early visual pathways Flashcards
what are m cells?
magnocellular layers with very large cells that get info from larger paracell RGC’s cells
what are p cells?
pavocellular layers with smaller cells that get info from midget RGC cells
how do m and p pathways differ?
mainly in their tuning to spatial frequency
what is spatial frequency/
refers to the number of repetions of a grating pattern in a fixed distance
what type of cell has larger light receptive fields at each retinal location?
m cells
do m and p cells differ in terms of temporal frequency?
yes
what is temporal frequency?
the number of repetitions of a periodic pattern in one second
what do p cells respond best to?
low Hz
what do m cells respond best to?
high Hz
do m and p cells have different responses to contrast?
yes, m cells have bigger responses at low contrast level
whats going on in those with dyslexia?
masking is making image unclear, they need more contrast and slower rate of change than others
what is causing saccadic eye movements?
a weak m system response may give less suppression and let info overlap causing masking
what is suppression?
a reduction in sensitivity to patterns that occurs during saccadic movements in the m system
what causes migraines?
deficient m and p pathways
what does our sensitivity to contrast depend on?
target size and spatial frequency
what is the receptive field?
the area of the image from which the cell collects responses to light
how do photoreceptors vary?
in convergence, spatial summation and spatial resolution
how do photoreceptors vary in convergence?
the number of photoreceptors that provide inputs from the receptive field varies
how do photoreceptors vary in spatial summation?
more PR’s allows more light to be collected and makes the retinal ganglion cells more sensitive to light
how do photoreceptors vary in spatial resolution?
fewer photoreceptors makes the receptive field smaller and allows the rational ganglion cell to respond to a small area of the image giving higher acuity
what type of organisation do receptive fields up to the first stage of processing have?
centre-surround organisation
what do spatial frequency channels do?
find units selective to different ranges of spatial frequency and orientation
when does max masking occur?
when the targets and masks have identical spatial frequency
what gets the message to the brain the quickest?
low spatial frequencies
