I think really digging into the neural nitty gritty may prove illuminative here. Dopamine release in nucleus accumbens (which is what drives reward learning and thus the updating of our predictions) is influenced by at least three independent factors:
1. A "state prediction error" or general surprise signal from PFC (either directly or via pedunculopontine nucleus and related structures). This provokes phasic bursting of dopamine neurons in the Ventral Tegmental Area.
2. The amount and pattern of GABAergic inhibition of VTA dopamine neurons from NAc, ventral pallidum, and local GABA interneurons. At rest, only a small % of VTA DA neurons will be firing at a given time, and the aforementioned surprise signal alone can't do much to increase this. What CAN change this is the hedonic value of the surprising stimulus. An unexpected reward causes not just a surprise signal, but a release of endorphins from "hedonic hotspots" in NAc and VP, and these endorphins inhibit the inhibitory GABA neurons, thereby releasing the "brake" on VTA DA neurons and allowing more of them to phasically fire.
3. It also seems acetylcholine may independently influence dopamine release in NAc independently of what's going on in VTA. This is less important for our purposes here, but it may help explain why cigarettes are addicting despite smoking not being particularly pleasurable.
Simplifying from 1 and 2 above, the unexpectedness of a stimulus affects the phasic firing rate of VTA DA neurons, and the hedonic value of the stimulus determines how many and which VTA DA neurons are allowed to phasically fire.
Now, what does the released dopamine do? In PFC (via the mesocortical pathway), it draws attentional resources to the surprising stimulus and its plausible causes, gating out the processing of other, less relevant stimuli. Simultaneously, in NAc, it strengthens connections between PFC inputs and the endorphin-releasing cells, thereby wiring together the hedonic features of the reward and the sensory features of any cues predictive of it. This imbues the cue with the ability to release the GABAergic brake on VTA DA neurons all by itself. Phenomenologically, it results in us "liking" the cue as much (or nearly as much) as we like the reward (this is what allows, e.g., animal trainers to reinforce behavior with only the sound of a clicker that has previously been paired with food).
But once the brain learns that a reward is reliably predicted by a cue, the reward ceases to elicit a surprise signal. This means it no longer increases VTA DA neuron firing rate. It may still cause endorphin release and thus keep the GABAergic brake off, but if there's no surprise signal driving phasic firing, dopamine release will be minimal.
That is to say: We still enjoy expected rewards; we just don't much *care* about our enjoyment of them. I don't think dopamine so much contributes a unique kind of happiness as it makes our happiness attention-grabbing, memorable, instructive, and motivating. *That* is what we lose when passionate love turns into companionate love.
The flipside of this is that we become very sensitive to unexpected *omissions* of reward. We take expected pleasures for granted as long as they keep coming, but woe betide anyone who suddenly threatens to take them away. This may add a certain kind of fragility to reliably pleasant relationships in the companionate love stage.
Abusive or otherwise volatile relationships keep partners engaged because they keep the good times unpredictable, thereby preserving their dopaminergic effects. Happiness on balance may be lower than in a more stable relationship, but partners over-learn from such happiness as there is, precisely because it is always surprising and thus significant.
But the physiological separability of the surprise signal and the pleasure signal suggests one may be able to keep a high baseline level of relationship bliss motivationally salient simply by being good to each other in surprising ways. So randomize (to an extent reasonable) gifts, dates, sexy times, vacations, and other fun things, along with their timing, and you should have at least some buffer against the decline of passionate love. Alas, this may be hard to do if your life is largely routinized by work, kids, or other commitments. It also needs buy-in from both partners or at least some degree of delegation to RNGesus.
I think really digging into the neural nitty gritty may prove illuminative here. Dopamine release in nucleus accumbens (which is what drives reward learning and thus the updating of our predictions) is influenced by at least three independent factors:
1. A "state prediction error" or general surprise signal from PFC (either directly or via pedunculopontine nucleus and related structures). This provokes phasic bursting of dopamine neurons in the Ventral Tegmental Area.
2. The amount and pattern of GABAergic inhibition of VTA dopamine neurons from NAc, ventral pallidum, and local GABA interneurons. At rest, only a small % of VTA DA neurons will be firing at a given time, and the aforementioned surprise signal alone can't do much to increase this. What CAN change this is the hedonic value of the surprising stimulus. An unexpected reward causes not just a surprise signal, but a release of endorphins from "hedonic hotspots" in NAc and VP, and these endorphins inhibit the inhibitory GABA neurons, thereby releasing the "brake" on VTA DA neurons and allowing more of them to phasically fire.
3. It also seems acetylcholine may independently influence dopamine release in NAc independently of what's going on in VTA. This is less important for our purposes here, but it may help explain why cigarettes are addicting despite smoking not being particularly pleasurable.
Simplifying from 1 and 2 above, the unexpectedness of a stimulus affects the phasic firing rate of VTA DA neurons, and the hedonic value of the stimulus determines how many and which VTA DA neurons are allowed to phasically fire.
Now, what does the released dopamine do? In PFC (via the mesocortical pathway), it draws attentional resources to the surprising stimulus and its plausible causes, gating out the processing of other, less relevant stimuli. Simultaneously, in NAc, it strengthens connections between PFC inputs and the endorphin-releasing cells, thereby wiring together the hedonic features of the reward and the sensory features of any cues predictive of it. This imbues the cue with the ability to release the GABAergic brake on VTA DA neurons all by itself. Phenomenologically, it results in us "liking" the cue as much (or nearly as much) as we like the reward (this is what allows, e.g., animal trainers to reinforce behavior with only the sound of a clicker that has previously been paired with food).
But once the brain learns that a reward is reliably predicted by a cue, the reward ceases to elicit a surprise signal. This means it no longer increases VTA DA neuron firing rate. It may still cause endorphin release and thus keep the GABAergic brake off, but if there's no surprise signal driving phasic firing, dopamine release will be minimal.
That is to say: We still enjoy expected rewards; we just don't much *care* about our enjoyment of them. I don't think dopamine so much contributes a unique kind of happiness as it makes our happiness attention-grabbing, memorable, instructive, and motivating. *That* is what we lose when passionate love turns into companionate love.
The flipside of this is that we become very sensitive to unexpected *omissions* of reward. We take expected pleasures for granted as long as they keep coming, but woe betide anyone who suddenly threatens to take them away. This may add a certain kind of fragility to reliably pleasant relationships in the companionate love stage.
Abusive or otherwise volatile relationships keep partners engaged because they keep the good times unpredictable, thereby preserving their dopaminergic effects. Happiness on balance may be lower than in a more stable relationship, but partners over-learn from such happiness as there is, precisely because it is always surprising and thus significant.
But the physiological separability of the surprise signal and the pleasure signal suggests one may be able to keep a high baseline level of relationship bliss motivationally salient simply by being good to each other in surprising ways. So randomize (to an extent reasonable) gifts, dates, sexy times, vacations, and other fun things, along with their timing, and you should have at least some buffer against the decline of passionate love. Alas, this may be hard to do if your life is largely routinized by work, kids, or other commitments. It also needs buy-in from both partners or at least some degree of delegation to RNGesus.