4. vision Flashcards
fo ran object to be visiible it must…
emit or reflect light
what is EMR
Electromagnetic radiation
light is simply one a part of this continuum
How is EMR generated
when an atom emits a particle called a photon
energy from photon s determine their wavelength
Human eye is only capable of detecting light within a narrow range of wavelengths
describe the continuum
> gamma rays (Shortest wavelength)
> X rays
> Ultraviolet light
> VISIBLE LIGHT >> Violet >> Blue >> green >> yellow >> red
> Infrared light
> Radar
> Radio waves (long wavelengths)
what are the two categories of photoreceptors in the retina
Cones and Rods
properties of Cones
- 6 million
- in fovea and periphery
- Fast dark adaptation
- low dark sensitivity
- yellow sensitive
- high acuity
properties of rods
120 million
periphery
slow dark adaptation
high dark sensitivity
Green sensitive
Low acuity
what did schultze propose about rods and cones
they are from two separate visual systems
> photopic = bright light vision via cones
> scotopic = dim light vision via rods
von kries observed that individuals without rods are …
night blind
von ries observed that individuals without cones were
day blind
able to see at night
retinal physiology
overview of connections between retina and visual cortex
> receptor cells (some connected by horizontal)
> bipolar cells (some connected by horizontal/Amacrine cells)
» ganglion cells (some connected by Amacrine cells
on average how many receptors connect to 1 ganglion cell
126 receptors conect to 1 ganglion cell
Massive convergence as move deeper into retina
what do connecting cells (horizontal or amacrine) allow
events at one location to influence events at another
describe receptive fields
ganglion calls have both excitatory and inhibitory connections within their receptive field
in centre surround receptive fields illumunation of
> centre = excititory
suround = inhibitory
or vise versa
what act as edge detectors?
retinal ganglion cells can act as edge detectors
integrate patterns of lightness over an area and
indicated whether that pattern of lightness within the area is different to that in an adjacent area (an edge)
hat are the three types of ganglion cells?
x-cells - exhibit centre-surround antagonisms
Y-cells - exhibit centre-surround antagonisms
W calls - don’t exhibit CS antagonism, fire regardless of which area of their receptive field is illuminated
X cells and y cells are associated with what
two different visual channels
Parvocellular (X cellls)
and
Magnocellular (Y cells)
describe the parvocellular system receptive field properties
Receptive Field size > small
Receptive field type > X
RF distribution > foveal
describe the parvocellular system properties > ganglion cell size > response type > movement > nerve transition
ganglion cell size > small
Response > sustained Movement > poor
Speed of nerve transmission > slow
describe the parvocellular system function
Detail, space and colour
describe the magnocellular system properties > ganglion cell size > response type > Movement > nerve transition speed
ganglion cell size > large response type > transient
Movement > good
nerve transition speed > fast
describe the magnocellular system receptive field properties
size > large
Type > Y
Distribution > Peripheral
describe the magnocellular system function
motion depth and time
what does LGN stand for
Lateral Geniculate nucleus
what are the properties of LGN cells
similar to those of ganglion cells
> centre-surround fields on the retina
> parvo and magno properties
> response simply to the presence of light
> receive input from the visual cortex (back projections) allowing feedback based on previously processed information
where does the tectopulvinar system received information from
> magno retinal ganglion cells
> receptive fields on retina of cells in the superior colliculi
> tectopulvinar system also received back projections from the cortex
describe the functions of the Tectopulivinar system
“old” in evolutionary terms
> localisation of objects in space
> guidance of eye movement
> gross pattern recognition
where are the structures of the tectopulvinar system located
in the thalamic region called the tectum
Schneider’s hamster study
no LGN = ___
No Superior colliculi =____
no LGN = unable to recognise patterns
no superior colliculi = able to recognise patterns but to able to accurately approach them
what are the two pathways of the tectopulvinar system
focal system for recognition
Ambient system for localisation
what happens when someone has damage to the focal system
Blindsight -
can still react to visual stimuli (point at it ) although claim they cant see them
describe the visual cortex
2mm thick
contains 100 mil. cell sin 6 main layers
LGN fibres enter at layer 4 where the neurons have centre surround receptive fields like those in LGN and Ganglion cells
outer layers have more complex behaviours
feature detectors in the primary visual cortex
simple cortical cells - respond to bars of a particular orientation
Complex cortical cells - response best to a correcty orientated bar moving across the entire receptive field
sometimes directive sensitive
hypercomplex cortical cells -
fire only to moving lines of a particular length or moving corners or angles
OVERALL cortical cells act as feature detectors
