Chapter 2: The Beginning of Perception Flashcards
What is the range of wavelengths for visible light?
- 370nm - 730nm
- Found within the electromagnetic spectrum
What’s your field of view?
- The portion of space surrounding you that you can see without moving your eyes
- We have 190 degrees of visibility from side to side
Why do we have slightly more visibility downwards compared to upwards?
- The brow bridge makes it difficult to see too high up
What’s the trade-off of having eyes on the side of your head?
- Have a greater field of view but worse depth perception (ex. alligators)
- Most predators have eyes on the front of their faces while the opposite is true for prey.
What are the three membranes that surround and protect the eye?
1) Sclera - tough, outer membrane. Protective covering, makes up the white part
2) Choroid - middle layer. Contains most of blood vessels. helps circulate oxygen and nutrients
3) Retina - Inner layer. Made up of neurons, including the photoreceptors
What are the extraocular muscles?
- 3 different pairs of muscles that surround the eye
- Superior and inferior rectus - up/down
- Medial (toward) and lateral (away) rectus - toward/away from center (center being the nose)
- Superior and inferior oblique - rotation clockwise/counterclockwise. Keeps eyes upright while you spin
What’s the cornea?
- A clear, protective bump at the front of your eye
- Sharply refracts light as it enters the eye (including side view)
- Helps focus the light onto the retina
- Fixed in place but still does about 80% of focusing power
What’s the purpose of the pupil?
- Note that it’s not really a structure
- Hole/opening in the center of the eye that allows for a certain amount of light to enter the eye.
- Dilation/constriction of the pupil is controlled by muscles in the iris
What’s the lens?
- An elastic, crystalline structure that also helps focus light onto the retina
- Does not do as much focusing as the cornea
- Main job is to fine-tune the light onto the retina
- Does about 20% of focusing
How does the lens change shape to focus light onto the retina?
- Performed by the ciliary muscles
- Becomes thinner to focus on distant objects (bends light less)
- Becomes thicker to focus close objects (bends light more)
What’s the state of the ciliary muscles and the zonule fibres when the lens is relaxed?
- Lens is more spherical
- Ciliary muscles are contracted (thinner). Anchored to the choroid membrane
- Zonule fibres are less taut
What’s accomodation?
- The process of changing the shape of the lens, so that light from different objects at different distances focuses correctly on the retina.
True or false: The retinal image is the exact same orientation as the perceived image that we see.
- FALSE: The retinal image is upside-down and backward at the fovea
What are the five cell layers of the retina?
*Ranked in the order that light moves through them
1) Photoreceptors (rods/cones)
2) Horizontal cells (send signals sideways)
3) Bipolar cells (make ‘through’ connections to ganglions)
4) Amacrine cells (send signals sideways)
5) Ganglion cells (axons combine to form the optic nerve)
What are rods?
- Photoreceptors responsible for being extremely receptive to light
- 120 million per eyeball
- Only found in the periphery
What are cones?
- Photoreceptors responsible for detecting colour (differences in wavelengths)
- Found primarily in the fovea
- Work best in bright environments
- No convergence, but better acuity
- Three different kinds (red, green, blue)
What are the implications of rods having convergence?
- While only a handful of rods may be activated by interactions with photons, they can combine their inputs with other receptors in order to reach the minimum threshold since they all come together at the same ganglion cell
- This also means they have lower acuity since the brain cannot detect where exactly the signal is being fired from.
- The complete opposite is true for cones
Why is the fovea centralis important?
- Where the majority of vision occurs
- Involved in directed looking
- highest density of photoreceptors
- Pretty much only cones in this area
- Rods are found in the periphery
Where’s the blind spot?
- The area where there are no receptors cause it’s where the optic nerve exits the eyeball and leads to the brain
- Brain ends ups filling in the missing info using the context of the surroundings
- The other eye will also attempt to fill in the missing field of view
What’s transduction?
- The transformation of a physical stimulus into a neural signal
What and where are the photopigment molecules?
- They are located in the outer segments of rods and cones, each receptor containing thousands of them
- When the retina absorbs a single photon, the photopigment molecules change shape (not the chemical makeup)
- This triggers a cascading chain reaction, causing the release of millions of charged molecules, and ultimately leading to the photoreceptor firing.
What term is used to describe when a photopigment molecule changes shape?
- Photoisomerization!
- Receptors have different regeneration rates
What does the dark adaptation curve tell us?
- There’s a progressive increase in sensitivity to light
- The dark adaptation curve shows how sensitivity to light changes over time
How was the dark adaptation curve developed?
- A participant stares at a black X while a light is shone off to the side
- While staring at the X, the participant uses the method of adjustment to determine minimum threshold for detection of light
- They continue to adjust the light over time