1. SENSATION
Week 2

2. The Story of Terry Byland
began having difficulty seeing in dim light, making driving at night a serious problem.
When he went to his eye doctor, he was given news no one is ever prepared to
hear.
His doctor told Terry he had an eye disease that would eventually lead to blindness

3. His vision worsened gradually over time, leaving him blind just 7 years later at the young age of 45.
After living in total darkness for 11 years, Terry volunteered to have experimental surgery in which a microchip with 16 electrodes was implanted into his eye.
By wearing a tiny camera built on a pair of glasses that carries signals to the damaged area of his eye, incredibly,

4. Terry gained vision. He can now see very rough forms of motion, lights, and objects. For example, he now sees light glowing from the chandelier when he flips on the light switch and action figures move across a TV screen.

5. Physical energy, in the form of light waves or heat or sound waves, stimulates your senses. An instant later you see a snowball whiz past your nose or feel the warmth of the sun on your face or hear a catchy tune on the radio. In that instant, a remarkable series of
events will have transpired as you detect, analyze, and interpret sensory information.

6. In an approach called psychophysics,physical energy (such as
sound waves or electromagnetic radiation) is measured and related to dimensions of the resulting sensations we experience (such as loudness or brightness).

7. Our eyes, for example, provide us with stunningly wide access to the world. In one instant you can view a star light-years away, and in the next, you can peer into the microscopic universe of a
dewdrop. Yet, vision also dramatically narrows what we can observe.
Like the other senses, vision acts as a data reduction system.Your senses send only the most important data to your brain

8. Transduction
Homeostasis

9. How does data reduction take place?
Thus, the eye transduces electromagnetic radiation, the ear transduces sound waves, and so on.
Many other types of stimuli cannot be sensed directly because we have no sense receptors to transduce their energy.
For example, humans cannot sense the bioelectric fields of other living beings, but sharks have special organs that can

10. Similarly, humans can transduce only visible light, which is a tiny slice of the
electromagnetic spectrum
In contrast, the eyes of honeybees transduce parts of the electromagnetic spectrum invisible to humans.

12. Absolute Thresholds
Just when does a stimulus become strong enough to be detected by our sense organs?
The answer to this question requires an understanding of the concept of absolute threshold.
An absolute threshold is the smallest intensity of a stimulus that must be present for it to be detected

13. Despite the “absolute” in absolute threshold, things are not so cut and dried. As the strength of a stimulus increases, the likelihood that it will be detected increases gradually. Technically, then, an absolute threshold is the stimulus intensity that is detected 50% of the time.
For example, it only takes three photons of light striking visual receptors at the back of the eye to produce a sensation.
A photon is the smallest possible “package” of light.

14. Some sensory systems have upper limits as well as lower ones.
we find that humans can hear sounds down to 20 hertz (vibrations per second) and up to about 20,000 hertz.
On the lower end, the threshold is as low as practical. If your ears could sense tones below 20 hertz, you would hear the movements of your own muscles.

15. It is fascinating to realize that “seeing” and “hearing” take place in the brain, not in the eye or ear. Information arriving from the sense organs creates sensations

16. VISION
As we have noted, various wavelengths of light make up the visible spectrum
Visible light starts at “short” wavelengths of 400 nano-meters which we sense as purple or violet.
Longer light waves produce blue, green, yellow, orange, and red, which has a wavelength of 700 nanometers

18. Structure of the Eye

19. Although the visual system is much more complex than any digital camera, both cameras and eyes have a lens to focus images on a light-sensitive layer at the back of a closed space.
In a camera, this layer is the digital image sensor. In the eye, it is a layer of photoreceptors(light-sensitive cells) in the retina, an area about the size and thickness of a postage stamp

21. Most focusing is done at the front of the
eye by the cornea, a clear membrane that bends light inward
The lens makes additional, smaller adjustments
Your eye’s focal point changes when muscles attached to the lens alter its shape.
This process is called accommodation.

22. Light Control
There is one more major similarity between the eye and a camera.
In front of the lens in both is a mechanism that controls the amount of light entering.
In the eye, this mechanism is the iris; in a camera, it is the diaphragm
The iris is a colored circular muscle that gives your eyes their blue or brown or green
color.

23. By expanding and contracting, the iris changes the size of the pupil(the opening at the center of the eye)

25. Having traveled through the pupil and lens, the image of the tree finally reaches its ultimate destination in the eye—the retina .
It is within the retina that the electromagnetic energy of light is converted to electrical impulses for transmission to the brain.
Note that, because of the physical properties of light, the image has reversed itself in traveling through the lens, and it reaches the retina upside down

26. Although it might seem that this reversal would cause difficulties in understanding and moving about the world, this is not the case. The brain interprets the image in terms of its original position.
The retina consists of a thin layer of nerve cells at the back of the eyeball
The names they have been given describe their shapes: rods and cones.

27. Rods are thin, cylindrical receptor cells that are highly sensitive to light.
Cones are typically cone-shaped, light-sensitive receptor cells that are responsible for sharp focus and color perception, particularly in bright light

29. Cones are concentrated on the part of the retina called the fovea.

30. The fovea is a particularly sensitive region of the retina
The fovea is a particularly sensitive region of the retina. If you want to focus on something of particular interest, you will automatically try to center the image on the fovea to see it more sharply
The rods and cones not only are structurally dissimilar but they also play distinctly different roles in vision. Cones are primarily responsible for the sharply focused perception of color, particularly in brightly lit situations; rods are related to vision in dimly lit situations and are largely insensitive to color and to details as sharp as those the cones are capable of recognizing.

31. The rods play a key role in peripheral vision—seeing objects that are outside the main center of focus—and in night vision.
Rods and cones also are involved in dark adaptation ,the phenomenon of adjusting to dim light after being in brighter light.
The speed at which dark adaptation occurs is a result of the rate of change in the chemical composition of the rods and cones

32. Although the cones
reach their greatest level of adaptation in just a few minutes, the rods take 20 to 30 minutes to reach the maximum level.

33. SENDING THE MESSAGE FROM THE EYE TO THE BRAIN
When light energy strikes the rods and cones, it starts a chain of events that trans-forms light into neural impulses that can be communicated to the brain.
What happens when light energy strikes the retina depends in part on whether
it encounters a rod or a cone?

34. Rods contain rhodopsin ,a complex reddish-purple substance whose composition changes chemically when energized by light
The sub-stance in cone receptors is different, but the principles are similar. Stimulation of the nerve cells in the eye triggers a neural response that is transmitted to other nerve
cells in the retina called bipolar cells and ganglion cells.

35. Bipolar cells receive information directly from the rods and cones and communicate that information to the ganglion cells.
The ganglion cells collect and summarize visual information, which is then moved out the back of the eyeball and sent to the brain through a bundle of ganglion axons called the optic nerve . Because the opening for the optic nerve passes through the retina, there are no
rods or cones in the area, and that creates a blind spot.

36. As the optic nerve leaves the eyeball, its path does not take the most direct route to the part of the brain right behind the eye. Instead, the optic nerves from each eye meet at a point roughly between the two eyes—called the optic chiasmwhere each optic nerve then splits.

37. When the optic nerves split, the nerve impulses coming from the right half of
each retina are sent to the right side of the brain, and the impulses arriving from the left half of each retina are sent to the left side of the brain.

39. PROCESSING THE VISUAL MESSAGE
By the time a visual message reaches the brain, it has passed through several stages of processing.
The ultimate processing of visual images takes place in the visual cortex of the brain, and it is here that the most complex kinds of processing occur.

40. Psychologists David Hubel and Torsten Wiesel won the Nobel Prize in 1981 for their discovery that many neurons in the cortex are extraordinarily specialized, being activated only by visual stimuli of a particular shape or pattern—a process known as feature detection

41. They found that some cells are activated only by lines of a particular width, shape,
or orientation. Other cells are activated only by moving, as opposed to stationary,
stimuli
Furthermore, different parts of the brain are involved in the perception of specific kindsof stimuli, showing distinctions, for example, between the perception of human faces, animals, and inanimate stimuli

42. If separate neural systems exist for processing information about specific aspects of the visual world, how are all these data integrated by the brain?
The brain makes use of information regarding the frequency, rhythm, and timing of the firing of particular sets of neural cells.
Furthermore, the brain’s integration of
visual information does not occur in any single step or location in the brain but
rather is a process that occurs on several levels simultaneously.