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Historical study of medicinal substances, their origins, preparation and effects; a predecessor and continuing descriptive component of pharmacology and pharmacognosy.

Descriptions of side effects can increase adverse experiences through a nocebo effect, but the appropriate response is accurate, supportive risk communication rather than withholding material information. Placebo mechanisms can complement effective treatment without deception or exaggerated promises. An expectation-dependent symptom is real; it need not be either fabricated or caused solely by the medicine's pharmacology.

A warning can focus attention on ordinary bodily sensations, raise anxiety and encourage attribution of those sensations to the drug. The resulting arousal may itself worsen discomfort, prompting further monitoring and avoidance. Previous adverse experiences can condition responses to the treatment setting, and observing others can transmit expectations. These routes predict effects on experience, reporting and adherence; they do not imply that every adverse drug reaction is psychological.

An illustrative study by Antonello Silvestri and colleagues gave the same beta-blocker to 96 men without pre-existing erectile dysfunction. Three groups of 32 differed in information: blinded to drug identity, informed of identity, or informed additionally of the sexual side effect. Reported dysfunction after three months was 3.1%, 15.6% and 31.2%, respectively. Because the pharmacological exposure was held constant, the contrast supports an information-dependent component. Small groups, self-report and the narrow population limit generalisation; it does not establish that the drug has no direct effect or that disclosure should be suppressed.^[ref]

Expectancy can also alter an active drug's experienced benefit. Ulrike Bingel and colleagues manipulated positive, neutral and negative expectations during a fixed-concentration remifentanil infusion and constant heat-pain stimulation. Positive expectation increased analgesia, whereas negative expectation counteracted the reported analgesic benefit; fMRI changes accompanied the subjective effects. Since drug concentration was held constant, a contextual influence cannot be explained simply by different administered amounts. Nevertheless, altered ratings and associated brain activity do not show that all pharmacological actions were reversed or that every illness responds similarly.^[ref]

Learning offers another route. In Luana Colloca's observational experiment, participants watched a demonstrator apparently benefit when a visual cue accompanied painful stimulation. Subsequent responses were compared with direct conditioning and verbal-suggestion conditions. The experiment shows that another person's apparent experience can establish an analgesic expectation without the observer first receiving effective treatment. It motivates attention to clinical demonstration and social information; it is not justification for staging deceptive testimonials in ordinary care.^[ref]

Practical communication should preserve informed consent while avoiding deterministic language. State absolute frequencies with a clear denominator and time interval, distinguish common transient symptoms from serious signs requiring action, and explain how symptoms will be assessed and managed. Giving both the event and non-event frequency can improve clarity: a hypothetical 10-in-100 risk also means 90-in-100 do not experience it. The two expressions describe the same probability, not a method for hiding risk. Ask about prior experiences, acknowledge uncertainty and agree on follow-up. Honest reassurance can reduce threat without suggesting that a patient's symptoms are imaginary.

The placebo effect must be separated from the placebo response. Improvement in a placebo group can include spontaneous change, regression to the mean, concurrent care and reporting effects. A credible estimate of the contextual intervention therefore needs a suitable comparison, often no treatment or matched usual care as well as an active-drug condition. Within-person experimental manipulations offer complementary evidence about expectancy. A symptomatic benefit does not by itself show alteration of the underlying disease process.

Open-label placebo tests whether concealment is necessary in at least some circumstances. Ted J. Kaptchuk's three-week randomized trial allocated 80 people with irritable bowel syndrome to openly described inert pills or no treatment with matched provider interaction. The open-label group reported greater improvement on the primary global scale, with endpoint means 5.0 versus 3.9. This is a proof of principle for a transparent contextual intervention, not evidence that placebo universally replaces effective medication. The trial was small, short, unblinded and reliant on subjective outcomes; reporting differences and the explanatory rationale remained possible contributors.^[ref]

Therapeutically, use a consistent supportive encounter, realistic positive expectations, a comprehensible rationale and effective care delivered with clear feedback. Learned treatment cues might eventually support carefully tested adjunctive regimens, but benefit cannot be assumed from conditioning theory alone. In routine communication, an adverse symptom should prompt assessment of drug effects, illness and contextual influences rather than automatic dismissal as nocebo. The goal is truthful disclosure with minimal avoidable expectation-induced harm, monitored benefits and harms, and evidence-based use of contextual effects as adjuncts.

Learning and memory provide powerful mechanistic accounts of some psychiatric symptoms, but they are not an adequate single-level definition of all psychiatric disorders. The most useful claim is that abnormal learning, valuation or retrieval can mediate particular symptoms and interact with biological vulnerability and environment. Calling every persistent symptom a “memory” without specifying a computation, mechanism and discriminating prediction would make the account unfalsifiable.

Several causal positions must be separated. A learning abnormality could precede illness and increase vulnerability, mediate the expression of an established biological disturbance, or follow from the illness, medication or adverse experiences. A cross-sectional task deficit does not distinguish these possibilities. Nor does a diagnosis identify one latent parameter: poor performance can result from altered motivation, perception, attention, beliefs or motor output as well as learning. Construct validity and external validity are therefore central, not afterthoughts.

For schizophrenia, an influential proposal links dopamine dysregulation to aberrant salience. Events or internal representations acquire inappropriate significance, and attempts to explain those experiences can contribute to delusions. This creates a possible bridge between neurochemistry and acquired belief. It is a theoretical framework rather than evidence that each psychotic belief is an ordinary reinforced association; psychosis is also not specific to schizophrenia. The account must explain which symptoms it predicts and which remain outside its scope.^[ref]

A pharmacological experiment provides a more direct test of one link. Healthy volunteers performed causal associative learning during randomized ketamine and placebo sessions with functional magnetic resonance imaging. The drug perturbed right-frontal responses related to violated expectations, and individual neural responses were related to subsequently assessed perceptual and belief-like drug effects. This supports an association between altered learning-related processing and psychotomimetic experience. It does not show that ketamine reproduces the developmental course of schizophrenia, or that the observed imaging signal is the unique mediator of a delusion. Pharmacology causally changes the drug condition; the relationship between an imaging response and a symptom remains a separate inferential step.^[ref]

Prospective evidence helps with temporal ordering. Elevated striatal dopamine synthesis capacity was observed before transition to psychotic disorder in a high-risk cohort. That finding is important biological evidence, but a PET synthesis-capacity estimate does not directly measure trial-wise reward prediction errors, and elevated capacity does not establish a specific learning algorithm. It also does not imply that every high-risk person or every diagnosed patient has the same abnormality. A learning account should connect this biological vulnerability to task-defined computations rather than substitute one measure for another.^[ref]

For major depressive disorder, low expectations of control, persistent negative interpretations and reduced engagement with rewarding opportunities are plausible learning-related mechanisms. Learned helplessness provides a specific model of how uncontrollability can change later behaviour; attributional style helps explain generalization and persistence in human accounts. These mechanisms are neither a moral judgment about the individual nor proof that all adverse circumstances are controllable. Sleep, bodily symptoms, neurodevelopmental factors and the diversity of depressive presentations remain part of the explanandum.

Behavioural evidence is more precise than a global claim that depressed people “cannot learn.” In a probabilistic reward task, unmedicated depressed participants showed reduced development of response bias toward a more frequently rewarded stimulus, with analyses implicating integration of reinforcement history rather than total absence of response to an individual reward. This separates one aspect of reward learning from subjective pleasure and from every component of anhedonia. The direction of causation remains open in a cross-sectional comparison.^[ref]

Treatment findings can also link biological and cognitive levels. An acute norepinephrine-reuptake manipulation changed emotional processing in depressed participants before subjective mood changed. That observation is consistent with pharmacologically altered experience processing contributing to later improvement; it does not prove that the early task change mediates every later clinical response. Learning and pharmacology can interact rather than constitute mutually exclusive treatments. A drug might alter the conditions under which corrective experiences are encoded, while behavioural intervention supplies those experiences.^[ref]

The learning perspective generates concrete treatment hypotheses. In threat disorders, extinction learning suggests improving safety learning and its retrieval across contexts, rather than merely suppressing an immediate response. A randomized pilot study combined D-cycloserine or placebo with virtual-reality exposure to heights in people with acrophobia. Greater improvement in the adjunct condition provided an early proof of concept for facilitating a specified learning experience. The small, specific trial does not establish a general cognitive enhancer for all diagnoses, and an intervention that facilitates learning need not selectively strengthen desirable information. Treatment timing and the quality of the learning episode are therefore empirical issues.^[ref]

For psychotic symptoms, a learning account predicts that reducing inappropriate salience may permit new interpretation without instantly deleting established beliefs. For depression, it motivates tests of changes in reward sensitivity, information integration and expectations, with functional recovery as an outcome. These are treatment-development hypotheses, not conclusions that a single task parameter should dictate care. Symptom improvement, correction of a proposed mechanism and prevention of relapse require separate measurements; changing a laboratory score is not sufficient evidence of benefit.

The broad claim becomes convincing only when models predict specific patterns, survive controls for performance confounds, and show temporally appropriate changes linked to symptoms. Comparisons with unaffected high-risk individuals, longitudinal designs and mechanism-sensitive randomized trials can distinguish vulnerability, mediation and consequence. Learning and memory are therefore valuable explanatory levels and possible treatment targets, especially for particular acquired responses and beliefs. They do not replace the biological, developmental and social causes of psychiatric illness, and they should not turn diagnostic heterogeneity into one assumed defect.

Catherine A. Marcinkiewcz and collaborators. Nature 537:97–101. Primary mouse circuit, pharmacology and recording experiments.

DOI: doi.org/10.1038/nature19318