Vision Eyes Optic Nerves Ganglion Cells Occipital Lobe

Vision Eyes, Optic Nerves, Ganglion Cells, Occipital Lobe… Its got it all!

What We See n Hue n n Brightness n n Visual experience specified by colour names and related to the wavelength of light. Lightness and luminance; the visual experience related to the amount of light emitted from or reflected by an object. Saturation n Vividness or purity of colour; the visual experience related to the complexity of light waves.

What We See n n n Hue Brightness Saturation

Physical Characteristics of Light and Sound Waves n Wavelength refers to the distance in one cycle of a wave, from one crest to the next n n With respect to vision, human can see wavelengths of about 400 to 700 nanometers Amplitude is the amount of energy in a wave, its intensity, which is the height of the wave at its crest n For light waves, amplitude determines its brightness

Brightness Explained n How much light reaches the retina? n Controlled by the PUPIL n. Dilated Pupil=DARK n. Contracted Pupil=LIGHT n Pupil Dilates in the Dark to expose more light waves to the photoreceptors n Many drugs can interfere with this by acting as agonists (replicating the neurotransmitters) involved in determining Pupil dilation and Contraction! (Primarily Dopamine and Endorphin)

A Typical Waveform and its Characteristics

How the Eye Works n n n The cornea covers the eye and is the clear covering through which light rays pass The light rays are further filtered by the pupil through the lens before being passed to the retina at the back of the eye The lens accommodates the light waves from objects of different distances directly on the retina n For nearsighted people, light rays from distant objects are focused in front of the retina, whereas for farsighted people, light rays from close objects are focused behind the retina

How the Eye Works n The retina is the light-sensitive layer of the eye and has three layers of cells: n n n The ganglion cells are the first layer through which light rays pass After which light rays pass through the bipolar cells And are finally processed in the receptor cells, which contain the visual receptor cells rods and cones The approximately 120 million rods are responsible for seeing in dim light and for peripheral vision n The approximately 5 million cones, located in the center of the retina, called the fovea, are responsible for seeing in bright light and in color n

How the Eye Works n After being processed in the retina, patterns of neural impulses describing the visual image are carried through the bipolar cells to the ganglion cells, which bundle together to form the optic nerve n n Where the optic nerve leaves the eye, there are no receptor cells, and thus we have a blind spot The optic nerve runs through the thalamus, which acts as a “relay station” to transmit sensory information to the correct part of the cerebral cortex Visual information is directed to the occipital lobe, where it is processed Feature detector cells recognize basic features of the stimulus, which are then coordinated to give it meaning (i. e. , to perceive it)

Visual Transduction n n Begins when light waves enter the eye Photoreceptors (rods and cones) on the retina interpret the waves n n n Rods work in the dark and are found all over the retina (nearly 18 X more rods than cones) Cones measure color and are found only on the fovea (area of most focus on the retina) Bipolar cells take info from rods and cones to the GANGLION CELLS (sense/afferent neurons) n n GC AXONS ARE the Optic Nerve. So… TRANSDUCTION occurs in the retina. !

How the Eye Works

The Structures of the Retina

Anatomy Key Elements: Pupil, Lens, Retina, Fovea, Photoreceptors- Rods Cones, Optic Nerve, & Occipital Lobe

n Cornea n n Lens n n Focuses on objects by changing shape. Iris n n Protects eye and bends light toward lens. Controls amount of light that gets into eye. Pupil n Widens or dilates to let in more light. An Eye on the World

An Eye on the World n Retina n n Rods n n Neural tissue lining the back of the eyeball’s interior, which contains the receptors for vision. Visual receptors that respond to dim light. Cones n Visual receptors involved in colour vision. Most humans have 3 types of cones.

Why the Visual System is not a Camera n Much visual processing is done in the brain. n n n Some cortical cells respond to lines in specific orientations (e. g. horizontal). Other cells in the cortex respond to other shapes (e. g. , bulls-eyes, spirals, faces). Feature-detectors n Cells in the visual cortex that are sensitive to specific features of the environment.

Hubel & Wiesel’s Experiment

How We See Colours n n Trichromatic theory Opponent process theory

How does the Visual System Create Color? n Color does not exist in the world, only in the mind---WHOA! n color is a sensation created when light waves are transduced (I have no idea if this is the correct past tense of transduction) and then processed in our visual cortex n Only CONES (visual receptor cells) can detect color. These are only found in the FOVEA

The Nature of Light n Electromagnetic spectrum- range of electromagnetic energy n Visible spectrum- part of the electromagnetic field that our brain can interpret in color n To interpret energy outside of the visible spectrum, we employ devices- radio’s, TVs, etc n Long waves- red n Medium waves- yellows/greens n Short waves- blues

Electromagnetic Spectrum

How We See Color n n The Trichromatic theory contends that there are three types of cones, each activated by a certain wavelength, which corresponds approximately to blue, green, and red The Opponent-Process theory assumes that there are three types of cell systems that help us see color, and these systems are located at the post-receptor level of processing n n The three types of cell systems are red-green and blue-yellow, as well as black-white (to detect brightness) If one color in a pair is stimulated, the other is inhibited

How We See Color n Both theories have validity, each at different levels of visual information processing n n The Trichromatic theory is correct in its account of how color information is processed by the cones The Opponent-Process theory is correct in its account of how color information is processed after it leaves the retina (and is processed by the bipolar, ganglion, and thalamic cells)

Color Blindness n n Color weakness- shades and pale colors are difficult to distinguish Color blindness- cannot see specific colors n Most red/green n Rarely blue/yellow n Only 500 ever have reported complete color blindness

Subtractive and Additive Mixtures

Demonstration of Complementary Afterimage

Demonstration of Complementary Afterimage

Test of Colour Deficiency

Depth Perception n Involves judging the distance of objects from us Binocular depth cues require the use of both eyes Monocular depth cues require only one eye n n Linear perspective refers to the fact that as parallel lines recede away from us, they appear to converge Interposition refers to the fact that if one object blocks our view of another, we perceive the blocking object as closer

Depth and Distance Perception n Binocular Cues: n n n Visual cues to depth or distance that require the use of both eyes. Convergence: Turning inward of the eyes, which occurs when they focus on a nearby object. Retinal Disparity: The slight difference in lateral separation between two objects as seen by the left eye and the right eye; refers to the fact that as the disparity between the two retinal images decreases, the distance from us increases (and vice versa)

The Ames Room n n n A specially-built room that makes people seem to change size as they move around in it The room is not a rectangle, as viewers assume it is A single peephole prevents using binocular depth cues

Visual Constancies n The accurate perception of objects as stable or unchanged despite changes in the sensory patterns they produce. n n n Shape constancy Location constancy Size constancy Brightness constancy Colour constancy

Shape Constancy n Even though these images cast shadows of different shapes, we still see the quarter as round

Visual Illusions n Illusions are valuable in understanding perception because they are systematic errors. n n Illusions provide hints about perceptual strategies. In the Muller-Lyer illusion (above) we tend to perceive the line on the right as slightly longer than the on the left.

The Ponzo Illusion n Linear perspective provides context Side lines seem to converge Top line seems farther away n But the retinal images of the red lines are equal!

Fooling the Eye n n n The cats in (a) are the same size The diagonal lines in (b) are parallel You can create a “floating fingertip frankfurter” by holding hands as shown, 5 -10” in front of face.