02. The Currency of Pursuit

An engine can be revved at full throttle while the car itself does not move an inch. The pistons fire, the tachometer climbs, fuel burns — but with the clutch disengaged, none of that energy reaches the wheels. Something like that split exists inside the brain's reward system, and for a long time it was missed, because the two things being split looked, from the outside, like a single feeling: wanting something, and enjoying it once you have it.

The chemical at the center of this story is dopamine, a molecule released by neurons and long associated with pleasure — it shows up in headlines as the "pleasure chemical," triggered by everything from chocolate to social media likes. Kent Berridge and Terry Robinson, working together at the University of Michigan, spent three decades building the case that this popular label is wrong, or at least badly incomplete. Their work began in 1989 with a strange observation in rats. Rats, like human infants, make specific facial expressions when they taste something sweet — a distinct orofacial reaction, a kind of reflexive "liking" face — and a different expression when the taste is bitter. If dopamine were the brain's pleasure chemical, then destroying the dopamine system should have wiped out the sweet-taste "liking" reaction. It did not. Rats with their dopamine severely depleted still made the same pleased face at sugar water. What changed instead was how hard they would work to get it.

Berridge and Robinson named the distinction wanting and liking. Liking is the raw hedonic hit — the moment-to-moment pleasure of an experience once it is happening. It turned out to be carried by a smaller, more fragile set of neural systems, and it is not dependent on dopamine. Wanting is something else: what they termed incentive salience, the pull an object or a goal exerts on attention and effort before it is obtained. That pull — the craving, the seeking, the getting up off the couch — is what dopamine tracks. An animal, or a person, can want something intensely without liking it very much once they have it, and can like something without wanting to go to any particular trouble to get more of it. The two systems usually move together in ordinary life, which is exactly why they were mistaken for one system for so long. It takes damage, or addiction, or depression, to pull them apart and see that they were never welded together in the first place.

The wanting/liking dissociation is now, in the authors' own words from their most recent review, "widely accepted" as basic neuroscience — the genuine scientific argument that continues, mostly within Berridge's own later writing on what he has called the "debate over dopamine incentive salience," is about the precise boundaries of how incentive-sensitization theory applies to addiction, not about whether wanting and liking are separable systems at all. Berridge and Robinson kept elaborating the theory across three decades, formalizing it in a 2016 review and returning to it once more in a 2025 retrospective marking thirty years since the original split was proposed. That distinction matters enormously once it is turned toward a clinical question: what actually happens when someone loses the capacity for pleasure.

Anhedonia is the clinical word for that loss — an inability to feel pleasure, listed as a core symptom of major depression alongside low mood itself. For decades it was described, and often still is described colloquially, as a kind of numbness: things that used to taste good, or feel good, or look good, simply stop registering. Under that description, the natural mental picture is a volume knob for pleasure, turned down toward zero across the board. The wanting/liking framework raises an uncomfortable and testable alternative to that picture. What if the deficit in depression is not primarily in liking at all — not a flattened hedonic response to the good things when they arrive — but in wanting: a failure of the specific system that gets a person to go and obtain them in the first place, leaving whatever pleasure remains on the far side of a task most days do not get done?

Michael Treadway and David Zald, at Vanderbilt University, built a tool to test that question directly rather than by self-report. It is called the Effort-Expenditure for Rewards Task, or EEfRT, and it works like a simple economic choice repeated many times over. On each trial, a participant picks between an easy button-pressing task that pays a small, guaranteed reward, and a harder task that pays more but takes more physical effort and is not always rewarded, the reward's probability and dollar value both varying from trial to trial. The task does not ask anyone how much pleasure they expect to feel. It measures what they are actually willing to work for, trial after trial, under changing odds. In the original 2009 validation study, published that August, 61 undergraduates completed the task alongside standard self-report measures of trait anhedonia and mood. The result was a clear inverse relationship: the more anhedonia a person reported, the more often they chose the easy, low-reward option over the harder, higher-paying one. Anhedonia, measured this way, looked less like a broken pleasure detector and more like a broken accelerator — a specific unwillingness to spend effort chasing a payoff, visible in a behavioral choice rather than inferred from a mood questionnaire alone.

That finding, in healthy undergraduates with varying trait anhedonia, needed to be tested in people who actually had major depressive disorder, not just people scoring higher or lower on a mood questionnaire. Treadway returned to the question with John Bossaller, Ronald Shelton, and David Zald in a 2012 study comparing EEfRT performance in currently depressed patients against healthy volunteers. Depressed participants chose the effortful, high-reward option less often than controls — consistent with the earlier finding — but the study went further. Their choices were also less sensitive to information about how likely a reward was, and how large it was. In other words, it was not simply that they wanted less; their decision-making was less responsive to the reward's actual value, as though the signal linking effort to expected payoff had been dampened rather than the payoff itself feeling smaller once obtained. The authors framed this as a translational model of motivational anhedonia — a bridge word, meaning a laboratory measure in humans built deliberately to mirror the same mechanism first isolated in Berridge and Robinson's rats.

It is worth being precise about how far each of these two studies actually reaches on its own. The original EEfRT validation was run in 61 healthy undergraduates, scored on a continuum of trait anhedonia rather than diagnosed with depression at all — a demonstration that the task can detect a real behavioral pattern, not yet a clinical finding. The 2012 study is what carries the claim into the clinic, and it does so with a direct comparison between diagnosed, currently depressed patients and healthy controls rather than a further student sample. That is a meaningful extension, but it is still one clinical study generalizing a pattern first shown in a much smaller, non-clinical group, not a large multi-site clinical trial. The finding is solid enough to build a chapter on. It is not yet so heavily replicated that it should be read as beyond question.

Read together, these two literatures — one in rodents tracing a chemical circuit, one in humans measuring a behavioral choice — support a specific and testable account of depressive anhedonia, though it is worth being honest about the shape of the inference. No single study places a depressed human under a brain scanner, watches dopamine wanting-signals directly, and simultaneously runs the EEfRT in the same sitting. The bridge between the rat data and the clinical data is a synthesis, built across two separate research programs that were never designed as one continuous experiment. What that bridge offers is not proof of an unbroken chain from molecule to symptom, but a falsifiable claim standing in place of a vague one: that when a depressed person describes not wanting to do the things that used to matter to them, the deficit under investigation is not a numbness to pleasure so much as a specific failure of the system that converts a goal into pursuit. That claim can be tested further, and refined, or broken. What it replaces — the assumption that depression simply flattens all feeling evenly — could not be tested at all, because it was never precise enough to fail.

The engine-revving image from the start of this chapter has a specific edge worth keeping. A stalled engine and a disengaged clutch look identical from the driver's seat: nothing moves either way. The wanting/liking distinction is the mechanic's answer to that appearance — a way of asking, before assuming the whole machine is broken, exactly which part has stopped doing its job.

Overturned if: a well-powered clinical study finds that depressed patients' EEfRT effort choices track normally with reward probability and magnitude once mood state is properly controlled — showing the deficit tracks general symptom severity rather than a specific wanting/pursuit mechanism.

Sources

• 2016 · Berridge KC, Robinson TE, "Liking, Wanting, and the Incentive-Sensitization Theory of Addiction" — American Psychologist, PDF
• 2009 · Treadway MT, Buckholtz JW, Schwartzman AN, Lambert WE, Zald DH, "Worth the 'EEfRT'? The Effort Expenditure for Rewards Task as an Objective Measure of Motivation and Anhedonia" — PLOS ONE
• 2012 · Treadway MT, Bossaller NA, Shelton RC, Zald DH, "Effort-based decision-making in major depressive disorder: a translational model of motivational anhedonia" — PubMed