primary visual pathway Flashcards
(30 cards)
what does visual information stimulate?
photoreceptors in retina
what is the fovea?
stimulated by centre of visual field
what is the visual pathway?
from retina travel via optic nerve to lateral geniculate nucleus (LGN) then primary visual cortex (PVC)
where is information from the right half field of vision?
left hemisphere
where is information from the left half field of vision?
right hemisphere
what are the information-processing stages in the primary visual cortex?
hierarchical processing
at beginning, only photoreceptors (detect presence of light and particular wavelengths)
PVC still code for small part of visual field but code for shapes and movements (still not actual visual experience)
what is the experimental strategy to reveal mechanisms of visual perception?
put electrodes at different stages and see how neurons respond to visual information
systematically doing this creates process systems
by studying the different neuronal responses and different stages of the visual pathway, may gain understanding of different stages of visual information processing that mediate visual perception
what is the experimental set up to record visual responses of neurons along the visual pathway?
provide stimulation to very small part of visual field in controlled manner and use electrodes at different points along visual pathway
who were David Hubel and Thorsten Wiesel?
Nobel prize in physiology or medicine 1981
for discoveries concerning information processing in the visual system
birth of neurobiology
what are the different types of photoreceptors?
rods
cones
what are rods?
more abundant
no colour (wavelength) discrimination
sensitive in low light levels
higher density in peripheral (don’t look directly at dim stars)
track high-rate speed changes (see flicker of 60Hz monitor from corner of your eye)
what are cones?
less abundant
three types discriminate different wavelengths
less sensitive to low light
higher concentration in fovea
cannot follow rapid changes (can’t see 60Hz flicker when directly looking at monitor)
what are the three types of cone?
S = blue
M = green
L = red
what is the difference between photoreceptors + bipolar cells and all subsequent cells?
photoreceptors and bipolar cells vary their voltage as they are stimulated (analogue signal)
all subsequent cells vary spike rate (all-or-nothing, digital signal)
what do bipolar cells do?
translate photoreceptor detection of light into excitation or inhibition of retinal ganglion cells
what are receptive fields of visual neurons?
portion of retina/visual field in which visual stimulation will evoke a change in the firing rate of a given visual neuron
what is the substructure of a receptive field?
a description of how visual stimuli need to be presented in the receptive field of a visual neuron in order to evoke firing-rate changes
what are retinal ganglion cells?
receive input from multiple photoreceptors (via bipolar cells)
ON-OFF centre-surround receptive fields
all have baseline firing rate
response rate of cell based on sum of stimulation in ON region minus stimulation in OFF region (enhancement of contrast and boundaries)
neurons in LGN respond to visual stimuli in similar ways to retinal ganglion cells
what are ON-OFF centre-surround receptive fields?
light presented in ON regions excites cells
light in OFF regions inhibits cell
ON and OFF regions organised in centre-surround fashion
what is the functional significance of centre-surround fields?
world has lots of things that stay constant and we don’t need to keep responding to them - what counts most are changes and boundaries - only responding to them is efficient
luminance of features represented relative to their surrounds - helps preserve appearance of objects regardless of light levels in environment, can result in illusions
what is colour sensitivity in retinal ganglion and LGN neurons?
receive input from cones and are sensitive to colour
have receptive fields that show centre-surround colour opponency
functional significance of colour-opponency not clear
colour-opponency, together with firing rate adaption (rebound effects), in retinal ganglion cells can explain negative afterimages
what is the primary visual cortex (striate cortex, V1)?
in human brain, sits between hemispheres (calcarine fissure)
position different in humans compared to other animals
called striate cortex because its stripey
what are orientation-selective cells in V1?
most neurons in V1 (PVC) respond to elongated stimuli with specific orientation
information assembled hierarchically
what are the different types of cells in PVC?
simple
complex
