Problem 1 The Eye
Wednesday, 11 January 2023 10:29
Helpful Videos: Keywords:
Sensation and Perception: Crash Course Psychology #5
Receptive Field: the area on the receptors that
influences the firing rate of the neuron. A specific
region of space in which an appropriate stimulus can
drive an electrical response in a sensory neuron.
Specificity Coding: Representation of particular objects
in the environment by the firing of neurons that are
tuned to respond specifically to that object.
There are single neurons that only respond to
specific stimulus. AKA Grandmother cell. They
Vision: Crash Course Anatomy & Physiology #18
are responding to concepts.
Distributed Coding: neurons respond to a number of
different stimuli, a particular object is represented not
by the firing of a single neuron but by the firing of
groups of neurons.
Representation of a particular object by the
pattern of firing of groups of neurons.
Sparse Coding: A particular object is represented by
the firing of a relatively small number of neurons.
Electromagnetic Spectrum: a continuum of
electromagnetic energy that is produced by electric
Name the different types of cells in the retina and their functions.
charges and is radiated as waves.
Make sure to not only talk about the different light receptors (an
important topic); also discuss other cells that play a role in the
Wavelength: the distance between the peaks of the
processing of information in the retina and the transmission of
electromagnetic waves. The wavelength of visible light
information from the light receptors to the brain.
is associated with the different colours of the spectrum.
◊ How does perception work via the eye?
After light has entered the eye, neural processing happens. It consists of
Visible Light: the energy w/in the electromagnetic
neural circuits.
spectrum that humans can perceive. Consists of
Neural Circuits:
photons.
▫ A simple neural circuit has no convergence, meaning it does not
synapse with more than one neuron into a single one. Another
Photon: the smallest packet of light energy.
type of neural circuit is the one involving convergence. Finally, a
type of circuit which involves inhibition signals. This one works
Accommodation: The changing of focus. The lens gets
best at a stimulus of medium length which only activates the
fatter as gaze is directed toward nearer objects. When
receptors w/out the inhibition signals. Neurons don't just
object is close to the eye. Prevents seeing the object
transmit signals, they also provide information about the
blurred. Happens unconsciously.
stimulus.
The ciliary muscles at the front of the eye tighten
and increase the curvature of the lens so that it
How is light focused correctly onto the retina so that we see
gets thicker. Therefore, the light rays bend, pass
sharp images (rather than blurry ones)?
through the lens to pull the focus point back to A
Light enters through the pupil and is focused by the cornea and lens to
to create a sharp image on the retina.
form sharp images of the objects on the which contains the receptors for
vision.
Far point: The distance at which the spot of light
◊ What are the processes involved ?
becomes focused on the retina.
1) Cornea
Excitation, Inhibition, Neural Responding
Absorb: To take up light, noise, or energy and not
Lateral inhibition: creates shadows. Helps us see in depth.
transmit it at all.
Ganglion cells receive excitatory input. The bipolar cells receive
inhibitory input hence decreasing the firing rate. Between the
Scatter: To disperse light in an irregular fashion.
photoreceptors.
Specificity Coding: The representation of particular objects in the
Reflect: To redirect smtg that strikes a surface to its
env by the firing of neurons that are tuned to respond specifically to
origin.
that object. Aka grandmother cell.
Accommodation: The lens changes its shape to help focus. With the
Transmit: To convey light from one place to another.
help of ciliary muscles.
Transduce: from light energy to neural electrical energy. Chemical
reactions in the outer segments of rods and cones transduce the light
Refract: 1) To alter the course of a wave of energy that
into electrical signals. (NOT VERY RELEVANT).
passes into smtg from another medium e.g water does
to light.
Accommodation: the lens changes its shape to accommodate to what
2) To measure the degree of refraction in a lens or
you are seeing. Ciliary muscles make accommodation possible.
eye.
Object in the distance: thin lens
Close object: thick lens
Near point and far point: distance on which the spot becomes clearer
on the retina
The light hits the outer segment, photoisomerization initiates a chain of
reactions, bipolar cells receive signals from photoreceptors, bipolar cells,
horizontal cells, amacrine cells, RGC and then to the optic nerve and then
it goes into the brain
2.4C Notes Page 1
, reactions, bipolar cells receive signals from photoreceptors, bipolar cells,
horizontal cells, amacrine cells, RGC and then to the optic nerve and then
it goes into the brain
◊ How do different eye conditions affect your vision?
Emmetropia: No refractive error. The refractive power of the eye is
perfectly matched to the length of the eyeball.
Presbyopia: distance of near point increases as a person gets older.
The lens hardens and the ciliary muscles become weaker. Can't see
things from up close. "Old Sight". It's bc of insufficient
accommodation. Muscles become too weak to accommodate for
anything. More general than hyperopia. Lens becomes less elastic
with age.
Myopia: near-sightedness, an inability to see distant objects clearly.
Parallel rays of light into focus at a point in front of the retina so
that the image reaching the retina is blurred. Can be due to the
shape of the eye and optic axis. The light stops in front of the retina.
Astigmatism: Unequal curving of one or more of the refractive
surfaces of the eye, usually the cornea. Vertical lines might be
focused slightly in front of the retina and horizontal lines are focused
slightly behind it (or vice versa).
Hyperopia: Farsightedness, light entering the eye is focussed
behind the retina and accommodation is required in order to see near
objects clearly. Eye is too small.
Cataract: An opacity of the crystalline lens.
Macular degeneration: fovea is damaged, only the peripheral
vision can be seen, centre is blind spot.
Detached retina: can be a result from injuries, becomes separated
from the rest, can lead to complete blindness, once disconnected they
cannot be connected again
Glaucoma: intraocular pressure (pressure of the fluids in the three
chambers of the eye, has to be greater than air pressure to keep the
eye from collapsing) so the pressure is too high for the eye
Diabetes: higher glucose levels, which can intervene with blood cells
and messages that are passed to optic nerve
Retinitis Pigmentosa: destroys retina, sight weakens
How are the created?
▫ Myopia: 1) Refractive Myopia: the cornea and/or the lens bends
the light too much. 2) Axial Myopia: the eyeball is too long.
Name the different parts of the eye and their function. Relate this
back to the figure in the pre-discussion.
◊ What is the anatomy and functions of the eye?
◊ Three Layers:
1) Fibrous Layer
a. Sclera
b. Cornea
2) Vascular Layer
a. Posterior Choroid (a membrane that supplies all of the layers
with blood).
b. Ciliary Muscle
3) Inner Layer
a. Ganglion and bipolar neurons
Visual Pigments: Light-sensitive chemicals contained in the visual
receptors of rods and cones, react to light and trigger electrical
signals.
Optic Nerve: Conducts signals towards the brain.
Cornea: The transparent covering of the front of the eye. Fixed in
place so can't adjust its focus. Contains no blood vessels, hence
transparency. It is made of fibres. Most powerful refractive surface.
○ The cornea accounts for about 80% of the eyes focusing power,
but it can't adjust its focus.
Lens: Can change its shape to adjust the eye's focus for stimuli
located at different distances. AKA Crystalline Lens. Thinner when
far, curvier/thicker when close.
Aqueous Humour: The watery fluid in the anterior chamber of the
eye. Supplies oxygen and nutrients, removes waste from the cornea
and the crystalline lens.
Pupil: The dark, circular opening at the centre of the iris. Where
light enters.
Iris: Muscular structure, coloured. Regulates the light entering the
eye by expanding and contracting the pupil.
Vitreous Humour: The transparent fluid that fills the vitreous
chamber in the posterior part of the eye. Space between the lens and
the retina. Light is being refracted for the fourth and final time. Gel-
like and transparent.
Retina: It's a layer of photoreceptors. A light-sensitive membrane in
the back of the eye that contains rods and cones, which receive an
image from the lens and send it to the brain through the optic nerve.
Fundus: The back layer of the retina, what the eye doctor sees
through an ophthalmoscope.
Optic Disc: The point where the arteries and veins that feed the
retina enter the eye, and where the axons of the ganglion cells leave
2.4C Notes Page 2
, retina enter the eye, and where the axons of the ganglion cells leave
the retina via the optic nerve.
How are the different types of light receptors distributed across
the retina? How does this affect what we see?
Photoreceptors: A light sensitive receptor in the retina.
○ Rod: A photoreceptor specialised for night vision. More
numerous. Only register a greyscale of black and white. Rule
peripheral vision. Info abt general shape, light or dark. There
are more rods than cones.
○ Cone: A photoreceptor: specialised for daylight vision, fine
visual acuity, and colour. Red, green and blue sensitive types.
Hit activation thresholds in bright conditions.
○ Closer to the retina centre.
○ Horizontal cells placed on the retina, help for communication.
○ Bipolar cells.
○ Ganglion cells.
Outer segment
Visual Pigments.
P-Ganglion Cell:
○ Feeds the Parvocellular LGN.
○ Small receptive field
○ Better resolution bc of cones
○ Receives excitatory input from single midget bipolar cells in the
central retina.
M-Ganglion Cell:
○ Feeds the Magnocellular layer of the LGN.
○ Sensitive to low light bc of rods.
○ Receive more input from photoreceptors so they have better
visual field.
○ Receives excitatory input from diffuse bipolar cells.
As the vision shifts from cones to the rods → we become more sensitive to
shorter wavelengths, such as blue and green colours.
Cones result in better detail vision, because they have less convergence •
Rods’ greater convergence: creates spatial summation.
What is lateral inhibition? How does it work? What is the function
of lateral inhibition?
Lateral inhibition helps us see depth and contrast, RGC receive excitatory
input, inhibitory input represses firing rate
Horizontal and amacrine cells are in charge of lateral inhibition
◊ How do optical illusions work?
1) Our photoreceptors can make see after images. Some stimuli such as
really brilliant colours, or brilliant lights are so strong that our
photoreceptors continue firing action potentials even after we close our
eyes.
2) Our cones can get tired. Receiving the bright stimulus for too long and
thus, stop responding. For example, American flag, blue and green ones
get tired, thus only the red ones can fire. The white screen includes all of
the colours and wavelengths of visible light, hence still receiving red, blue
and green light but only the red cones were able to respond. Hence, seeing
the red stripes.
3) Rods also got tired. Dark replaced with light. Hence, black stars and
stripes turned white.
If they are not next to each other you will receive 2 different lights.
White's illusion: The rectangle under A looks darker than the
rectangle under B. They have the same amount of light. Part B is
surrounded by more white, it receives more lateral inhibition.
Therefore it should appear darker than A but it doesn't. That's why
this illusion is not explained by lateral inhibition. Instead, it can be
explained by the principles of belonging.
○ Principle of belonging: an area's appearance is influenced by
the part of the surroundings to which the area appears to
belong. Rectangle B is affected by the dark bars bc it appears to
be resting on them -> dark bars make B appear lighter.
Mach band: the dark side appears lighter and the light side appears
darker, explained by lateral inhibition
○ Mach bands: seeing borders more sharply.
○ • Each of the 6 receptors send signals to bipolar cells.
○ • Each bipolar cell sends lateral inhibition to its neighbours on
both sides.
• A, B and C – intense illumination = appear lighter
2.4C Notes Page 3
, ○ • A, B and C – intense illumination = appear lighter
○ • D, E and F – receive dim illumination = appear darker
○ • The light intensity is the same from A-C and D-F.
Hermann grid: intersection, can be explained by lateral inhibition
○ Hermann grid: grey ghost images at the intersection of white
areas.
○ • All five points are stimulated.
○ • They send signals to bipolar cells.
○ • C & E send inhibition to A.
○ • B & D send excitatory signals to A.
○ Inhibition signals as "to not see white"
○ Because the response is associated with receptor A (receptor at
the intersection) is smaller than the response associated with
receptor D (in the corridor between black squares) the
intersection should appear than the corridor → we perceive grey
images at the intersection.
3. Simultaneous contrast: when our perception of the brightness
or colour of one area is affected by the presence of an adjacent
surrounding area.
○ • Lightness contrast → objects identical in light intensity
appear different in lightness.
○ • Lightness constancy → different physical intensities
appear in same lightness.
○ • Simultaneous contrast → occurs when our perception of the
brightness or colour of one area is affected by the presence of an
adjacent or surrounding area.
• Left dark square is less intensely stimulated + receives more lateral
inhibition = the grey square appears darker.
• However, the simultaneous contrast cannot only be explained by lateral
inhibition. Because if we start at the edge of the centre square on the left
and move toward the middle of the square, the lightness appears to be the
same all across the squares.
2.4C Notes Page 4
