Vision II Overview of Topics - University of...

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Vision IIRetinal Processing & Early Vision

Chapter 9 in Chaudhuri Text

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• Anatomy of the retina

• Photoreceptors: Rods vs. Cones

• Electrical circuits in the retina: Ganglion cells and their supporting cast

• Perceptual phenomena arising from retinal processing

Overview of Topics

Review: Anatomy of Eye

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Anatomy of the Retina

Macula

FoveaOptic Disc

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PeripheralRetina

PeripheralRetina

PeripheralRetina

Layers of the Retina

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front of eye is down

Rods & Cones

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• Rods (1 type) and Cones (3 types)

• Rods are larger and more numerous120 million rods vs. about 6 million cones

• Rods specialized for night vision, Cones specialized for day vision

• Cones types are short λ, medium λ and long λ, or S-cone, M-cone, and L-cone.

Four Types of Photoreceptors

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Distribution of Rods & Cones Across Retina

• Fovea consists solely of cones, no rods. Cones here are at maximum density

• Cone density drops rapidly as we move out towards peripheral retina

• Rods maximally dense in macula, with density again dropping towards peripheral retina

• No rods or cones at optic disc.

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Distribution of rods & cones across the

midline of the retina

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• Optic disc - place where optic nerve leaves the eye. There are no receptors here.

• Q: So why isn’t there a “hole” in our vision?

• A: There is! But...

• One eye covers the blind spot of the other

• It is located in the periphery, where acuity is low anyway

• The brain “fills in” the spot

The Blind Spot

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Specialization Across the Retina

• Fovea is specialized for detailed vision

• Try reading the first word of this sentence while looking at the last word of it.

• This is why we look directly at objects of interest.

• Periphery specialized for low-light vision

• Averted vision: Look slightly away from things at night to see them better

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Questions

• How many types of photoreceptors in the human eye? Name them.

• Describe the distribution of each photoreceptor type across the retina.

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Phototransduction

• The process of transducing light energy into bioelectric signals in neurones

• Similar to the transduction processes we’ve seen in other sense

• However, it has some odd, even backward, aspects

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Fundamental Concept:Transducers

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Transduction of Light

• Photoreceptors have inner and outer segments.

• Outer segments contain visual pigment:

• Rods: Rhodopsin

• Cones: 3 types of Photopsin (aka “Cone Rhodopsin”, a bit of misnomer)

• These opsins are G-protein coupled receptors (recall we’ve seen GPCRs in other senses)

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Rhodopsin(& Photopsins)

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The Dark Current• In darkness:

Sodium ions (Na+) flow into outer segment Potassium ions (K+) flow out of inner segment

• Along with sodium-potassium pumps this keeps potential at a steady -40 mV.

• However, sodium inflow can only happen with help of cGMP

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The Phototransduction Cascade

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• Light absorption by an opsin isomerizes it into an active form, which...

• …activates the coupled G-protein, which...

• …activates an enzyme that converts cGMP into plain old GMP, which...

• ...makes for less cGMP, meaning the sodium (Na+) channels shut down

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• Thus more light means lower positive inflow

• The end result of all this is greater polarization! Down to -70 mV from -40 mV!

• Weird! In every other transducer we’ve seen, more stimulus means depolarization and (in many cases) action potentials.

• Here it’s the complete opposite

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It Gets Weirder...• More light = less

neurotransmitter release from the photoreceptors!

• And it’s glutamate, which is normally excitatory!

• So more light = less excitation?

• All of this is completely backwards to other sensory systems

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Questions

• “Light absorption activates rhodopsin, which activates a G-protein, which converts cGMP to GMP” What’s missing in this sequence?

• What is the significance of cGMP getting converted to GMP?

• Name two weird things about how the receptors work.

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Rods, Cones, & The Spectrum of Light

• Only photons absorbed by an opsin are transduced into neural signals.

• Number of photons absorbed depends on:

• Number of photons hitting retina (more photons present = more photons absorbed)

• Wavelength of photons: Each photoreceptor has a λ it absorbs best

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Absorption Spectra of Photopigments

• Shine a laser (i.e. light of a single λ) through vial of rhodopsin

• Measure how much is transmitted. We can ∴ determine how much it absorbs

• Do this across the range of λs (400-700 nm). Gives us the absorption spectrum

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Absorption Spectrum of Rhodopsin

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Absorption Spectra of Cone Photopsins

• Repeat the procedure we executed with rhodopsin, but using the 3 cone photopigments

• We find that each has a unique absorption spectrum

L-cone pigmentM-cone pigmentS-cone pigment

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Absorption Spectra of Cone Photopsins

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Absorption Spectra & Human Sensitivity Spectra

• Do the absorption spectra relate to variations in human sensitivity across the spectrum?

• To answer this, we have to measure two human spectral sensitivity functions (SSFs):

• Scotopic (night vision, rods)

• Photopic (day vision, cones)

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Absorption Spectra & Human Sensitivity Spectra

• An SSF is a psychometric function, relating two physical units

• It shows sensitivity as a function of wavelength

• Here, we are not referring to d’, but rather sensitivity (S) as the inverse of threshold (T)! ! ! ! S = 1/T ∴ T = 1/S

• Like all psychophysical functions, SSFs are derived from thresholds derived from multiple psychometric functions

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Scotopic & Photopic Sensitivity Spectra

• Scotopic SSF: To measure rod function in isolation from cones, we:

• Do the measurements in low light levels

• Place the stimulus just off fixation (why?)

• Photopic SSF: To measure cone function in isolation from rods, we:

• Make the stimulus small

• Place it directly at fixation (why?)

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Scotopic & Photopic SSFs

Scotopic Photopic

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Absorption Spectra & Human Sensitivity

• Now we can compare the absorption spectra of the various photopigments to the SSFs

• For night vision, the rod absorption function matches the scotopic SSF very well

• For day vision, the three cone functions must be combined in some way to explain the SSF

• Specifically, it seems that M & L cones combine to produce the photopic SSF, with little input from S cones

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Rod Absorption Spectrum Cone Absorption Spectra

Scotopic Spectral Sensitivity Photopic Spectral Sensitivity

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Absorption Spectra of Photoreceptors

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The Purkinje Shift

• Rods and cones have different spectral sensitivity curves.

• The three cones, taken together, are most sensitive to light around 550 nm (yellowish)

• The rod type is most sensitive to light around 500 nm (bluish green)

• Purkinje Shift: At twilight/night, blue/green things look brighter, while reddish/yellowish look darker

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Purkinje ShiftDayTwilightNight

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Questions

• What is the peak of the rhodopsin absorption curve?

• What is the peak of the scotopic sensitivity function?

• Which cone type is least involved in producing the photopic SSF?

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Rods & Cones: Like Night and Day

• Rods are specialized for night vision whereas cones deal with day vision.

• Rods are much more sensitive to light than cones, but cones provide much better acuity

• Cones also provide colour sense, whereas rods do not.

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Rods & Cones: Like Night and Day

• When light levels are photopic (> 1 cd/m2) rods are bleached (all photopigment is isomerized) and only cones function

• When light levels are scotopic (< .01 cd/m2) rod photopigment is regenerated and they become functional

• Cones’ photopigments are available under scotopic conditions, but cones are not sensitive enough to detect such low light levels

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• Between scotopic and photopic light levels is the mesopic level (.01 to 1 cd/m2)

• Here both sets of receptors are a work, but neither works particularly well.

• Colour vision is odd due to all 4 receptor types working at once.

• Perhaps part of the reason that twilight is considered a mystical and strange time of day.

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Dark & Light Adaptation

• Your visual system dark adapts when you (e.g.):

• Turn off the bedroom lights at night. Illumination from indoor lights is about 1000x what you get from moonlight

• Come indoors on a sunny day. Illumination from indoor lights is between 100x and 10 000 000x less than outdoors

• Not primarily due to pupil dilation, which only varies from ≈ 3 mm2 to 40 mm2 (i.e., ≈10x).

• Instead, it is mainly a retinal phenomenon

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Dark & Light Adaptation

• Your visual system light adapts when you (e.g.):

• Turn on the bedroom lights at night after being in darkness for a while. Illumination again is changing by a factor of about 1000x.

• Go outdoors on a sunny day. Illumination changes by a factor of between 100x and 10 000 000x.

• As with dark adaptation, mainly a retinal phenomenon, with limited input from pupil size

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Time-Course of Dark Adaptation

• Dark adaptation is the process of switching from cone vision to rod vision.

• Full dark adaptation takes 20 mins or more

• It is a two-phase process, starting with cone adaptation (rapid, about 5 mins.) and then rod adaptation (slow, about 20 mins.).

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• How do we measure the time-course?

• 3 separate experiments are used

• Basic method used in all three experiments:

• Observer is light adapted: Exposed to a bright light so that cones and rods are bleached

• Then lights are turned off and we measure the absolute threshold for light brightness at various time intervals thereafter.

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Experiment 1: Rods & Cones

• Stimulus is placed off fixation (Why?)

• Results show threshold decreases in two stages

• Threshold decreases rapidly for ≈5 minutes

• Then threshold levels off for a few minutes

• Threshold begins to decrease again at about minute 10, finally levelling off for good at about the 20 minute mark.

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Stimulus is to the side of fixation so that both rods and cones are stimulated (remember, fovea has only cones)

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Det

ectio

n T

hres

hold

High

Low


• Experiment II: Cones Only

• Test small light right at fixation (why?)

• Results: Threshold levels off after ≈5 mins.

• Experiment III: Rods Only

• Test a rod monochromat participant (why?)

• Results: Threshold decreases steadily for 20 minutes (with no “break” at 5-10 minutes).

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Det

ectio

n T

hres

hold

High

Low

One Photon!

• Hecht et al. determined the absolute threshold for detecting a light under full dark adaptation

• Determined how many rod cells the threshold level light would affect

• Results showed that only one photon was needed to excite a rod!

• Note that this is the smallest amount of light possible in the universe!

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Hecht et al.’s Experiment

7 photons hit a patch of ≈350 photoreceptors, so most likely each is absorbed by a different rod cell (7 balls vs. 350 cups analogy)

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Light Adaptation

• Time-course of light adaptation is much shorter, usually less than a minute

• This is because it only requires that pigments be bleached (i.e., “used up”)

• Dark adaptation takes much longer because pigments must be regenerated after being bleached, a more intensive process

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Retina after full dark adaptation

Rod pigment partlybleached after 30 s

light exposure

Rod pigment fullybleached after 60 s

light exposure

Light Bleaches Rod Pigment in Frog Retina

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20 min1 min

Photopigment levels in rods and cones at different points in dark adaptation.

5 min2 min 10 min

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PhotopicScotopic

Photopigment levels in rods and cones after complete adaptation to different light levels.

Mesopic

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Questions

• About how long is the first stage of dark adaptation? The second?

• Why does this two-stage adaptation occur?

• Define the following: Photopic, mesopic, scotopic.

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Questions

• Why do cones provide colour vision but not rods?

• What is the peak wavelength sensitivity of the three cones taken together? What about rods?

• Define threshold and sensitivity.

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Neural Circuits in Retina

• Overview of Topics

• General principles of neural circuits

• Retinal ganglion cells (RGCs) and their centre-surround receptive fields (RFs)

• How the general principles of neural circuits are embodied in the centre-surround RFs of RGCs

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Neural Circuits

• A neural circuit is a group of neurones connected by excitatory and inhibitory links

• Such circuits solve specific perceptual problems

• We have a good understanding of how simple ones called feature-detector circuits work

• More complex ones (e.g., for face recognition) must exist, but our understanding of their organization is incomplete at best

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General Principles of Neural Circuits

• Linearity: High acuity, low sensitivity

• Convergence: Low acuity, high sensitivity

• Lateral Inhibition: Sharpens tuning

• Centre-Surround circuits incorporate all of the above to detect features

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• A linear circuit is simply a chain of neurones, one stimulating the next

• Input into each receptor has no effect on the output of neighbouring cells

• Each circuit can only indicate single spot of stimulation

Linearity

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7 LinearCircuits

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0 +1 +2 +3–1–2–3

±1 ±2 ±3 0

• Input from each receptor summates into the next neurone in the circuit

• Output from convergent system varies based on input

• Output of circuit can indicate single input & increases output as stimulus increases in some characteristic (e.g., length, bandwidth)

Convergent Circuit

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• Figure 3.17 Circuit with convergence added. Neuron B now receives inputs form all of the receptors, so increasing the size of the stimulus increases the size of neuron B’s response.

Convergent Circuit

3 2 1 0 1 2 3

0 1 2 3

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0 +1 +2 +3–1–2–3

±1 ±2 ±3 0

• Adjacent receptors inhibit one another, resulting in:

• Strong response for stimuli having a given narrow range of a particular characteristic, such as size (or frequency, sweetness, etc.)

• Weaker response for stimuli that are close to the range

• No response for stimuli outside the range

Lateral Inhibition

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• Figure 3.18 Circuit with convergence and inhibition. Because stimulation of the receptors on the side (1, 2, 6, and 7) sends inhibition to neuron B, neuron B responds best when just the center (3 - 5) are stimulated.

Lateral InhibitionCircuit

3 2 1 0 1 2 3

0 1 2 3

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0 +1 +2 +3–1–2–3

±1 ±2 ±3 0

Characteristic Width

(A little more about...)

Convergent Circuits

• Greater convergence (e.g., more receptors feeding into one neurone) leads to:

• Higher sensitivity to faint signals (soft sounds, dim lights, gentle pressure)

• Less specificity/acuity of signal detail (size of light dot, sound pitch, precise location, etc.)

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• Higher convergence of rods than cones

• Average of 120 rods to one ganglion cell

• Average of 6 cones to one ganglion cell

• Cones in the fovea have 1 to 1 relation to ganglion cells (no convergence; linearity)

Convergence in the Retina

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• Rods collectively are more sensitive to light than cones. Two reasons:

• Rods individually take less light to respond

• Rods have greater convergence, resulting in summation of inputs of many rods into individual ganglion cells

• Trade-off is that rod system cannot distinguish detail (Note: nor can peripheral cones)

Convergence Leads to Sensitivity

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• Figure 3.2 The wiring of the rods (left) and the cones (right). The spot and arrow above each receptor represents light that stimulates the receptor. The numbers represent the number of response units generated by the rods and the cones in response to a spot of intensity of 2.0.

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• Linear foveal cone circuits results in high visual acuity

• Trade-off is that foveal cones need much more light to respond than rods

• Convergent rod (and peripheral cone) circuits lead to greater sensitivity at the expense of acuity.

Convergence Reduces Detail

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Questions

• What are the advantages/disadvantages of convergent vs. linear circuits?

• Which kinds of photoreceptors exhibit a great deal of convergence?

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Retinal Ganglion Cells

• Retinal Ganglion Cells (RGCs) are a type of neurone, found throughout the retina

• Photoreceptors (rods & 3 cones) ultimately send signals to RGCs via neural circuits

• There are ≈ 1.25 million RGCs and ≈ 125 million photoreceptors, so average convergence is 100:1

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Receptive Fields of RGCs

• Almost all RGCs have centre-surround receptive fields

• The RFs vary in size, small near the fovea, large in the peripheral retina

• This is a primary reason why visual acuity is so much greater near the centre of one’s visual field

• About half are ON/OFF and half are OFF/ON

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Projecting Receptive Fields

A visual receptive field canbe projected out into spaceor onto a surface.

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Centre-Surround RFs

• RGCs fire maximally to a stimulus of just the right size

• Fire less if stimulus is smaller or bigger

• Thus we already are processing information about spatial layout of light

• Note that for OFF/ON RGCs we would need a dark spot on a light background

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RGCs• Two major types of RGCs:

• Midget: Small cells with small RFs (70%)

• Parasol: Big cells with big RFs (10%)

• We will see that these ultimately feed into two distinct visual pathways

• But how is it that RGCs have these centre-surround RFs? What is the neural circuitry linking photoreceptors to RGCs?

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Inner Nuclear Layer Cells

• Between the photoreceptor layer and the RGC layer is the inner nuclear layer

• It contains bipolar, horizontal, and amacrine cells

• All of these cells are set up in such a way as to give the RGCs a centre-surround receptive field

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Bipolar Cells• Carry information “vertically” from receptors

to RGCs

• Come in two varieties:

• ON bipolars respond to light (they, weirdly, fire more when they get less glutamate)

• OFF bipolars respond to dark (they fire more when they get more glutamate)

• Mediate the centre response of RGCs’ RFs

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Horizontal Cells

• Integrate information horizontally across many receptors over a large area of retina

• Some are inhibitory, others excitatory

• Help to create the surround response of RGCs

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Amacrine Cells

• Carry information horizontally across ganglion cells

• May play a role in surround response, but this is poorly understood

• May also play a role in higher-order lateral inhibition between ganglion cells (speculative)

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+

-

– + + + –

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Receptors

Inner Nuclear Cells

RGCs

– + + + –

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Receptors

Inner Nuclear Cells

RGCs

– + + + –

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Receptors

Inner Nuclear Cells

RGCs

– + + + –

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Receptors

Inner Nuclear Cells

RGCs

– + + + –

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Receptors

Inner Nuclear Cells

RGCs

+ – – – +

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Receptors

Inner Nuclear Cells

RGCs

Foveal (note: not center/surround)

Parafoveal

Macula

Peripheral Retina (may involve hundreds of receptors)

Retinal Receptive Fields Come In Many Sizes

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Questions

• What is a receptive field? In which sensory modalities do they exist?

• What is a centre-surround receptive field?

• If light is placed just on the centre of an on-centre/off-surround RF, what happens? What about an off-centre/on-surround RF?

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Acuity

• Refers to the ability to detect fine detail in vision (recall tactile acuity)

• Acuity is best at the fovea because it has

• Densest possible receptor mosaic

• Smallest possible RFs

• Best optical quality image

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Acuity

• As in touch, we can measure acuity in terms of two-point threshold or with gratings

• But the most common clinical test is a letter chart such as the old Snellen chart or the more modern ETDRS chart

• Here the unit of measurement is the Minimum Angle of Resolution (MAR)

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ETDRS* vs. Snellen*Early Treatment of Diabetic Retinopathy Study

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ETDRS vs. Snellen

ETDRS SnellenEqual steps in letter size Apparently arbitrary steps in

letter size

Same number of letters per line

Number of letters per line varies non-systematically

10 Specific letters chosen for roughly equal discriminability

Letters apparently chosen arbitrarily

Each line equated for difficulty

Lines not equated for difficulty

Letters equally spaced relative to letter width

Letters not equally spaced

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An Aside: Visual Angle

5’ 5

”

5’ 5

”

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The “rule of thumb” method of determining the visual angle of an object. When the thumb is at arm’s length, whatever it covers has a visual angle of about 2 degrees. At 57.3 cm, 1 cm = 1°.

Visual Angle Estimation

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The Sun & Moon are " The Size of Your Thumb

The moon’s disk almost exactly covers the sun during an eclipse because the sun and the moon have (almost) the same visual angles.

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Visual Angle Calculation

• A = 2 x arctan((S / 2) / D)

• A is Angle; S is size; D is distance

• S and D must be in the same units.

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A = 2 x arctan((S / 2) / D)0°

30°

60° 90°

-30°

-60°

-90°

A°

D

S

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Visual Angle Calculation: Step by Step

1. Find half the size of the object. (S/2)

2. Divide the result of step 1 by the distance. (S/2) / D

3. Take the arctan (using a calculator) of step 2. arctan((S/2)/D)

4. Multiply step three by 2. 2 x arctan((S/2)/D)

5. Pat yourself on the back.

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Example: My HeadSize ≈ 20 cm, Distance ≈ 400cm

1. Find half the size of the object. (S/2) = 10

2. Divide the result of step 1 by the distance. (S/2) / D = 10/400 = .025

3. Take the arctan (using a calculator) of step 2. arctan((S/2)/D) = arctan(.025) = 1.43°

4. Multiply step 3 by 2. 2 x arctan((S/2)/D) = 2.86°

5. You should be able to almost hide my head with your thumb at arm’s length.

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Questions

• What are 3 reasons why we have best acuity at the fovea?

• If a person walked away from you and their visual angle didn’t change, what (bizarrely) would you be forced to conclude?

• What is the visual angle subtended by a 10 cm wide object at a distance of 57.3 cm?

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Now, Back to Acuity...

• Acuity is measured as Minimum Angle of Resolution (MAR)

• 20/20 vision (6/6 in metric) is defined as being able to resolve 1 minute of angle (1/60th of a degree).

• 20/60 means that you can resolve only 3’

• Legal blindness is 20/200 (MAR = 10’) or worse

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Grating Acuity

• Letter charts are quick and easy, but a more precise and thorough way to measure acuity is with gratings of different spatial frequencies

• Recall that we discussed spatial frequency in terms of tactile grating acuity

• Here we use the same concept, but in term of # of cycles/degree of visual angle

• We measure the minimum contrast needed to detect a grating across the spatial frequency spectrum

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Contrast Sensitivity

• Measuring the contrast detection threshold across the spatial frequency spectrum gives us the Contrast Sensitivity Function

• CSF is a psychophysical function linking spatial frequency and contrast

• Tells us about the sensitivity of vision across a range of sizes of stimuli, instead of just the minimum size detectable (acuity)

107Spatial Frequency

Con

tras

tH

iLo

Lo Hi108

CSF, SF, & RGC RFs

• Our ability to detect gratings at different spatial frequencies (the CSF) arises because there are no RGCs with RFs...

• ...large enough to optimally detect frequencies below 8-10 cycles/degree

• ...small enough to detect (at all) frequencies above 60 cycles/degree

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The Nyquist Limit

• The best acuity one can theoretically have is related to the spacing of receptors

• In our fovea, there are about 60 cones per degree

• Our upper SF detection limit is ≈60 cycles/degree

• So we have the best possible acuity we can have! ≈ 0.5°

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Factors Affecting CSF

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Questions

• What is the MAR for a person with 20/40 vision?

• What is the CSF?

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Centre-Surround Effects

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Lateral Inhibition

• Adjacent sensory receptors send signals that inhibit one-another in a competitive matrix

• Occurs in all sensory systems, resulting in, among other things, centre/surround receptive fields.

• Lateral inhibition may seem counter-productive, but in fact aids in detection of perceptual features (changes in stimulus across space or time)

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Relative vs. Absolute Magnitude

• Lateral inhibition also helps emphasize relative differences or changes more than absolute values

• For instance, we see differences in brightness more accurately than absolute brightness

• Why might this be?

• Differences are the important info, adaptively speaking

• Enables us to deal with large range of absolute values

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• Centre-surround lateral inhibition helps with:

• Emphasis on relative lightness, rather than absolute brightness. Results in lightness constancy & simultaneous contrast illusion.

• Emphasis on edges rather than fills. Results in better object detection, & the Mach Band illusion

• Other illusions may also arise from lateral inhibition, such as the Hermann Grid

Lateral Inhibition in Vision

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Lightness Constancy

Emits 100 Units

Reflects 90 units Reflects 10 units

Emits 100 000 Units



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Lightness Contrast

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• Centre-surround RFs emphasize edges, enabling better figure-ground segregation and object recognition

• Mach Bands Illusion:

• People see enhanced lightness and darkness at borders of light and dark areas

• Actual physical intensities indicate that this is not in the stimulus itself

Edge Enhancement

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Mach Bands

Reality (cd/m2)

Perception

• People see an illusion of gray images in intersections of white areas

• Effect can be (partly) explained by centre-surround antagonism.

Hermann Grid

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• Note that the preceding explanation is at best partial. It is controversial and cannot be the whole story.

• See http://tinyurl.com/5v5oq for a detailed critique of the lateral inhibition explanation of the Hermann Grid illusion

Hermann Grid

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• Benary Cross & White’s Illusion: May be due to a combination of lateral inhibition and Gestalt “belongingness” principle.

• Shadow Effects: Contrast changes interpreted as being due to illumination differences instead of reflectance differences.

• Exact physiological mechanisms unknown

“Higher Order” Lightness Illusions

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The Benary Cross

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White’s Illusion

Shadow Effects: Are Squares A and B Different Shades of Grey?

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Questions

• What is lateral inhibition?

• What is the advantage of a system with lateral inhibition?

• Name three illusions that might be explained by lateral inhibition.

• Why does the Hermann grid illusion occur?

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Vision II Overview of Topics - University of...

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