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Summary 2.4 - Perception
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Summary 2.4 – Perception2.4.1 – The eye
Shape and size
• Diameter of 24 mm
• Spherical à allows for rotation and
movement
• Optic axis à imaginary line from front to
back of eye and passes through center of
the lens, each eye points in the direction
defined by optic axis
Three membranes
• Sclera à outer membrane, tough
protective covering, visibly seen as white
of the eye and transparent cornea at the
front of the eye
• Choroid à middle membrane, lines
interior of sclera, contains most vessels
that supply eye with oxygen/nutrients
• Retina à inner membrane, made up of
neurons, and receptors that covert light
into neural signals
Cornea à transparent membrane at the front of the eye
• Light first enters here and is refracted (bent), this focuses light on the retina
• Rigid, cannot adjust how much light is refracted + can’t adjust focus
• 80% of eye focusing power
Iris and pupil
• Iris à colored part of the eye
• Donut shaped muscle, opening in the middle is the pupil
• Iris controls size of pupil by contracting/relaxing in response to light intensity
o Pupillary reflex à automatic process in which the iris responds to light by
contracting/relaxing, changing the size of the pupil
, § Intense light à iris contracts, pupil gets smaller à reduces amount of
light that can enter
§ Dim light à iris relaxes, pupil gets larger à increases amount of light
• Pupil à can range in diameter (2mm to 8mm)
o Gets smaller = constricts
o Gets larger = dilates
• Irises work in tandem; both constrict dilate simultaneously + to the same degree
Three chambers
• Iris lies behind cornea and in front of the lens
• Anterior chamber à space between cornea and iris
• Posterior chamber à space between iris and lens
o Both filled with aqueous humor (clear, thin fluid)
• Vitreous chamber à main interior portion of eye
o Filled with vitreous humor (more gel like fluid)
• Both humors slightly refract light, but amount of refraction performed cannot be
adjusted
• Intraocular pressure à pressure of fluids in three chambers
o Must be greater than air pressure to prevent collapse
Lens and accommodation
• 20% of eye focusing power
• After passing through cornea, light passes
into lens à transparent structure that
further refracts light to ensure it focuses
properly on retina
• Focal length à distance from the lens at
which an image of an object is in focus
when the object is far away from the lens
o Determined by power of any lens to
refract light
o Weak lens doesn’t refract much
light à thin, flat, and long focal
length
o Strong lens refracts light sharply à
thick, rounded, and short focal
length
• Diopters à units used to express power of a lens
, o Defined as 1/(focal length) in meters
o Eg. lens with focal length of 0.5 m has a power of 2.0 diopters
§ 1/0.5 = 2
• Accommodation à in order to focus, mammalian eye adjusts the shape of the lens to
focus on objects at different distances from the eye
o In the eye, shape of lens in ellipsoidal because its edges are stretched by zonule
fiber
o Zonule fibers connect lens to choroid
o Ciliary muscles also attached to choroid
§ When ciliary muscles are relaxed, choroid can pull on zonule fibers =
stretches lens à weak lens (thin, flat shape + long focal length), good for
focusing light from distant objects
§ When ciliary muscles contract, can oppose pull by choroid on zonule
fibers = strong lens (thick, round shape + short focal length), good for
focusing light from nearer objects
Retina
• Transforms image into neural signals
• The retinal image à clear image on the retina of the optic array
• Innermost of three membranes enclosing the eye
• Made up of different classes of neurons which perform distinct functions
o Classes have types/subtypes
• Main layers à outer nuclear layer, inner nuclear layer, and ganglion cell layer
o Nuclear layers = contain nuclei of retinal neurons
§ Are separated by synaptic layers
o Outer nuclear layer à photoreceptors (but not inner and outer segments)
o Inner nuclear layer à bipolar, horizontal, amacrine cells
o Ganglion layer à retinal ganglion cells (RGCs)
§ Axons of RGCs exist eye at optic disk (blind spot, no photoreceptors) and
form optic nerve
• Synaptic layers à outer and inner
o Retinal neurons make synapses with each other
o Outer synaptic layer à contains synapses among photoreceptors, bipolar cells,
horizontal cells
o Inner synaptic layer à contains synapses among bipolar cells, amacrine cells,
and RGCs
• Photoreceptors à inner and outer segments located nearest to the back of the eye
o Retinal neurons that transduce light into neural signals
o Two classes à rods and cones
, o Transduction occurs are embedded in pigment epithelium (layer of cells attached
to choroid)
Fovea
• Optic axis passes through fovea at the center of the retina
• Where light from objects at center of gaze strike retina
• No rods, cone density is very high
• Cones are thinner, packed into hexagonal grid
• Ganglion cell/inner nuclear layers are pushed off to side of fovea à light can reach
fovea cones without being scattered
è Maximized high-acuity vision at center of gaze
Pathways of neural signals in retina
1. Incoming light passes through a) other layers of neurons in retina and b) outer parts of
photoreceptors à transduced into neural signals (changes in membrane potential of
photoreceptors)
2. Changes in photoreceptor membrane potential alter neurotransmitter molecules that
photoreceptors release
3. “Through pathway” transmits signals from photoreceptors to RGCs via bipolar cells
(main flow of signals through retina)
a. Flow of neurotransmitters released by rods/codes affects membrane potential of
bipolar cells and changes their release of neurotransmitters
b. Change in bipolar cell release affects membrane potential of RGCs, which affects
firing rates
4. “Lateral pathway” involves horizontal and amacrine cells, allows presence of light in one
location on retina to affect
responses of photoreceptors,
bipolar cells, and RGCs at
adjacent locations on retina
a. Horizontal cells receive
signals from
photoreceptors and other
horizontal cells, send
signals back to both
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