Visual System Flashcards
Light
Electromagnetic waves of photons
Amplitude
Determines intensity of light; large amplitude yields brighter light while small amplitude yields dull light
Frequency
Determines color of light; long wavelength yields low frequency and warm colors while short wavelength yields high frequency and cool colors
Reflection
Light waves encounter a surface that bounces the waves away
Absorption
Light waves encounter a surface that absorbs to make an object appear a certain color or opacity
Refraction
The bending of light that allows an image to focus
Iris
A colored muscle around the pupil that regulates light entry by controlling pupil dilation
Pupil
The opening of the eye that allows light to enter
Cornea
The transparent outer layer of the eye that is primarily responsible for refracting light to form a clear image on the retina
Lens
An eye structure that changes shape to focus an image
Flat lens
For far images, ciliary muscles and ligaments stretch to give the lens a more stretched, flat shape
Fat lens
For near images, ciliary muscles and ligaments relax to give the lens a more oval, fat shape
Accommodation
The process where ciliary muscles adjust the lens shape to bring objects into focus
Fovea
The central portion of the retina packed with photoreceptors that is the central point of vision
Sclera
The white outer layer of the eye that helps maintain its circular shape
Blindspot
The area of the retina that has no photoreceptors where ganglion cell axons forming the optic nerve exit the eye
First layer of the retina (next to vitreous humor)
Ganglion cells
Second layer of the retina
Bipolar cells
Third layer of the retina
Photoreceptors
Photopic vision (receptors, number per eye, sensitivity, location, and receptive field size/ acuity)
Cones
4 million
Low (needs strong stimulation)
In and near the fovea (less dense throughout the retina)
Small field in fovea, larger outside fovea, high acuity
Scotopic vision (receptors, number per eye, sensitivity, location, and receptive field size/ acuity)
Rods
100 million
High (stimulated by weak light, allows for night vision)
Outside the fovea
Larger field, low acuity
Phototransduction definiton
The conversion of light into a change in the membrane potential
Phototransduction Pathway
- Light reaches rods and cones
- Na ion channels close for rods and open for cones
- Na ions cannot enter rods, hyper polarizing the cell membrane, Na enter cones, depolarizing the cell membrane
- In cones, a threshold is met to fire an action potential, sent via the optic nerve to the brain
**vice versa in darkness
Cellular convergence
Neural connections allow for many cells to send signals to a single cell
What is the cellular convergence in cone-fed circuits?
Low; 1 cone feeds 1 bipolar cell feeds 1 retinal ganglion cell
What is the cellular convergence in rod-fed circuits?
High; multiple rods feed multiple bipolar cells feed 1 retinal ganglion cell
What’s the trade-off for visual acuity?
Because there’s so little light going to rods, more rods are needed to reach a threshold, which allows greater sensitivity to faint light but less visual acuity
Signal transduction of rods in light
- Light bleaches rhodopsin molecules
- Na ion channels close
- Na ions cannot enter, hyperpolarizing the rods
- Glutamate release is inhibited
Signal transduction of rods in darkness
- Rhodopsin molecules are inactive
- Na ion channels are kept open
- Na ions flow into the rods, depolarizing them
- Rods continuously release glutamate
Foveal Indentation
A depression in the retina leaving only photoreceptors in the center
Why is foveal indentation important?
Because photoreceptors are the last layer of the retina, the fovea indents so that there’s less retinal cells to travel through, thus creating a sharper image
Retina-geniculate system
Neural projections from the retina pass through the lateral geniculate nucleus in the thalamus to the primary visual cortex
How are right and left visual fields represented?
Contralaterally
Nasal hemiretina sends axons how?
Through the LGN contra laterally via the optic chiasm
Temporal hemiretina sends axons how?
Through the LGN ipsilaterally
What does the retinotopic map mean?
Two stimuli presented to adjacent areas of the retina excite adjacent neurons at the LGN and V1
What’s different about the fovea’s representation in the brain?
The fovea takes up a larger area of cortical neurons than the peripheral field
M pathway
Input from rods are received by larger cell-bodied neurons at the bottom LGN layers that are particularly responsive to rapidly-changing stimuli and movement with good temporal but poor spatial resolution
P pathway
Input from cones are received by smaller cell-bodied neurons at the top LGN layers that detect color, detail, and still or slow objects with good spatial but poor temporal resolution
Receptive field characteristics of on-center/ off-surround cells
On response when light is in the field’s center
Off response when light is in the periphery
Little response when full receptive field is lit
Best respond to contrast where light completely illuminates the center
Receptive field characteristics of off-center/ on-surround cells
On response when light is in the periphery
Off response when light is in the field’s center
Little response when full receptive field is lit
Best respond to contrast where light completely illuminates the surround
Mach banding
An exaggeration of the contrast between edges of slightly differing shades due to the brain adjusting, allowing us to see enhanced contrast
Lateral inhibition
Interconnected neurons inhibit their neighboring neurons to produce contrast on the edges of the regions (evolutionarily adaptive to discern different objects)
What does it mean when a retinal cell has a lower firing rate?
There’s more lateral inhibition and more contrast detected
What does it mean when a retinal cell has a higher firing rate?
There’s less lateral inhibition and more contrast detected
Receptive fields of single cortical cells
Rectangular shape to respond to a straight line of certain orientation and direction
Receptive fields of complex cortical cells
Rectangular shape to respond to a bar of a particular width and orientation in motion (made up of multiple simple cortical cells)
V4 of the extra striate cortex
Responds to color to recognize an object but not its movement
Cerebral achromatopsia
Losing the ability to perceive color from lesions to V4, but can still recognize movement
V5/ MT of the extra striate cortex
Responds to movement to recognize an object’s motion but not the object itself since it does not respond to color
Cerebral akinetopsia
Losing the ability to perceive visual motion from lesions to MT, but can still recognize color
Fusiform facial area
FFA; association cortex specialized for facial recognition
Prospagnosia
The inability to recognize faces from lesions to FFA
Dorsal stream of visual processing
Specializes in visual spatial recognition
Ventral stream of visual processing
Specializes in visual pattern recognition
Blindsight
Visualizing a moving object in space without recognizing it, caused by lesions to the ventral stream
Spectral sensitivity
Three types of cones respond to three pigments (red, green, blue) where each have a different spatial sensitivity
Trichromatic theory of Helmholtz
Human eyes only perceive red, green, and blue light and every other color is a ratio of mixtures of the three
Dichromia
Red and green colorblindness where males have a tough time distinguishing between red and green
Why is Dichromia more common in males?
Genes encoding photopigments are carried on the X chromosome, so females can compensate for a defective gene with a normal copy
Color constancy
The ability to perceive colors as relatively constant over varied lighting
Explain the blue and black dress phenomenon
Our eyes are able to assign fixed colors to objects under widely different lighting, so if the dress is perceived to be in shadows, it appears white and gold, but if the dress is perceived to be in light, it appears blue and black
