Task 2 - The retina Flashcards
Electromagnetic spectrum
is a continuum of electromagnetic energy that is produced by electric charges and is radiated as waves
Wavelength
the distance between the peaks of the electromagnetic waves – a.k.a. the energy in the electromagnetic spectrum
Visible light
the energy within the electromagnetic spectrum that humans can perceive
Cornea
the transparent covering of the front of the eye – it accounts for about 80 percent of the eye’s focusing power, but like the lenses in eyeglasses, it is fixed in place so can’t adjust its focus
Lens
supplies the remaining 20 percent of the eye’s focusing power, can change its shape to adjust the eye’s focus for objects located at different distances
Accommodation
the change in the lens’s shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker
Near point
the distance at which your lens can no longer accommodate to bring close objects into focus
Presbyopia
(“old eye”) - The distance of the near point increases as a person gets older
Presbyopia
Two solutions to this problem:
- Hold reading material further away (about arm length)
- Wear reading glasses to replace the focusing power that can no longer be provided by the “Old,” poorly accommodating lens
Myopia
nearsightedness - an inability to see distant objects clearly
Two factors for the causes: (Myopia / nearsightedness)
- Refractive myopia - in which the cornea and/or the lens bends the light too much
- Axial myopia - in which the eyeball is too long
Solutions (Myopia)
- Move the object
- Corrective eyeglasses or contact lenses
- Laser surgery
Hyperopia
farsightedness - can see distant objects clearly but has trouble seeing nearby objects
–> In the hyperopic eye, the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short
Solutions (hyperopia)
corrective lenses that bring the focus point forward onto the retina
Transduction
the transformation of one form of energy into another form of energy
–> Occurs in the receptors for vision: the rods and cones
Visual pigments
have two parts:
- a long protein called opsin and
- a much smaller light sensitive component called retinal
Isomerization
the change of shape → When a visual pigment molecule absorbs one photon of light, the retinal changes its shape, from being bent, to straight
Dark adaptation
the process of increasing sensitivity in the dark
- Experiments have shown that rod receptors and cone receptors adapt to the dark at different rates and that these differences occur because of differences in their visual pigments
Fovea
small area that contains only cones
Peripheral retina
includes all of the retina outside the fovea, contains both rods and cones
Macular degeneration
Destroys the cone rich fovea and a small area that surrounds it — one cannot see what one is directly looking at
Retinitis pigmentosa
degeneration of the retina that is passed from one generation to the next (although not always affecting everyone in the family) — causes loss of vision in the periphery, resulting condition is sometimes called “tunnel vision”
Blind spot
absence of receptors where the optic nerve leaves the eye
most important reason that we don’t see the blind spot is that some
mechanism in the brain “fills in” the place where the image disappears
Dark adaptation curve
- the function relating sensitivity to light to time in the dark, beginning when the lights are extinguished
- first step in measuring a dark adaption curve is to have the observer look at a small fixation point while paying attention to a flashing test light that is off to the side
- high threshold corresponds to low sensitivity
Light-adapted sensitivity
The sensitivity measured in the light — called like that because it is measured while the eyes are adapted to the light
Dark-adaptation sensitivity
the sensitivity at the end of dark adaptation
Rod monochromatic
people, who have no cones due to a rare genetic defect
Their all-rod retinas provide a way for us to study rod dark adaptation without interference from the cones
Rod-cone break
The place where the rods begin to determine the dark adaptation curve
Visual pigment bleaching
(light causes the retinal part of the visual pigment molecule to change its shape. Eventually, after this shape change, the retinal separates from the opsin part of the molecule) —> This change in shape and separation from the opsin causes the molecule to become lighter in color
Visual pigment regeneration
(When the pigments are in their lighter bleached state, they are no longer useful for vision. In order to do their job of changing light energy into electrical energy, the retinal needs to return to its bent shape and become reattached to the opsin) —> It is the this process of reforming the visual pigment molecule
Detached retina
when a person’s retina becomes detached from the pigment epithelium, a layer that contains enzymes necessary for pigment regeneration — can occur as a result of traumatic injuries of the eye or head, as when a baseball player is hit in the eye by a line drive
Spectral sensitivity
the eye’s sensitivity to light as a function of the light’s wavelength
Spectral sensitivity curve
the relationship between wavelength and sensitivity
Monochromatic light
light of a single wavelength
Purkinje shift
A shift from cone vision to rod vision occurs at dusk because your eye begins dark adapting in low light levels, so the rods begin to influence vision. This enhanced perception of short wavelengths during dark adaptation
Absorption spectrum
a plot of the amount of light absorbed versus the wavelength of the light
Lateral Inhibition
inhibition that is transmitted across the retina – the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction
Lightness
the perception of shades ranging from white grey to black
Hermann grid
Each bipolar cell has an initial response of 100. Bipolar cells B, C, D, and E each send 10 units of inhibition to bipolar cell A, as indicated by the red arrows. Because the total inhibition is 40, the final response of bipolar A is 60
Mach bonds
illusory light and dark bands near a light-dark border
calculate lateral inhibition
- Start with the initial response of each bipolar cell
- Determine the amount of inhibition that each bipolar cell sends to its neighbor on each side
- Determine the output of each cell by starting with its initial response and subtracting the amount of inhibition received from the left and from the right
Simultaneous contrast
occurs when our perception of the lightness or color of one area is affected by the presence of an adjacent or surrounding area
White’s illusion
right rectangle seems lighter than the left one (the bars resting either in the white or black bars) —> this cannot be explained by lateral inhibition — opposite happens
Belongingness
which states that an area’s appearance is influenced by the part of the surroundings to which the area appears to belong
Receptive field
the area that caused the neuron to fire the nerve fiber’s
Center-surround organization
in which the area in the “center” of the receptive field responds differently to light than the area in the “surround” of the receptive field
Excitatory area
a spot of light to the center increasing firing
Inhibitory area
stimulation of the surround causes a decrease in firing
Center-surround antagonism
when one covers the whole receptive fields it decreases the firing
Simple cortical cells
cells with these side-by-side receptive fields - cells in the striate cortex with receptive fields that, like center- surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas
Orientation tuning curve
the relationship between orientation and firing is indicated by a neuron’s orientation tuning curve, which is determined by measuring the responses of a simple cortical cell to bars with different orientations
Complex cells
respond best to bars of a particular orientation – most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field
End-stopped cells
a type of cell that fires to moving lines of a specific length or to moving corners or angles
