The eye* The eye is a slightly asymmetrical globe, about an inch in diameter. The front part of the eye (the part you see in the mirror) includes:

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1 The eye* The eye is a slightly asymmetrical globe, about an inch in diameter. The front part of the eye (the part you see in the mirror) includes: The iris (the pigmented part) The cornea (a clear dome over the iris) The pupil (the black circular opening in the iris that lets light in) The sclera (the white part)

2 The eye* Just behind the iris and pupil lies the lens, which helps to focus light on the back of the eye. Most of the eye is filled with a clear gel called the vitreous Light projects through the pupil and the lens to the back of the eye The inside lining of the eye is covered by special lightsensing cells that are collectively called the retina.

3 The eye* The retina converts light into electrical impulses. Behind the eye, the optic nerve carries these impulses to the brain. The macula is a small extra-sensitive area within the retina that gives central vision It is located in the center of the retina and contains the fovea, a small depression or pit at the center of the macula that gives the clearest vision.

4 What happens at the retina?* The retina contains two types of photoreceptors, rods (about 120 million) and cones (6-7 million). The rods and cones have very different functions.

5 What happens at the retina?* In general terms the cones function in both color vision and the highest visual acuity. Visual examination of a small detail involves focusing light from that detail onto the fovea. The rods are responsible for night vision, our most sensitive motion detection, and our peripheral vision..

6 What happens at the retina?- cones* The experimental evidence suggests that among the cones there are three different types of color reception. Response curves for the three types of cones have been determined. They provide the eye's color sensitivity. (more about that in a moment) The cones are concentrated in the fovea.

7 Colour vision* Theories of Colour Vision There are two major theories that explain and guide research on colour vision: the trichromatic theory also known as the Young-Helmholtz theory, (wave lengths) and the opponent-process theory. (photons) These two theories are complementary and explain processes that operate at different levels of the visual system.

8 Trichromatic Theory* Individuals with normal colour vision needed three different wavelengths (i.e., primaries) to match any other wavelength in the visible spectrum. This finding led to the hypothesis that normal colour vision is based on the activity of three types of receptors, each with a different peak sensitivity. Consistent with the trichromatic theory, we now know that the overall balance of activity in S (short wavelength), M (medium wavelength), and L (long wavelength) cones determines our perception of colour.

9 Trichromatic Theory* The difference between each of these cones is down to a single protein called an opsin. So if we follow a molecule of light (photon) it passes through your cornea, lens, the vitreous humours, three layers of brain cells, nerves and blood vessels to reach the very back of the eye where the opsins sit, bound to the cones.

10 Trichromatic Theory* The opsin molecules capture the photons. This capture alters the molecular shape of the opsins which trigger a neuronal response of the cones (in the form of an electrical impulse) which travel down the other end of the cone cells and together travel down nerves that collectively form the optic nerve!!!

11 Opponent-Process Theory* the opponent-process theory states that the cone photoreceptors are linked together to form three opposing colour pairs: blue/yellow, red/green, and black/white. Activation of one member of the pair inhibits activity in the other. That is there are cells that respond in one direction (e.g. increased firing) to one colour and in the opposite direction (e.g. decreased firing) to its complementary colour.

12 Colour Theory Colour Constancy* Colour constancy refers to the fact that the perceived colour of an object remains relatively constant under varying illumination conditions. The world would be a confusing place if the color of a surface changed with every change in the wavelength composition of the light reflected from it. We would be unable to categorize colorrelated properties in the same way, and color would cease to be an efficient biological signaling mechanism.

13 What happens at the retinaback to the other type of receptor? the rods* The rods are the most numerous of the photoreceptors, some 120 million, and are the more light sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark-adapted, or scotopic, vision.

14 What happens at the retinaback to the other type of receptor? the rods* While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors Since the rods predominate in the peripheral vision,(away from the fovea) and rods are more light sensitive, you can see dimmer objects in your peripheral vision. So, in a darkened movie theatre, find your seat by using peripheral vision

15 What happens at the retina?- back to the other type of receptor the rods* Imagine a rose at twilight During a sunny day, the color-sensitive cones are predominant and we see a brilliant red rose with somewhat more subdued green leaves. But At twilight, the lesssensitive cones begin to shut down for the night, and most of the vision comes from the rods. The rods pick up the green from the leaves much more strongly than the red from the petals, so the green leaves become brighter than the red petals!

16 Visual pathway* Via the optic nerve (or tract) the visual information (can t call it an image) ends up in the right and left thalamus (lateral geniculate). The thalamus is an egg shaped structure in the middle of the brain that serves as an integrative distribution centre for all of our senses (except olfactory) From the thalamus, the information travels along increasingly divided neural streams (optic radiations) Eventually reaching the visual cortex at the back of the brain If you put your hand on the back of your head you are within a quarter of an inch of a most amazing part of your brain

17 Specialization of the visual cortex Some clusters of neurons respond only to diagonal lines and only to specific ones e.g. certain cells respond to a line tilted at 40 degrees but not tilted at 45 degrees) Other process colour information Others edges and Others only motion and Some that respond to an edge moving in a particular direction The pioneer work in this field was carried out in the 60 s by Hubel and Weisel who got the Nobel prize for this amazing research.

18 Specialization of the visual cortex* Now the next step is to reassemble this fragmented information and integrate it so that we see an image, a colour, a face, a movement etc. This occurs via two main neural streams that go to particular parts of the brain termed visual association areas where visual signals are further interpreted and given additional meaning Each stream has a particular role

19 The Ventral Stream* This stream passes through the secondary visual cortex and extends downward into the inferior temporal cortex. The processing done by neurons in this pathway allows us to recognize faces and objects based on their size, shape and colour. This is the WHAT system i.e. what the objects are.

20 The Dorsal Stream* This stream passes through the secondary visual cortex and extends upward to the parietal cortex. Neurons in this pathway provide information about the motion of objects. This is the WHERE system i.e. where the objects are and are they moving.

21 Cézanne progression from Impressionists* Impressionists, painted what the camera left out - instead of static image they painted the transience of light Cézanne wanted to reveal the moment was more than its light He felt visual forms are mental inventions that we unconsciously impose onto our sensations- the very substance of todays lecture

22 Blind spot* At the point on the retina where the axons come together and leave the eye as the optic nerve there are no rods or cones and thus it is impossible for an image to activate any nerves. This creates a blind spotbut we don t notice a missing part of the stimulus. Why?

23 Eye versus brain and perceived reality The two eyes-one image dilemma* The human brain combines an image from the left and right eye to create three-dimensional perception. The difference between the two images is known as disparity. The closer an object is to the eyes, the larger the disparity between two retinal images.

24 What happens at the retina- cones* About 3 fixations occur every second via very quick eye movements called saccades. Then, incredibly, the visual system in the brain integrates the information from the preceding fixations to produce a wide-angled, high acuity, richly coloured perception. It is because of this temporal integration that the world doesn t vanish momentarily each time we blink