perception 2

visual pathway

visual pathway

• The information from the left and  right visual fields go to contralateral sides of the brain.

• Each Eye sends information to both  the contralateral and ipsilateral side  of the brain.

• Information from the retina travels  along a specific pathway to arrive at the visual cortex.

contralateral vs ipsilateral

contralateral: opposite side

ipsilateral: same side

what is the visual pathway retina - V1

1.Retina

2.Optic Chasm

3.Lateral Geniculate Nucleus (LGN;  90%) and Superior Colliculus  (10%)

4.Primary Visual Cortex (V1)

The Visual Pathway: Lateral  Geniculate Nucleus

The Lateral Geniculate Nucleus serves as a

“relay station” to the visual cortex.

• It receives 90% of the signals sent from the  retina.

• It is arranged in alternating layers that receive signals from the contralateral or ipsilateral  eye.

• The dark layers (Magnocellular) receive input  from rods. The lighter layers (Parvocellular)  from mostly cones.

what are the dark layers of the LGN called and what do they get input from

Magnocellular layers get their input from rods

what are the light layers of the LGN and where do they get input from

Parvocellular layers get their input from cones

LGN: Passing an electrode straight through  the layers

will contact cells with  overlapping receptive fields on the  retina.

LGN: Passing an electrode across a layer

will pass through cells with  receptive fields next to each other on  the retina.

how much information does the LGN receive from the retina vs the cortex

• receives more information from the

retina than it sends to the cortex

• receives more information from the

cortex than from the retina.

Receptive Fields In the LGN Neurons

The Receptive Fields of neurons in the

LGN are identical to those in the retina.

• LGN neurons share the center-surround  antagonistic structures of the ganglion  retinal cells.

The Visual Pathway: The Visual Cortex

•The Primary Visual Cortex (V1) is the first area within the visual cortex that processes  information from the eye.

• Most of V1 is located on the inner surfaces of  the Occipital Lobe, of both hemispheres.

whats another name for the primary visual cortex and why

called the Striate Cortex because it has a striped appearance and is divided into layers.

visual cortices

V1: catalogs input

V2: relays signals

V3a: motion

V3: form

V4: colour and form

V5: motion

VP: relays and signals

what marks the initial stages of neural processing

when Information arrives to layer 4 of the visual  cortex, from the LGN, is still before it converges  in the other layers.

simple cortical cells

Cells within the Primary Visual Cortex  (V1), have side-by-side structures

Receptive Fields of  Simple Cortical Cells

Like the receptive fields of the Retinal Ganglion cells and the neurons within the  LGN, receptive fields of V1 neurons have inhibitory and excitatory areas.

• side by side: inhibitory - excitatory - inhibitory

• sensitive to light line orientation and position

• layers 4 & 6 of V1

what determines the specific orientation preference of a cell

position of receptive field

do all cells in the visual cortex respond to spots or stationary bars of light

no

receptive field of complex cells

respond best to light in lines moving in a specific orientation, direction and motion

• the receptive field of the complex cell is  likely the result of neural convergence of simple cells onto a complex cell

• layers 6, 2 & 3 of V1, V2 & V3

receptive fields of hyper complex or end stopped cells

most responsive to light lines of a certain length, motion, direction and orientation

• layers 2 & 3 of V1

what are simple, complex hypercomplex cells

feature detectors

Ecological valance theory

states that preference for a given colour is determined by the combined valance of all objects and events associated with the colour.

perceptual segregation

the perceptual separation of one object from another,

separating one from one another.

figure-ground segregation

When we see a separate object, it is usually seen as a figure that stands out from its background, which is called the ground.

• The figure is more “thinglike” and more memorable than

  the ground

• EX: sitting at your desk, you would probably see a book or papers on your desk as a figure and the surface of your desk as ground, or stepping back from the desk, you might see the desk as figure and the wall behind it as ground.

evolutionary origins of colour vision

Colour vision may have evolved through natural selection because it increased the odds of survival. Colour vision aids:

• Identifying threats more easily.

• Identifying ripe fruit and spoiled meat.

• Identifying objects more easily.

colour and light: Newtons famous experiment

The second prism does not change the perceived colour of light.

• Each of the “beams of light” showed a different amount of “bending” as they passed through the prism. - We now understand this to be related to wavelength.

colour and light: wavelength

The colour that you perceive depends on the specific wavelength(s) of light that reach(es) your eye.

reflection and transmission of light

chromatic colours and achromatic colours

chromatic colours

colours that occur because of selective reflectance or selective transmission

• EX: Blue, Greens, yellows, and Reds

achromatic colours

colours that occur when all parts of the spectrum are reflected equally to the eye.

• equal reflection of all wavelengths

selective reflection

aspect of chromatic colour perception occurs when only some of the wavelengths are reflected back to the eye

• only long wavelengths reflect back to the eye

selective transmission

aspect of chromatic colour perception when only some of the wavelengths pass through an object to the eye.

• only long wavelengths

reflectance curves

plot the percentage of light at each wavelength that is reflected from an object.

• similar to transmission curves

how much light do vantablack absorb

99.995%

what does colour depend on

colour depends of the combination or types of wavelengths that are reflected or transmitted that hit the retina

how does the perception of white occur

when all the wavelengths of light are reflected and hit the retina

• long, medium and short wavelengths reflected

how does the perception of clack occur

occurs when all the wavelengths are absorbed by the object

making new colours by mixing paints - subtractive colour mixtures

All paints are made from an ingredient known as a pigment

• Mixing paints is an example of subtractive colour mixture, because you are subtracting wavelengths.

• After mixing paints, only the wavelengths that the two paints reflect in common are reflected from the new mixture.

pigment in paints is defined as

a material that changes the reflected or transmitted light as a result of selectively absorbing specific wavelengths.

making new colours by mixing lights - additive colour mixture

Mixing lights on a is an example of additive colour mixture, because you are adding wavelengths.

• Mixing Blue and Yellow lights results in L, M, and S wavelengths being reflected and, therefore, the perception of white.

the perceptual dimensions of colour

spectral colours & non-spectral colours

• hue, saturation, value of colours

spectral colours

Colours that appear in the spectrum of visible light.

• red, orange, yellow, green, blue, and violet.

non-spectral colours

are mixtures of the spectral colours and do not appear in the spectrum of visible light.

• E.g., Magenta, Mauve, Purple, Brown, pink, etc.

hue of a colour

describes the dominant colour.

E.g., a “reddish hue” has a quality of redness.

saturation

The intensity/pureness of the colour.

• De-saturated colours appear washed out.

value of a colour

:The degree of lightness or darkness in the colour

theories of colour vision

Trichromatic theory

Opponent processing theories

trichromatic theory + history

In 1802, Thomas Young proposed that each colour sensitive nerve fibre must consisted of three portions - one for each primary colour

• Hermann von Helmholtz built further on this theory during the 1850-60’s.

• James Clerk Maxwell conducted some of the first research to support the idea that three mechanisms for colour vision were necessary. - Herman von Helmholtz published similar research five years later.

• The theory is still called the Young-Helmholtz theory of colour vision.

• Physiological evidence came 100 years later: Micro spectrometry

micro spectrometry as physiological evidence to trichromatic theory

Using micro-spectrometry, a narrow beam of light can be presented to the photoreceptor.

• The amount of light absorbed is detected by determining the difference between the amount of light presented and the amount that passes through the photoreceptor and hits the sensor behind.

• This is done for many different wavelengths.

• Using techniques like this, and others, three types of cones were found in the retina.

absorption rates of the 3 types of cones

• Short (S) or “Blue” Cones peak at approx. 420nm.

• Medium (M) or “Green” Cones peak at approx. 534nm.

• Long (L) or “Red” Cones peak at approx. 564nm.

are cone types evenly distributed equally in the retina

no - they are not

• there is a lack of S or Blue cones in the centre

Images of the retina: adaptive optical imaging

Allows images of photoreceptors to be taken in the retina of living people.

The technique accounts for the distortion (abberations) caused by the lens in the eye and the fact that the eye is constantly in motion (micro-saccades).

process of identifying S or “blue” cones

• Bleach retina and take image.

• Dark adapt to regenerate all visual pigment (darken retina)

• Take picture

• Expose retina to a wavelength that isn’t absorbed by the S cones (550 nm).

• S cones will not “bleach”, remaining dark.

the human retina contains what according to trichromatic theory

• contains 1 type of rod which is involved in producing the experience and 3 types of cones that are sensitive to different parts of the spectrum of visible

percentages of each type of cone in the retina

S/ “blue” cones = 2-7% of all cones

M/ “green” cones = 32% of all cones

L/ “red” cones = 64% of all cones

trichromatic theory: Metamerism

Metamers’ are stimuli that are perceived to be the same but are physically different.

example of metamerism

The perception of yellow light can be created by either :

1. a single wavelength of 580nm

2. a combination of 530nm and 620nm wavelengths

• Both situations result in the exact same response from the S, M, and L cones.

Monochromacy

true colour blindness

• result of having no functional cones

• only able to see shades of lightness (whites, greys and blacks)

monochromacy & colour matching

Only Stimulus Intensity (dark/light) can be matched by an individual with Monochromacy.

• Additional lights make no difference.

• When matching stimulus intensity, they are simply changing the amount of photons given off by the light.

how do we know there is not only a single cone type involved in colour vision

If we only had “green” cones, both a 475nm light and a 610nm light (of equal brightness) would produce the same response from the cone, despite being perceptually different.

what is a photon

the smallest unit of light energy

principle of univariance

states that after the photoreceptor absorbs the light, the wavelength is irrelevant.

• One photoreceptor cannot tell the difference between a change in wavelength and a change in stimulus intensity.

dichromacy

colour blindness/deficiency

• the result of having only two functional Cone types.

• 3 types of dichromacy: protanopia, deuteranopia, tritanopia

what cone is missing in the 3 different types of dichromacy

1. protanopia = missing L cones - red/green colour blindness

2. deuteranopia = missing M cones - red/green colour blindness

3. Tritanopia = missing S cones - blue-yellow colour blindness

what is the principle behind trichromatic theory

Color perception depends on the ratio of firing in three cone types (S,M, L).

• in this, response to a 500 nm light results in the medium cone absorbing most of the light, and firing most of the light.

• Followed by the Long wavelength cone, and then finally by the Short wavelength cone.

opponent process theory

Ewald Hering’s observations about colour perception:

1. Blue, Yellow, Green, and Red appear to be “Pure Colours”. They are not combinations of other colours.

2. Some colour combinations do not seem possible (Bluish Yellow, Greenish Red).

3. Colour afterimages are reliable.

• Staring at blue produces a yellow after image

• Staring at green produces a red after image

opponent colours

or complementary colours are those colours that are directly across from each other on Hering’s colour circle/wheel.

• opponent colours create colour mixes that don’t work - EX: Reds and green are opponent colours and red can have a yellow or bluish tinge, but not a greenish tinge.

opponent process theory: what are the 3 opponent mechanisms that colour vision is based on

• Blue / Yellow

• Red/Green

• Black/White

opponent process theory: psychophysical evidence - Hue cancellation

• Yellow cancels Blue: Can I take a wavelength that corresponds to “greenish blue” and remove all traces of blue by adding yellow light?

• Green cancels Red: Can I take away the red from reddish yellow by adding green?

hue cancellation experiments

provided evidence for the existence of Opponent processes in colour vision.

• Hurvich & Jameson (1957) measured the strength of each component to the opponent process cells by determining the amount of opponent light needed to cancel the other colour at each wavelength.

• opponent process curve

opponent process curve

plots the relative strength of each part of the opponent colour mechanism at each point along the spectrum.

• The crossover points represent unique hues of Blue (472), Green (492), Yellow (573), and Red (outside of visible light).

• At each wavelength, you can determine the amount red, blue, green, or yellow components present.

• The point in which the curves of the two mechanisms (blue yellow & Red Green) cross over indicate equal amounts of Blue and Green light = Cyan & Yellow and Red light = Orange

opponent process theory: physiological evidence: opponent neurons

neurons that show an excitatory response from light in one part of the spectrum and an inhibitory response to light from another part of the spectrum.

• Retinal Ganglion Opponent Cells (type I one and type II)

• LGN opponent Cells (type I one and type II)

• Cortical opponent Cells (Side by side structure, far right)

opponent process theory: physiological evidence: single -opponent vs double opponent cells

• single-opponent cells purpose is to respond to large areas of colour and to the interior of objects

• double-opponent cells purpose is to recognize colour boundaries, patterns and textures

receptive fields of opponent neurons

Receptive fields in the fovea are very small.

• There is little to no convergence in foveal cones.

• The center of a receptive field may only consist of a single cone.

• The circuits are similar to those seen in the earlier center-surround set-ups.

trichromacy vs opponent processing - which is right

it seems that trichromatic and oponent process theories work together

neural circuits for colour vision

Theoretical Neural Circuits for the Red-Green and Blue-Yellow mechanisms.

• +L –M , + M -L cells, +S –ML, and –S+ML circuits

how can opponent neurons can compare wavelengths; +L-M

the brain might use different information in the opponent cell’s rate of fire to differentiate between colours

colour constancy

we perceive the colour of objects as relatively constant under changing Illumination.

perceiving the colour green under different lighting

Somewhere Around 500 nm, we start to perceive greens.

light bounces off of an object, and the colour of the object is determined by the wavelengths that are reflected (and not absorbed) by the object.

The three lighting scenarios result in unequal amounts of “Green” light being reflected from the shirt.

3 lighting scenarios in colour constancy

• Sunlight has fairly equal amounts of light each wavelength (white light).

• Incandescent bulbs emit more light from the longer wavelength part of the spectrum.

• LED bulbs emit relatively more light from the shorter wavelength part of the spectrum.

colour constancy: chromatic adaptation

occurs with prolonged exposure to light of a specific wavelength. After prolonged exposure, sensitivity is reduced to that wavelength and it has less of an effect on the overall perception of colour.

lightness constancy

refers to the phenomenon of perceiving whites, greys, and blacks as roughly the same shade under different illuminations.

lightness constancy: Ratio Principle

As long as the ratio of light reflected by two surfaces remains the same, the perceived lightness will remain the same.

Lightness Constancy: Uneven illumination

The Visual system also accounts for the difference between the opposite sides of reflectance edges and illumination edges.

A bit of Top-Down Processing followed by a little inference.

reflectance edges

separate areas that are physically different and reflect different amounts of light.

illumination edges

separate areas that are the same but illuminated with different amounts of light.

In shadows, how does the visual system know the difference between a reflectance edge and an illumination edge

A bit of Top-Down Processing followed by a little inference.

The visual system may recognize shadows by the presence of the penumbra

penumbra

he partially shaded outer region of the shadow cast by an opaque object.

• outside edge of show that is lighter

lighting constancy: Simultaneous Contrast

refers to the way two colours (or shades) affect each other.

• First noted by the 11th Century physicist Hasan Ibn al-Haythan - Reported that red spots of paint appeared almost black when presented on a white surface.

Top Down Processing: Does knowledge and Experience affect Colour Perception?

language and culture has an interesting influence on colour perception

how language & culture affects colour perception - The Namibian Himbas

their culture describes colour as 5 categories:

• Serandu: reds, browns, oranges, some yellows

• Dambu: Greens, reds, beige, yellows, and the term for Caucasian people

• Zuzu: most dark colours

• Vapa: some yellows and white

• Buru: collection of greens and blues

languages affect in reaction time for colours

Language seems to create reaction time differences in the recognition of colours

how language & culture affects colour perception: Russians

Russian speakers have different categories for light blues (“goluboy”) and dark blues (“siniy”).

• Russian and English speakers were asked to pick the blue that matches the top square.

• Russian’s were better able, than English speakers, to discriminate the “blues” when they fell into different categories (i.e., “Goluboy” or “Siniy”).

• While English speakers have difficulty discriminating blues that are closer together, Russian speakers cannot help but do so because of linguistic categorization.

different languages and the colour blue

In Russian, Spanish, and other languages, there are names for types of blue (light blue, dark blue) but no overall term for blue.

• Spanish: Celeste: Light Blue - Azul: Dark Blue

• Russian: light blues goluboy and dark blues siniy

Blue vs Green in Japanese language

The Japanese call green traffic lights and green apples “Blue”.

• “Ao” describes things that are green and blue. ao-ringo(Green apple)

• ao-shingō(Green light)

• ao-zora(Blue Skies)

why is blue a new colour

• InThe Iliad, Homer refers to the ocean as “the wine dark sea.”

• Skies were described as “Bronze”.

• William Gladstone pointed out that black, red, yellow, and green were mentioned to varying degrees, but blue never appeared.

• The ancient Greeks, and many ancient civilization never had a word for Blue.

what is one of the earliest civilizations to have a word for blue

The ancient Egyptians were one of the earliest civilizations to have a word for Blue.

how did the Egyptians having a word for blue help

Having the vocabulary helps with categorization of colours.

Categorization means that we can more easily identify differences.

what is the universality of black & white

All languages share the same terms