IntroductionIntroduction

Modular 5: Sensation

Electric Billboard in the Brain

Can Katie see without her eyes?Katie lost her eyesight at the age of 22 to glaucomaAt 42 she gold wires placed in her occipital lobe in the

back of her brainWhen activated she saw colors of lightThey could adjust the brightness and size of the

flashesThe wires formed a rectangular grid and researchers

could apply current to particular wires in the grid to form patterns of flashing dots

Katie is officially blind but she can see flashing color and dots when her brain is activated

Three Characteristics of All Senses

Eyes, nose, skin, and tongue are complex, miniaturized, living sense organs that gather information about your environment

All senses follow three characteristics: Transduction– process in which a sense organ changes, or

transforms, physical energy into electrical signals that become neural impulses– maybe sent to the brain

Adaptation– the decreasing response of the sense organs, the more they are exposed to a continuous level of stimulation

Sensation versus perceptions– Gathering information involves two things sensations and perceptions Sensations are relatively meaningless bits of information that result

when the brain processes electrical signals that come from the sense organs

Perceptions are meaningful sensory experiences that result after the brain combines hundreds of sensations

Stimulus: Light WavesEach sense organ can only receive a certain kind

of stimulus

The reason you can’t see radio waves is that they are not the right length

Only small specific wave length can be seen by the eye

Stimulus: Light Waves Invisible– too short– gamma

rays, X-rays, and ultraviolet rays are too short and can be seen by some birds and insects

Visible– The visible spectrum is a segment of electromagnetic energy that we can see because these waves are the right length to stimulate receptors

You see something because light waves from the visible spectrum bounce off the object an return to your eyes

Stimulus: Light Waves Invisible—too long– long

wavelengths such as radio and television waves are invisible because they are too long to stimulate receptors in the eye

Stimulus-- for someone to see something reflected light waves must be gathered and changed into electrical signals, and for that process—transduction must happen

Structure and FunctionHow can you see a giraffe?

The eyes must gather and focus light waves into a precise area at the back of your eye and then this area absorbs and transforms light waves into impulses

Image reverse– The image is upside down in the back of the eye (the brain turns things right side up for us)

Light Waves– the problem with light waves is that after they strike an object, they are reflected back in a broad beam– the cornea and the lens brings the image into focus like a camera does

Structure and Function Cornea- the light first

comes through- this is the rounded transparent covering over the front of your eye– the light bends and focuses as it passes through this curved surface into a narrower beam

Pupil– round opening at the front of your eye that allows light to pass into the eye’s interior

Structure and Function Iris—pupil is surrounded

by the iris– circular muscle that surrounds the pupil and controls the amount of light entering the eye Dim light iris relaxes

allowing more light in Pupil dilates in bright

light- constrict allowing less light to enter

The Iris also includes the pigment that gives you eye color

Structure and Function Lens—transparent, oval structure

with curved surface bends and focuses light waves into a an even narrower beam– lens is attached to muscles that adjust the curve of the lens which adjusts focusing Distant objects light needs less

bending (focusing), so muscles automatically stretch the lens so its surface is less curved

Near objects, light needs more focusing, so muscles relax and allow the surface to be come very curved

Retina– Transduction happens in the retina– thin film that contain cells that are extremely sensitive to light (photoreceptors absorb the light waves)

Eyeball’s Shape and Laser Eye Surgery

Normal vision– If the eyeball is shaped so that objects are perfectly focused on the back of the retina then there will be no vision problems

Nearsighted– eyeball is too long so that objects are focused at a point in front of the retina (near objects are clear, distant objects appear blurry)

Farsighted– eyeball is too short so objects are focused at slightly behind the retina (distant objects are clear but near objects appear blurry)

Eye Surgery– LASIK is a procedure in which the surface of the eye is folded back and a laser reshapes the exposed cornea so that light waves correctly bend and focus on the retina)

Retina: Miniature Camera—Computer

The retina is a combination of a video camera and process of transduction

The retina has three layers of cells– it contains two kinds of photo receptors that begin the process of transduction– changing light waves into electrical signals (rods are located primarily in the periphery of the retina and the cones are located in the center of the retina in an area known as the fovea)

Retina: Miniature Camera—Computer

There are 60 million rods located in the periphery (Rods are photoreceptors that contain rhodopsin which is activated by small amounts of light)

Rods allow us to see in dimly lit areas but only in shades of black, white, and gray

Retina: Miniature Camera—Computer

There about 3 million cones (located in the fovea)

Cones contain three chemicals called osins which are activated in bright light and allow us to see color

Cones are wired individually to neighboring cells and allow us to see fine details

Retina: Miniature Camera—Computer

Transduction begins when chemicals in the rods and cones break down after absorbing light waves

The breakdown generates a tiny electrical force that triggers nerve impulses in neighboring ganglion cells

Retina: Miniature Camera—Computer

Nerve impulses generated in ganglion cells exit through the optic nerve and carry the impulse to the brain

The blind spot is the point where the optic nerve exits the eye and it is unnoticed because the eyes are continually moving

In order to see something impulses must reach the visual areas of the brain

Visual Pathways: Eye to Brain

Nerve impulses flow through the optic nerve from the back of the eye

The optic nerve goes through the hypothalamus for some initial processing

The thalamus sends the impulses to the back of the occipital lobe in the back of the right and left hemispheres

Visual Pathways: Eye to Brain

Primary visual cortex is located at the back of the occipital lobe and it transforms nerve impulse into simple visual sensations

Research has shown that 25% of the cortex is devoted to processing visual information

We know that different cells in the primary visual cortex responds to specific kinds of visual stimuli

Visual Pathways: Eye to Brain

The visions that Katie reported were not meaningful images because neurons in the primary visual cortex are simple visual sensations

For a person to see detail the sense must be sent from the primary visual cortex to the visual association areas

Visual Pathways: Eye to Brain

The primary visual cortex sends visual sensations to association areas

The association areas receive sensations of texture, line, movement, orientation, and color and assembles them into a meaningful image

If damaged a person would experience visual Agnosia– can not piece together objects into meaningful images

Visual Pathways: Eye to Brain

The visual association areas are involved in many visual activities such as reading, writing, and perceiving objects, animals, people and colors

Color: VisionDebra was born with cataracts (film over the

eye)

After seeing color for the first time she stated that you “can’t imagine what colors are until you’ve seen them”

Objects are not colors they reflect light waves whose different wavelengths are transformed by your visual system into what you see as color

Making Colors from Wavelengths

1. Sunlight is called white light because it contains all the light waves in the visible spectrum

2. Light waves can be broken into the light spectrum or in nature by a rain drop

3. Our visual system transforms light waves of various lengths into millions of different colors Humans see shorter wavelengths as shades of violet,

blue, and green and longer ones as yellow, orange, and red

There are two theories on how our visual system transforms light waves into color (the trichromatic and opponent process theories)

Trichromatic TheoryTrichromatic theory says that there are three

different kinds of cones in the retina– each containing one of three different light-sensitive chemicals

Each of the three ospins are most responsive to wavelengths that correspond to each of the three primary colors, blue, green, and red

The different wavelengths are absorbed by their specific cone

Genes affect the ability you have to see different shades of color also (red for example)

Opponent-Process TheoryAfterimage is a visual sensation that continues after

the original stimulus is removed

Edward Hering suggested that the visual system codes color by using two complementary pairs– red-green and blue-yellow

Hering’s idea became known as the opponent-process theory that ganglion cells in the retina and cells in the thalamus of the brain respond to two pairs of colors– red-green and blue-yellow– when the cells are exited they respond with one color of the pair and when inhibited, they respond to the complementary pair

Theories CombinedTo see colors, it involves both the opponent-process

and trichromatic theories

Trichromatic states there are three different kinds of cones (as many as nine—genes)

When electrical signals reach the ganglion cells in the retina and neurons in the thalamus, they use opponent-process theory which involves pairs of colors

The nerve impulses carry this color information to the visual cortex where neurons respond and give us colors

Color Blindness1 in 20 people in America have color blindness

Color Blindness is the inability to distinguish two or more shades in the color spectrum

There are several kinds of color blindnessMonochromats– total color blindness– looks like a

black and white movie (only have rods)Dichromats-- usually have trouble distinguishing

red from green because they have just two kinds of cones