Download The retina contains two types of photoreceptors, rods (about 120

What happens at the retina?*
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 light‐
sensing cells that are collectively called the retina. What happens at the retina?*
The density curves for the rods and cones on the retina show an enormous density of cones in the fovea. On the other hand, the rods are absent from the fovea.
At a few degrees away from it their density rises to a high value and spreads over a large area of 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
.
What happens at the retina?*
In general terms the cones function in both color vision and the highest visual acuity. Visual examination of 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..
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 . 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.
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,
• and the opponent‐process theory.
• These two theories are complementary and explain processes that operate at different levels of the visual system. 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.
•
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 color‐
related properties in the same way, and color would cease to be an efficient biological signaling mechanism.
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 What happens at the retina?‐
back 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. 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 The Ventral Stream*
The Dorsal 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.
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.
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
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 spot‐
but we don’t notice a missing part of the stimulus. Why?
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.
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
So, is seeing believing??