Lecture 6 Flashcards
Receptive fields and light inhibition are to be studied on iPad
go to iPad, many slides with notes there
who first mapped out the receptive fields of individual retinal ganglion cells
Kuffler on cats
explain how on centre cells work
ON-center ganglion cells—excited by light that falls on their center and inhibited by light that falls in their surround.
explain how off centre cells work
OFF-center ganglion—inhibited when light falls in their center and excited when light falls in their surround
does the size of the light matter when interacting with the on/off receptive feilds
yes, if not perfect it will fire less and less
what are mach bands
equal physical intensity of a gradient (grey gets light and lighter each block)
what is th ePERCEIVED brightness intensity of mach bands
looks like it is brighter on the left (next to the darker block) than on the right (next to the lighter block)
what is edge enhancements
The purpose of the center-surround organisation is to help us perceive the edges of objects
why does one side of a block in the act bands look brighter than the other when they are actually they exact dame
the receptive fields that overlap with the edges are centre surround but overlap with some of the darkness AND brightness so lateral (horizontal) cells are inhibited and it makes some areas seem brighter near the edges when contrasting with another shade
what is the Hermann grid illusion
the many black boxes (grid) with while lines and the intersections look like there are black dots when there arents
how does the Hermann grid illusion work
the lines remain white because the receptive fields have centre light and the surrounding is inhibited by the black lines so all is good
the INTERSECTIONS look as though there are black/grey dots because the centre is light but the surround is also light so the horizontal cells are inhibited and thus so is the cell in general making the contrast difficult and thus having a grey dot
if you fixate on an intersection the illusion vanishes in the hermann grid, why
receptive field in foveal vision is much smaller than in peripheral so the light is actually just the right size to make it look all white
whatadoes the size of the receptor field depend on
type of bipolar cell
what are the types of bipolar cells
Diffuse bipolar cell:
Midget bipolar cell:
what are Diffuse bipolar cell
Receives input from multiple photoreceptors.
what are Midget bipolar cell:
Receives input from a single cone.
are Diffuse bipolar cells mostly in fovea or periphery
periphery
are Midget bipolar cell mostly in fovea or periphery
fovea
what are the 2 types/pathways of ganglion cells
Parvocellular vs Magnocellular
P ganglion cells are connected where
Connect to the parvocellular pathway.
M ganglion cells are connected where
Connect to the magnocellular pathway.
P ganglion cells: Connect to the parvocellular pathway.
Receive input from where
midget bipolar cells (in fovea)
M ganglion cells: Connect to the magnocellular pathway.
Receive input from where
diffuse bipolar cells (in peripheral)
Parvocellular pathway is involved in what
fine visual acuity, color, and shape processing; poor temporal resolution but good spatial resolution
Magnocellular (“large cell”) pathway is involved in what
motion processing; excellent temporal resolution but poor spatial resolution
can you have acuity and light sensitivity
no it is Acuity vs light sensitivity trade-off
more convergence of rods/cones to ganglion cells causes what
More convergence causes:
More light sensitivity
Less acuity
why does More convergence cause
More light sensitivity
Less acuity
for light sensitivity, there is more chance that the light will be detected if the ganglion cell is connected to many rods or cones (higher receptive field)
for acuity, the ganglion cells are set up the same way: if light activates a photoreceptor with lots of convergence of the ganglion cell, the receptive field is so large and it cannot tell the brain exactly where in the receptive field the light is, just that it is there. And vice versa: if no convergence, there is a smaller receptive field and the ganglion cell can tell the brain exactly where the light is when activated thus higher acuity
defineAcuity
The smallest spatial detail that can be resolved.
There are several ways to measure visual acuity. give example
Eye doctors use distance to characterize visual acuity, as in “20/20 vision.”
explain the numbers in “20/20 vision.”
Your distance/Normal vision distance
The patient is typically placed at 20 feet from the letters
20/20 means that, when placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 20 feet
20/15 means that, when placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 15 feet
20/40 means that, when placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 15 feet
how do Vision scientists define acuity
Smallest visual angle of a cycle of grating.
Vision scientists: Smallest visual angle of a cycle of grating.
thus, The smaller the visual angle at which you can identify a cycle of a grating…
the better your vision
basically this can be tested by having black and white alternating lines (either hormonally or vertically this is a grate) and the smaller and smaller the scientists makes it, you will eventually just see grey and this help to determine your acuity and visual angle
for grating:
Vision scientists: Smallest visual angle of a cycle of grating.
The smaller the visual angle at which you can identify a cycle of a grating, the better your vision.
The limit is determined primarily by what
the spacing of photoreceptors on the retina.
how many cones do you need to determine the difference between the black and white grating
at least 2 cones
what is Spatial frequency
Cycles of a grating per unit of visual angle (in degrees).
Another way to think of spatial frequency is as the number of times a pattern repeats per unit area
what kind of graph is used to display spatial frequency
sine waves/grafs
so the iage is a blurry grate (blurry white and black lines) because it is not really a sharp up and down that happens
what are Gabor patches
another way to display the sin graph (for spatial frequency)
the farther spread the ripples, the lower the spatial frequency and vice versa
Why sine gratings?
for visual acuity
Patterns of stripes with fuzzy boundaries are quite common.
(reality) Trees in a forest, books on a bookshelf, pencils in a cup.
Any black-and-white image can be described in terms of a weighted combination of different frequencies with different orientations.
Low frequencies display what
general image
High frequencies display what
details
do Ganglion cells with center-surround receptive fields respond preferentially to certain frequencies.
absolutely
Ganglion cells with center-surround receptive fields respond preferentially to certain frequencies.– explain the response of lw frequency
weak response
will look like very large blurry line
Ganglion cells with center-surround receptive fields respond preferentially to certain frequencies.– explain the response of medium frequency
strong response
will look more solid…
Ganglion cells with center-surround receptive fields respond preferentially to certain frequencies.– explain the response of high frequency
weak response (will look like very small and blurry lines)
Ganglion cells with center-surround receptive fields respond preferentially to certain frequencies.
what else is also important
Phase
what is Phase
The phase of a grating refers to its position within a receptive field
define Contrast:
intensity difference between the lightest and darkest portions of the patch
is it easier to see gabor patches with high or low contrast
high
what is Contrast sensitivity function
Visibility of a pattern as a function of spatial frequency and contrast
