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
How does the dark adaptation curve change if the participant stares directly at the light?
- Cone adaptation occurs, but since cones don’t have very good convergence they can only detect so much light
- Takes cones approximately 6 minutes to regenerate after photoisomerization. Once they all regenerate, it means that at any given time, there will be some available to react to photons. This is where almost an equilibrium is reached.
What does the dark adaptation curve look like for someone who is a rod monochromat?
- Initially, when you move into a dark room, there aren’t many rods available for use
- Take approximately 25 minutes for rods to regenerate, that’s why after 25 minutes occurs, there’s a major increase in threshold at this time
What does spectral adaptation signify?
- The relationship between sensitivity to light and the wavelength of light
- For humans, the highest sensitivity to light is around 560nm (yellow neon)
- Differences in spectral sensitivity are due to differences in absorption spectra of pigments
What’s the Purkinje Effect?
- An explanation for why rods have a lower peak wavelength compared to cones
- Rods’ peak sensitivity is at 500nm which is thought to be because rods are used primarily for seeing in the dusk (note: they still do not identify colour)
What are the three types of cones?
- Short-wavelength ‘blue’ cones
- Medium-wavelength ‘green’ cones
- Long-wavelength ‘red’ cones
*The ratio of types of cones varies from person to person
If there’s only three types of cones, how do we respond to the whole colour spectrum?
- Each colour/hue has its own specific cone response pattern for each wavelength
- Each cone will absorb different amounts of light, and then the brain puts together info from all three cones and then perceives a colour to represent the firing pattern
What’s strabismus?
- Disorder of the extraocular muscles
- The two eyes do not coordinate to point in the same direction, resulting in a double-image and impaired depth perception
Is strabismus the same as lazy eye?
- No it is not, although if left untreated it can lead to ‘lazy eye’ also known as amblyopia where signals from one eye become completely ignored by the brain
What’s myopia?
- Also known as nearsightedness
- Light from distant objects is focused in front of the retina (i.e., can’t see far objects)
- Often caused by slight elongation of the eye
- The longer the eye, the worse the myopia
- The lens will focus light where it thinks it should be, not were it actually fovea is
What’s hyperopia?
- Also known as farsightedness
- Light from nearby sources is focused behind the retina (i.e., can’t see objects up close)
- Often caused by a shortened eye length
- Can also be caused by aging where the lens loses elasticity (presbyopia), cannot bend light to the same degree
What’s astigmatism?
- The lens or cornea is misshapen, causing a misplaced focal point and/or more than one focal point on the retina
- Causes blurriness and/or double vision
What’s cataracts?
- Progressive clouding of the lens, caused by the breakdown of proteins
- Associated with aging, pupils start to appear grey
What’s macular degeneration?
- Damage to the photoreceptors in the central areas of the retina (fovea and parafovea)
- The damage may spread due to a number of reasons
What’s retinitis pigmentation?
- An inherited (genetic) condition of gradual degeneration of photoreceptors, starting in the periphery
- Often leads to night blindness and tunnel vision
- Field of view also diminishes but doesn’t always lead to total blindness
How do night vision devices generally work?
- The few photons in a dark environment that interact with the device are funnelled into an intensifier tube, which are then converted to electrons
- The electrons strike a phosphorous screen which glow depending on the electron energy levels
- More photons = more electrons interacting with the phosphorous = brighter area
