It's definitely not a single parameter. I've been down this rabbit hole a bunch.
There's actually 5 main subtypes of the dopamine receptor. They express in different concentrations in different brain regions, and have different functions.
DRD2 is kind of the heavy lifter when it comes to goal seeking. If you had to boil it down to one idea, I'd call it "go/no go". It might be tempting to try to map D2 to something like a coefficient, perhaps even the "run coefficient" in run and tumble. But it's not at all like that, even on its own. It's very nonlinear. Furthermore, the action of D1 modulates D2 in a concentration-dependent way. So you have not just D2 signaling, D1 signalling, but also the differential signal as well. We don't know exactly how it maps out, but we know it plays a roll in weighing risk/reward ratios. Screwing with this pathway often results in "low risk low reward" mild manner folks switching to "high risk high reward", often going into gambling, manic spending, hypersexuality, and grandiose thinking. This can be seen occasionally with e.g. Parkinson's drugs such as L-DOPA.
Actually my headcannon is there are some parallels between Parkinson's and control theory. Net loss of dopaminergic neurons results in lower bandwidth on your control loop. In PID loops, one of the symptoms of insufficient I/D gain is ringing, where the controller constantly hunts for the setpoint and overshoots. Tremor is a common symptom of Parkinson's (though it affects much more than just tremor), but if you notice, it's usually not a continuous tremor, it's only when trying to execute motor control, especially fine motor control. The body knows kinematically where it wants to be in space, but the feedback loop rings constantly as it isn't able to get sufficient bandwidth to error correct.
Anyway, I think the point of TFA is not to suggest that this loop is a single parameter, it's just to model it as such. It's a spherical cow reduction. That kind of reduction lets you build practicable theories and test hypotheses. Such as: if Parkinson's is the result of reduced bandwith, can we treat the symptoms by increasing dopamine gain? And in fact, we can, that checks out.
There's actually 5 main subtypes of the dopamine receptor. They express in different concentrations in different brain regions, and have different functions.
DRD2 is kind of the heavy lifter when it comes to goal seeking. If you had to boil it down to one idea, I'd call it "go/no go". It might be tempting to try to map D2 to something like a coefficient, perhaps even the "run coefficient" in run and tumble. But it's not at all like that, even on its own. It's very nonlinear. Furthermore, the action of D1 modulates D2 in a concentration-dependent way. So you have not just D2 signaling, D1 signalling, but also the differential signal as well. We don't know exactly how it maps out, but we know it plays a roll in weighing risk/reward ratios. Screwing with this pathway often results in "low risk low reward" mild manner folks switching to "high risk high reward", often going into gambling, manic spending, hypersexuality, and grandiose thinking. This can be seen occasionally with e.g. Parkinson's drugs such as L-DOPA.
Actually my headcannon is there are some parallels between Parkinson's and control theory. Net loss of dopaminergic neurons results in lower bandwidth on your control loop. In PID loops, one of the symptoms of insufficient I/D gain is ringing, where the controller constantly hunts for the setpoint and overshoots. Tremor is a common symptom of Parkinson's (though it affects much more than just tremor), but if you notice, it's usually not a continuous tremor, it's only when trying to execute motor control, especially fine motor control. The body knows kinematically where it wants to be in space, but the feedback loop rings constantly as it isn't able to get sufficient bandwidth to error correct.
Anyway, I think the point of TFA is not to suggest that this loop is a single parameter, it's just to model it as such. It's a spherical cow reduction. That kind of reduction lets you build practicable theories and test hypotheses. Such as: if Parkinson's is the result of reduced bandwith, can we treat the symptoms by increasing dopamine gain? And in fact, we can, that checks out.