do bipolar cells synapse with
ganglion cells
what do horizontal cells connect to
different receptors or different bipolar cells
what do amacrine cells connect with
different bipolar or different ganglion cells
what do the connecting cells allow
horizontal and amacrine cells allow events at one location to influence events at another
what happens as we move deeper into the retina
convergence
- average of 126 receptors connect to 1 ganglion cell
what does a ganglion cell do
acts like a lens as it captures an image over an area of the retina
- receptive field
it can also enhance the image in its local area
what type of connections do ganglion cells have within their receptive field
excitatory and inhibitory
what is the first thing the photoreceptors identify
an edge
what is an on centre off surround ganglion cell
a Ganglion cell that responds best when it receives stimulation in the centre of the visual field and gets suppressed when it receives light outside of the visual field
what is an off centre on surround ganglion cell looking for, when does it function best
The cell is looking for a dark spot in a bright background. Functions best when the light is outside of the visual field
what does the retinal ganglion cell act as
edge detectors
integrate pattern of lightness over an area
indicate whether that pattern of lightness within an area is different to that in an adjacent area (an edge)
what are ganglion cells which exhibit centre-surround antagonism called
x cells or y cells
what are the x and y cell visual channels called
parvocellular and magnocellular systems
what is the magnocellular system
older system
by passers the brain and goes straight to lower brain structure and sometimes directly to regions that enable us to respond to the info fast
what is the parvocellular system
projects to the high cortical areas and tends to be used for information processing
parvocellular system characteristics
small in size
sustained response
Receptive Field small x cells distributed in the foveal
poor movement
slow speed of nerves
magnocellular characteristics
large in size
transient response
Receptive Field large y cells in the peripheral
good movement
fast nerve speed
what is the magnocellular good at
perceiving movement
where is movement perceived best
in the peripheral
what is the function of the parvocellular system
detail, space and colour
what is the function of the magnocellular system
motion, depth and time
passage of the visual signal
information from the left visual fields of both eyes travels to the right side of the brain to be processed. Information from the right visual field of both eyes is processed in the left side of the brain
where do the optic nerves cross over
the optic chiasm
what is the purpose of the tectopulvinar pathway
the basic relay system that doesnt need to go through the brain. Used when you need a response really fast eg, ducking for safety
where are the tectopulvinar system n1 located
in the thalamic region called the tectum
what do cells in the superior colliculi receive their input from
the magno retinal ganglion cells
what does the tectopulivnar system receive
back projections from the cortex
how are the tectopulvinar cells organised
in topograpgic maps
what are the 2 pathways in the tectopulvinar system n2
focal system and ambient system
what is the focal system for
recognition
what is the ambient system for
localisation
what happens if we have no LGN
unable to recognise patterns
what happens if we have no superior colliculi
able to recognise patterns but not able to accurately approach them
what happens if you have damage to the focal system (blindsight)
can still react to stimuli eg. point to them but they cannot see them
(cortically blind)
what cells act as feature detectors
cortical cells
