1. Sensation and Perception
Chapter 3

2. Sensing the World Around Us
Learning Outcomes
Define absolute thresholds
Explain the difference threshold and Weber’s law
Discuss sensory adaptation

3. Absolute Thresholds
Absolute threshold: the smallest intensity of a stimulus that must be present for it to be detected
Our senses are extremely responsive to stimuli. For example, our sense of touch is so sensitive that we can feel a bee’s wing falling on our cheek when it’s dropped from a distance of 1 cm.
Of course, ‘absolute thresholds’ are measured under ideal conditions. Normally, our senses can’t detect stimulation quite as well because of noise. Noise is background stimulation that interferes with the perception of other stimuli. It’s not necessarily auditory stimuli, but any unwanted stimuli that interfere with other senses.

4. Difference Thresholds
Difference threshold (just noticeable difference)
Weber’s law
Psychologists have discussed this comparison problem in terms of the difference threshold, the smallest level of added (or reduced) stimulation required to sense that a change in stimulation has occurred). Thus, the difference threshold is the minimum change in stimulation required to detect the difference between two stimuli, and so it also is called a just noticeable difference (Nittrouer & Lowenstein, 2007)
The relationship between changes in the original value of a stimulus and the degree to which a change will be noticed forms one of the basic laws of psychophysics: Weber’s law. Weber’s law (with Weber pronounced “vay-ber”) states that a just noticeable difference is a constant proportion of the intensity of an initial stimulus.

5. Sensory Adaptation
Sensory adaptation: an adjustment to sensory capacity when stimuli in the environment are unchanging; “getting used to” a sensory stimulus so that you no longer have the same reaction to it as you initially did
One example of adaptation is the decrease in sensitivity that occurs after repeated exposure to a strong stimulus. If you were to hear a loud tone over and over again, eventually it would begin to sound softer. For example, check-in staff at many hotels in Las Vegas often don’t even notice the flashing lights, buzzers and sounds coming from the casinos in their lobbies. They’ve become accustomed to the stimulus and can tune it out.
Sensory nerve receptors can’t fire off messages to the brain indefinitely. These receptor cells respond to changes in stimulation; constant stimulation is not effective in producing a sustained reaction.

6. Vision: Shedding Light on Our Eye
Learning Outcomes
Explain the basic structure of the eye
Compare and contrast color vision with color blindness

7. The Structure of the Eye
Light passes through the cornea, pupil, and the lens before reaching the retina: converts the energy of the light to electrical impulses for transmission to the brain
Rods
Cones
Optic Nerve
Feature Detection
Despite the similarities between the eye and a traditional camera, vision involves processes that are far more complex and sophisticated than those of any camera. Once an image reaches the neuronal receptors of the eye, the eye/camera analogy ends. In the processing of the visual image in the brain, a comparison to a computer is more appropriate.
The eye’s lens focuses light by changing its own thickness, a process called accommodation: It becomes flatter when viewing distant objects, and rounder when looking at closer objects.

8. Figure 2
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. Normally, however, this absence of nerve cells does not interfere with vision because you automatically compensate for the missing part of your field of vision.
Most 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.
Researchers have found that some cells are activated only by lines of a particular width, shape, or orientation. Others are activated only by moving, as opposed to stationary, stimuli.

9. Color Vision and Color Blindness
Trichromatic theory of color vision: three kinds of cones exist in the retina (one most responsive to blue-violet, one to green, & one to yellow-red)

10. Figure 5
Stare at the dot in this flag for about a minute and then look at a piece of plain white paper. What do you see? Most people see an afterimage that converts the colors in the figure into the traditional red, white, and blue U.S. flag. If you have trouble seeing it the first time, blink once and try again.

11. Color Vision and Color Blindness
Opponent-process theory of color vision: receptor cells are linked in pairs (blue-yellow, red-green, & black-white), working in opposition to each other

12. Hearing and the Other Senses
Learning Outcomes
Describe how we sense sound
Discuss smell and taste
Distinguish the skin senses

13. Sensing Sound
Sound: movement of air molecules brought about by vibration (sound waves)
Semicircular canals: movement of fluid here affects our sense of balance
Although many of us think primarily of the outer ear when we speak of the ear, that structure is only one simple part of the whole. The outer ear acts as a reverse megaphone, designed to collect and bring sounds into the internal portions of the ear.
Wave patterns in the air enter each ear at a slightly different time, which enables us to have a clue as to the sound’s point of origin. In addition, the two outer ears delay or amplify sounds of particular frequencies to different degrees.

14. Smell and Taste Smell (olfaction) Taste (gustation)
Molecules enter the nasal passages and pass over olfactory cells (receptor neurons); responses sent to brain, where they are combined for recognition of particular smells
Taste (gustation)
Receptor cells (taste buds) respond to four basic stimulus qualities: sweet, sour, salty, and bitter
Instructor notes: Suggest starting a discussion with the class by asking how their lives would be different if they lost their sense of smell. We don’t often think about it, but you can point out all the ways our olfactory senses affect our lives, such as detecting danger (smelling smoke before seeing it), influencing the level of desire for certain foods, influencing the level of desire for other people, and being a memory cue/conditioned stimulus that reminds you of another person.
More than 1,000 receptor cells, known as olfactory cells, are spread across the nasal cavity. The cells are specialized to react to particular odors. The human sense of smell permits us to detect more than 10,000 separate smells.
Ultimately, every taste is simply a combination of the basic flavor qualities, in the same way that the primary colors blend into a vast variety of shades and hues. The receptor cells for taste are located in roughly 10,000 taste buds which are distributed across the tongue and other parts of the mouth and throat.

15. The Skin Senses
Skin senses: touch, pressure, temperature, and pain; receptor cells in skin distributed unevenly throughout the body
Gate-control theory of pain
Some people are more susceptible to pain than others. Women typically experience painful stimuli more intensely than men. In addition, certain genes are linked to the experience of pain, so that we may inherit our sensitivity to pain.
Gate-control theory: particular nerve receptors in the spinal cord lead to specific areas of the brain related to pain. When these receptors are activated, a “gate” to the brain is opened, allowing us to experience the sensation of pain.

16. Perceptual Organization
Learning Outcomes
Explain the gestalt laws of organization
Identify top-down and bottom-up processing
Define perceptual constancy
Explain depth perception
Relate motion perception to daily life
Determine the importance of perceptual illusions

17. Perceptual Organization
Figure-ground organization: we usually perceive objects as a figure standing out against a background
You’ll be able to see a face of a woman if you look at the drawing long enough.

18. The Gestalt Laws of Organization
Principles that describe how we organize pieces of information into meaningful wholes (gestalts = patterns)
Closure
Proximity
Similarity
Simplicity
Some of the most basic perceptual processes can be described by a series of principles that focus on the ways we organize bits and pieces of information into meaningful wholes.
We do not just passively respond to our retinas. Instead, we actively try to organize and make sense of what we see.
Although gestalt psychology no longer plays a prominent role in contemporary psychology, its legacy endures. One fundamental gestalt principle that remains is that two objects considered together form a whole that is different from the simple combination of the objects.

19. Figure 2
Organizing these various bits and pieces of information into meaningful wholes constitutes some of the most basic processes of perception, which are summed up in the gestalt laws of organization. Do you think any other species share this organizational tendency? How might we find out?

20. Top-Down and Bottom-Up
Top-down processing: perception is guided by higher-level knowledge, experience, expectations, and motivations
Bottom-up processing: processing information by progressing from the individual elements of a stimulus and moving up to the perception of the whole

21. Perceptual Constancy
Physical objects are perceived as unvarying and consistent despite changes in appearance or changes in the physical environment
Ex.: the image on your retina of a person far away from you is very small, but you understand (perceive) her to be of “normal” size

22. Depth Perception
Depth perception: the ability to view the world in three dimensions and to perceive distance
Binocular disparity
To get a sense of binocular disparity for yourself, hold a pencil at arm’s length and look at it first with one eye and then with the other. There is little difference between the two views relative to the background. Now bring the pencil just six inches away from your face, and try the same thing. This time you will perceive a greater difference between the two views.

23. Motion Perception How do we perceive motion?
Movement of an object across the retina is perceived relative to an unmoving background
If a stimulus is coming toward you, the image on the retina will expand in size, filling more of the visual field, but we assume the stimulus is approaching rather than it’s growing in size
We factor information about our head and eye movements with information about changes in the retinal image
It’s not just the movement of images across the retina that brings about the perception of motion. If it were, we’d perceive the world as moving every time we moved our heads!

24. Perceptual Illusions
Visual illusions: physical stimuli that consistently produce errors in perception
Muller-Lyer illusion
Cultural differences are reflected in depth perception.
The misinterpretations created by visual illusions are ultimately due to errors in both fundamental visual processing and the way the brain interprets the information it receives.
Visual illusions, by illustrating something fundamental about perception, become more than mere psychological curiosities. There is a basic connection between our prior knowledge, needs, motivations, and expectations about how the world is put together and the way we perceive it.