Introduction to Sensing & Perceiving (OLI)

The senses provide our brains with information about the outside world and about our own internal world.
Even single-celled organisms have ways to detect facts about their environment and they typically have the ability to use this information either to find nutrients or to avoid danger. For more complex organisms, certainly for humans, many sources of information about the external and internal world are necessary to allow us to survive and thrive. The systems we have throughout our bodies that allow us to detect information and transform energy into neural impulses are called the senses or sensory systems.

Detection of food or danger is generally not enough to permit an organism to respond effectively for survival. The world is full of complex stimuli that must be responded to in different ways. Organisms generally use both genetically transmitted knowledge and knowledge derived from experience to organize and interpret incoming sensory information. This process of organization and interpretation is what we refer to as perception.


In this unit we discuss the strengths and limitations of these capacities, focusing on both sensation—awareness resulting from the stimulation of a sense organ, and perception—the organization and interpretation of sensations. Sensation and perception work seamlessly together to allow us to experience the world through our eyes, ears, nose, tongue, and skin, but also to combine what we are currently learning from the environment with what we already know about it to make judgments and to choose appropriate behaviors.

The study of sensation and perception is exceedingly important for our everyday lives because the knowledge generated by psychologists is used in so many ways to help so many people. Psychologists work closely with mechanical and electrical engineers, with experts in defense and military contractors, and with clinical, health, and sports psychologists to help them apply this knowledge to their everyday practices. The research is used to help us understand and better prepare people to cope with such diverse events as driving cars, flying planes, creating robots, and managing pain.

We begin the unit with a focus on the six senses of seeing, hearing, smelling, touching, tasting, and monitoring the body’s positions, also called proprioception. We will see that sensation is sometimes relatively direct, in the sense that the wide variety of stimuli around us inform and guide our behaviors quickly and accurately, but nevertheless is always the result of at least some interpretation. We do not directly experience stimuli, but rather we experience those stimuli as they are created by our senses. Each sense accomplishes the basic process of transductionthe conversion of stimuli detected by receptor cells to electrical impulses that are then transported to the brain—in different but related ways.

Each of your sense organs is a specialized system for detecting energy in the external environment and initiating neural messages—action potentials—to send information to the brain about the strength and other characteristics of the detected stimulus. For example, the eyes detect photons (individual units of light) and photosensitive (light sensitive) cells in the back of the eye react to the photons by sending an action potential down a series of neurons all the way to the occipital cortex in the back of the brain. Each of the senses has a specific place in the brain where information from that particular sense is processed. Very often, the information from these sense-specific brain areas is then sent to other parts of the brain for further analysis and integration with information with other senses. The result is your experience of a rich and constantly changing multisensory world, full of sights, sounds, smells, tastes, and texture.

After we have reviewed the basic processes of sensation, we will turn to the topic of perception, focusing on how the brain’s processing of sensory experience allows us to process and organize our experience of the outside world. However, the perceptual system does more than pass on information to the brain. Perception involves interpretation and even distortion of the sensory input. Perceptual illusions, discussed at the end of the unit, allow scientists to explore the various ways that the brain goes beyond the information that it receives.

Did I get this


Stimulation of the cells in the back of the eye by light energy (photons) is an example of
sensation OR perception
Visual sensation is the process of detecting light energy from the outside world and initiating neural impulses in response to that energy.


You are alone in your house at night and hear a loud noise. You check to see if someone is trying to break into the house. Your response of checking for an intruder is based directly on your ___________.
sensation OR perception
Not only did you detect sound--sensation--but you interpret that sound as possibly coming from an intruder. That interpretation is a key to perception.


True or False: All of the senses are specialized to pick up information coming from the outside world.
true OR false
Most of the senses--seeing, hearing, tasting, smelling, and touch--involve specialized detectors for energy coming from the world around us. But proprioception uses internal information about the position of our own body, so it makes the sentence false.


Which of the two processes--sensation or perception--is more likely to be influenced by our beliefs and expectations?
sensation OR perception
Most of our sense organs are specialized to detect energy coming from the outside world, and they generally produce neural responses regardless of our thoughts. Perception uses sensory information along with knowledge and beliefs to interpret the world.

What Senses Are There?

Odd as it may seem, there is disagreement about the exact number of senses that we have. No one questions the fact that seeing uses the visual sensory system and hearing uses the auditory sensory system. There is some disagreement, however, about how to categorize the skin senses, which detect pressure and heat and pain, and the body senses, which tell our brains about body position. For our purposes, we discuss these senses:
  1. Vision
  2. Audition
  3. Smell (olfaction)
  4. Taste (gustation)
  5. Touch
  6. Balance

What Is Transduction?

Transduction is the process of turning energy detected around us into nerve impulses. Remember from the brain unit that a nerve impulse is called an action potential, so the result of transduction is always an action potential along a nerve going to the brain. Even though some action potentials start in the retina of the eye and other action potentials start in the cochlea in the inner ear, all action potentials are the same. At the neural level, there is no difference between an action potential coming from the eye or the ear or any other sensory system. What makes sensory experiences different from one another is not the sense organ or the action potential coming from a sense organ. Sensory experiences differ based on which brain area interprets the incoming message.

Transduction: Energy Initiates Action Potential

Each of our senses is specialized to detect a certain kind of energy and then to send a message to the brain in the form of action potentials in nerves that run from the sense receptor to specific parts of the brain. Let’s consider what kind of energy or information each sense receptor picks up:
  1. Vision detects photons, which are units of light.
  2. Hearing (audition) detects pressure waves in the air that we experience as sound.
  3. Smell detects molecules floating in the air.
  4. Taste detects molecules in the substances touching our tongues.
  5. There are four different kinds of detectors in our skin, but taken together they detect light pressure (touch), heat (or cold), heavy pressure, and possible damage (pain).
  6. We also have several kinds of detectors for how our body is positioned and how it is moving. We will concentrate here on our sense of balance, which is detected by fluids in our ears that move in response to gravity.

How Does Transduction Occur in Each of the Senses?

There are three major types of transduction:
     1.   Photoreaction: Photo means “light,” so one type of transduction is the reaction of a chemical in a nerve cell. This is the basis of vision.
     2.   Mechanical Reactions: Physical movement of attachments to some nerve cells or physical pressure against others starts transduction in three systems:
  • Our sense of hearing works by fluids in the inner ear brushing against hair cells attached to neurons. The movement of the fluid is produced by sound waves.
  • Our sense of balance also works in a very similar way to our sense of hearing: a gelatin-like mass in the semicircular canals of your inner ear brush against hair cells attached to neurons.
  • Our skin senses have four different kinds of detectors, and three of them respond to physical pressure. We have a soft touch detectors and strong pressure detectors. Pain detectors respond to strong and potentially damaging physical contact. Finally, we have hot/cold detectors that respond to the temperature.
      3.   Molecular Reactions: Two of our senses are sensitive to molecules in the world around us.
  • Our sense of taste is based on taste buds on our tongue that respond to molecules in the substances that touch the tongue. The taste buds also sort the molecules into different types, giving us the variety—like sweetness and bitterness—that we are able to taste.
  • Our sense of smell is based on specialized detectors in our nasal cavity that detect different kinds of molecules floating in the air. Like taste, smell categorizes the molecules into different types to give of the wonderful range of odors we can detect.

Where Do the Nerves from the Senses Go?

There is no difference between an action potential coming from one sense (e.g., the eye) and an action potential coming from a different sense (e.g., the ear). The way your brain knows if it is processing visual information or sound is by the location that receives the signal. If the action potential ends in the occipital lobe, your brain experiences it as visual information. If the action potential ends in the temporal lobe, then the brain interprets it as sound information.

You learned that different parts of the brain serve different functions. Let’s see where in the brain each of the senses sends its messages. First, all of the senses—except the sense of smell—send action potentials to the THALAMUS, in the middle of the brain, deep under the cortex. Then the different senses go to different parts of the brain.
  1. Visual signals go down the OPTIC NERVE to the THALAMUS and then to the OCCIPITAL LOBES.
  2. Sound signals go down the COCHLEAR NERVE to the THALAMUS and then to the TEMPORAL LOBES.
  3. Smell signals, the only ones that do not first pass through the thalamus, go down the OLFACTORY NERVE directly to the LIMBIC SYSTEM under the cortex.
  4. Taste signals go down the GUSTATORY NERVE to the THALAMUS and then to the FRONTAL LOBES.
  5. Skin sensations go down the SENSORY NERVES to the THALAMUS and then to the PARIETAL LOBES.
  6. Balance sensations go down the VESTIBULAR NERVE to the THALAMUS and then to the CEREBELLUM at the rear of the brain.











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