Sensation and Perception Although intimately related, sensation and perception play two complimentary but different roles in how we interpret our world . Sensation refers to the process of sensing our environment through touch, taste, sight, sound, and smell. This information is sent to our brains in raw form where perception comes into play. Perception is the way we interpret these sensations and therefore make sense of everything around us.
Sensation Sensation is the process by which our senses gather information and send it to the brain . A large amount of information is being sensed at any one time such as room temperature, brightness of the lights, someone talking, a distant train, or the smell of perfume. With all this information coming into our senses, the majority of our world never gets recognized .
O n e o f t h e w a y s t o o r g a n i z e the different senses in your mind is by thinking about what they gather from the outside world . Vision, hearing and touch gather energy in the form of light, sound waves, and pressure, respectively. Think of these three senses as energy senses . Taste and smell gather chemicals and therefore are chemical senses . Two more senses, vestibular and kinesthetic , help us with body position and balance. My Seven senses
Psychophysics is the study of the interaction between the sensations we receive and our experience of them. Researchers who study psychophysics try to uncover the rules our minds use to interpret sensations .
We don't notice radio waves, x-rays, or the microscopic parasites crawling on our skin. We don't sense all the odors around us or taste every individual spice in our gourmet dinner. We only sense those things we are able too since we don't have the sense of smell like a bloodhound or the sense of sight like a hawk; our thresholds are different from these animals and often even from each other.
Absolute Threshold The absolute threshold is the point where something becomes noticeable to our senses. It is the softest sound we can hear or the slightest touch we can feel. Anything less than this goes unnoticed. The absolute threshold is therefore the point at which a stimuli goes from undetectable to detectable to our senses.
The technical definition for absolute threshold is the minimal amount of stimulus we can detect 50 percent of the time, because researchers try to take into account individual variation in sensitivity and interference from other sensory sources. Stimuli below our absolute threshold is said to be subliminal .
Some companies claim to produce subliminal message media that can change unwanted behavior. Psychological research does not support their claim. In fact, a truly subliminal message would not, by definition, affect behavior at all because if a message is truly subliminal, we do not perceive it!
Research indicates some messages called subliminal (because they are so faint that we do not report perceiving them) can sometimes affect behavior in subtle ways. QQQQQQQQQQQ QQQQQQQQPQQ QQOQQQQQQQQ QQQQQQQQQQQ
Difference Threshold Once a stimulus becomes detectable to us, how do we recognize if this stimulus changes. When we notice the sound of the radio in the other room, how do we notice when it becomes louder. It's conceivable that someone could be turning it up so slightly that the difference is undetectable. The difference threshold is the amount of change needed for us to recognize that a change has occurred . This change is referred to as the just noticeable difference.
The reason many of us would not is because the change required to detect a difference has to represent a percentage. In the first scenario, one pound would increase the weight by 20%, in the second, that same weight would add only an additional 2%. This theory, named after its original observer, is referred to as Weber's Law . Weber's Law This difference is not absolute, however. Imagine holding a five pound weight and one pound was added. Most of us would notice this difference. But what if we were holding a fifty pound weight ? Would we notice if another pound were added?
Perception Theories Psychologists use several theories to describe how we perceive the world. These perceptual theories are not competing with one another. Each theory describes different examples or parts of perception. Sometimes a single example of the interpretation of sensation needs to be explained using all of the following theories.
Situations like this can make it difficult to focus on any particular stimulus , like the conversation we are having with a friend. We are often faced with the daunting task of focusing our attention on certain things while at the same time attempting to ignore the flood of information entering our senses . When we do this, we are making a determination as to what is important to sense and what is background noise. This concept is referred to as signal detection because we attempt detect what we want to focus on and ignore or minimize everything else. Signal Detection Theory All of us have been in a crowded room with lots of people talking.
Signal detection theory takes into account how motivated we are to detect certain stimuli and what we expect to perceive. These factors together are called response criteria (also called the receiver operating characteristics) . By using factors like response criteria, signal detection theory tries to explain and predict the different perceptual mistakes we make. A false positive is when we think we perceive a stimulus that is not there. For example, you may think that you see a friend of yours in a crowded room and end up waving at a total stranger. A false negative is not perceiving a stimulus that is present. You may not notice directions at the top of an exam that instruct you not to write on the test. Do Not Write On This Exam
When we use top-down processing , we perceive by filling in gaps in what we sense. For example, try to read the following sentence: I _ope yo_ _et a 5 on t_ _ A_ e_am. I hope you get a 5 on the AP exam. Top-Down Processing Top-down processing occurs when you use your background knowledge to fill in gaps in what you perceive. Our experience creates a schemata , mental representations of how we expect the world to be. Schemata can create a perceptual set , which is a predisposition to perceiving something in a certain way. Examples include: shapes in clouds and backmasking (hidden messages in music played backwards).
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Bottom-Up Processing Bottom-up processing , also called feature analysis, is the opposite of top-down processing. Instead of using our experiences or knowledge to perceive an object as we do in top-down processing, we use only the features of the object itself to build a complete perception in bottom-down processing (e.g. from letters to words to phrases).
Sensory Adaptation The last concept refers to stimuli which has become redundant or remains unchanged for an extended period of time. Ever wonder why we notice certain smells or sounds right away and then after a while they fade into the background? Once we adapt to the perfume or the ticking of the clock, we stop recognizing it. This process of becoming less sensitive to unchanging stimulus is referred to as sensory adaptation , after all, if it doesn't change, why do we need to constantly sense it?
Perception As mentioned in the introduction, perception refers to interpretation of what we take in through our senses. The way we perceive our environment is what makes us different from other animals and different from each other. In this section, we will discuss the various theories on how our sensations are organized and interpreted, and therefore, how we make sense of what we see, hear, taste, touch, and smell .
Gestalt Principles of Grouping The German word " Gestalt " roughly translates to "whole" or "form," and the Gestalt psychologist's sincerely believed that the whole is greater than the sum of its parts. In order to interpret what we receive through our senses, they theorized that we attempt to organize this information into certain groups. This allows us to interpret the information completely without unneeded repetition. For example, when you see one dot, you perceive it as such, but when you see five dots together, you group them together by saying a " row of dots ." Without this tendency to group our perceptions, that same row would be seen as "dot, dot, dot, dot, dot," taking both longer to process and reducing our perceptive ability. The Gestalt principles of grouping include four types: similarity, proximity, continuity, and closure .
In the first figure below, we tend to see two columns of apples and two columns of pears. We tend to group the objects according to similar fruits. In the second figure, we are more likely to see four rows of fruits. Similarity Similarity refers to our tendency to group things together based upon how similar to each other they are. Describe what you see in each of the two figures below.
Proximity In the next figure, we tend to perceive three columns of two lines each rather than six different lines. The lines are grouped together because of how close they are to each other , or their proximity to one another. Proximity
Continuity Continuity refers to our tendency to see patterns and therefore perceive things as belonging together if they form some type of continuous pattern . The figure below seems like it is two lines that are crossed. You perceive line 1,2 and line 3,4 because your eyes follow the lines a straight path. However, the lines could as easily be drawn as 1,3 and 2,4.
Closure is the tendency to complete familiar objects that have gaps in them . Even at first glance, we perceive a picture of a person. Closure
Identify each of the Gestalt Principles
Our visual system simplifies the visual scene into a figure that we look at and a ground which is everything else and forms the background. Figure and Ground
What you see in the figures depends on the order in which you look at the figures:
If you scan from the left, see an old woman
If you scan from the right, see a woman’s figure
This tendency is exploited in reversible figure-ground figures like at the bottom of this page.
Perceptual Constancy Imagine if every time an object changed we had to completely reprocess it . The next time you walk toward a building, you would have to re-evaluate the size of the building with each step, because we all know as we get closer, everything gets bigger . The building which once stood only several inches is now somehow more than 50 feet tall.
Luckily, this doesn't happen. Due to our ability to maintain constancy in our perceptions, we see that building as the same height no matter what the distance. Perceptual constancy refers to our ability to see things differently without having to reinterpret the object's properties. There are typically three constancies discussed, including size, shape, and brightness .
Size constancy refers to our ability to see objects as maintaining the same size even when our distance from them makes things appear larger or smaller . This holds true for all of our senses. As we walk away from our radio, the song appears to get softer. We understand, and perceive it as being just as loud as before. The difference being our distance from what we are sensing. Size Constancy
Everybody knows that a quarter is shaped in the form of a circle. When we see that same quarter from an angle, however, it looks more like an ellipse. Shape constancy allows us to perceive that quarter as still being a circle even though the angle from which we view it appears to distort the shape . Shape Constancy
Proximal stimuli are often distorted, shifting representations of distal stimuli in the real world. If you look directly down at a small, square piece of paper on a desk (a), the distal stimulus (the paper) and the proximal stimulus (the image projected on your retina) will both be square. But as you move the paper away on the desktop, as shown in (b) and (c), the square distal stimulus projects an increasingly trapezoidal image on your retina, making the proximal stimulus more and more distorted. Nevertheless, you continue to perceive a square.
Brightness constancy refers to our ability to recognize that color remains the same regardless of how it looks under different levels of light. That deep blue shirt you wore to the beach suddenly looks black when you walk indoors. Without color constancy, we would be constantly re-interpreting color and would be amazed at the miraculous conversion our clothes undertake. Brightness Constancy The Hermann Grid is an example of the perceived brightness depending on the surrounding area. The gray spots at the intersection of the white strips do not exist, but are a perceptual effect of the surrounding black squares.
Depth Cues One of the most important and frequently investigated parts of visual perception is depth. Without depth perception , we would perceive the world as a two-dimensional flat surface, unable to differentiate between what is near and far. E.J. Gibson’s visual cliff experiment found that an infant old enough to crawl will not crawl across a visual cliff, implying that the child has depth perception.
Monocular Cues Monocular cues are clues about distance based on the image in either eye alone. Some of the most common monocular cues include: linear perspective texture gradient relative size light and shadow interposition height in plane
___________________ Parallel lines that run away from the viewer seem to get closer. ___________________ A texture is coarser for near areas and finer for more distant ones. ___________________ The shapes of near objects overlap or mask those of more distant ones. ___________________ If separate objects are expected to be of the same size, the larger ones are seen as close. ___________________ Near objects are low in the visual field; more distant objects are higher up. ___________________ Patterns of light and dark suggest shadows that can create an impression of three-dimensional forms. Identify each of the following monocular depth cues: interposition relative size height in plane linear perspective light and shadow texture gradient
Identify the cues to depth used in this painting texture gradient interposition linear perspective relative size height in plane light and shadow
Monocular cues may involve motion parallax. Motion parallax is when images of objects at different distances moving across the retina at different rates.
Binocular Cues Other cues for depth result from our anatomy . We see the world with two eyes set a certain distance apart, and this feature of our anatomy gives us the ability to perceive depth. Each of our eyes sees any object from a slightly different angle . The brain gets both mages. It knows that if the object is far away, the images will be similar, but the closer the object is, the more disparity there will be between the images coming in from each eye.
With binocular vision we experience convergence. Convergence is when when our eyes converge toward each other as they focus on a target. To demonstrate convergence do the following: Hold up your finger and focus on it. Now, change your focus and look at the wall behind your finger. Then look at your finger again. You can feel the muscles changing the shape of your lens as switch your focus.
Binocular cues include retinal disparity - objects within 25 feet project images to slightly different locations on the left and right retinas; thus each eye sees a slightly different view of the object. This perspective of the cyclist is from the eyes of one person. Can you pick out some of the differences between the two images?
A simple way to understand this principle is to hold your thumb up at arms length and close one eye. Then try closing the other eye. As you switch between open eyes you should see your thumb "jumping" back and forth against the background.
3D Viewing If you've ever used a View-Master or a stereoscopic viewer , you have seen your binocular vision system in action. In a View-Master, each eye is presented with an image. Two cameras photograph the same image from slightly different positions to create these images. Your eyes can then correlate these images automatically because each eye sees only one of the images.
How do stereograms work? In normal vision, when looking at a page of text for example, both eyes look at exactly the same place. With a flat page both eyes see practically the same thing and hence the brain concludes that the page is indeed flat. Now an stereogram picture is basically made up of a pattern which repeats across the width of the page. When you diverge your eyes to "see" it each looks at adjacent repeats of the pattern, but the brain is fooled into believing that both eyes are still looking at exactly the same thing. Since the pattern is not just copied but is subtly distorted on each repeat, (in accordance with the three dimensional image represented), the two eyes see slightly different images. At this point, human perception takes over and the brain concludes that the differing images arise from looking at a three dimensional object, whose form it decodes in an instant. Hence the 3D illusion occurs.
Effects of Culture on Perception Research indicates that some of the perceptual rules psychologists once thought were innate, are actually learned. For example, cultures that do not use monocular depth clues , such as linear perspective, in their art, do not see depth in pictures using these cues.
Also, some optical illusions are not perceived in the same way by people from different cultures. The carpentered world hypothesis states that people who live in cultures in which straight lines, right angles, and rectangles predominate should be more susceptible to illusions, such as the Müller-Lyer illusion, involving straight lines, right angles, and rectangles than are people who live in noncarpentered cultures. Which of the following straight lines, a or b, appears longer to you?
The colored surface of this box can be either the outside front surface or the inside back surface, but cannot simultaneously be both. This shows how our brain can interpret the ambiguous cues two different ways Ambiguous Figure
The same physical stimulus can be interpreted differently. We use other cues in the situation to resolve ambiguities Is this the letter B or the number 13?