Vision

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

What We See Hue

Visual experience specified by colour names and related to the wavelength of light.

Brightness Lightness and luminance; the visual experience

related to the amount of light emitted from or reflected by an object.

Saturation Vividness or purity of colour; the visual experience

related to the complexity of light waves.

What We See

Hue Brightness Saturation

Physical Characteristics of Light and Sound Waves

Wavelength refers to the distance in one cycle of a wave, from one crest to the next 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 For light waves, amplitude

determines its brightness

Brightness Explained How much light reaches the retina?

Controlled by the PUPILDilated Pupil=DARKContracted Pupil=LIGHT

Pupil Dilates in the Dark to expose more light waves to the photoreceptors

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 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 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 The retina is the light-sensitive layer of the eye and

has three layers of cells: 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 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

How the Eye Works 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 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 Begins when light waves enter the eye Photoreceptors (rods and cones) on the retina

interpret the waves Rods work in the dark and are found all over the retina

(nearly 18X 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) GC AXONS ARE the Optic Nerve. So… TRANSDUCTION occurs in the retina.!

How the Eye Works

The Structures of the Retina

AnatomyKey Elements: Pupil, Lens, Retina, Fovea,

Photoreceptors- Rods Cones, Optic Nerve, & Occipital Lobe

An Eye on the World Cornea

Protects eye and bends light toward lens.

Lens Focuses on objects by

changing shape. Iris

Controls amount of light that gets into eye.

Pupil Widens or dilates to let

in more light.

An Eye on the World Retina

Neural tissue lining the back of the eyeball’s interior, which contains the receptors for vision.

Rods Visual receptors that respond to dim light.

Cones Visual receptors involved in colour vision. Most

humans have 3 types of cones.

Why the Visual System is not a Camera

Much visual processing is done in the brain. 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

Cells in the visual cortex that are sensitive to specific features of the environment.

Hubel & Wiesel’s Experiment

How We See Colours Trichromatic theory Opponent process theory

How does the Visual System Create Color?

Color does not exist in the world, only in the mind---WHOA! 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

Only CONES (visual receptor cells) can detect color. These are only found in the FOVEA

The Nature of Light Electromagnetic spectrum- range of

electromagnetic energy Visible spectrum- part of the electromagnetic

field that our brain can interpret in colorTo interpret energy outside of the visible

spectrum, we employ devices- radio’s, TVs, etcLong waves- redMedium waves- yellows/greensShort waves- blues

Electromagnetic Spectrum

How We See Color 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

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 Both theories have validity,

each at different levels of visual information processing 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 Color weakness- shades and pale colors

are difficult to distinguish Color blindness- cannot see specific

colors Most red/green Rarely blue/yellow 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 Involves judging the distance

of objects from us Binocular depth cues

require the use of both eyes Monocular depth cues require only one eye

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 Binocular Cues:

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 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 The accurate perception of objects as stable

or unchanged despite changes in the sensory patterns they produce. Shape constancy Location constancy Size constancy Brightness constancy Colour constancy

Shape Constancy Even though these images cast shadows of

different shapes, we still see the quarter as round

Visual Illusions

Illusions are valuable in understanding perception because they are systematic errors. 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 one on the left.

The Ponzo Illusion Linear perspective

provides context Side lines seem to

converge Top line seems

farther away But the retinal

images of the red lines are equal!

Fooling the Eye

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.