Retina Slides Flashcards
Eye parts
Cross section of the human retina. In this figure, light would reach the receptors from below (i.e., through the other cells in the retina).
What does the Retina do for us?
- ) Encode the retinal image in space, time, and wavelength
- ) Amplify edges relative to uniform areas of retinal image
- ) Provide a wide field of view and high spatial resolution
- ) Adjust for changes in ambient light level
Schematic of neurons and connections in the Retina
There are several types of ganglion cells. Most are midget (parvo) ganglion cells. Next most common are parasol (magno)
Recording from single neurons in the primary visual cortex
Basic receptive field properties of ganglion cells.
“Discharge patterns and functional organization of the mammalian retina.”
Herring ladder. Each stripe is uniform gray, but they appear darker near the light boundary and lighter near the dark boundary. The receptive field properties of ganglion cells may contribute to the perceptual effect.
Can mach bands be explained in a similar fashion to the Herrind Ladder?
Yes.
Sine wave grating patterns at five different contrasts. The contrast is defined to be the amplitude of the sine wave divided by the mean intensity (mean luminance). Sine wave grating stimuli are often used to measure receptive field properties as well as the visual performance of organisms in behavioral tasks. Sometimes contrast is also expressed in percent rather than proportion. Sine wave sitmuli are useful for studying sensory systems. If a neuural or mechanical system can be described in terms of addition and subtractions, then measuring the responses using sine wave stiumuli can be used to predict the responses to arbitrary stimuli.
What is the Formula for a sine wave grating?
L(x) = Asin (2π ƒx + ø) + Lbar
L(x) = luminance as a function of position
x = position
A = amplitude
ƒ = frequency
ø = phase
Lbar = mean luminance
C = A/L = Contrast
Examples of sinewave stimuli used to measure the contrast sensitivity function (CSF).
Contrast sensitivity Function
Contrast sensitivity function of the human visual system. The contrast threshold is the stimulus contrast required for the organism to detect a pattern with some criterion level of accuracy (e.g., 75% correct). The contrast sensitivity is defined to the one divided by the contrast threshold: the lower the contrast threshold the higher the contrast sensitivity. The contrast sensitivity function plots contrast sensitivity as a function of the test target’s spatial frequency.
What is this?
Demo of contrast sensitivity function. Contrast is constant along any horizontal line. Noice that vertical bars are tallest in the middle frequencies.
More contrast Sinewave images
Chart of contrast sensitivity functions for humans and macaque monkeys.
Relative contrast sensitivity functions of several different species of animal
Comparison of cone responses in the primate and mouse retina to the same image.
Contrast sensitivity functions for brieft stimuli and for long stimuli. Typically the eye jumps from one location to another 3-4 times per second, so the contrast sensitivity function that is most representative of normal conditions does not have much roll-off at the low spatial frequencies.
What are the competing goals for visual system design?
Maximize spatial resolution
Maximize field of view
Minimize neural resources
Densities of cones and ganglion in the human retina as a function of eccentricity along the horizontal median. Ganglion cell density falls precipitously with eccentricity; faster than the falloff in cone density.
Look for/at the fovea
Illustration of the variable resolution of the human visual system with eccentricity
Receptive field profiles of P-type (midget) ganglion cells in the macaque monkey at difference retinal eccentricities, as determined from measurements with sine wave gratings.
Visual search display: A ‘T” embedded in a field of “L”s
Stimulated response of the P cells in the retina for the search stimulus in the previous figure, when fixation is in the lower-left corner. Brightness above medium gray indicates firing rate of on-center P cells. Darkness below the medium gray indicates the firing rate of off-center P cells.
What is this?
Test patterns in Modelfest Phase ONE.
Fit of a simple model of contrast detection threshold to the data obtained with the targets in the previous figure. This model directly based upon macaque and human retinal ganglion cell response properties.
Color coding in the Retina. Diagram of different bipolar and ganglion cell types.
What is this?
Receptive field types in the primate retina.
Light and darker Adaption: Solving the Dynamic range Problem
The pupil opens up at low light levels, closes down at high light levels.
There are two photoreceptor systems: the rod system (for low light levels) and the cone system (for high light levels)
The photoreceptors adjust their individual sensitivities based upon the ambient light level.
The other retinal neurons adjust their sensitivities based upon the ambient light level.
Natural light levels cover a very lage range, which poses a serious probem for neural coding. These are some of the mechanism visual systems use to solve the dynamic range problem.
What is this?
A simplified illustration of the consequences of retinal processing (not including color processing). A given difference in surface reflectance in the environment produces a difference in retinal response that is approximately constant independent of the ambient light level in the environment.
