eyesight 2

Question 1

Transduction occurs in:

Answer 1

photoreceptors

Photoreceptors are the light absorbing elements of the retina

Photoreceptors are in the outermost layer of the retina

2 kinds of photoreceptors: rods and cones (thus humans have duplex retinas)

Question 2

Organization of the Retina

Answer 2

layered sheet of neurons

Outermost layer is made up of photoreceptors. Why: retinal pigment epithelium helps regenerate photo pigment once it gets bleached (in photoreceptors, which regulate retinal pigment epithelium)

Activity in photoreceptors stimulates neurons in the intermediate layer, which connect with ganglion cells in the innermost layer

Question 3

Rods - photoreceptors

Answer 3

Cylindrical outer segment

90 million

Periphery of retina

One single photopigment (colour blind)

Specialized for night vision

More sensitive

Use graded potentials (not AP, small axons), small hyper and depolarizations

-closest to retinal pigment epithelium

Question 4

Cones - photoreceptors

Answer 4

Conical outer segment

4-5 million

Mostly in fovea (behind pupil)

Specialized for day vision

Fine visual acuity

Three photopigments (colour vision)

-contain photopigment comb-like invagination segments

Question 5

Distribution of rods and cones in the retina

Answer 5

1. Cone density is highest in the fovea
   o Cones get larger and sparser away from fovea
   o Specialized for detailed vision
   o Fovea is directly behind the pupil and subtends a visual angle of ~1°
2. Rod density is highest in periphery
3. Retinal ganglion cell axons leave the eye at the optic disc (blind spot)

Question 6

Humans having a fovea with no rods means

Answer 6

that under dim illumination, we are effectively blind in the central 1° of our visual field (we see stars better in our periphery, use rods, than if you look directly at them and only use cones)

Question 7

nasal and temporal retina cone concentration vs foveal centre/retinal eccentricity

Answer 7

nasal and temporal retina: more rods than cones, cones get larger and sparser as we get farther

foveal centre/rettinal eccentricity: highet cone density, no rods

Question 8

Rods and cones have different light sensitivities

SCOTOPIC (dim light conditions):

Answer 8

Rods are more efficient than cones at converting photon absorption to neural signals → thus rods, and not cones, are active at low light levels

Question 9

Rods and cones have different light sensitivities

MESOPIC (intermediate lighting)

Answer 9

Cones and rods activated. Rods are only active at low light levels; above this level, photopigment cannot be activated any more → bleaching

Question 10

Rods and cones have different light sensitivities

PHOTOPIC (brighter conditions)

Answer 10

Only cones. Cones have mechanisms to prevent bleaching at high light intensities

Question 11

How does the visual system adjust to changes in illumination?

Answer 11

1. Rods and cones have different ranges: Rods are very sensitive but get overwhelmed by moderate light levels. Cones are less sensitive but have a broader operating range.
2. Photopigment must be regenerated: Lots of photopigment is available in dim light.
   When a photopigment is bleached (used up), the molecule must be regenerated again → thus, not all photons are capture
3. Pupil size is adjustable: Pupil diameter changes in response to light. A 4-fold increase in diameter = 16-fold increase in ability to capture photons. Very fast response. Darkness = contract, brightness = constrict
4. Ganglion cells respond best to contrast (spot of light next to dark background), not diffuse light: Ganglion cells – the output cells of the retina – are most sensitive to differences in light intensity between the centre and surround of their receptive field

Question 12

Dark adaptation experiment: determine the detection (absolute) threshold for a small spot of light at various intervals after bleaching (exposure to bright light)

Answer 12

How long does it take cones to recover from bleaching? Related to regeneration of photopigment (usually less time, 5 mins)

How long does it take rods to recover from bleaching? Rods take approximately 20-25 minutes to recover from bleaching and return to their maximum sensitivity. This recovery process involves the regeneration of rhodopsin, the light-sensitive pigment in rods

How does recovery from bleaching correlates with photopigment activity? Cones react quicker from photobleaching, rods recover slower from photobleaching. Threshold continues to lower after 5 mins due to rods

**theres a graph in notes

Question 13

Visual pigment (aka photopigment): made in

Answer 13

made in the inner segment and stored in the outer segment of photoreceptor cells

Question 14

Each photopigment consists of:

Answer 14

a protein plus a chromophore

Protein (opsin): structure determines which wavelengths of light the pigment molecule absorbs

Chromophore (retinal): absorbs light. The first event in phototransduction is capture of light photons by retinal (absorption of light and change of shape)

Question 15

how many photopigments in rods vs cones

Answer 15

Cones = 3 photopigments, Rods = 1

Question 16

Spectrophotometry

Answer 16

measures how much of the incoming light is absorbed by a protein

Measure light that is not absorbed
Remaining light is absorbed

-Amount of light absorbed by a photoreceptor depends on the intensity of light and its wavelength

500nm wavelength is most absorbed

Question 17

Rod Vision: Absorption spectrum and spectral sensitivity

Answer 17

Absorption Spectra:
Light absorption by rod photopigment is best at 500 nm (bluish-green light)

Spectral Sensitivity:
Human sensitivity to light in dim conditions (i.e., spectral sensitivity function) also shows a peak sensitivity (detection) at 500 nm
Spectral density experiments should be run in dim conditions (a while for adaptation) so that rods have the lowest sensitivity. do not detect color.

Experiment measures absolute thresholds at different wavelengths (we are most sensitive to wavelengths with lowest thresholds). Shine a flashlight in the periphery to measure rod sensitivity.

Question 18

Cone Opsins: Absorption spectrum and spectral sensitivity

Answer 18

Absorption Spectra (How each cone type responds to different wavelengths)
-3 types of cones, each with own characteristic absorption spectrum (ex: blue - short 440nm, green - middle 530 nm, red -long 560 nm)
-What makes each cone type unique is the “opsin” protein it contains: Opsins tune the light sensitivity of the cone to a specific part of the spectrum. This tuning shifts how much light (photons) each cone absorbs at different wavelengths.

Spectral Sensitivity (How the brain interprets color)
-Brain combines input from 3 types of cones to create colour vision
-Tested by measuring detection threshold at different wavelengths
-Sensitivity function is a combo of inputs from multiple cone opsins
-Conditions: central vision
- 550 nm peak in humans
- Researchers measure spectral sensitivity by finding the lowest light intensity (threshold) needed for detection at each wavelength.

The resulting sensitivity function is a combined response of all three cones.

In central vision (i.e., fovea), where cones are densely packed, humans show a peak sensitivity around 550 nm — close to green-yellow light.

check notes for diagram (more clear)

Question 19

Distribution of cone photopigments

Answer 19

Cone photopigments are not distributed equally among the cones

5-10% = short wavelength-sensitive cones (S-cones)

Remainder = 2:1 L-cones : M-cones

Fovea has mainly red+green cones (less blue)

Question 20

Comparing the scotopic and photopic spectral sensitivity curves

Photochromatic interval and purkinje shift

(look at graph in notes)

Answer 20

Photochromatic interval: difference between just seeing a light and being able to tell its colour - the difference between the threshold for detecting light (rods) and the threshold for detecting color (cones).

Purkinje shift: difference in perceived brightness of objects due to spectral shift (scotopic -> photopic) our perception of brightness changes from day to night:

In bright light (photopic), yellow/red wavelengths appear brightest.

In dim light (scotopic), blue/green wavelengths appear brighter than reds.

What happens when you increase the intensity of a subthreshold light at 450 nm? Rods activated till we reach cone threshold

What happens when you look at a visual scene and the overall illumination decreases? Decreases = lightness, things w/diff colours will change brightness

Photopic sensitivity is higher only at very long wavelengths: Exception: nyctalopia (night blindness) - photopic (cone-based, bright light)

Scotopic: blue is brighter, red is less bright (rod-based, dim light)

Question 21

Retinal Information Processing

The dark current - Rhodopsin is Inactive

Answer 21

1. In the dark, a molecule called cyclic GMP (cGMP) binds to ion channels permeable to Na+ and Ca2+
   * Keeps them open
   * Dark current → flow of cations into the outer segment in the dark
2. K+ leaves the cell through K+ leak channels in the inner segment
3. The Na+/K+ pump maintains the concentrations of Na+ and K+ inside and outside the cell

chat definition: In the dark, photoreceptors are active — depolarized by a steady flow of cations (dark current).

This unusual state allows them to continuously release neurotransmitter (glutamate).

Light will reduce cGMP, close the channels, hyperpolarize the cell, and reduce neurotransmitter release — the basis of phototransduction.

Question 22

As a result of the dark current…

In the dark, the membrane potential of a photoreceptor is

Answer 22

~-40 mV (How does this compare to most neurons? Most neurons are -70mV (more negative)) photoreceptors are less negative (more depolarized).

So in darkness, glutamate is continuously released at the synaptic terminals.

This is opposite to typical neurons, which release neurotransmitters only during action potentials.

The neurotransmitter glutamate is constantly being released from photoreceptor terminals (darkness = glutamate release)

Question 23

Phototransduction ) light is converted into an electrical signal in the retina) Steps:

Answer 23

1. Absorption of light by retina
2. Rhodopsin changes conformation → activated
3. Activated rhodopsin activates a G-protein called transducin
4. G-protein activates an enzyme called PDE
5. PDE breaks down cGMP → GMP
6. cGMP-gated channels close ( Na⁺ and Ca²⁺ stop entering.
   → This causes hyperpolarization (the cell becomes more negative).less glutamate release, opposite of what happens in the dark (where cGMP is high and the channels stay open).

Question 24

Outcome of phototransduction:

What happens to the membrane potential when cGMP-gated ion channels close?

AND How does this compare to the receptor potential generated in other sensory systems?

What is the effect on neurotransmitter release from the synaptic terminal?

Answer 24

More negative (start at -40mV and gets more negative as K+ leaves and is not balanced by Na+ going in)

how does this compare?
opposite: light causes hyperpolarization - not depolarization

Less glutamate (NT) released

Question 25

Neural processing beyond the photoreceptor

Answer 25

Photoreceptors, bipolar cells, and ganglion cells form a vertical (radial) pathway in the retina Bipolar cells synapse with either rods or cones and pass signals onto retinal ganglion cells (RGCs)

Question 26

RGCs are:

Answer 26

1. The only neurons whose axons leave the eye! 2. The only retinal neurons that generate action potentials 3. A part of parallel visual streams

Question 27

Signal convergence in the periphery

Answer 27

100 million photoreceptors vs. 1.25 million ganglion cells Signals from multiple photoreceptors must converge onto a single RGC Small part of visual field has smaller dendritic and receptive fields (more detail) Larger part of retina responds to larger parts of receptive field (less detail)

Question 28

Midget vs Diffuse bipolar cell

Answer 28

Midget: few photoreceptors converge upon this single cell (in the fovea this ratio is 1:1, 1 cone: 1 bipolar cell: 1 retinal ganglion) Diffuse: convergence of multiple signals, pool info from many photoreceptors, larger dendritic fields

Question 29

Visual acuity

Answer 29

a measure of the finest detail that can be resolved by the eyes

Question 30

Compare the sensitivity of the left 3 RGCs (G) to the sensitivity of the right RGC to stimulus 1 (dim diffuse light).

Answer 30

1 RGC: activity from all photoreceptors adds up and allows for better input (dim light), more sensitive ganglion cells. Convergent input means weak signals from dim light add up (summate). This makes it more sensitive — it can detect dim light better. (likely in peripheral retina). 3 RGC: wont activate receptor. Each receives input from fewer photoreceptors (likely in central/foveal retina). Without enough input (because the light is dim), they may not reach activation threshold. So, they are less sensitive in dim light.

Question 31

Compare the ability of the RGCs on the left and right to discriminate between 3 separate light spots (stimulus 2)

Answer 31

3 RGC: more detail (sample smaller part of visual field), 1 RGC: would know where spot of light is, large receptive field

Question 32

Foveal vision allows us to see fine details because,

Answer 32

1. Retinal neurons (except cones) are shifted to one side to allow light unimpeded access to cones (no bypass) 2. Cones are tightly packed (outer segments are small, can pack tightly) 3. Cones and bipolar cells are connected to each other in a 1:1 ratio (no mixed info, stays separate) * High spatial sampling

Question 33

2 types of bipolar cells (Each foveal cone actually contacts 2 types of bipolar cells):

Answer 33

Bipolar cell responses are explained by the presence of different glutamate receptors on bipolar cell Dendrites (photoreceptors release glutamate) OFF bipolar cells are hyperpolarized by light, just like photoreceptors (turned off by light) ON bipolar cells are depolarized by light (light excites them, excited by glutamate depolarization)

Question 34

Bipolar cells connect to:

Answer 34

ganglion cells 2 types ( P and M ganglion)

Question 35

P-ganglion cell (petit/small)

Answer 35

* Receives input from midget bipolar cells * Project to parvocellular LGN layers * Small size * Small dendritic field * Most numerous (~70% of all RGCs) * Especially high density in fovea

Question 36

M-ganglion cell (massive/large)

Answer 36

* Receives input from diffuse bipolar cells * Project to magnocellular LGN layers * Large size * Large dendritic field * 2nd most numerous (~10% of all RGCs) * periphery of retina

Question 37

How did Stephen Kuffler study RGC responses in living cats.

Answer 37

1. Microelectrode records electrical activity from an RGC in the retina. 2. Electrical signals are amplified and displayed on a monitor

Question 38

All RGCs have a certain level of spontaneous activity (action potential firing).

Answer 38

Firing rate can either increase or decrease in response to light (go above or below baseline) Flooding the retina with light doesn’t change spontaneous firing (spots of light on dark background activates light) Retina doesn’t just detect light, rather it detects differences in light in adjacent parts of the retina.

Question 39

Retinal ganglion cell receptive fields

Answer 39

area of the retina (or visual field) that influences the neuron, either by inhibition or excitation * Receptive field is roughly circular * 2 concentric zones: excitatory + inhibitory = centre-surround antagonism Shape: The receptive field is roughly circular. Structure: It has two concentric zones: Center: Either excitatory or inhibitory. Surround: Opposite effect of the center. This interaction is called centre-surround antagonism. This center-surround design helps the visual system detect contrast and edges, making it easier to distinguish objects in our visual environment — especially under varying lighting conditions.

Question 40

Responses of an ON-centre ganglion cell to light: 1. What causes the increase in firing rate? 2.Why does this light stimulus cause a faster/ more vigorous firing rate? 3. Why is the firing rate reduced compared to above? 4. Why does diffuse light have no effect on the firing rate?

Answer 40

1. Cell has excitatory centre (on centre cell) 2. Spot of light is bigger/takes up entire excitatory region 3. Light is in inhibitory region (cancels out effect of light the centre) 4. Same amount of inhibition and excitation, cancel eachother out refer to image in notes

Question 41

Consequences of centre-surround receptive fields

Answer 41

RGCs exhibit centre-surround antagonism → the two subregions of the receptive field oppose each other * The antagonistic interaction between the centre and surround is known as lateral inhibition and is mediated by horizontal cells (have lateral connections) RGCs act as a filter to respond best to stimuli that are just the right size (preferentially excited by specific size light) * Respond less to stimuli that are smaller/larger (bright centre on really dark background = best firing rate response [contrast]) RGCs are optimized for detecting contrast * RGCs are most sensitive to differences in the intensity of light in the centre and its surround

Question 42

Perceptual consequences of centre-surround effects

Answer 42

1. | Lightness contrast What it is: The perceived brightness (lightness) of an object depends on its background. Why it happens: Due to center-surround antagonism, the same patch of light can look brighter or darker depending on the light level of surrounding areas. Example: A gray square looks lighter on a dark background and darker on a light background — even though the square’s luminance doesn’t change. ➡️ This effect helps the visual system enhance contrast and detect edges more clearly 2. | Lightness constancy What it is: Our perception of an object's lightness remains stable, even when the overall lighting changes. Why it happens: If the whole scene (object + background) gets brighter or darker equally (like when a cloud covers the sun), the brain adjusts for the lighting shift. Result: We still perceive a white shirt as white whether indoors or in sunlight. ➡️ This effect allows for stable perception in dynamic environments.

Question 43

Mach Bands (vertical bars with uniform colour/brightness): Centre Surround effects

Answer 43

Tend to see light and dark bands where the intensity changes abruptly False impression of a narrow dark band immediately to the left and a narrow light band immediately to the right of each boundary **refer to notes for image Mach Bands are an optical illusion where we perceive illusory light and dark bands at the edges between adjacent regions of different brightness — even though no such bands actually exist. Why Do We See Them? → Centre-Surround Antagonism Retinal ganglion cells respond to local contrast, not absolute light levels. Each cell has a receptive field with a center (excitatory or inhibitory) and surround (opposite effect). At edges, the difference in stimulation between center and surround causes exaggerated responses: Cells on the bright side near the edge get more excitation → perceived as a lighter band. Cells on the dark side near the edge get more inhibition → perceived as a darker band.