4.2 Vision I: The Eyes and the Brain Convert Light Waves to Neural Signals Flashcards
What is Visual Acuity?
Defined as the ability to see fine detail
Who designed the 20/20 vision measurement?
Hermann Snellen;
It is the smallest line of letters that a typical person can read from a distance of 20 feet.
What is Visible Light?
The portion of the electromagnetic spectrum that we can see, (which is an extremely small slice)!
What is the range, in nanometres which Visible Light falls under?
400-700nm
What are the shortest waves on the Visible Light spectrum?
Ultraviolet Rays
What are the longest waves on the Visible Light spectrum?
Infrared Waves
What does “wavelength” mean?
The distance between peaks of waves
What aspect of a wave determines the colour a human will see?
The LENGTH of a lightwave determines hue/color
What aspect of a wave determines the brightness a human will see?
The AMPLITUDE (height of the peaks) of a wave
What aspect of a wave determines the saturation or richness of colour a human will see?
The PURITY (number of distinct wavelengths that make up the light)
What’s the first part of the eye which light passes through?
The Cornea, which is a clear, smooth, outer tissue.
The Cornea bends the light coming in,
Where does the light go after it’s past the Cornea?
The Pupil, which is a hole in the coloured part of the eye
What are the three properties of light waves?
Purity
Wavelength
Amplitude
What is the name of the part of the eye which controls the size of the pupil? Why does it do this?
The Iris. By changing the size of the Pupil, the amount of light that can enter the eye changes
Retina
A layer of light-sensitive tissue lining the back of the eyeball
Define “Accomodation”:
The process whereby the eye maintains a clear image on the retina; the process by which infants revise their schemas in light of new information
What’s “Myopia”?
If the eyeball is too long, images are focused in front of the retina, leading to nearsightedness
You can see objects near to you clearly, but objects farther away are blurry
What’s “Hyperopia”?
If the eyeball is too short, images are focused behind the retina, and the result is farsightedness
You can see objects far away clearly, but objects up close are blurry
Two types of photoreceptor cells in the retina contain light-sensitive pigments that transduce light into neural impulses
What are the two types of photoreceptor cells in the retina, and what do they do?
Cones
Rods
They contain light-sensitive pigments that transduce light into neural impulses
These photoreceptor cells allow us to detect colour, operate under normal daylight conditions, and focus on fine detail
Cones
What do “Rods” do for our vision?
They become active under low-light conditions for night vision
What shades of colour do Rods produce?
None. They all contain the same pigment, which is perceived to us as shades of grey
Which photoreceptor is more sensitive?
Rods are more sensitive
However this comes at a cost, as they produce no information about colour
An area of the retina where vision is the clearest and there are no Rods at all
Fovea
How many Rods are distributed more or less evenly around each retina except in the very centre?
120 million
How many Cones are contained inside each Retina? Is it less or more than Rods?
6 Million, which is much less than Rods (120 million)
How can a lack of Rods in the Fovea influence how astronomers view bright stars?
By looking slightly off to the side of a target (star), the image won’t fall on the Rod-free Fovea.
Looking directly at a reduced-light target would make things blurry
Why are objects in our Peripheral Vision less clear?
Cones are densely packed in the Fovea, but are much more sparse in areas of the Retina off to the side. This affects our Visual Acuity
Light reflecting from those peripheral objects is less likely to land in the Fovea, making the resulting image less sharp
What kind of neurons can be found in the outermost layer of the Retina?
Retinal Ganglion Cells (RGC’s)
What cells connect neural signals from photoreceptor cells and transmit them to RGC’s?
Bipolar Cells
What do Retinal Ganglion Cells do?
They organize signals and send them to the brain
How is the Optic Nerve formed by RGC’s?
The axon of a Retinal Ganglion Cell connects with other RGC axons
How many RGC axons can be found in a single eye?
1.5 Million
A location in the visual field that produces no sensation on the retina
Blind Spot
How does light register in the brain? What is the process?
Streams of action potentials (neural impulses) containing information encoded by the retina travel to the brain along the optic nerve
Where is the Lateral Geniculate Nucleus located?
It’s a relay centre located in the Thalamus
Where does the Optic Nerve travel to in the brain?
Lateral Geniculate Nucleus, and then on to the Visual Cortex (which is part of the Occipital Lobe)
What is the relationship between our visual field and the retina stimulation?
Objects in our left visual field will stimulate the right half of our retina’s and vice versa
Where does visual information end up in our brains relative to our eyes?
Info from our left visual field will travel into the right hemisphere of our brains (via the Thalamus)
What are the three types of cones, and what are they each good for?
The three types are long, medium, and short wavelength. Each is used for blueish, greenish, and reddish light, respectively
Each type is most sensitive to a narrow range of wavelengths in the visible spectrum
What percentage of men inherit conditions in which either the red or the green photoreceptors do not transduce light properly
Around 5%
What is Colour-opponent System?
States that pairs of visual neurons work in opposition: red-sensitive cells against green-sensitive and blue-sensitive cells against yellow-sensitive). The colour-opponent system explains colour aftereffects
Describe the VENTRAL stream:
Travels across the occipital lobe into the lower levels of the temporal lobes and includes brain areas that represent an object’s shape and identity—in other words, what it is, essentially a “what” pathway…
Describe the DORSAL stream:
travels up from the occipital lobe to the parietal lobes (including some of the middle and upper levels of the temporal lobes), connecting with brain areas that identify the location and motion of an object—in other words, where it is. Known as the “how” pathway
Allows us to perceive spatial relations
Define “Visual Agnosia”
The inability to recognize objects by sight
