AP Psychology Unit 3 Notes: Sensation and Perception

February 12, 2024
AP Psychology Study Notes: Unit 3

Get ready for the AP Psychology exam by reviewing what is covered in Unit 3, including an overview and key terms. These AP Psychology study notes should be used to supplement what you’re learning in your AP Psych class. More study strategies and expert tips can be found in our latest AP Psychology Test Prep Book.

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AP Psychology: Unit 3 Summary

Content in this chapter is closely related to some of the material in the Biological Bases of Behavior unit .In that unit, you learned about how neurons function and the different roles of parts of the brain. As you study sensation and perception, you’ll learn how specialized neurons in our sensory organs operate and how sensations are transformed into perceptions in the brain. 


Right now as you read this, your eyes capture the light reflected off the page or emitted by the screen in front of you. Structures in your eyes change this pattern of light into signals that are sent to your brain and interpreted as language. The sensation of the symbols on the page and the perception of these symbols as words allow you to understand what you are reading. All our senses work in a similar way. In general, our sensory organs receive stimuli. These messages go through a process called transduction, which means the signals are transformed into neural impulses. These neural impulses travel first to the thalamus, then on to different cortices of the brain (you will see later that the sense of smell is the one exception to this rule).  

Sensory Adaptation & Sensory Habituation 

What we sense and perceive is influenced by many factors, including how long we are exposed to stimuli. For example, you probably felt your socks when you put them on this morning, but you stopped feeling them after a while. You probably stopped perceiving the feeling of your socks on your feet because of a combination of sensory adaptation (decreasing responsiveness to stimuli due to constant stimulation) and sensory habituation (our perception of sensations is partially due to how focused we are on them). What we perceive is determined by what sensations activate our senses and by what we focus on perceiving. We can voluntarily attend to stimuli in order to perceive them, as you are doing right now, but paying attention can also be involuntary. If you are talking with a friend and someone across the room says your name, your attention will probably involuntarily switch across the room. (This is sometimes called the cocktail-party phenomenon.) 

These processes are our only way to get information about the outside world. The exact distinction between what is sensation and what is perception is debated by psychologists and philosophers. For our purposes, though, we can think of sensation as activation of our senses (eyes, ears, and so on) and perception as the process of understanding these sensations. 

AP Psychology: Unit 3 Key Terms & People

Below, we describe some of the Unit 3 key terms and people you should review ahead of the AP Psychology exam.

  • Accommodation: Through a process called accommodation, light that enters the pupil is focused by the lens; the lens is curved and flexible in order to focus the light.
  • Transduction: The term transduction refers to the translation of incoming stimuli into neural signals. This term applies not only to vision but also to all our senses. In vision, transduction occurs when light activates the neurons in the retina.
  • David Hubel and Torsten Wiesel: Perception researchers David Hubel (1926–2013) and Torsten Wiesel (1924–present) discovered that groups of neurons in the visual cortex respond to different types of visual images. The visual cortex has feature detectors for vertical lines, curves, motion, and many other features of images. What we perceive visually is a combination of these features.
  • Trichromatic theory: Competing theories exist about how and why we see color. The oldest and simplest theory is trichromatic theory (also called the Young-Helmholtz Trichromatic [three color] theory). This theory hypothesizes that we have three types of cones in the retina: cones that detect the different colors blue, red, and green (the primary colors of light). These cones are activated in different combinations to produce all the colors of the visible spectrum.
  • Opponent-process theory: The opponent-process theory states that the sensory receptors arranged in the retina come in pairs: red/green pairs, yellow/blue pairs, and black/white pairs. If one sensor is stimulated, its pair is inhibited from firing.
  • Place theory: Place theory holds that the hair cells in the cochlea respond to different frequencies of sound based on where they are located in the cochlea. Some bend in response to high pitches and some to low. We sense pitch because the hair cells move in different places in the cochlea.
  • Gate-control theory: Gate-control theory helps explain how we experience pain the way we do. Gate-control theory explains that some pain messages have a higher priority than others. When a higher priority message is sent, the gate swings open for it and swings shut for a low priority message, which we will not feel. Of course, this gate is not a physical gate swinging in the nerve; it is just a convenient way to understand how pain messages are sent.
  • Absolute threshold: Research shows that while our senses are very acute, they do have their limits. The absolute threshold is the smallest amount of stimulus we can detect. For example, the absolute threshold for vision is the smallest amount of light we can detect, which is estimated to be a single candle flame about 30 miles (48 km) away on a perfectly dark night.
  • Signal detection theory: Signal detection theory investigates the effects of the distractions and interference we experience while perceiving the world. This area of research tries to predict what we will perceive among competing stimuli.
  • Top-down processing: When we use top-down processing, we perceive by filling in gaps in what we sense. Top-down processing occurs when you use your background knowledge to fill in gaps in what you perceive. Our experience creates schemata, mental representations of how we expect the world to be.
  • Bottom-up processing: Bottom-up processing, also called feature analysis, is the opposite of top-down processing. Instead of using our experience to perceive an object, we use only the features of the object itself to build a complete perception. We start our perception at the bottom with the individual characteristics of the image and put all those characteristics together into our final perception.
  • Figure-ground relationship: One of the first perceptual decisions our mind must make is the figure-ground relationship. What part of a visual image is the figure and what part is the ground or background? Several optical illusions play with this rule. One example is the famous picture of the vase that if looked at one way is a vase but by switching the figure and the ground can be perceived as two faces.
  • Gestalt rules: At the beginning of the twentieth century, a group of researchers called the Gestalt psychologists described the principles that govern how we perceive groups of objects. The Gestalt psychologists pointed out that we normally perceive images as groups, not as isolated elements. They thought this process was innate and inevitable.

    Several factors influence how we will group objects:
  • Extrasensory perception: Someone claiming to have “extrasensory perception” is claiming to perceive a sensation “outside” the senses discussed in this chapter. Psychologists are skeptical of ESP claims primarily because our senses are well understood, and researchers do not find reliable evidence that we can perceive sensations other than through our sight, smell, hearing, taste, touch, and vestibular/balance systems. Researchers who test ESP claims using rigorous experiments, such as double-blind studies, find other more likely explanations for supposed extrasensory phenomena. Usually ESP claims are better explained by such things as deception, magic tricks, or coincidence.

Next, test your AP Psychology Unit 3 knowledge using our free Key Terms Worksheets!