Chapter 2- The First Steps in Vision: From Light to Neural Signals Flashcards
Wave
An oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium.
Photon
A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties.
Hue
The perceptual attribute of colors that enables them to be classified as similar to red, green, or blue, or something in between.
The electromagnetic energy spectrum
The visible spectrum is 400-700 nm.
Absorb
To take up something- such as light, noise, or energy, and not transmit it at all.
Scatter
To disperse something- such as light- in an irregular fashion.
Reflect
To redirect something that strikes a surface- especially light, sound, or heat- usually back toward its point of origin.
Transmit
To convey something (e.g., light) from one place or thing to another.
Refract
1) To alter the course of a wave of energy that passes into something from another medium, as water does to light entering it from the air.
2) To measure the degree of refraction in a lens or eye.
Image
A picture or likeness.
Cornea
The transparent “window” into the eyeball.
Transparent
Referring to the characteristic of a material that allows light to pass through it with no interruption such that objects on the other side can be clearly seen.
Aqueous Humor
The watery fluid in the anterior chamber of the eye.
Lens
The structure inside the eye that enables the changing of focus.
Pupil
The dark, circular opening at the center of the iris in the eye, where light enters the eye.
Iris
The colored part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil.
Vitreous Humor
The transparent fluid that fills the vitreous chamber in the posterior part of the eye.
Retina
A light-sensitive membrane in the back of the eye that contains photoreceptors and other cell types that transduce light into electrochemical signals and transmit them to the brain through the optic nerve.
Accommodation
The process by which the eye changes its focus (in which the lends gets fatter as gaze is directed toward nearer objects).
Focal Distance
The distance between the lens (or mirror) and the viewed object, in meters.
Diopter (D)
A unit of measurement of the optical power of a lens. It is equal to the reciprocal of the focal length, in meters. A 2-diopter lens will bring parallel rays of light into focus at 1/2 meter (50 cm).
Presbyopia
Literally “old sight”; the age-related loss of accommodation, which makes it difficult to focus on near objects.
Cataract
An opacity of the crystalline lens.
Emmetropia
The condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball.
Refractive Error
A very common disorder in which the image of the world is not clearly focused on the retina. the most common refractive errors are myopia, hyperopia, astigmatism, and presbyopia.
Myopia
Nearsightedness, a common condition in which light entering the eye is focused in front of the retina, and distant objects cannot be seen sharply.
Hyperopia
Farsightedness, a common condition in which light entering the eye is focused behind the retina, and accommodation is required in order to see near objects clearly.
Astigmatism
A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea.
Transduce
To convert from one form of energy to another (e.g., from light to neural electrical energy, or from mechanical movement to neural electrical energy). Neurons use electrical signals in their communication.
Fundus
The back layer of the retina: what the eye doctor sees through an ophthalmoscope.
Photoreceptor
A light-sensitive receptor in the retina.
How does our visual system deliver a focused image onto our retina?
The optics involved include a mechanisms for regulating the amount of light (the iris) and a lens for adjusting focal length so that both near and distant objects an be focused on the retina.
Optic Disk
The point where the arteries and veins that feed the retina enter the eye and where the axons of ganglion cells leave the eye via the optic nerve. This portion of the retina contains no photoreceptors, and consequently is blind.
What do we not normally notice our blind spot?
You don’t normally notice it because we have two eyes, and objects whose images fall into the blind spot of one eye can be seen by the other eye.
Fovea
A small pit located near the center of the macula and containing the highest concentration of cones and no rods. It is the portion of the retina that produces the highest visual acuity and servs as the point of fixation.
Macula
The pigmented region with a diameter of about 5.5 mm near the center of the retina. it is sometimes referred to as the macula lutea because of its yellow appearance.
Rod
A photoreceptor specialized for daylight vision, fine visual acuity, and color.
Cone
A photoreceptor specialized for daylight vision, fine visual acuity, and color.
Eccentricity
The distance between the retinal image and the fovea.
Duplex
In reference to the retina, consisting of two parts: the rods and cones, which operate under different conditions.
Vascular Tree
The branched blood vessels that spread out across the retina but stop short of the fovea.
Optical Coherence Tomography (OCT)
This is a noninvasive imaging technique that uses low-coherence light to capture high-resolution images form within light-scattering media (like the retina).
What layers does light have to travel through?
When photoreceptors sense light, they can stimulate neurons in the intermediate layers, including bipolar cells, horizontal cells, and amacrine cells. These neurons then connect with the frontmost layer of the retina, made up of ganglion cells, whose axons pass through the optic nerve to the brain.
Visual Angle
The angle that an object subtends at the eye.
What lighting conditions do rods and cones operate under respectively?
Rods function relatively well under conditions of dim (scotopic0 illumination, but cones require brighter (photopic) illumination to operate efficiently.
Properties of the fovea in human vision:
Photoreceptor Type: Mostly Cones
Bipolar Cell Type: Midget
Convergence: Low
Receptive-Field Size: Small
Acuity (Detail): High
Light Sensitivity: Low
Properties of the periphery in human vision:
Photoreceptor Type: Mostly rods
Bipolar Cell Type: Diffuse
Convergence: High
Receptive-Field Size: Large
Acuity (Detail): Low
Light Sensitivity: High
Retinal Pigment Epithelium (RPE)
This is where rods and cones get their energy. It lies below the retina. Recent work suggests that rods and cones burn glucose, converting the leftovers into lactate, which they feed back to this layer, which in turn uses the lactate for it’s energy.
What are the four primary ways in which the visual system adjusts to changes in illumination?
1) Pupil Size
2) Photopigment Regeneration
3) The Duplex Retina
4) Neural Circuitry
Receptive Field
The region on the retina in which visual stimuli influence a neuron’s firing rate.
Age-Related Muscular Degeneration (AMD)
A disease associated with aging that affects the macula. It gradually destroys sharp central vision, making it difficult to read, drive, and recognize faces. There are two forms of this- wet and dry.
Retinitis Pigmentosa (RP)
A progressive degeneration of the retina that affects night vision and peripheral vision. It commonly runs in families and can be caused by defects in a number of different genes that have recently been identified.
What are the five major classes of neurons that the retina contains?
1) Photoreceptors
2) Horizontal Cells
3) Bipolar Cells
4) Amacrine Cells
5) Ganglion Cells
Outer Segment
The part of the photoreceptor that contains photopigment molecules.
Inner Segment
The part of the photoreceptor that lies between the outer segment and the cell nucleus.
Synaptic Terminal
The location where axons terminate at the synapse for transmission of information by the release of a chemical transmitter.
Chromophore
The light-catch8ing part of the visual pigments of the retina.
Rhodopsin
The visual pigment found in rods.
Melanopsin
A photopigment that is sensitive to ambient light.
Photoactivation
Activation by light.
Hyperpolarization
A change in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface.
Graded Potential
An electrical potential that can vary continuously in amplitude.
Melanopsin-Containing Retinal Ganglion Cells (MRGCs) or intrinsically photosensitive retinal ganglion cells (ipRGCs)
They send signals to the suprachiasmatic nucleus, the home of the brain’s circadian clock, which regulates 24-hour patterns of behavior and physiology. The signals may also influence pupil response.
The Photopic System
Photoreceptor: 4-5 Million Cones
Location in Retina: Throughout retina, with highest concentration close to fovea.
Spatial Acuity (Detail): High
Light Sensitivity: Low
Response Speed: Fast
Saturation: No saturation.
Dark Adaptation: Recovery in ~5 minutes.
Light Adaptation: Fast (Weber’s Law)
Color Vision: Trichromatic
The Scotopic System
Photoreceptor: 90 million rods.
Location in Retina: Outside of fovea
Spatial Acuity (Detail): Low
Light Sensitivity: High
Response Speed: Slow
Saturation: Saturate at ~ twilight levels.
Dark Adaptation: Recover in ~40 minutes.
Light Adaptation: Slow (square root law)
Color Vision: None
Horizontal Cells
A specialized retinal cell that contacts both photoreceptor and bipolar cells.
Lateral Inhibition
Antagonistic neural interaction between adjacent regions of the retina.
Amacrine Cell
A retinal cell found in the inner nuclear layer that makes synaptic contacts with bipolar cells, ganglion cells, and other amacrine cells.
Bipolar Cell
A retinal cell that synapses with either rods or cones (not both_ and with horizontal cells and then passes the signals on to ganglion cells.
Diffuse Bipolar Cell
A bipolar retinal cell whose processes are spread out to receive input from multiple cones.
Sensitivity
1) The ability to perceive via the sense organs. 2) Extreme responsiveness to radiation, especially to light of a specific wavelength.
3) The ability to respond to transmitted signals.
Visual Acuity
A measure of the finest detail that can be resolved by the eyes.
Midget Bipolar Cell
A small bipolar cell in the central retina that receives input from a single cone.
ON Bipolar Cell
A bipolar cell that depolarizes in response to an increase in light captured by the cones.
OFF Bipolar Cell
A bipolar cell that hyperpolarizes in response to an increase in light captured by the cones.
Ganglion Cell
A retinal cell that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells) and transmits information to the brain and midbrain.
P Ganglion Cell
A small ganglion cell that receives excitatory input from single midget bipolar cells in the central retina and feeds the parvocellular layer of the lateral geniculate nucleus.
M Ganglion Cell
A ganglion cell resembling a little umbrella that receives excitatory input from diffuse bipolar cells and feeds the magnocellular layer of the lateral geniculate nucleus.
Koniocellular Cell
A neuron located between the magnocellular and parvocellular layers of the lateral geniculate nucleus. The layer is known as the koniocellular layer.
ON-Center Cell
A cell that increases firing in response to an increase in light intensity in its receptive-field center.
OFF-Center Cell
A cell that increases firing in response to a decrease in light intensity in its receptive-field center.
Filter
An acoustic, electrical, electronic, or optical device, instrument, computer program, or neuron that allows the passage of some frequencies or digital elements and blocks the passage of others.
Contrast
The difference in luminance between an object and the background or between lighter and darker parts of the same object.
What are the two important functional consequences of the center-surround organization?
1) Each ganglion cell will respond best to spots of a particular size (and will respond less to spots that are either bigger or smaller). In this way, retinal ganglion cells act as a filter by responding best to stimuli that are just the right size and less to stimuli that are larger or smaller.
2) Ganglion cells are most sensitive to difference sin the intensity of the light in the center and in the surround and they are less affected by the average intensity of the light.
