The term external stimuli (singular: external stimulus) refers to objects or events that impact an organism and evoke a sensory, psychological, or behavioral response.
External stimuli are commonly described as stimuli that impact upon the 5 sensory modalities: hearing, taste, touch, sight, and smell.
External Stimuli Definition
The role of external stimuli in affecting human behavior is a central component of behaviorism, espoused by Pavlov (1927) and Watson (1913).
Behaviorism is a theory of learning which postulates that animal and human behavior is a result of associations with environmental stimuli, called learning by association.
This concept is also known as classical conditioning and is based heavily on Pavlov’s research on stimulus associations.
Pavlov’s research identified that learning takes place when an organism associates the occurrence of two stimuli. In his studies, Pavlov demonstrated that a dog could be trained to salivate in response to a bell because it had been previously associated with food.
The food naturally triggers salivation, but because the dog had heard the bell so many times just prior to being given food, the dog began to salivate when it heard the bell only, regardless of the presence of food or not.
Two types of stimuli exist in a classical conditioning scenario: the unconditioned stimulus (UCS) and the conditioned stimulus (CS).
- The unconditioned stimulus is a stimulus that naturally triggers a response in the organism. For example, the presence of food naturally makes a dog salivate.
- The conditioned stimulus is a stimulus which has no inherent meaning to the organism. But, because it has been repeatedly associated with the UCS, it begins to also evoke the organism’s response.
The (External) Stimulus and Response Mechanism
Classical conditioning places emphasis on external stimuli and behavior. This can be represented by a model depicted as S-R (stimulus-response).
Early behaviorists rejected references to mental processes within the organism because they could not be directly observed or measured. If something could not be measured, then it could not be studied scientifically.
As Watson stated,
“Psychology, as the behaviorist views it, is a purely objective, experimental branch of natural science which needs introspection as little as do the sciences of chemistry and physics” (p. 176).
This S-R model was later expanded to include consequences (S-R-C) based on Thorndike’s (1898; 1905) Law of Effect and Skinner’s work on operant conditioning (1965).
The Law of Effect is defined as:
“Responses that produce a satisfying effect in a particular situation become more likely to occur again in that situation, and responses that produce a discomforting effect become less likely to occur again in that situation” (Gray, 2007, p. 106).
Skinner (1965) researched how different schedules of reinforcement affected behavior, which eventually led to the principles of operant conditioning.
Some schedules of reinforcement produce frequent and lasting behavior, while other schedules produced low rates of behavior.
However, as the field of psychology advanced, acceptance of internal stimuli and cognitive processes became more prevalent.
Bandura’s (1977) social learning theory and his studies on observational learning helped usher in the inclusion of mental processes as a legitimate area of scientific study. The addition of mental processes expanded the model further to
S-O-R-C (as presented below):
- Stimulus: Visual, auditory, olfactory, tactile, taste, classical conditioning
- Organism: Cognition, emotions, personality factors, homeostatic processes
- Response: Salivation, startle reflect, approach, avoidance, fight-or-flight
- Consequence: Rewards, punishments, reinforcement schedules, operant conditioning
Today, studying the mental processes that occur within the organism is referred to as cognitive psychology, or cognitive science.
Cognitive psychology has been valuable in the understanding of various psychological disorders such as depression, in addition to more mundane aspects of life related to the formation of attitudes and persuasion in consumer behavior.
Other organismic stimuli include homeostatic processes such as those involved in regulating body temperature and blood glucose levels. These can also function as internal stimuli and inform the organism to seek shelter from heat or ingest nutrients.
Internal vs External Stimuli
The fundamental difference between internal and external stimuli is the point of origin. Internal stimuli emanate from within the organism, even though they are sometimes the result of processing external stimuli.
External stimuli include temperature, odors, visual images, objects that can be ingested such as food and liquid, and tactile stimuli. Bacteria and viruses are also external stimuli.
Each of these stimuli originate externally; they are encoded through sensory modalities which then create biologically-based perceptions, cognitions, and emotions.
Internal stimuli on the other hand, primarily originate from within. This includes cognitive processes such as those involved in problem-solving, planning, or executive functions related to attentional control and self-regulation.
Emotions are also internal stimuli, as well as homeostatic processes mentioned above that indicate when the body needs nourishment or temperature modulation.
External Stimulus Examples
- Activation of the Startle Reflex: A sudden loud noise or bright flash of light will evoke the startle reflex in most mammals. This is a hard-wired, immediate response to external stimuli.
- The Power of Suggestion: Walking past a pizzeria can include both visual and olfactory stimuli that may make a person crave pizza, even after eating a meal when hunger should be satiated.
- Facial Expressions: Specific facial expressions such as those associated with happiness or anger can create similar, albeit subtle emotional reactions in the viewer. This occurs via mirror neurons that are reactive to facial stimuli.
- Activation of the Fight-or-Flight Response: The fight-or-flight response occurs when a person encounters a life-threatening stimulus such as a dangerous wild animal or fast-approaching object that could cause significant bodily harm.
- Soft, Silky Fabric: Gently touching certain fabrics such as silk creates a pleasant sensation.
- Sugar: Foods that contain sugar naturally taste good. The sugar molecules impact the taste buds located on the tongue and create a pleasurable sensation.
- Message Appeals: External stimuli can also be informational. Advertisements often contain information that is designed to be persuasive and influence the purchase behavior of consumers.
- Activation of the Immune System: When bacteria or virus penetrates the organism it can cause an immune system response that attempts to destroy the invading external stimuli.
- Activation of Nausea: The ingestion of certain types of external stimuli in the form of spoiled food or poisonous compounds can cause a violent expulsion of the substance, commonly referred to as vomiting.
- Money: Monetary reward represents an external stimulus that is highly desirable and motivating for most human beings.
Applications of External Stimulus Research
1. In Meditation
Meditation is a great way to relax and become grounded, particularly after a long and stressful day. It has also been used in therapy to help patients suffering from anxiety disorders.
In trying to create an environment that is conducive to meditation, novices and practitioners incorporate a variety of external stimuli.
This can include playing soft music, presenting photos and images of nature, and even filling the air with aromas from essential oils.
Each of these external stimuli will help a person focus their attention on their breathing and internal states.
2. In Treatment of Substance Abuse
In treating a patient with substance abuse issues, there are many factors that can be addressed. The specific form of treatment depends on the theoretical orientation of the therapist.
One therapist might focus on addressing underlying inner conflicts. Another therapist might spend more time looking at triggers in the environment that are associated with the destructive behavior.
For example, certain places the person used to drink have become so associated with drinking, that whenever they find themselves in that environment, they have a strong desire to drink.
The situation serves as an external stimulus that triggers the craving to drink. Therefore, the removal of that stimulus will help decrease the likelihood of this destructive behavior.
3. In Treatment of Phobias
For someone with a phobia, an external stimulus that has been associated with a traumatic event can generate intense feelings of fear and anxiety.
For example, if a patient has a fear of spiders, the mere black and white photo of a spider can create fear and anxiety. In treatment, the therapist will teach the patient how to develop a relaxed bodily response to that stimulus.
This is accomplished by training the patient to control their heart and respiration rate by practicing progressive relaxation.
Once that stimulus is no longer fear-inducing, the therapist will present a slightly more intense version of the stimulus, such as a larger, colorful photo or a very small toy spider placed far away on the floor.
This process will continue until the patient no longer feels anxious in response to each stimulus. Eventually, over a period of weeks or months, they might even be able to hold a real spider in the palm of their hand.
4. In Treatment of Bed-Wetting (Nocturnal Enuresis, NE)
Formally defined, bed-wetting is “involuntary urination while asleep after the age at which staying dry at night can be reasonably expected” (Mayo Clinic). It affects 5-7 million children in the U. S. (Sleep Foundation, 2022).
The most common treatment for NE utilizes an automated alarm system that is activated whenever liquid is detected in the bed or on the child’s clothing.
As Adler (2011) reports, although the underlying mechanism behind the effectiveness of the alarm system is still unknown,
“There is a high level of evidence for using an alarm to treat NE and this has well-supported efficacy…which increases with the duration of therapy” (p. 17).
5. To Enhance Physical Exercise
External stimuli in the form of music can enhance physical exercise by giving a person an added boost of energy. This can be seen at any local gym. Many people wear earphones during their workout that plays music they find personally motivating.
Most likely, the music selected is fast-paced with an engaging rhythm or lyrics that the listener finds inspiring in some way.
The music serves as an external stimulus that increases motivation and energy.
External stimuli are events or objects that impact the organism and evoke a response. There are five main types of external stimuli, each corresponding to a sensory modality.
Some external stimuli, such as a sudden loud noise or the encounter of a deadly predator, activate a reflexive response in the organism.
Understanding how external stimuli can affect an organism can has led to treatments for various phobias, anxiety disorders, and substance abuse.
Some types of external stimuli, such as music, can generate a relaxed response useful in meditation or a boost of energy for strenuous physical exertion.
The relation between external stimuli and responses represents two components of a larger model of human behavior that also takes into account the role of the organism and the effect of positive and negative consequences.
Alder, R. (2011). Problems with toilet training and bed-wetting (Nocturnal Enuresis). Journal of the New Zealand College of Clinical Psychologists, 21(2), 13-20.
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Ferster, C. B., & Skinner, B. F. (1957). Schedules of reinforcement. New York: Appleton-Century-Crofts.
Gray, P. (2007). Psychology (6th ed.). Worth Publishers, NY.
Pavlov, I.P. (1927). Conditioned reflexes. London: Oxford University Press.
Skinner, B. F. (1965). Science and human behavior. New York: Free Press.
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