The human body is constantly bombarded with stimuli. Simply entering a room can bring a barrage of sights, sounds and smells, all of which the brain takes in. These stimuli fall into two basic categories: environmental or attended. When a particular stimulus out of many is focused upon, it changes from an environmental stimulus into an attended stimulus.
Environmental stimuli are all the stimuli that the brain could possibly perceive and process. At any point, environmental stimuli could trigger a reaction from one of the five senses: taste, touch, hearing, smell or sight. However, simply because a stimulus is occurring does not mean it is actively being perceived. For example, if you are sitting in a movie theater, stimuli could include the sound and sight of the film, the sound of people talking or eating, the smell of popcorn, the feel of the theater seat and the taste of candy. All of these environmental stimuli occur at once. However, until a person chooses one or more of the stimuli to focus on, the stimuli are not perceived by the brain.
When a person decides to actively focus on a particular stimuli, it becomes an attended stimuli. From the movie theater example, if you are focused on the sound of the film, it's possible that the brain is not perceiving someone talking several rows back. A moment later, your attention may shift to the sound of the words behind you, transforming the environmental stimuli into an attended stimuli. In this way, though environmental stimuli may remain constant in a particular situation, attended stimuli may change with time as a person's focus shifts from one stimuli to another.
The brain is able to perceive stimuli via receptors found on the ends of sensory neurons. There are five types of receptors: chemoreceptors, mechanoreceptors, electromagnetic receptors, thermoreceptors and pain receptors. Chemoreceptors are able to detect ions and molecules, allowing humans to smell and taste. Mechanoreceptors are able to sense changes in pressure and acceleration. These types of receptors help with stimuli such as sound, which impact the ear bone and muscles in the form of pressure changes. Electromagnetic receptors respond to changes in the electromagnetic spectrum, such as infrared and visible light. In particular, they respond to photons of light that allow us to see variations in color.
Thermoreceptors respond temperature, with nerves responding directly to sudden changes in hot and cold. Pain receptors, also called nociceptors, can also detect severe changes in temperature as well as chemicals released by damaged tissue, indicating an injury.
Sensory Processing Disorder
Sensory Processing Disorder (SPD) occurs when the brain cannot interpret stimuli in a typical way. For example, a person with SPD may not react to extreme cold or heat. SPD can also enhance problems with other disorders, such as Attention Deficit Hyperactivity Disorder. For people with both disorders, it can be a challenge to determine which environmental stimuli need to become attended stimuli. They may be distracted by excess environmental stimuli, resulting in difficultly focusing.
- Sensation and Perception
- University of California-Irvine: Chapter 1 Introduction
- State University of New York: Sensory Systems
- Sensory Processing Disorder Foundation: About SPD
- Sensory Processing Disorder Foundation: Response to "Living with ADHD and SPD" — An OT Perspective
- University of Minnesota: Temperature Perception Experiment
- Photo Credit Alexander Beck/Hemera/Getty Images
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