The relationship between PFC dopamine and cognitive performance is not linear. It is an inverted U. Too little dopamine in the PFC produces poor working memory and inattention. Too much dopamine in the PFC produces noise, disorganization, and impaired signal-to-noise. Only the middle of the curve supports optimal cognition.
This single principle explains an extraordinary amount of clinical neuropsychiatry. Hold it carefully, because it gives you a unified way of thinking about ADHD, schizophrenia, stimulant effects, and antipsychotic effects.
On the left side of the curve (too little dopamine), you get the ADHD phenotype: distractibility, poor working memory, difficulty initiating tasks, failure to sustain attention. This is also part of the cognitive symptom profile of schizophrenia, since mesocortical dopamine is reduced in that disorder.
On the right side of the curve (too much dopamine), you get the psychotic phenotype: noisy thinking, disorganization, aberrant salience attribution. Excessive PFC dopamine produces firing patterns that lack the focused signal that supports coherent thought.
Stimulants for ADHD work by raising PFC dopamine. The patient who was on the left side of the curve is moved toward the middle, and cognition improves. The same medication, given to someone whose PFC dopamine is already optimal, will move them rightward off the peak and produce worse cognition — which is why stimulants do not help (and may hurt) people without ADHD.
Stimulants in someone vulnerable to psychosis can push them off the right side of the curve into full psychotic disorganization. This is part of why stimulant-induced psychosis is a recognized syndrome, and why stimulants are used with caution in patients with bipolar or psychotic disorders.
Antipsychotics reduce dopamine signaling. In a patient whose mesolimbic dopamine is excessive, this is therapeutic. But in the PFC, if the patient is already on the left side of the curve, antipsychotics push them further left, worsening cognition and motivation. This is the cognitive cost of antipsychotic therapy, and it is why dose selection matters — finding the level that quiets mesolimbic over-signaling without further depleting mesocortical signal.
The inverted U is also why different patients need different doses. Each patient sits at a different baseline position on the curve. The same dose of stimulant that is therapeutic for one patient may be subtherapeutic or overshoot for another. Effective prescribing is iterative dose adjustment guided by the patient's response — moving them along their personal curve toward the peak.
When you next prescribe a stimulant or an antipsychotic, picture the inverted U. Ask yourself: where on the curve does this patient sit? Where will the medication move them? The answer is not always the same direction for the same medication.