Lecture 5 Flashcards
The First Steps in Vision:
From Light to Neural Signals
The Man Who Could Not Read
Imagine not being able to read, drive, or recognize faces. For some people, a specific type of retinal disease can rob them of their high acuity central vision
AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma
Age-related Macular Degeneration (AMD)
A disease associated with aging that affects the macula.
AMD gradually destroys sharp central vision.
AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma
Age-related Macular Degeneration (AMD)
A disease associated with aging that affects the macula.
AMD gradually destroys sharp central vision.
AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma.
There are 2 varieties of AMD:
Wet and Dry
Macula
The central part of the retina containing the fovea
AMD
Loss in central vision —> blindness
1.75 million Americans
Double prevalence by 2020
Caused by formation of new veins or thinning of tissue.
AMD Details
Loss in central vision —> blindness
1.75 million Americans
Double prevalence by 2020
Caused by formation of new veins or thinning of tissue.
Wet AMD
Abnormal blood vessels grow under the macula and can leak blood and fluid, raising the macula and impairing central vision.
As the macula is displaced, straight lines may look wavy
Dry AMD
More common.
Occurs when macula cones degenerate.
Sometimes dry AMD turns into wet AMD.
Once dry AMD is advanced, no treatment can reverse the loss of vision.
Age-related macular degeneration (AMD)
Two varieties:
Wet AMD:
Abnormal blood vessels grow under the macula and can leak blood and fluid, raising the macula and impairing central vision.
As the macula is displaced, straight lines may look wavy
Dry AMD: More common. Occurs when macula cones degenerate. Sometimes dry AMD turns into wet AMD. Once dry AMD is advanced, no treatment can reverse the loss of vision.
HOWEVER: medical treatment can slow or stop further loss of vision
Some treatments can delay and possibly prevent AMD from progressing.
Studies show taking high-dose anti-oxidants and zinc significantly reduces risk of advanced AMD and associated vision loss.
Studies show taking high-doses of _______ significantly reduces risk of advanced AMD and associated vision loss.
anti-oxidants and zinc
Convergence
There is greater convergence of rods than cones onto ganglion cells
Thus, there is greater summation of rod signals,
and less stimulation per rod is required to obtain a response.
Cone vision can see finer details than rod vision.
Greater convergence of rods than cones onto ganglion cells limits their spatial sensitivity.
horizontal pathway
Horizontal cells: Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells.
Responsible for lateral inhibition, which creates the center–surround receptive field structure of retinal ganglion cells
Amacrine cells: These synapse horizontally between bipolar cells and retinal ganglion cells.
Have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time).
Horizontal cells
Specialized retinal cells that run perpendicular to the photoreceptors and make contact with photoreceptors and bipolar cells.
Responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells
vertical pathway
The retina’s vertical pathway: Photoreceptors, bipolar cells, and ganglion cells
Bipolar cell: A retinal cell that synapses with one or more rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells
vertical pathway
The retina’s vertical pathway: Photoreceptors, bipolar cells, and ganglion cells
Bipolar cell: A retinal cell that synapses with one or more rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells
P ganglion cells: Connect to the parvocellular pathway
M ganglion cells: Connect to the magnocellular pathway
Midget bipolar cell
A small bipolar cell that receives input from a single cone
Bipolar cell
A retinal cell that synapses with one or more rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells.
Diffuse bipolar cell:
A bipolar cell that receives input from multiple photoreceptors
Midget bipolar cell:
A small bipolar cell that receives input from a single cone
P ganglion cells
Connect to the parvocellular pathway.
Receive input from midget bipolar cells.
Parvocellular (“small cell”) pathway is involved in fine visual acuity, color, and shape processing.
Poor temporal resolution but good spatial resolution
M ganglion cells
Connect to the magnocellular pathway
Receive input from diffuse bipolar cells
Magnocellular (“large cell”) pathway is involved in motion processing.
Excellent temporal resolution but poor spatial resolution
Receptive Field
The region on the retina in which stimuli influence a neuron’s firing rate.
Lateral Inhibition
The response properties of retinal ganglion cells can be explained by the neural circuit depicted above.
Note that the neuron being recorded receives excitatory inputs from one group of receptors and inhibitory inputs from receptors in surrounding regions
Lateral Inhibition
The response properties of retinal ganglion cells can be explained by the neural circuit depicted above.
Note that the neuron being recorded receives excitatory inputs from one group of receptors and inhibitory inputs from receptors in surrounding regions.
Kuffler
Kuffler mapped out the receptive fields of individual retinal ganglion cells in the cat retina
ON-center ganglion cells:
Excited by light falling on center, inhibited by light falling surround.
OFF-center ganglion cells:
Inhibited by light falling on center, excited by light falling on surround
ON-center ganglion cells
Excited by light falling on center, inhibited by light falling surround
OFF-center ganglion cells
Inhibited by light falling on center, excited by light falling on surround
Why center-surround receptive fields?
Each ganglion cell will respond best to spots of a particular size (and respond less to spots that are too big or too small).
Retinal ganglion cells act like a filter for information coming to the brain.
Retinal ganglion cells are most sensitive to differences in the intensity of light between center and surround and are relatively unaffected by the average intensity.
Luminance variations tend to be smooth within objects and sharp between objects.
Thus, center-surround receptive fields help to emphasize object boundaries.
One consequence of Center-Surround architecture:
Mach bands
Illusory stripes that emphasize differences in luminance.
The stripes are a product of your visual system’s center-surround receptive fields
P and M ganglion cells revisited
P ganglion cells: Small receptive fields, high acuity, work best in high light situations, sustained firing.
Provide information mainly about the contrast in the retinal image
M ganglion cells: Large receptive fields, low acuity, work best in low light situations, burst firing.
Provide information about how an image changes over time
Hermann Grid
An Illusion:
Why do spots appear at the junctions, and why do they disappear when a junction is fixated?
Point of fixation
Spots do not appear at the fixated junctions because receptive fields in the fovea are smaller than in more peripheral regions.
Cells with receptive fields at the junction receive more inhibition (2 more minus signs), so that the junctions appear darker.
Dark and Light Adaptation
One of the most remarkable things about the human visual system is the incredible range of light levels we can adjust to.
2 mechanisms for dark and light adaptation:
Pupil Dilation
Photoreceptors and their replacement
Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels.
The amount of photopigment available in photoreceptors changes over time.
The more light entering the retina, the faster the photopigments are used up, and the fewer photopigments there are to process more light.
The less light entering the retina, the more slowly photopigments are used up, and the more photopigments there are to process what little light is there
The visual system regulates the amount of light entering the eye, and ignores whatever variation in overall light level is left over
In bright light, the pupil constricts, letting in less light
Next, the number of photopigments in the photoreceptors decreases over a few minutes
Being light-adapted means that even though there are more photons coming into the eye, there are fewer photopigments available to process them, so much of the light is unused.
Retinitis pigmentosa (RP)
Family of hereditary diseases involving the progressive death of photoreceptors and degeneration of the pigment epithelium.
The Man Who Could Not See Stars
Many people may not notice the onset of retinitis pigmentosa at first because it primarily affects peripheral vision.
