Psychology Class Notes > Sensation & Perception

Sensation & Perception - When we smell a fragrant flower, are we experiencing a sensation or a perception? In everyday language, the terms "sensation" and "perception' are often used interchangeably.

However, as you will soon see, they are very distinct, yet complementary processes. In this section, we will discuss some concepts central to the study of sensation and perception and then move on to discuss vision and the perception of pain (it is not possible in the scope of these notes to discuss all the senses).

I. Sensations and Perceptions

Sensations can be defined as the passive process of bringing information from the outside world into the body and to the brain. The process is passive in the sense that we do not have to be consciously engaging in a "sensing" process.Perception can be defined as the active process of selecting, organizing, and interpreting the information brought to the brain by the senses.


1) Sensation occurs:

a) sensory organs absorb energy from a physical stimulus in the environment.

b) sensory receptors convert this energy into neural impulses and send them to the brain.

2) Perception follows:

a) the brain organizes the information and translates it into something meaningful.

B) But what does "meaningful" mean? How do we know what information is important and should be focused on?

1) Selective Attention - process of discriminating between what is important & is irrelevant (Seems redundant: selective-attention?), and is influenced by motivation.

For example - students in class should focus on what the teachers are saying and the overheads being presented. Students walking by the classroom may focus on people in the room, who is the teacher, etc., and not the same thing the students in the class.

2) Perceptual Expectancy - how we perceive the world is a function of our past experiences, culture, and biological makeup.For example, as an American, when I look at a highway, I expect to see cars, trucks, etc, NOT airplanes. But someone from a different country with different experiences and history may not have any idea what to expect and thus be surprised when they see cars go driving by.

Another example - you may look at a painting and not really understand the message the artist is trying to convey. But, if someone tells you about it, you might begin to see things in the painting that you were unable to see before.


C) Psychophysics can be defined as, the study of how physical stimuli are translated into psychological experience.

In order to measure these events, psychologists use THRESHOLDS.

1) Threshold - a dividing line between what has detectable energy and what does not.

For example - many classrooms have automatic light sensors. When people have not been in a room for a while, the lights go out. However, once someone walks into the room, the lights go back on. For this to happen, the sensor has a threshold for motion that must be crossed before it turns the lights back on. So, dust floating in the room should not make the lights go on, but a person walking in should.

2) Difference Threshold - the minimum amount of stimulus intensity change needed to produce a noticeable change.

the greater the intensity (ex., weight) of a stimulus, the greater the change needed to produce a noticeable change.

For example, when you pick up a 5 lb weight, and then a 10 pound weight, you can feel a big difference between the two. However, when you pick up 100 lbs, and then 105 lbs, it is much more difficult to feel the difference.

3) Signal-Detection Theory - detection of a stimulus involves some decision making process as well as a sensory process. Additionally, both sensory and decision making processes are influenced by many more factors than just intensity.

a) Noise - how much outside interference exists.

b) Criterion - the level of assurance that you decide must be met before you take action. Involves higher mental processes. You set criterion based on expectations and consequences of inaccuracy.

For example - at a party, you order a need to pay attention so that you will be able to detect the appropriate signal (doorbell), especially since there is a lot of noise at the party. But when you first order the pizza, you know it won't be there in 2 minutes, so you don't really pay attention for the doorbell. As the time for the pizza to arrive approaches, however, your criterion become more focused on the doorbell and less on extraneous noise.


A) the visual system works on sensing and perceiving light waves. Light waves vary in their length and amplitude:

a) wave length (also referred to as frequency, since the longer a wave, the less often/quickly it occurs) - affects color perception (ex., red=approx 700, yellow approx 600)

b) wave amplitude (this is the size/height of the wave) - affects brightness perception.

B) Structure of The EYE:

1) Cornea - the round, transparent area that allows light to pass into the eye.

2) Lens - the transparent structure that focuses light onto the retina.

3) Retina - inner membrane of the eye that receives information about light using rods and cones. The functioning of the retina is similar to the spinal cord - both act as a highway for information to travel on.

4) Pupil - opening at the center of the iris which controls the amount of light entering the eye. Dilates and Constricts.

5) Rods & Cones - many more rods (approximately 120 million) than cones (approx 6.4 million).

a) cones - visual receptor cells that are important in daylight vision and color vision.

the cones work well in daylight, but not in dim lighting. This is why it is more difficult to see colors in low light.

most are located in the center of the retina...called the FOVEA, which is a tiny spot in the center of the retina that contains ONLY cones...visual acuity is best here.

SO...when you need to focus on something you attempt to bring the image into the fovea.

b) rods - visual receptor cells that are important for night vision and peripheral vision.

the rods are better for night vision because they are much more sensitive than cones.

in addition, the rods are better for peripheral vision because there are many more on the periphery of the retina. The cones are mostly in and around the fovea but decrease as you go out.

to see best at night, look just above or below the object...this keeps the image on the rods.

C) Seeing In Color - we can see many colors, but only have 3 types of cones that receive information about color. We have cones that pick up light waves for red, green, and blue.

Color Vision Theories:

1) Trichromatic Theory - this theory indicates that we can receive 3 types of colors (red, green, and blue) and that the cones vary the ratio of neural activity (Like a projection T.V.). The ratio of each each color to the other then determines the exact color that we see.

2) Opponent-Process Theory - color perception depends on the reception of pairs of antagonist colors. Each receptor can only work with one color at a time so the opponent color in the pair is blocked out. Pairs = red-green, blue-yellow, black- white (light-dark).

Note: Most every Introductory Psychology book has a demonstration on the Opponent-Process theory. Please look for the one in your book and give it a try.

DOES COLOR EXIST? People just assume that because we see colors, that they actually exist in the world. In other words, that when they see the color red, that red is a real, physical, tangible, "thing". But is it, or is color just a matter of our perception? If we had different types of nervous systems, we would see things differently (literally) and so wouldn't we think those other things we saw were the real "things"? Let's examine this question of perception a bit further.


Much of our understanding of how and why we perceive things comes from Gestalt Psychology

For example - one of the most well known Gestalt principles is the Phi Phenomenon, which is the illusion of movement from presenting stimuli in rapid succession. When you see a cartoon or running Christmas lights, you see movement (although none actually exists) because of this principle.

A) Gestalt Principles of Perceptual Organization

1) figure-ground - this is the fundamental way we organize visual perceptions. When we look at an object, we see that object (figure) and the background (ground) on which it sits. For example, when I see a picture of a friend, I see my friends face (figure) and the beautiful Sears brand backdrop behind my friend (ground).

2) simplicity/pragnanz (good form) - we group elements that make a good form. However, the idea of "good form" is a little vague and subjective. Most psychologists think good form is what ever is easiest or most simple. For example, what do you see here: : > )

do you see a smiling face? There are simply 3 elements from my keyboard next to each other, but it is "easy" to organize the elements into a shape that we are familiar with.

3) proximity - nearness=belongingness. Objects that are close to each other in physical space are often perceived as belonging together.

4) similarity - do I really need to explain this one? As you probably guessed, this one states that objects that are similar are perceived as going together. For example, if I ask you to group the following objects: (* * # * # # #) into groups, you would probably place the asterisks and the pound signs into distinct groups.

5) continuity - we follow whatever direction we are led. Dots in a smooth curve appear to go together more than jagged angles. This principle really gets at just how lazy humans are when it comes to perception.

6) common fate - elements that move together tend to be grouped together. For example, when you see geese flying south for the winter, they often appear to be in a "V" shape.

7) closure - we tend to complete a form when it has gaps.

B) Illusions - an incorrect perception caused by a distortion of visual sensations.

1) Muller-Luyer Illusion

2) Reversible Figures - ambiguous sensory information that creates more than 1 good form. For example, the picture of two faces looking toward each other that is also a vase. I am sure most every Introductory Psychology book has this example.

3) Impossible Figures - objects that can be represented in 2-dimensional pictures but can not exist in 3-dimensional space despite our perceptions. You know the artist, Escher who draws the pictures like...the hands drawing each other, the waterfall that goes down and stays level at the same time, etc...


Pain is an unpleasant yet important function for survival: warning system (but not all pain is needed for survival).

There are two different pathways to the brain on which pain can travel - information brought from free nerve endings in the skin to the brain via two different systems:

1) fast pathways - registers localized pain (usually sharp pain) and sends the information to the cortex in a fraction of a second. EX. - cut your finger with a knife.

2) slow pathways - sends information through the limbic system which takes about 1-2 seconds longer than directly to the cortex (longer lasting, aching/burning).

Factors in Pain Perception - not an automatic result of stimulation:

1) expectations - research shown that our expectations about how much something will hurt can effect our perception.

Melzack - indicated that believing that something will be very painful helps us prepare for it.

For example - child birth: Lamaze method falsely leads us to believe it won't be painful. Maybe if we know it will be bad we can adequately prepare to handle it.

another example - placebo effect - if we believe pain has stopped, it may.

2) personality - people with negative types of personalities often have more pain. E

For example - a very uptight person may experience muscle pains, back pains, etc.

3) mood - bad moods, angry, unhappy, etc, can lead to the experience of increased pain.

For example - study manipulated moods of subjects then asked them to complete questionnaires of pain perception. Those in negative mood group reported significantly more pain than other subjects.

So, it seems that our brains can regulate, control, determine, and even produce pain.


1) Gate Control Theory (Melzack & Walls, 1965) - incoming pain must pass through a "gate" located in the spinal cord which determines what information about pain will be sent to the brain. So, it can be opened to allow pain through or closed to prevent pain from being perceived.

The Gate - actually a neural network controlled by the brain. Located in an area of the spinal cord called the Substansia Gelatinosa. There are two types of nerve fibers in this area:

a) large - sends fast signals and can prevent pain by closing the gate.

b) small - sends slower signals which open the gate. So - when pain occurs it is because the large fibers are off and the small are on, opening the gate.

Since the gate is controlled by the brain, he factors discussed earlier (expectations, mood, personality) influence the functioning of the gate.

Contradiction to Gate Control Theory:

1) endorphins - the body's own pain killers (morphine-like). May explain acupuncture, acupressure, pain tolerance during last two weeks of pregnancy, etc.

BUT- endorphins may work with the gate control theory - maybe pain is perceived, endorphins are released, so the brain no longer needs the signals and closes the gate.


ability to feel pain, pressure, temperature, and many other types of sensations including pain in a limb that does not exist (either amputated or born without).

the feelings and the pain are sometimes so life-like that person attempts to pick things up with phantom hand, step with phantom foot or leg, etc. Often person feels phantom moving in perfect coordination with the rest of the body - some report a missing arm extending outward at a 90 degree angle so they turn sideways when going through a doorway.

may occur right after amputation or not until years later.

often felt as part of the body (belonging to the rest of the body). EX. - with a missing leg, some report having a phantom foot but not the rest of the leg. Still, the foot feels as though it is part of the body.

Go to Phantom Limb Pages - includes case studies


1) the neuroma explanation - remaining nerves in the stump grow into nodules (neuromas) at the end of the stump continue to fire signals. Signals follow the same pathways the brain as when the appendage existed.

2) the spinal cord explanation - neurons in the spinal cord that are no longer receiving information from the lost appendage continue to send information to the brain.

Problem - studies have shown that when areas in the spinal cord are severed often feelings still being perceived from areas that meet the spinal cord in lower areas (below separation in spinal cord).

3) the brain explanation - signals in the somatosensory circuits of the brain change when the limb is lost which produce the phantom...the brain compensates for the loss or altered signals. This has been expanded - brain contains a network of fibers that not only respond to stimulation but continually generates a pattern of impulses that indicate that the body is intact and functioning. Thus, the brain creates the impression that the limb exists and is al right. This system may be prewired.

4) the hardwired explanation - we may have a biological makeup to be born with all of our appendages. So, when we are born w/o one or lose one, the nerves are still there and are still going to send the information.

Please note that we can not cover ALL the senses in class so make sure you read about taste, touch, hearing, and smell in the book.


Muller-Lyer explained

Although many theories exist for this illusion, there is no certain explanation. One theory is based on eye movement. When the arrows point inwards, our gaze rests inside the angles formed by the arrows. When they point outwards, our eyes demarcate the entire perspective and our gaze rests outside the angles. The outward pointing arrows make the figure more open and so the horizontal line appears longer.

The illusion takes its name from Franz Carl Müller-Lyer (1857-1916), who studied medicine in Strasbourg and served as assistant director of the city's psychiatric clinic. Müller-Lyer's main works were in the field of sociology. He himself attempted to explain the illusion he had discovered as follows: "the judgment not only takes the lines themselves into consideration, but also, unintentionally, some part of the space on either side." He published two articles on the illusion bearing his name. ('Optical Illusions' 1889, and 'Concerning the Theory of Optical Illusions: on Contrast and Confluxion' 1896)

Favreau (1977) studied a number of textbooks in which Müller-Lyer presented and measured the figures. He noticed that in many cases, the figure had been drawn the wrong way round so that the illusion appeared more forceful!