Summary Perception to Conciousness Part 1 (Lecture 1 - 8)

Summary Perception to Consciousness
Part 1
Marissa Verbokkem – 11335882
February 2023


1 LECTURE 1: THE RETINA

1.1 RETINA
- The retina processes signals
- Contains rods and cones, sensitive to different
wavelengths
o They sample the image
o Red cones: long wavelength
 Red / yellow
o Green cones: medium wavelength
 Green (less to yellow)
o Blue cones: short wavelength
 Blue
- Cones give you colour vision
o Are mainly in the central part of visual
field
 Mainly in the fovea
- Rods and cones have protein with different
shapes:
o Rhodopsin
o Light hits rhodopsin  sodium(Na+)
channel closes  hyperpolarisation  signal
goes to bipolar cell (via horizontal cell)
o Humans have 4 types of rhodopsin
 3 for cones, 1 for fords
 Some animals have more
 Results in seeing more colours
- Retina pre-processes rod and cone signals via bipolar
cells
o Bipolar cells go to ganglion cells
o Ganglion cells pass signals to the brain
 Through optic nerve


1.2 RETINAL COLOR BLINDNESS
- Missing a certain cone type
- Can be tested with Isihara plates
- 8/100 white males suffer color blindness
o Less in Asians and Africans
o Less in females

,1.3 FOVEA
- Fovea: part of the retina with the highest density of
photoreceptors
- Mainly cones
o Leads to sharpest vision and color vision
- Cup shaped
- Ophthalmologists also look at the macula:
o Area around fovea
o Most important part of retina
- Fundoscopy: studying the back of the eye
o Shows that light has to pass many
obstacles to reach photoreceptors
 Veins, vitreous body particles
- Blood vessels run through our retina
o They ‘obscure’ the visual field but we do not
perceive them
- Blind spot: part where optic nerves leave the retina
- Optic disc: optic nerve leaving the eye
- Macular degeneration:
o Typical old age disease
o Leads to loss of central vision and acuity
 Distortion and black holes in visual field
o Cause: pigment epithelium receptors are lost, due to accumulation of toxic
products in brain (or eye)
o Wet macular degeneration: blood / bloody patches underneath the macula
o Dry macular degeneration: yellow deposits in the macula


1.4 PHOTORECEPTORS
- Each eye has +/- 130.000.000 photoreceptors
- Photoreceptors are embedded in a layer of heavily pigmented cells: pigment
epithelium
o Pigment epithelium prevents light from scattering (by absorbing it)
o Makes sure light only hits one photoreceptor
o Cats have reflective pigment epithelium
 Better dark vision, but less sharp (also in the dark)
- Light has to pass through the retinal network to reach photoreceptors
o Because pigment epithelium is in the back of the eye
- Blind spot: part where retinal-ganglion cell fibres pass through the optic disc to leave
the eye
o No receptors here
- Light on photoreceptor: Na+ channel closes (hyperpolarisation)
o Response to dark: Na+ channels open (depolarisation)
o Graded potential signal

,1.5 GLAUCOMA
- Glaucoma: disease where eye pressure gets too high and presses on optic nerve
- Pressure leads to damage of retinal ganglion cells (RGC’s)
- First loss of peripheral vision
- Treatment:
o Eye drops, surgery
o All lost ganglion cells are lost forever :(


1.6 FROM RETINA TO THE OPTIC NERVE
- Retina has 130 million photoreceptors per eye
o Only 1.000.000 nerve fibers in optic nerve
o Input has to be compressed
- Retina does data compression
- Retina to optic nerve:
o Photoreceptor signal: graded potential
o Translated into on/off signal in bipolar cell
 Glutamate between photoreceptor and bipolar cell
 Based on amount of polarization in photoreceptor, more/less
glutamate
 Bipolar cell picks up glutamate
o ON bipolar cell: sign inverting synapses
 Sign inverting synapse translates hyperpolarization into action
potential
 Light on = action potential
o OFF bipolar cell: sign conversing synapses
 Sign conversing synapse translates depolarization into action potential
o Horizontal cells:
 Receive input from many photoreceptors
 Inhibit photoreceptors: provide negative feedback
 Gives bipolar receptive field a center-surround profile





 Light in center: photoreceptors respond
 Light in surround: horizontal cells inhibit
 Result: summed signal = 0

, o Ganglion cells:
 Optic nerve fibers are ganglion cell axons
 On-center: responds to light in the center and dark in the surround
 Off-center: responds to dark in the middle and light in the surround
 Code for contrast, discard luminance
 This is why humans struggle perceiving luminance of a scene
 Leads to luminance illusions
 Respond the same to uniform patches
 Color (white, gray, black) doesn’t matter
- Herman Grid illusion: side effect of this data compressing

Photoreceptor Ganglion cell
Graded potential Action potential
- Because of long axons
Responds to luminance Responds to contrast
- Because inhibition from horizontal
cells
Rods and cones On and off cells




1.7 HOW DO RODS WORK?
- Rods connect to rod-bipolar cells
o These do not connect directly to ganglion cell, but via amacrine cells
o Rod  Rod-bipolar cell  amacrine cell  cone driven bipolar cell (on and
off) ganglion cell
- Larger receptive fields than cones
o Because rod-bipolar cells receive input from several rods
o Also makes vision less sharp
- Bipolar cells and ganglion cells have overlapping receptive fields from rod and cone
input
- Horizontal cells also give rods the center-surround profile
- 100x more sensitive to light than cones
o Rods can detect a single photon of light
o Primary source of visual information at night (scotopic vision)
- Sensitive to 1 wavelength
o Not useful for seeing color
- Rods are mainly in the periphery (also makes vision less sharp)

1.7.1 Dark adaptation
- Dark adaptation: the process of adapting vision to darkness
- In the dark rods take over vision
- Bleaching: rods become less sensitive to bright light, making it possible for cones to
mediate vision in bright light
- It takes about 30 minutes to become completely adapted to the dark
1. Pupils dilate in the dark
2. Cones adapt (to see better)
3. Bleaching becomes undone (after 10/11 minutes)
a. Rods take over
4. Less receptor signal leads to less negative feedback from horizontal cells
a. Making the signal better