SOR theory proposes that behavior unfolds in three stages: a stimulus reaches the organism, the organism processes it through internal biological and psychological states, and a response emerges. Developed in the 1970s to explain how store environments influence shoppers, the Stimulus-Organism-Response framework has since migrated into neuroscience, stress physiology, and cognitive research, where it provides a surprisingly clean skeleton for understanding how brains and bodies convert sensory input into action. The “organism” layer turns out to be far richer than the original model imagined, stretching from neurotransmitter dynamics and gut microbiota to evolutionary capacities for learning itself.
From Retail Floors to Neurobiology
The SOR model was formalized by Albert Mehrabian and Philip Russell, who argued that sensory variables in an environment and the overall level of uncertainty in that setting influence a person’s emotional responses, which then drive them to either approach or avoid the space.1Journal of Business Research. Stimuli–organism–response framework: A meta-analytic review in the store environment Lighting, music, layout, crowd density: all of these act as stimuli, the shopper’s emotional state is the organism, and buying or leaving is the response.
That three-part logic, though, is not confined to shopping malls. Any system in which an external event must be filtered through internal states before it produces a behavioral outcome follows the same basic pattern. Researchers studying digital products, for example, have used SOR to show that design aesthetics and media richness of an app positively influence users’ perceived cultural value, satisfaction, and cultural identity, which in turn predict whether they continue using it.2PLOS ONE. Applying the Stimulus – Organism – Response framework for exploring use intention – a case study for a digital cultural and creative product, The Forbidden City 365 app And during the COVID-19 pandemic, the model was applied to understand how crisis-related stimuli triggered emotional states in university students, including academic anxiety, fear, and mysophobia, which then drove behavioral changes such as panic buying and a shift to e-learning.3PubMed Central. Psychological impact of covid-19 crises on students through the lens of Stimulus-Organism-Response (SOR) model But the most productive expansion of SOR has been downward into biology, where the “organism” layer can be traced to identifiable circuits, chemicals, and physiological systems.
Sensory Gating and What Counts as a Stimulus
Before a stimulus can trigger any internal processing, it has to survive the brain’s filtering mechanisms. Sensory gating is the process by which the nervous system suppresses its response to repetitive or irrelevant input, and it operates by presenting repetitious stimuli and measuring the degree of neural inhibition that occurs.4PubMed Central. Sensory gating: a translational effort from basic to clinical science You experience this every time a ticking clock fades from awareness after a few minutes: the auditory system decides the sound carries no new information and stops forwarding it for conscious processing.
The auditory thalamus, specifically the medial geniculate body, serves as an obligatory relay station for hearing. When adaptive filtering breaks down at this level, it can produce a range of auditory dysfunctions.5PubMed. Sensory gating functions of the auditory thalamus: Adaptation and modulations through noise-exposure and high-frequency stimulation in rats The SOR implication is important: the “S” in the framework is not raw reality. It is a curated version of reality that the nervous system has already decided is worth attending to. Two people in the same room may effectively receive different stimuli because their sensory gating profiles differ.
The Neural Architecture Behind the Organism
The “O” is where the framework gets biologically dense. Cortico-limbic circuits, particularly pathways connecting the prefrontal cortex to deeper emotion-processing regions, provide the neural substrate for adaptive behavioral and emotional responses. These prefrontal-limbic pathways are especially interesting because they facilitate interactions among emotion, cognition, and decision-making, all of which are disrupted in psychiatric disorders.6PubMed Central. Cortico-Limbic Interactions Mediate Adaptive and Maladaptive Responses Relevant to Psychopathology
Within this circuitry, the amygdala and orbital prefrontal cortex play complementary roles. The amygdala helps acquire predictive information about the probable value of future events. The orbital prefrontal cortex updates that information and shapes behavior accordingly. Context and the organism’s current physiological state act as important moderators on these learning-based predictions.7PubMed Central. The human amygdala and orbital prefrontal cortex in behavioural regulation In other words, whether you feel hungry or full, rested or exhausted, safe or threatened shifts how your prefrontal cortex weighs the same piece of incoming information.
These physiological and motivational states are sometimes called internal states, and they are a prominent focus of neuroscience research across animal species. Fear, arousal, hunger, motivation, and aggression are all partially hidden variables that profoundly shape perception, cognition, and action.8PubMed Central. The emergence and influence of internal states The SOR framework captures this neatly: the organism is never a blank slate passively registering stimuli. It arrives at each moment already colored by internal conditions that precede the current stimulus.
Neurochemistry in the Moment of Decision
Neurotransmitters provide a chemical readout of what the organism is doing in real time. Recordings taken directly from the human striatum during perceptual decision-making have shown that serotonin signaling tracks sensory uncertainty: shortly after a visual stimulus appeared, serotonin transiently increased when the stimulus was ambiguous and decreased when it was clear. Dopamine did not show this same relationship with uncertainty.9PubMed Central. Sub-second Dopamine and Serotonin Signaling in Human Striatum during Perceptual Decision-Making This was the first evidence in humans that serotonin plays this role during brief sensory judgments.
For SOR, this finding deepens the organism layer considerably. The internal state isn’t just an emotional temperature; it includes chemical signals that encode how confident the brain is about the stimulus it just received. When you glance at a dark shape on a hiking trail and are unsure whether it is a stick or a snake, serotonin circuits are part of the machinery registering that uncertainty before your response (freeze, relax, or step back) takes shape.
Cognitive Appraisal and Stimulus Complexity
Not all organism-level processing is subcortical or chemical. Cognitive appraisal, the conscious or semi-conscious evaluation of an event, adds a deliberative layer. According to Richard Lazarus’s framework, people first evaluate whether an event is benign, threatening, or irrelevant to their well-being, and then assess whether they have the resources to cope. The first evaluation drives the intensity and emotional tone of the response, while the second determines which specific emotion appears.10PubMed Central. Appraisals, emotions and emotion regulation: An integrative approach This is why two people hearing the same layoff announcement can have different emotional reactions: one who has savings and marketable skills appraises the situation differently from one who does not.
The stimulus itself also shapes how effectively the brain can process it. Neural recordings during working-memory tasks have revealed that complex stimuli trigger a multiplexed neural code, where neurons represent both visual and value-related features before switching to encode the upcoming behavioral choice. Simpler stimuli, by contrast, produce a single-dimension code that emerges quickly but decays over time, ultimately leading to worse performance.11Nature / Communications Biology. Working memory performance is tied to stimulus complexity Rich, complex stimuli give the brain more to grip, creating a sturdier memory trace. The “S” and the “O” are not independent: stimulus complexity directly affects the organism’s capacity to retain and use information.
Stress as the Full SOR Chain in Action
Stress research provides one of the clearest views of the SOR chain unfolding across multiple body systems at once. Laboratory studies have shown that psychological stressors heighten cardiac sympathetic activation, elevate plasma catecholamine concentrations, and affect the cellular immune response. Chronic stress layers additional effects: elevated blood pressure, raised ACTH levels, and diminished production of the inflammatory mediator interleukin-1 beta.12PubMed. Autonomic, neuroendocrine, and immune responses to psychological stress: the reactivity hypothesis
Over weeks and months, repeated activation of these compensatory mechanisms produces what researchers call allostatic load, the accumulated wear and tear on the body from chronically mobilizing stress responses. Allostatic load can accelerate aging, impair health, and reduce longevity.13PubMed Central. Chronic stress, allostatic load, and aging in nonhuman primates In SOR terms, the response to a stimulus (elevated cortisol, increased heart rate) becomes a new internal condition of the organism that alters how it processes the next stimulus. The chain doesn’t reset to baseline between events; it accumulates.
This feedback loop explains why chronic stress feels qualitatively different from a bad day. The organism itself has been reshaped by prior responses, so the same stimulus that once felt manageable now feels overwhelming. The SOR model handles this because the “O” is not fixed. It is a moving target that carries the history of prior cycles.
Why the Same Stimulus Hits People Differently
One of the persistent puzzles in stress physiology is why the same event can produce a cardiac spike in one person and barely register in another. Personality traits and daily hassles moderate the relationship between stressors and physiological responses, with immune system parameters showing especially variable flexibility across individuals. Traits like aggression and external locus of control were identified as significant moderators.14PubMed. Moderation of physiological stress responses by personality traits and daily hassles: less flexibility of immune system responses
In applied settings, personality differences show up even in how people decide to adopt new technology. When researchers studied decision-makers’ willingness to use generative AI, conscientiousness was the only personality trait that significantly moderated the link between trust in the technology and the intention to adopt it. Agreeableness and openness to experience did not have the same effect.15Technological Forecasting and Social Change. Stimulus-organism-response framework of decision-makers intention to adopt generative AI to replace entry-level jobs: The moderating impact of personality traits Disciplined, structured individuals appeared better at converting internal trust into a concrete behavioral response, adding a layer of trait-based modulation that the original SOR model alluded to but never fully developed.
The Organism Beyond the Brain
Reducing the organism to neural circuits misses major contributors. The gut-brain axis is a bidirectional communication network linking the central nervous system to the gastrointestinal tract through neural, hormonal, immune, and metabolic pathways. Gut microorganisms influence brain function and behavior through vagus nerve signaling, neurotransmitter production, inflammation regulation, and the production of short-chain fatty acids, directly and indirectly affecting learning and memory.16PubMed Central. How the gut microbiome shapes learning and memory: A comprehensive review
This adds a remarkable wrinkle to SOR: the organisms living inside your gut are literally part of the “organism” that mediates your cognitive and emotional responses. Shifts in gut microbial composition from diet, antibiotics, or illness can alter mood, memory, and inflammatory tone, which in turn change how you respond to the next environmental stimulus. The boundary between the organism and its environment blurs considerably once you factor in the trillions of bacteria that negotiate the interface.
At an even more fundamental level, organisms adjust to environmental demands through phenotypic plasticity, where the same genome can produce different traits depending on context. Some well-documented examples have been traced to specific changes in DNA transcription rates, RNA translation rates, and protein expression patterns.17Trends in Ecology & Evolution. Phenotypic plasticity: linking molecular mechanisms with evolutionary biology In SOR terms, the organism can physically reconfigure itself in response to sustained stimuli, not merely react and return to baseline.
Predictive Processing and the Bayesian Brain
A more recent theoretical development challenges the assumption that organisms passively receive stimuli and then react. Bayesian brain theory proposes that the brain encodes a probabilistic model of its environment and continually generates predictions about incoming sensory input. When the prediction matches what arrives, processing is efficient. When it doesn’t, the mismatch generates a prediction error that demands attention and learning.18PubMed. Bayesian brain theory: Computational neuroscience of belief
This flips the SOR arrow, at least partially. The organism is not waiting for a stimulus and then constructing a response. It is constantly projecting expectations outward and updating them when reality diverges. Predictive processing has been extended to internal physiological states as well, with interoceptive inference models suggesting that the brain predicts its own bodily signals and adjusts autonomic regulation based on the error between predicted and actual states.19Open MIND. The cybernetic Bayesian brain: from interoceptive inference to sensorimotor contingencies The organism, in this view, is simultaneously the predictor and the predicted.
Evolutionary Transitions in Learning
The capacity to process stimuli and produce adaptive responses did not emerge all at once. A framework for evolutionary transitions in learning identifies five major shifts across animal phylogeny: learning in non-neural organisms, limited elemental associative learning that required neural centralization, unlimited associative learning that entailed hierarchical brain organization with dedicated memory and value networks, imaginative planning through mental simulation of events, and finally human symbol-based cognition and cultural learning.20PubMed Central. Evolutionary transitions in learning and cognition
Each transition represents a qualitative change in how neural information is integrated, stored, and used. On this account, the “organism” in SOR has grown progressively more sophisticated across evolutionary history, from basic habituation in sponge-like ancestors to the symbol-wielding, culturally transmitting minds of modern humans. The SOR chain is not uniquely human. Even simple organisms show stimulus-response loops. What makes the human version distinctive is the extraordinary depth of the organism layer, which can represent abstract symbols, simulate counterfactual futures, and communicate the resulting insights across generations.
Pharmacological Modulators of the Organism
If the organism layer is a biological system, it can be pharmacologically altered, and psychotropic drugs do exactly that. These substances interact with the entire lived body, influencing emotional processing, perception, and the sense of embodied selfhood. They can produce emotional blunting, shift temperamental dispositions, and alter motor function and sensory experience. These changes reshape the patient’s embodied engagement with their environment, which in turn feeds back into the broader system.21Karger. The Embodied Mind as Pharmacological Target: Towards a Phenomenology of Psychopharmacological Interventions
This is worth pausing on. When someone takes an antidepressant or an anxiolytic, the drug doesn’t change the external stimuli they encounter. It changes the organism that encounters them. The same morning commute, the same work email, the same social interaction now passes through a neurochemically different internal environment and may produce a fundamentally different behavioral response. In SOR terms, pharmacology operates exclusively on the “O,” and yet it can transform the entire chain. That single observation may be the most vivid demonstration of why the organism layer deserves the attention it gets: it is the fulcrum on which stimulus and response pivot, and nudging it even slightly can redirect everything that follows.